Biomedical Applications of Microencapsulation [1 ed.] 9780849354403, 9780367201036, 9780367202521, 9780429260469, 9780429536076, 9780429550775, 9780429522604

Table of contents :

1. An Overview of Pharmaceutical Applications


Louis Luzzi and Anthony Palmierri, III







2. Release Characteristics of Microcapsules


Joseph Robert Nixon




3. Biodegradeable Microspheres for Parental Administration


Curt Thies and Marie-Christine Bissery




4. Microencapsulation of Progesterone for Contraception by Intracervical Injection


Norbert S. Mason et al.







5. Artificial Cells


Thomas Ming Swi Chang




6. Free Thyroxine Assay: An Application of Microencapsulated Antibodies


Robert J. Buehler et al.




7. Reusable Microencapsulated Enzymes for the Clinical Laboratory


Richard D. Moss and Franklin Lim




8. Microencapsulation of Living Cells and Tissues – Theory and Practice


Franklin Lim

Citation preview

Biomedical Applications of Microencapsulation Editor

Franklin Lim, Ph.D. Associate Professor of Pathology Medical College of Virginia Virginia Commonwealth University Richmond, Virginia

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PREFACE

It has been said that the process of microencapsulation started with nature's creation of the first living cell. Man, on the other hand, did not consciously attempt to copy this ingenious process of nature until about 40 years ago. Often regarded by many as the founding father of microencapsulation technology, Barry Green of the National Cash Register Company (NCR), developed the first process of microencapsulation by coacervation, a process in colloid science first described by Bungenberg de Jong in the 1930s. Once started, the technology of microencapsulation slowly gained momentum, as more and more scientists and engineers entered this fascinating field and started producing new processes and new applications. Today, microencapsulation has truly come of age. Four International Symposia and one American Chemical Society symposium on mi­ croencapsulation were held in various parts of the world in the last 9 years. A wide variety of processes for producing microcapsules together with a large number of significant and new applications were described. However, as from the early days of development of this unique technology, truly detailed or in-depth presentations are still very much lacking in the scientific literature. Most of the technical information regarding the practice and appli­ cations of microencapsulation can only be found in the patent literature.* Applications of microencapsulation encompass a field as wide as the human imagination from graphic arts to agriculture to medicine. It would be extremely difficult to comprehen­ sively cover such a large variety of disciplines in a single volume. Therefore, for this volume, the publishers at CRC Press have chosen to present information on just one important area, namely, the biomedical field, where much progress in the application of microencapsulation has been made in recent years. Since one of the natural functions of microcapsules is to serve as microreservoirs, it is unavoidable, and at the same time not surprising, that some of the chapters in this volume deal with controlled or sustained release type of applications. In the first chapter, Professors Palmieri and Luzzi review the various types of microcapsules and their uses in the field of pharmaceutical sciences. A truly in-depth analytical characterization of drug releasing mi­ crocapsules is found in the chapter by Professor Nixon. Another analytical characterization dissertation is provided by Professor Thies and Dr. Bissery in the chapter on parenteral type of microcapsules. Presented in a short chapter is Professor Sparks’ description of a new type of contraceptive microcapsule which is injectable. The remaining four chapters deal with nondrug-releasing type of microcapsules which have semipermeable membrane. Professor Chang who pioneered the development of semipermeable microcapsules contributes a chapter where he reviews and updates his "artificial cells" research and development. The original ideas and developments of the work described in the last three chapters came from my laboratory. All of the work was supported by research grants from Damon Corporation of Needham Heights, Massachusetts. The chapter on the use of microencapsulated antibodies in radioimmunoassay systems represents the result of genuine collaborative research and development undertaking. When I first postulated my theoretical considerations of the free hormone (free thyroxine as the prime example) with all the dozens of differential equations, it was Dr. Moss, my research associate, who was responsible for the analytical solutions and elucidations through the computer technique of numerical integration. Subsequently, Dr. Buehler, formerly the director of research at Damon Diagnostics, took over the final phase of the development work including the clinical cor­ relation studies. Eventually this microencapsulated antibody project evolved into a com­ * 1. Gutcho, M. H., Microcapsules and Microencapsulation Techniques, Noyes DataCorp., New Jersey, 1976. 2. Gutcho, M. H., Microcapsules and Other Capsules, Advances Since 1975, Noyes Data Corp., New Jersey, 1979.

mercial product under the trade name Liquisol. The work described in the chapter on microencapsulated enzymes came from long-term studies involving 4-year-old “ used” mi­ croencapsulated enzymes. The last chapter, hopefully, contains sufficient technical infor­ mation on a new process of microencapsulation to enable both experts and novice in microencapsulation to microencapsulate any variety of living cells and tissues of their choice. I would like to thank Damon Corporation for the research support; Beverly Lockwood and Caroline Coe for their technical assistance and manuscript reading; and Linda Sylte and her word processing staff in our Pathology Department for typing most of the manuscripts. Franklin Lim, Ph.D. 1983

THE EDITOR Franklin Lim, Ph.D., is an Associate Professor of Pathology at the Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia. Dr. Lim graduated from Silliman University, Philippines, with a B.S. degree in Chemistry and an A.B. degree in Mathematics. He obtained his M.S. (1958) and Ph.D. (1960) degrees from Purdue University with majors in Biochemistry and minors in Physical Chemistry and Pharmaceutical Chemistry. Since 1960 Dr. Lim has been involved in the research and development of new approaches and innovations in laboratory diagnostic methodologies and instrumentation. Microencap­ sulation technology and other areas of biotechnology have occupied a major part of Dr. Lim’s professional activities in the last 20 years. Dr. Lim is the author of over 50 publications, presentations, patents, and pending patents in the fields of laboratory instrumentation, diagnostic procedures, clinical biochemistry, microencapsulation, and biotechnology. Dr. Lim was a runner-up in the Medicine and Science category in Next magazine’s selection of “The Most Powerful People for the 80s” in 1980. He was also one of the winners of the 1981 “Technology 100” awards from Technology magazine (now High Technology).

CONTRIBUTORS

Marie-Christine Bissery Laboratoire de Pharmacie Galenique Universite de Paris-Sud Paris, France Robert J. Buehler Damon Diagnostics Needham Heights, Massachusetts Thomas Ming Swi Ching, M.D., Ph.D., F.R.C.P.(C) Director Artificial Cells and Organs Research Centre Professor of Physiology and of Medicine Faculty of Medicine McGill University Montreal, Canada D. V. S. Gupta, Ph.D. Department of Chemical Engineering Washington University St. Louis, Missouri David W. Keller, M.D. Department of Obstetrics and Gynecology Washington University St. Louis, Missouri Franklin Lim, Ph.D. Associate Professor of Pathology Medical College of Virginia Virginia Commonwealth University Richmond, Virginia Louis A. Luzzi, Ph.D. Dean, College of Pharmacy University of Rhode Fsland Kingston, Rhode Island

Norbert S. Mason, Ph.D. Senior Research Associate Biological Transport Laboratory Department of Chemical Engineering Washington University St. Louis, Missouri Richard D. Moss, Ph.D. Department of Pathology Medical College of Virginia Virginia Commonwealth University Richmond, Virginia Joseph Robert Nixon, Ph.D. Senior Lecturer of Pharmaceutics Department of Pharmacy Chelsea College University of London London, England Anthony Palmieri Associate Professor of Pharmaceutics School of Pharmacy University of Wyoming Laramie, Wyoming Robert E. Sparks, D. Eng. Department of Chemical Engineering Washington University St. Louis, Missouri Curt Thies, Ph.D. Biological Transport Laboratory Washington University St. Louis, Missouri Robert S. Youngquist, D.V.M. School of Veterinary Medicine University of Missouri St. Louis, Missouri

TABLE OF CONTENTS

Chapter 1 An Overview of Pharmaceutical Applications..................................................................... 1 Louis Luzzi and Anthony Palmieri, III Chapter 2 Release Characteristics of Microcapsules............................................................................19 Joseph Robert Nixon Chapter 3 Biodegradable Microspheres for Parenteral Administration...............................................53 Curt Thies and Marie-Christine Bissery Chapter 4 Microencapsulation of Progesterone for Contraception by Intracervical Injection........... 75 Norbert S. Mason, D. V. S. Gupta, David W. Keller, Robert S. Youngquist, and Robert E. Sparks Chapter 5 Artificial Cells......................................................................................................................... 85 Thomas Ming Swi Chang Chapter 6 Free Thyroxine Assay:AnApplicationof MicroencapsulatedAntibodies........................... 105 Robert J. Buehler, Franklin Lim, and Richard D. Moss Chapter 7 Reusable Microencapsulated Enzymes for the ClinicalLaboratory................................... 119 Richard D. Moss and Franklin Lim Chapter 8 Microencapsulation of Living Cells and Tissues — Theory and Practice.......................137 Franklin Lim Index...................................................................................................................................... 155

1 Chapter 1 AN OVERVIEW OF PHARMACEUTICAL APPLICATIONS

Louis Luzzi and Anthony Palmieri, III TABLE OF CONTENTS

I.

Introduction...................................................................................................................2

II.

Examples of Pharmaceutical Applications...................................................................2 A. Analgesics......................................................................................................... 2 B. Steroids .............................................................................................................3 C. Vitamins............................................................................................................ 4 D. Sedatives and Hypnotics...................................................................................4 E. Antibiotics......................................................................................................... 5 F. Miscellaneous Pharmaceuticals....................................................................... 5 G. Aerosol Formulations........................................................................................7 H. Antidote and Artificial Kidney Uses............................................................... 7 I. Chemotherapeutic Agents............................................................................... 7 J. Prostaglandins...................................................................................................8 K. Radiopharmaceuticals ......................................................................................8 L. Veterinary Uses................................................................................................8 M. Health and Beauty Aids...................................................................................8 N. Summary...........................................................................................................9

III.

Encapsulation Methods as Applied to Pharmaceuticals..............................................9 A. Simple and Complex Coacervation.................................................................9 B. Polymerization................................................................................................10 C. Silicone Encapsulation...................................................................................11 D. Biodegradable Coatings................................................................................. 11 E. Mechanical Methods......................................................................................12

IV.

Clinical Studies and Significance of Microencapsulated Pharmaceuticals........... 12

V.

Possible Future Applications......................................................................................13

References.............................................................................................................................. 14

2

Biomedical Applications of Microencapsulation I. INTRODUCTION

The science and technology of microencapsulation is a good example of how one can extrapolate an application from a field other than pharmacy to pharmaceutical sciences. The first practical use of microencapsulation was the National Cash Register Company’s No Carbon Required (NCR) copying paper used as an improvement over carbon paper copying. In this process two dyes were coated with a clay and when ruptured the dyes formed a colored imprint. With this successful use of microencapsulation many researchers adapted microencapsulation techniques and the process to specific allied sciences such as agriculture, advertising, pharmaceuticals, oil industries, and food industry uses. Microencapsulation in pharmacy has many applications such as (1) obtaining solid entities from oils, (2) to control odor or taste, (3) to protect drugs from moisture, heat, or oxidation, (4) to alter solubility, (5) to delay volatilization, (6) prevent incompatibilities, (7) to handle toxic materials, (8) enhance flow characteristics, and (9) produce sustained or slow release medication. It is of course, the last-mentioned application, that of producing sustained-release or slowrelease dosage forms, where microencapsulation technology has had the major impact in the pharmaceutical industry. This chapter includes discussions on examples of pharmaceutical applications, microencapsulation methods used in pharmacy, and significant clinical appli­ cations of the technique. Many applications of microcapsules in pharmacy are not of practical use. Some techniques used often do not carry over economically and practically very well to large-scale manu­ facturing processes so that while the dose form may have potential use, the actual commercial use may be limited. Examples of both instances will be presented. II. EXAMPLES OF PHARMACEUTICAL APPLICATIONS

Since it became apparent that microencapsulation could be successfully used as a method to prepare sustained-release dosage forms, a large body of literature has been generated dealing with pharmaceuticals. These applications are in some instances practical, other instances esoteric. Both categories are discussed here with the understanding that the dis­ cussion is by example and cannot possibly be exhaustive. A. Analgesics Probably the product that has received the most attention is aspirin. As well as being one of the earliest encapsulated pharmaceuticals, it is the subject of many patents. It has been encapsulated using cellulose derivatives such as ethylcellulose and cellulose acid phthalate with the resultant product being insoluble in stomach fluid while soluble in the intestines.1 Boncey et al.2 produced water-insoluble microcapsules by using low-molecular-weight amino acids, sugars, and sugar alcohols, singularly and in combination. Two patents by Holliday et al., assigned to the Sterling Drug Company, encapsulate aspirin with ethylcel­ lulose.34 These researchers went to further describe in their patents the tableting process for the encapsulated drug as well as describing some of the comparative dissolution characteristics. Microencapsulated aspirin has also been shown to produce significantly less gastric ul­ ceration in rats than plain aspirin. Also, in human studies, aspirin microcapsules in older patients did not increase occult bleeding when given in doses of 3g for 4 consecutive days.5-6 Measurin®, a commercially available microencapsulated aspirin product has been the subject of extensive clinical investigation in adults and children.710 One of the major cited advantages of microencapsulated aspirin is a dramatic reduction in side effects for both short-term and long-term therapy. Aspirin presents a case of special interest to the microencapsulation scientist for a variety

3 of reasons. It is one of the many microencapsulations which resulted directly from a patient need, that is rheumatoid arthritis therapy. The sustained-release aspirin provides the clas­ sically constant release rate and is especially useful in providing steady blood levels of salicylate for nighttime relief. Microencapsulated aspirin also illustrates a problem for the pharmaceutical technologist in that due to its relatively large dose, the patient is required to take two tablets or capsules rather than one. Usually, this problem, or that of the relatively short biological half-life of aspirin would exclude it from being considered a good candidate for microencapsulation. Most analgesics have potential for microencapsulation because of the long duration of the desired therapeutic effect. Because of the need for a rapid onset, the dose form usually also contains a conventional, fast-acting amount of drug. Indomethacin, l-(/?-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic acid, an anti-in­ flammatory analgesic has been the subject of microencapsulation. Using an ethylcellulosepolyethylene system and also by complex coacervation, Morse has encapsulated indometh­ acin successfully claiming prolonged release and possibly less ulceration.12 Microencapsulation of indomethacin has recently been the subject of a bioavailability study in beagle dogs.13 In this study, Takeda et al. encapsulated the active ingredient in a gelatin-acacia system after the drug had been suspended in soybean oil. The authors concluded that the encapsulated product had better bioavailability than did the plain indomethacin. Unfortunately, many studies dealing with microencapsulation report only the technical method of encapsulation or the bioavailability when compared to the conventional dose form. The study by Takeda and co-workers reported both technique and bioavailability. B. Steroids As might be expected, drugs which are to be used as long-term therapy are prime candidates for microencapsulation. Contraceptive drugs are a good example of this technique. In 1979 Gupta et al.14 demonstrated that a progesterone microcapsule injection could be used as a sustained-release system in the cervix. The microcapsules were developed using poly(lactic acid) and the in vitro results were promising. In a further, more extensive report, Gupta and Sparks developed a mathematical model for in vitro release of these microcapsules15 and claimed that the model could be applied to in vivo situations if the external layer of the microcapsule was the rate-limiting step. After presenting a rigorous mathematical devel­ opment, the authors compared their predictions to the actual case as well as the classical Higuchi model16 for drug diffusion through a matrix. While allowing that their model did not predict exactly, they claimed superiority over the Higuchi method. The model proposed by Gupta and Sparks allowed for a diffusion boundary layer and more importantly a timevariant concentration whereas the Higuchi model assumed a sink condition and no diffusion boundary. Because of this, the superiority of their method when applied to this system was not surprising. As well as encapsulating steroids to prolong release, it may also be used to mask the taste of drugs. Calanchi reported that Eurand developed an encapsulated steroid product which was easily tolerated in a 5-mg dose whereas 200-pg doses of uncoated drug were unacceptable.17 The patent literature also contains examples of encapsulated steroids. Morishita et al.18 patented a method of microencapsulating water-insoluble drugs using gastric-soluble wall material. The wide variety of exemplary pharmaceuticals successfully encapsulated by this method included: cortisone, hydrocortisone, prednisone, dexamethasone, and testosterone derivatives as well as a variety of antibiotics. A novel method of encapsulating steroids was reported by Price and Palmieri.14 In that report, prednisone and hydrocortisone individually were suspended in oil and then microen­ capsulated using complex coacervation. In vitro dissolution profiles were obtained using 0.1 N HC1 and both systems displayed sustained-release characteristics. In a subsequent paper,

4

Biomedical Application of Microencapsulation

it was demonstrated that the prednisone microcapsules were also sustained release when exposed to conditions simulating the changing pH of the entire GI tract.20 The pH of the dissolution media was changed by adding varying amounts of powdered TRIS at different times in the dissolution process. C. Vitamins Microencapsulation of drugs is also done to those that are used to overcome deficiencies and as such must be administered for long periods of time in a relatively uniform dosage. Vitamins are a large class of drugs in this group which has received much attention. Since most of this class are relatively water insoluble, they lend themselves nicely to microen­ capsulation, especially by simple or complex coacervation. Vitamins are also encapsulated because of their usually unpleasant taste. One of the earliest applications of microcapsules for pharmaceuticals was an article by Luzzi and Gerraughty, in which while encapsulating various oils, they mentioned that encapsulation could be employed to protect vitamin A and K from moisture and light.21 One of the commercially successful applications is that of Diffuspan® by Eurand which combines encapsulated vitamin B complex and calcium pantothenate.17 The Diffucap® sus­ tained release dosage form of vitamin C was the subject of a comparative bioavailability study.22 When microcapsules of vitamin C were administered, maximum concentration of drug in the plasma (Cpmax) occurred at 2 hr, whereas when the encapsulated form was administered the Cpmax was 8 hr. Total absorption was approximately equal.22 As expected, much of the information on vitamin microencapsulation appears in the patent literature. Kondo and Nakano patented a process where ascorbic acid is microencapsulated using an oil and fat coacervation, a technique not often employed to produce microcapsules for pharmaceutical use.23 In a recent text covering microcapsule patents since 1975, Gutcho discusses this process.24 Briefly, the coacervation of oil and fat occurs at a temperature well below the dissolving temperatures of the chemicals involved. The coacervation occurs as the temperature is dropped and continues even after the system reaches room temperature. Riboflavin has been successfully encapsulated by preparing empty cellulose microcapsules and then filling them with riboflavin.25 This is another unusual technique of encapsulation of pharmaceuticals since the capsule can be prepared empty and then filled. Many of the vitamin oils are cohesive or “ sticky” and this can often cause a technical problem for the pharmaceutical formulator. An answer to this problem was proposed by Murakami et al.26 In this patent a method of producing vitamin A acetate microcapsules which are noncohesive is described. Calcium lactate pentahydrate is the encapsulating ma­ terial. Vitamin A has also been encapsulated in corn flour.27 In this patented process the encapsulating agent, corn flour, and vitamin A are mixed followed by gelatinization or polymerization of the encapsulating agent. Extrapolating from these examples it becomes apparent that most vitamins could be successfully microencapsulated by coacervation or polymerization for the purpose of improving stability, delaying or sustaining release or, as is often the case, for taste abatement. D. Sedatives and Hypnotics Sedatives and hypnotics are another pharmacological class of drugs that have received much attention from microencapsulators. This is especially true of the barbiturate subclass. Two of the earliest reports of microencapsulation use in pharmaceuticals used complex coacervation to sustain-release pentobarbituric acid and reported the effect of changing conditions during the encapsulation process.2*29 In a later study, sodium pentobarbital was encapsulated using nylon polymerization and when compressed into tablets, an inverse relationship was found to exist between tablet hardness and release rate of the sodium pentobarbital.30 In a similar study, Nixon et al., using ethylcellulose as the encapsulating

5 agent, successfully encapsulated sodium phenobarbitone and then tableted the material and studied dissolution characteristics.31 Spray polycondensation has also been used to microencapsulate phenobarbital.32The effect of pH, viscosity, and curing conditions were reported. Like many microencapsulation reports, only in vitro information, not in vivo information, was presented. This causes a void in much of the literature since in vivo data is greatly needed and these variables are not often discussed in the majority of microencapsulation reports. This lack of in vivo information may be attributed to the fact that the studies are conducted by physical chemists rather than clinical scientists. As an editorial note, it appears that this is one of the many areas where basic scientists and clinical scientists can interface with mutual benefit. E. Antibiotics Antibiotics are another pharmacologic class which has been encapsulated successfully. This group is most often encapsulated to improve taste. The sulfa analog series of this group has become a classic example. A 1967 paper by Nixon et al.33 discussed the preparation of microcapsules containing sulfamerazine using coacervation with gelatin, sodium sulfate, and ethanol. In a later report, Nixon and Walker, using similar simple coacervation technique, successfully encapsulated sulfadiazine and discussed the comparative dissolution of the microencapsulated form with the conventional dose form.34 Sodium sulfathiazole has been the core for nylon gelatin encapsulation.35 This report suggested an improved method to encapsulate water-soluble drugs and claimed a better process than that previously used by Luzzi et al.30 In a later report,36 the same authors discuss sodium sulfathiaziole, sodium salicylate, diazepam, phenytoin, morphine sulfate, and other drugs as to their partitioning behavior between various types of microcapsule systems and an organic phase. The spray-drying technique of producing microcapsules was used by Takenaka et al. to encapsulate sulfamethoxazole.37 Using ammonium solutions of sulfamethoxazole and cel­ lulose acetate phthalate, the system was spray dried and the characteristics were delineated. An important contribution of this paper was the development of a flow-type dissolution apparatus where the pH was changed continuously from 1.2 to 7.0 to approximate pH conditions the tablets and microcapsules would be exposed to in the GI tract. Ampicillin is a common antibiotic that has been successfully encapsulated.38 This report by de Sabata contained a discussion of a clinical study comparing ampicillin powder and two microencapsulated products. The encapsulated formulations exhibited less adverse ef­ fects, were better absorbed, and were more thoroughly absorbed from the GI tract. Seager claimed a patent for encapsulation of (3-lactam antibiotics using a spray-drying technique.39 The coating agents disclosed were hydroxypropylethylcellulose, polyvinylpyr­ rolidone, sodium carboxymethylcellulose, and gelatin. No bioavailability information was disclosed. Erythromycin is another often used antibiotic which appears in the patent literature on microcapsules.40 The reason for encapsulation of erythromycin derivatives is to prevent inactivation of the antibiotic by the gastric juices. Egg albumin was the coacervate material in this example and the patent claimed equivalent or better bioavailability of the drug when compared to conventional erythromycin ethyl succinate. F. Miscellaneous Pharmaceuticals Probably every pharmacological class of drugs has at least one example of a microen­ capsulated dosage form. Some of these examples are purely academic, some are in the transition from academic to practical, and some are totally in the practical (economically feasible) stage. Propranolhydrochloride, a (3-adrenergic blocking agent used to treat hypertension, has

6

Biomedical Applications of Microencapsulation

been encapsulated using ethylcellulose and plasticizer in an extrusion process.41 Digitoxin has been successfully made into a sustained release product by using a shellac, ethylcellulose, zein coating.42 The patent of Morishita et al.18 discussed earlier, claims a number of active cores including antibiotics, steroids, diazepam, phenacetin, hexobarbital, and folic acid as well as a number of others. This approach is quite common in the patent literature on microencapsulated pharmaceuticals since it is most often the process or the shell-wall material that is patented and these are suitable or claimed suitable for a number of drugs. Umezawa has patented a spray process for microencapsulation of pepstatin to be used in patients for long-term therapy in treating ulcers.43 The innovative aspect of these microcap­ sules is that carbon dioxide is produced causing the capsules to remain in the stomach for a longer period of time. Silica microcapsules have been reported as being successful in prolonging in vitro release of codeine.44 This report also discussed ephedrine, atropine, and chlorpromazine microcap­ sules. In the same text, Koishi and Kasai used ethylcellulose coacervation to prepare mi­ crocapsules of magnesium aluminum hydroxide as an antacid.45 Sodium salicylate has been encapsulated and is a good example of encapsulation for watersoluble medications.46 By using a capillary tubule the authors were able to prepare gelatin coacervate microcapsules. Interestingly, the hardened microcapsules exhibited first-order dissolution up to about 70% release of active ingredient, while the unhardened microcapsules did not follow first-order kinetics but rather approximated zero-order release. The same drug was also encapsulated by Deasy et al.47 These authors, using ethylcellulose and paraffin wax as a sealant, found release to be linear when graphed on a probability-log time scale. Simple coacervation, using gelatin and sodium sulfate, was used by Phares and Sperandio in one of the first reports of applying microencapsulation to pharmaceuticals.48 These re­ searchers claimed successful coating of riboflavin, cod liver oil, procaine penicillin G, carbon tetrachloride, charcoal, a micronized ion-exchange resin, castor oil, aspirin, and acetanilid. Since this was a very early application of the method, it is understandable that dissolution data were not presented. Eprazinone, a mucolytic drug, has been encapsulated using complex coacervation with gluteraldehyde as the cross-linking agent.49 This report was an important contribution to the literature, not only for the encapsulation of eprazinone, but for the development of equations relating coating thickness calculations to the diffusion of the drug through the microcapsule shell wall. Thiabendazole microcapsules have been prepared by Nixon and Hassan using the classical complex coacervation process.50 Encapsulation was used in an attempt to reduce the side effects of dizziness, nausea, vomiting, and hypersensitivity usually associated with the drug. Clofibrate has been the subject of extensive microencapsulation research in an attempt to mask the unpleasant odor and taste as well as increasing the release time of the drug.51-52 Phenacetin has been microencapsulated using egg albumin by the heat coagulation method.53 Potassium chloride, a commonly used electrolyte replenisher was encapsulated using various systems including cellulose acid phthalate, wax, and ethylcellulose in various combinations as well as the classical complex coacervation system.54 An extensive report by Nixon dealt with in vitro and in vivo evaluation of chlorothiazide microcapsules.55 Gelatin-acacia coacervation was employed and the encapsulated drug was then tableted. In vitro dissolution exhibited a classical pattern of initial rapid release due to some drug not being encapsulated followed by a slow steady release of chlorothiazide. In vivo dissolution was studied using 27 subjects each being given plain chlorothiazide tablets, microcapsules, or tableted microcapsules and a urine assay was employed. Both sustainedrelease forms were found to result in significantly less chlorothiazide in the urine than the

7 classical dose form. This indicated either retention of the encapsulated form in the body or very possibly it is an indication of poor absorption from the encapsulated form. Phenylpropanolamine and dextromethorphan have been encapsulated with an ion-exchange resin.56 Ethylcellulose was the barrier film studied and results indicated that desired plasma concentration profiles could be obtained by mixing different types and amounts of ionexchange resin. G. Aerosol Formulations While most applications of microcapsules are oral, topical, or parenteral, Feinstein and Sciarra microencapsulated dexamethasone for use in an aerosol formulation.57 After mi­ croencapsulation with ethylcellulose and formulation of the aerosol using the traditional method with fluorinated hydrocarbons, the drug was administered to rabbits for bioavaila­ bility comparisons. Microcapsules have also been successfully used to produce a sustainedrelease product for ophthalmic drugs as an ocular insert using biodegradable polymers.58 H. Antidote and Artificial Kidney Uses A potential major use of microencapsulation in pharmacy has been a carrier for antidotal drugs or in some cases as a “ one-way trap” for toxins. One such use is where a chelating compound is encapsulated within liposomes which help transfer the chelating agent across cell membranes.59 Chang has reported on use of microcapsules for patients with kidney failure, drug intox­ ication, and hepatic coma.60 Sparks et al. have also reported using microencapsulation techniques for the development of an artificial kidney.61 The removal of uremic wastes from biological systems have also been studied by Kim et al.62 The use of microencapsulated activated carbon for hemoperfusion was first proven by Chang63 as early as 1964 and has received much attention although this application has not yet proven a viable alternative to presently accepted methods of removing organic metabolites. The discussion of these tech­ niques here is limited since they are not true pharmaceutical examples but rather ancillary to this discussion. I. Chemotherapeutic Agents Because of extremely rapid clearance and toxic side effects anticancer drugs are good candidates for encapsulation. Chemotherapeutic agents have been encapsulated successfully. Microcapsules of methylglyoxal and derivatives have been prepared using gelatin and poly­ methyl methacrylate.64 The potential for ethylcellulose microcapsules of mitomycin, an anticancer drug was the subject of a study by Kato et al.65 Administered by percutaneous catherization, the chem­ otherapeutic agent was found to have an effective response rate of 77% along with a reduction in systemic toxicity effects. An interesting new application of microencapsulation is one where electromagnetic forces are used to concentrate the encapsulated drug at a specific desired site of action. Gordon described a method of coating 5-fluorouracil with ferromagnetic material which in turn is coated with ferric hydroxide as well as a method of using magnetism to direct the injected drug to a specific area.66 The antineoplastic agent, Doxorubicin has been prepared and encapsulated in magnetic microspheres for in vivo study.67 Using the hydrochloride salt in the drug, encapsulation was successful by emulsion polymerization and in vivo studies in rats gave promising results. Further study was done on this novel drug delivery system to determine specific cell binding using staphlylococcal protein-A magnetic microspheres.68 Undoubtedly, the use of magnetically directed microcapsules will receive great attention in coming years since it allows high therapeutic concentrations at targeted sites. This is es-

8

Biomedical Application of Microencapsulation

pecially important with drugs having a low margin of safety, since it still allows for a low, comparatively safe concentration throughout the remainder of the body. J. Prostaglandins Because of recent pharmacological advances, prostaglandins have also received attention as a core for microcapsules. Silicone rubber has been a primary coating media for this type of drug. The prostaglandins Dinoprost (prostaglandin F2°o) and Carboprost have been studied in encapsulated form for possible use in an intrauterine or vaginal dose form.69 Silicone rubber was also used to prepare silastic microcapsules of dexamethasone in what the authors claim was the first use of silicone rubber microcapsules.70 K. Radiopharmaceuticals Using the same rationale that lead to chemotherapeutic agents being good microencap­ sulation candidates, radioactive pharmaceuticals should then also be prime possibilities for encapsulation. A process of preparing parenterally administered radioactive drugs for di­ agnostic and/or therapeutic purposes was described in the patent literature by Evans.71 Zolle described a process using 123I-labeled insulin as a diagnostic tool72 wherein albumin was used as the encapsulating agent. As with the chemotherapeutic drug class, it is extremely likely that radioisotope drugs will find increased favor as a core for microencapsulation. L. Veterinary Uses Ancillary uses of microencapsulated pharmaceuticals are the veterinary, beauty aid, and food applications. Many veterinary uses of microencapsulation later result in human use of the same or similar product. The veterinary applications vary widely and include use as a food supplement for cattle,73 encapsulation of undecanovanillylamide as a coyote deterrent when sprayed on sheep or other livestock,74 and for administering drugs such as methinonine,73 ferrous sulfate,76 and tetracycline.76 A general discussion of the FDA interest in microencapsulated animal drugs is available.77 M. Health and Beauty Aids Health and beauty aids are areas where microencapsulation has helped the pharmaceutical industry. As well as use in cosmetics, many health related items have been improved by microencapsulation techniques. As with many important disclosurers, much of this infor­ mation is in the patent literature. Because of the problems sometimes associated with access to patent literature, the reader is directed to two excellent references for microencapsulation found in the patent literature.78 79 Both of these texts discuss patented methods of encap­ sulation as well as applications related to pharmaceuticals, veterinary applications, food, copying systems, laundry products, agricultural uses, pigments, and other lesser known uses. Uses of microencapsulation techniques for cosmetics has led to a variety of patents. A skin cream using an encapsulated base in an oil-in-water emulsion, wherein improved softness and smoothness was claimed has been patented.80 “ Wet-look” lipstick is now available because of oil containing microcapsules81 as are soaps that have encapsulated oil for a lubricating effect.82 Perfumes have also been encapsulated.83 84 Other varied uses that appear for health and beauty aids include microencapsulated nail polish remover83 86 aerosol containing hair dye,87 a sanitary napkin containing encapsulated deodorant,88 and encapsulated flavors for tooth polish,89 90 to name a few. Microencapsulation has also been used to produce dry microcapsules of flavoring oils for use in chewable tablets and other dose forms as well as being used as a protective coating to keep the oils from oxidation or other decompositions.91

9 N. Summary The period beginning with 1965 has seen the technology of microencapsulation applied to many sciences. It has become a science in itself and has transcended areas of technology that seemingly are unrelated. As for the pharmaceutical applications of microcapsules, there has been wide use and general acceptance of microencapsulation as a means of protecting drugs from their environment, preparing sustained-release medicinals, and as an important tool for ancillary uses such as dentifrices, cosmetics, and veterinary use. It appears that these present applications of microcapsules will continue to expand and to create more uses for this technology as it applies to pharmaceuticals. III. ENCAPSULATION METHODS AS APPLIED TO PHARMACEUTICALS

Pharmaceuticals have been encapsulated using many different types of shell-wall materials. The choice of encapsulation method is often dependent on the material characteristics. As well as the materials meeting the usual requirement of being able to encapsulate the drug, the encapsulating chemical must be nontoxic and if possible, biodegradable. Among other characteristics, it must be readily digestible or removable from the body, nonirritating, have no unpleasant odor or taste, and in most instances, it should not form a complex with the encapsulated drug. A. Simple and Complex Coacervation Coacervation was the earliest method of encapsulation for pharmaceuticals. The reader is referred to the classical text by Bungenberg de Jong for a detailed discussion on both simple and complex coacervation.92 According to Bungenberg de Jong coacervates can be considered either “ simple” or “complex” . If the reduction in solubility is not due to the change in charge on the mac­ romolecule but rather to nonionized forms, it can be considered simple coacervation. If the reduction in solubility is the result of salt bond formation, the process is considered to be complex coacervation. Bungenberg de Jong specifically compares the simple and complex coacervation process between gelatin and gum arabic. Simple coacervation can occur above the isoelectric point of gelatin when both colloids possess a negative charge, or below the isoelectric point when opposite charges exist. Simple coacervation of acacia and gelatin is only possible on mixing concentrated sols and coacervation disappears on addition of water. A water deficit in the total system is necessary for simple coacervation. Addition of indifferent salts (salts not entering into chemical reaction with the colloid) promotes simple coacervation in some cases and will never suppress the process. Simple coacervation is most often approached through two methods. First, if two colloids are employed, one would have a high affinity for water and be in a concentration from 20 to 40%. Alternatively, and more commonly, less concentrated colloid solutions are used and a strongly hydrophilic substance is added to cause two phases to be formed. The formation of a water deficit in the vicinity of the colloidal molecules is the major force behind simple coacervation. In discussing complex coacervation, Bungenberg de Jong92 makes the following obser­ vations. Complex coacervation can occur only below the isoelectric point of gelatin where the colloids have opposite charges and the opposing charge on the two colloids is the cause of coacervate formation. Complex coacervation will not occur in concentrated sols, but rather only after dilution with water. Other differences between simple and complex coacervation are that complex coacervation droplets will not exhibit a tendency to disintegrate in a direct current field, and in complex coacervation the addition of indifferent salts always suppresses the coacervation process.

10

Biomedical Applications of Microencapsulation

Although complex coacervation can be distinguished by the previously mentioned attri­ butes, it is primarily pH dependent. The classical explanation for complex coacervation was offered by Bungenberg de Jong. Complex coacervation of gelatin with acacia is only possible at a pH below the isoelectric point of gelatin since this type of coacervation is fundamentally due to salt bond formation. At values below the isoelectric pH, the gelatin containing both carboxyl and amino groups becomes positively charged, while acacia remains negatively charged containing only carboxyl groups susceptible to ionization. Simple chemistry dictates that the optimum pH for coacervation is that which renders equivalent opposite charges on the gelatin and acacia, since this is when the greatest number of salt bonds are formed. Lower than optimum pH values have been shown to reduce the charge on the acacia by suppressing carboxyl ion dissociation and therefore lower the coacervate volume. Indomethacin,12steroids,19 20 barbiturates,28-30 sulfa drugs,33-34 cod liver oil,48 procaine penicillin G,48 eprazinone,49 thiabendazole,50 clofibrate,51 52 and chlorothia­ zide,55 have all been encapsulated using simple and/or complex coacervation. While many drugs have been the choice for microencapsulation by coacervation, these systems do not often adapt well to large-scale manufacturing, are often difficult to reproduce, and are expensive processes. Because of this, coacervate microcapsules seldom are com­ mercially viable. B. Polymerization Nylon polymerization has been used extensively to microencapsulate drugs. Chang et al. used semipermeable nylon shells containing an erythrocyte hemolysate as an extra corporeal shunt which allowed plasma to pass through the shell and into the microcapsule for contact with the encapsulated material.93 Luzzi et al., have prepared nylon microcapsules of sodium phenobarbital.30The procedure used 25 m€ each of methylcellulose and sodium pentobarbital hexamethylene-diamine solutions; after mixing, 165 m€ of sebacyl chloride was added to complex the nylon production. The resultant microcapsules settled and then were spray dried or the wet slurry was subjected to flash evaporation. Sulfathiazole sodium was encapsulated using a modification of the nylon producing proc­ ess.35 In this process, the researchers used nylon to coat a drug-gelatin matrix and after treatment with formaldehyde, the capsules remained at 5°C for 24 hr to allow the micro­ capsules to harden. Jenkins and Florence prepared nylon 6-10 microcapsules using interfacial condensation of hexane diamine with sebacyl chloride.94 Using electron microscopy, the surface characteristics were investigated and the microcapsules were found to have a con­ tinuous structure and not a thin polymer membrane surrounding a liquid core as originally posited. These researchers later used the same method to encapsulate trifluoperazine as the embonate salt for use as a long-acting intramuscular injection as a tranquilizer.95 Polymerization was also used to encapsulate the narcotic antagonist, naltrexone, and its pamoate salt.96 Although no human studies have been reported, the author desired in vivo release for 30 days and used di-poly(lactic acid) as the encapsulating polymer. Using a twostep interfacial polycondensation reaction, Lim and Moss have produced microcapsules using a series of diamino compounds polymerized with sebacyl chloride or terephthaloyl chloride.97 These authors used the prepared systems to encapsulate enzymes. A potential use for mi­ crocapsules is as a detoxifying agent by concentrating aqueous solutions; the core material was prepared by polymerizing arylamide and methylene-bis-aerylamide. This material was then encapsulated successfully using a chemical interfacial method.98 Microcapsules have also been prepared by polymerization of poly (Na,N€-L-lysinediylterephthaloyl) which have potential as a barrier coating for pharmaceuticals.99 Gupta and Sparks used poly(lactic acid) capsules to sustain-release progesterone as a possible intracervical injection for fertility control.1415 While polymerization of polyamides often result in acceptable, economically feasible

11

processes, drugs which are affected by acid cannot be encapsulated by this method since in the polymerization process hydrochloric acid is released by the poly condensation reaction.63 Use of a reaction system that forms polyurethanes is usually an acceptable alternative for these acid-unstable drugs. An advantage of most polymerization reactions is that the film formed is usually very thin and durable, which is often not true with coacervate encapsulation. Polymerization using many different polymers, especially of the nylon series, has received much attention recently and will probably be the encapsulating system of choice for many more drugs. C. Silicone Encapsulation Porous silica as an encapsulating means has recently been investigated by Rupprecht et al.44 These researchers used methyl-polysilocane and n-octylmethyldichlorosilane to encap­ sulate codeine. Simply, the drug was dispersed in liquid poly(ethoxysiloxane) and hydro­ lytically polycondensed forming solid particles that were dehydrated and dried. Deng and Luzzi have also produced silicone rubber microcapsules to encapsulate dexamethasone.70 The capsules or microspheres as the researchers called them were prepared by first emulsifying the drug into the silastic polymer by simple admixture. The silicone polymer, drug, and filler were emulsified with silicone oil and stannous octate was added to vulcanize the mixture. Filtration followed and the resultant product was a tacky substance. The mi­ crospheres were categorized microscopically, sized, and the effects of Carbopol® and lactose on in vitro release were discussed. Prostaglandins have also been encapsulated using silicone rubber.69 Methylesters of Dinoprost and Carboprost were suspended in silicone monomer and then catalyzed with stannous octate. The mixture was then placed in a stainless steel mold and cured overnight at room temperature under an acrylic resin plate used to insure a smooth surface. Release profiles of the prostaglandins were developed and the release rate correlated with the lipophilicity of the selected prostaglandin in the silicone rubber. D. Biodegradable Coatings One of the newer promising types of microcapsule coatings are the biodegradable poly­ mers. For purposes of discussion, biodegradable coatings are those which are decomposed by their immediate environment. Preferably, the biodegradable chemicals are natural prod­ ucts. Along with the usual concerns with microencapsulating agents, in the case of biode­ gradable products, the toxicity of the biodegraded material is an important consideration. These types of polymers are especially useful for surgical implants since it is undesirable to have a second surgical process to remove the empty polymeric drug delivery device. Kim et al. discussed three classes of biodegradable polymers: (1) water-soluble polymers rendered insoluble by hydrolytically unstable cross-linking agents, (2) water-insoluble pol­ ymers that become soluble by hydrolysis but retain their backbone, and (3) water-insoluble polymers that become soluble by backbone cleavage.100 Poly(lactic acid) has received the most attention of this group, especially for release of contraceptives. For example, proges­ terone has been microencapsulated with poly-L-C-l-lactide and after a 5% suspension of the encapsulated drug was injected into rabbits, blood level studies showed steady release of the drug for 16 to 92 days.101 Release from biodegradable microcapsules is dependent on diffusion through the polymer and polymer degradation. If, during the desired release time, polymer degradation is con­ siderable, the release rate may not be predictable and may be erratic due to breakdown of the microcapsule wall. For this reason, the formulator desires an encapsulation system where the polymer is biodegraded much slower than the rate at which the drug is released.

12

Biomedical Application of Microencapsulation

E. Mechanical Methods A mechanical method is often the most commercially successful system for preparing drug microcapsules since the process often adapts well to large-scale manufacturing and is more reliably reproducible than other encapsulation techniques. Probably the best known of these methods is the Wurster fluidized-bed coating appara­ tus.102 106 Luzzi described the process as follows: ...that the apparatus may consist of a vertical, somewhat conical column. A gas, carrying the coating material, is introduced at the base or constricted part of the column at a velocity high enough to suspend the particles. The gas velocity in the Hared part of the column is greatly decreased, so the particles cannot be supported in this region and they fall outward and downward into the constricted region where they are again lifted by the gas How. The wall material is dissolved in a solvent (usually volatile) and is sprayed onto the supported particles, in a fine mist, from a nozzle located near the bottom of the column. The solution coats the suspended particles, and heated air drives off the solvent. When the particles are sufficiently dry, the air How is cut off and the coated product falls to the bottom of the apparatus for collection. The amount of wall material applied is generally proportional to the atomizing time, since the coating material is sprayed at a uniform rate and the particles are uniformly exposed to the spray. The time and air velocity required to coat the particles depend upon: (a) the starting surface area of the particles to be coated (the smaller the particle, the greater the surface area per pound of material to be coated and, therefore, the longer the time needed to coat); (b) the desired thickness of the coating; (c) the weight of particles coated per batch; and (d) the rate of How of the coating liquid.107

Hall and Pondell have discussed the Wurster process, applications, advantages, disad­ vantages, and the equipment in great detail.108 Aspirin is one of the many products coated by this process.109110 The Wurster process has also found application to food and feed industries, agriculture, and the chemical industry. Excellent drying conditions and the ability of the process to handle almost any shape particle as well as being able to encapsulate hydroscopic particles are the primary advantage of this system. Major disadvantages of the Wurster process are its inability to form capsules below 75 p,m as a final size, volatile substances are difficult to encapsulate, and liquids must first be converted to solids.108 Another mechanical method used counter-rotating disks with the inner disk producing droplets or small particles of the core. While the drug is pushed outward, the outer disk is lined with shell-wall material and coats the drug.111 This system has not proved commercially acceptable for pharmaceuticals. IV. CLINICAL STUDIES AND SIGNIFICANCE OF MICROENCAPSULATED PHARMACEUTICALS

While there have been numerous reports of successful microencapsulation in the literature, there are relatively few studies reporting on in vivo human pharmacokinetics. One can speculate a variety of reasons for this. First, since encapsulation applied to pharmaceuticals is still relatively a new discipline; the mechanics and possibilities of microencapsulation applied to drugs has not been fully explored. Because of this we find researchers still interested in applying encapsulation to different drug classes rather than attempting to refine applications already developed. Secondly, as discussed earlier, many of the procedures used to microencapsulate drugs do not adapt well to large-scale commercial processes either mechanically or economically and because of this the possibility of commercial application are not good. There is often a more economical process of preparing a sustained-release dose form than microencapsulation even though the process might not be as experimentally interesting as that of encapsulation. Another reason, correlated to the economic difficulties of producing microcapsules, is that of new drug application requirements of the FDA. Here, many pharmaceutical firms, especially in a “tight” economy find that the governmental regulations are burdensome and inhibit the development of new dosage forms. In short, the

13 economic feasibility as well as the scale-up problems often make microencapsulation a questionable economic process. Continuing the discussion with the above-mentioned in mind, a review of the literature shows that some clinically significant studies have been reported. Silicone capsules of four progesterone products were prepared by Lifchez and Scommenga for injection into rats.112 Analysis of the data showed good in vivo correlation with a modified Higuchi equation for release through a homogeneous matrix. As mentioned earlier, some clinical studies on microencapsulated aspirin have been reported.113 In this report salicylate blood level curves obtained after dosing with microencapsulated drug gave virtually flat curves as opposed to the “ saw-tooth” effect seen after repeated dosing with conventional tablets. Pancrelipase has been microencapsulated and has been used in congenital pancreatic insufficiency as well as insufficiency due to surgery.114 Marketed as Pancrease® (Johnson & Johnson), this product was found to be comparable or better than conventional tablets with no side effects.115 117 Microencapsulation of pancrelipase was employed since its en­ zymatic activity is destroyed by gastric juices. Encapsulation of potassium chloride for use as an oral potassium supplement has been successful in offsetting urinary loss of potassium during diuretic treatment.118 Micro-K Extentabs® (Robins) was administered in daily doses of 1 to 12 capsules for periods of up to 12 months and was well tolerated and effective in that potassium serum levels were normal. Eurand of Italy has developed a number of microencapsulated products and is employed by manufacturers to produce such products. This company has successfully microencap­ sulated aminophylline, marketed as “ Aminomal Retard” by Malesci, an Italian pharma­ ceutical company. Superior bioavailability was claimed for this product. Eurand has also successfully encapsulated potassium chloride, phenylbutazone, aminopropylon, ampicillin, nitrofurantoin, phenacetin, methaqualone, ASA, methionine, clofibrate, cod liver oil, disulfuram, dimethicone, ferrous sulfate, lithium carbonate, and caffeine, all of which are commercially available. de Sabata discussed microencapsulated vitamin C using urine and blood level data in humans finding that the encapsulated product, known as Diffucaps® gave absorption qual­ itatively and quantitatively similar to repeat dosing with raw material.38 The microencap­ sulated form gave a Cp ma* of 8 hr post dose while the plain dose form has a Cp max of 2 hr. de Sabata also discussed other published reports on the relative bioavailability of 2,5 isosorbide dinitrate as well as quinidine extentabs, ampicillin, and aspirin. In all cases, the microencapsulated products compared favorably with repeated doses of conventional drugs although he questioned the validity of some of the experimental design. Nixon has also prepared gelatin-acacia coacervate microcapsules of chlorothiazide and studied in vivo re­ lease, finding that significantly less chlorothiazide appeared in the urine when the drug was microencapsulated.55 While there may be other in vivo studies, the relative limited number of these studies indicates that there is much more research to be conducted in the clinical evaluation of microencapsulated products. While most drugs can be encapsulated successfully, the largescale manufacture is sometimes difficult and this often further limits the clinical study of microencapsulated dose forms. V. POSSIBLE FUTURE APPLICATIONS

While the pharmaceutical applications of microcapsules have been researched for more than 20 years, new encapsulation techniques have greatly increased the possibility for their use in dosage forms. As previously discussed, the use of microcapsules to deliver drugs to

14

Biomedical Applications of Microencapsulation

target sites by magnetization and to remove toxic wastes are applications currently receiving much attention from pharmacy researchers. While these applications have much potential it is too early to judge their full value. Certainly use of microencapsulation to deliver drugs for long-term therapy needs and for drugs administered to correct deficiencies in the body will enjoy continued success. In many instances, these techniques are already available and the process is one of simply finding economically feasible alternatives in the future. While the quest for new drugs continues, many companies are also researching novel dose forms and the use of microencapsulation techniques is certainly to increase. The relatively recent developments of “ superpotent” drugs, active in extremely small amounts, should increase the potential for microencapsulation since these drugs will allow for a physically smaller dose form. This will allow a larger dose to be incorporated into the same size capsule. The pharmaceutical applications of microencapsulation are many-fold in that almost any chemical compound can be encapsulated. While the question of encapsulation of a specific chemical often remains one of economics, microencapsulation has become a technique often applied successfully in the pharmaceutical industry.

REFERENCES

1. 2. 3. 4. 5.

Kitajima, M., Tsuneoka, Y., and Kondo, A., U S. Patent 3,703,576, 1972. Boncey, G. A., Hedge, M. J., and Henderson, J. R., U S. Patent 3,882,228, 1975. Holliday, W. A., Berdick, M., Bell, S. A., and Kiritis, G. C., U S. Patent 3,488,418, 1960. Holliday, W. A., Berdick, M., Bell, S. A., and Kiritis, G. C.,U.S. Patent 3.524,910, 1970. Lechat, P., Ganter, G., Gontagne, J., and Flouvat, B., Etude experimentale de tolerance gastrique d’une apirine en micrograins enrobes, Thercipie, 22, 403, 1966. 6. Vignalou, J. and Beck, H., Etude clinique et biologique de deux formes d’aspirine. Estimation de la tolerance gastrique, Thercipie, 22. 967, 1967. 7. Cass, L. J. and Frederik, W. S., A clinical evaluation of a sustained release aspirin, Curr. Ther. Res., 1,638,1965. 8. Bell, S. A., Berdick, M., and Holliday, W. M., Drug blood levels as indices in evaluation of a sustained release aspirin, J. New Drugs, 6, 284, 1966. 9. Gotoff, S. P., McCue, S. A., and Wendell, P. W., Sustained release aspirin in children, J. Pediatr., 73, 127, 1968. 10. Hollister, L. E., Measuring measurin: problems of oral prolonged medications, Clin. Pharmacol. Ther. 13. 1. 1972. 11 Rotstein, J., Estrin, I., Cunningham, C., Gilbert, M., Jordan, A., Lamstein, J., Safrin, M., Wimer, E., and Silson, J., The use of a sustained release aspirin preparation in the management of rheumatoid arthritis and osteoarthritis, J. Clin. Pharmacol., 7, 97, 1967. 12. Morse, L. D., U.S. Patent 3,557,279, 1971. 13. Takeda, Y., Nambu, N., and Nagai, T., Microencapsulation and bioavailability in beagle dogs on indomethacin, Chem. Pharm. Bull., 29. 264, 1981. 14. Gupta, D. V. S., Sparks, R. E., Mason, N. S., and Keller, D. W., Microencapsulation of progesterone for contraception by intracervical injection, in 4th Int. Symp. Microencapsulation, Key West, Fla., 1979. 15. Gupta, D. V. S. and Sparks, R. E., Mathematical model for progesterone release from injectable poly (lactic acid) microcapsules in vitro, in Controlled Release of Bioactive Materials, Baker R., Ed., Academic Press, New York, 1980. 189. 16. Higuchi, Mechanism of sustained action medication: theoretical analysis of rate of release of solid drug dispensed in solid matrices, J. Pharm. Sci., 52. 1145, 1963.

15 17. Calanchi, M., New dosage forms, in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker, 1976, chap. 7. 18. Morishita, M., Inaba, Y., Fukushima, M., Hattori, Y., Kobari, S., and Masutda, T., U S. Patent 3,960,757, 1976. 19. Price, J. C. and Palmieri, A.,Microencapsulation of drugs suspended in oil, preparation and evaluation of prednesone and hydrocortisone microcapsules, in Microencapsulation New Techniques and Application, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979. 20. Palmieri, A., Dissolution of prednisone microcapsules in conditions simulating the pH of the gastrointes­ tinal tract. Can. J. Pharm. Sci., 12, 88, 1977. 21. Luzzi, L. A. and Gerraughty, R. J., Effects of selected variables on the extractability of oils from coacervate capsules, J. Pharm. Sci., 53, 429, 1964. 22. Daniel, J. W., Capeland, I. D., and Rycroft, D., Vitamin C: crossover studies in human volunteers, in Life Science Research, Stock, Essex, England, 1974. 23. Kondo, S. and Nakano, H., U.S. Patent 4,102,806, 1978. 24. Gutcho, M. H., Microcapsules and Other Capsules, Advances Since 1975, Noyes Data Corporation, Park Ridge, N.J., 1979, 97. 25. Morse, L. D., Walker, W. G., and Hammes, P. A., U.S. Patent 4,123,382, 1978. 26. Murakami, M., Kawada, H., Ohmura, T., and Sugiura, H., U.S. Patent 4,013,773, 1977. 27. Katzen, S., U.S. Patent 3,962,416, 1976. 28. Luzzi, L. A. and Gerraughty, R. J., Effect of additives and formulation techniques on controlled release of drugs from microcapsules, J. Pharm. Sci., 56, 1967, 1974. 29. Luzzi, L. A. and Gerraughty, R. J., Effects of selected variables on the microencapsulation of solids, J. Pharm. Sci., 56, 634, 1967. 30. Luzzi, L. A,, Zoglio, M. A., and Maulding, H. V., Preparation and evaluation of the prolonged release properties of nylon microcapsules, J. Pharm. Sci., 59, 338, 1970. 31. Nixon, J. R., Jalsenjak, I., Nicolaidou, C. F., and Harris, M., Release of drug from suspended and tabletted microcapsules. Drug Dev. hid. Pharm., 4, 117, 1978. 32. Voellmy, C., Speiser, P., and Soliva, M., Microencapsulation of phenobarbital by spray polycondensation, J. Pharm. Sci., 66, 631, 1977. 33. Nixon, J. R., Khalil, S. A. H., and Carless, J. E., Gelatin coacervate microcapsules containing sulphamerazine: their preparation and the in vitro release of the drug, J. Pharm. Pharmacol., 20, 528, 1968. 34. Nixon, J. R. and Walker, S. E., The in vitro evaluation of gelatin coacervate microcapsules, J. Pharm. Pharmacol.. 23, 1475, 1971. 35. McGinity, J. W., Combs, A. B., and Martin, A. N., Improved method for microencapsulation of soluble pharmaceuticals, J. Pharm. Sci., 64, 889, 1975. 36. McGinity, J. W., Martin, A., Cuff, G. W., and Combs, A. B., Influences of matrixes on nylonencapsulated pharmaceuticals, J. Pharm. Sci., 70, 372, 1981. 37. Takenaka, H., Kowashima, Y., and Lin, S., Preparation of enteric-coated microcapsules for tableting by spray-drying technique and in vitro simulation of drug release from the tablet in GI tract, J. Pharm. Sci., 69, 1388, 1980. 38. de Sabata, V., Bioavailability from microencapsulated drugs, in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker. New York, 1976, chap. 13. 39. Seager, H., U.S. Patent 4,016,254, 1977. 40. Farahdich, B., U.S. Patent 3,922,379, 1975. 41. McAinsh, J. and Rowe, R. C., U.S. Patent 4,138,475, 1979. 42. Pescetti, A., U.S. Patent 3,939,259, 1976. 43. Umezawa, H.,U.S. Patent 4,101,650, 1978. 44. Rupprecht, H., Unver, K., Kramer, H., and Dircher, W., Controlled release of silica embedded drugs, in Microencapsulation, New Techniques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979, 107. 45. Koishi, M. and Kasai, S., Preparation of ethylcellulose coacervate microcapsules containing magnesium aluminum hydroxide hydrate and their application to acid-neutralization, in Microencapsulation, New Tech­ niques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979. 46. Madan, P. H., Jani, R. K., and Bartilucci, A. J., New method of preparing gelatin microcapsules of soluble pharmaceuticals, J. Pharm. Sci., 67, 409, 1978. 47. Deasy, P. B., Brophy, M. R., Ecanow, B., and Joy, M. M., Effect of ethylcellulose grade and sealant treatments on the production and in vitro release of microencapsulated sodium salicylate, J. Pharm. Phar­ macol., 32, 15, 1980. 48. Phares, R. E., Jr., and Sperandio, G. S., Coating pharmaceuticals by coacervation, J. Pharm. Sci., 49. Si-Nang, L., Carlier, P. R., Delor, P., Gazzola, J., and LaFont, D., Determination of coating thickness of microcapsules and influence upon diffusion, J. Pharm. Sci., 62, 452. 1973.

16

Biomedical Application of Microencapsulation 50. Nixon, J. R. and Hassan, M. A. M., The effect of pH on the release characteristics of thiabendazole microcapsules. Drug Dev. Ind. Pharm., 7, 305, 1981. 51. Madan, P. L., Madan, D. K., and Price, J. C., Clofibrate microcapsules preparation and release rate studies, J. Pharm. Sci., 65, 1476, 1976. 52. Madan, P. L., Clofibrate microcapsules.il. mechanism of release, Drug Dev. Ind. Pharm., 6, 629, 1980. 53. Ishizaka, T., Endo, K., and Koishi, M., Preparation of egg albumin microcapsules and microsphere, J. Pharm. Sci., 70, 1981. 54. Harris, M. S., Preparation and release characteristic of potassium chloride microcapsules, J. Pharm. Sci., 70, 391, 1981. 55. Nixon, J. R., In vitro and in vivo release of microencapsulated chlorothiazide, J. Pharm. Sci., 70, 376, 1981. 56. Raghunathan, Y., Amsel, L., Hinsvark, O., and Bryant, W., Sustained release drug delivery system. I. Coated ion-exchange resin system for phenylpropanolamine and other drugs, J. Pharm. Sci., 70, 379, 1981. 57. Feinstein, W. and Sciarra, J. J., Development and evaluation of a dexamethasone time-release aerosol formulation, J. Pharm. Sci., 64, 408, 1975. 58. Michaels, A. S., U.S. Patent 3,962,414, 1976. 59. Rahman, Y. E., U.S. Patent 4,016,290, 1977. 60. Chang, T. M. S., Semipermeable microcapsules as certificial cells: clinical applications and prespectives. in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976, 57. 61. Sparks, R. E., Mason, N. S., Goldenhersh, K. K., and Huang, W. N., Microcapsules for augmenting artificial kidney function, in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976, 113. 62. Kim, B. C., Choi, P. S. K., Baytos, W. C., and Gardner, D. L., Removal of uremic waste products from gastrointestinal tract using microencapsulated sorbents, in Microencapsulation, New Techniques and Applications, Kondo, T.. Ed., Techno Books, Tokyo, Japan, 1979, 377. 63. Chang, T. M. S., Semipermeable aqueous microcapsules, Science, 146, 524, 1964. 64. Nozawa, Y. and Fox, S. W., Microencapsulation of methylgloxal and two derivatives, J. Pharm. Sci., 70, 385. 1981. 65. Kato, T., Nemoto, R., Mor, H., Takahashi, M., Tamakawa, Y., and Harada, M., Arterial chemoembolization with microencapsulated anticancer drug, JAMA, 245, 1123, 1981. 66. Gordon, R. T., U.S. Patent 4,106,488, 1978. 67. Senyei, A., Reich, S., Gonczy, C., and Widder, K., In vivo kinetics of magnetically targeted low-dose doxorubicin, J. Pharm. Sci., 70, 389, 1981. 68. Widder, K., Senyei, A., Ovadia, H., and Paterson, P., Specific cell binding using staphylococcal protein A magnetic microspheres, J. Pharm. Sci., 70, 387, 1981. 69. Roseman, T. J., Larion, L. J., and Butler, S. S., Transport of prostaglandins through silicone rubber, J. Pharm. Sci., 70, 562, 1981. 70. Deng, C. and Luzzi, L. A., Silastic microspheres-preparation and evaluation, in Microencapsulation, New Techniques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979. 71. Evans, R., U.S. Patent 3,663,687, 1972. 72. Zolle, I., U.S. Patent 3,937,668, 1976. 73. Scott, T. W. and Hills, G., U.S. Patent 4,073,960, 1978. 74. Palmieri, A., Microencapsulation and dissolution parameters of undecenovanillyamide — a potential coyote deterrent, J. Pharm. Sci., 68, 1561, 1979. 75. Sibbald, I. R., Loughheed, T., and Linton, J. H., U. S. Patent 3,541,204, 1970. 76. Bavert, K. and Hoff, D., U.S. Patent 3,880,990, 1975. 77. Arnold, R. G., U.S. food and drug administration interest in microencapsulated animal drugs, in Microen­ capsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976. 78. Gutcho, M. H., Microcapsules and Microencapsulation Techniques, Noyes Data Corporation, Park Ridge, N.J., 1976. 79. Gutcho, M. H., Microcapsules and Other Capsules, Advances Since 1975, Noyes Data Corporation, Park Ridge, N.J., 1979. 80. Barnett, G., Gershaw, N., and Mausner, J. J., U.S. Patent 4,087,555, 1978. 81. Murphy, J. H. and Lieberman, G., U.S. Patent 3,947,571, 1976. 82. Jedzinak, J. E., U.S. Patent 4,124,521, 1978. 83. Kanda, M. and Saeiki, U.S. Patent 3,897,578, 1975. 84. Whitaker, J. G. and Crainick, V. A., Jr., U.S. Patent 3,578,482, 1971. 85. Charle, R., Zviak, C., and Kalopissis, G., U S Patent 3,729,569, 1973. 86. Charle, R., Zviak, C., and Kalopissis, G., U.S. Patent 3,686,701, 1972. 87. Charle, R., Zviak, C., and Kalopissis, G., U.S. Patent 3,691,271, 1972.

17 Charle, R., Zviak, C., and Kalopissis, G., U S. Patent 3.691,271, 1972. Grimm, J. E., Ill, U. S. Patent 4,071,614, 1978. Grimm, J. E., Ill, U.S. Patent 3,957,964, 1976. Nixon, J. R. and Nouh, A. The oxidation of microencapsulated oils, in Microencapsulation, New Tech­ niques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979, 313. 92. Bungenberg de Jong, H. G., in Colloid Science, Vol. 2, Kruyt, H. R., Ed.. Elsevier, Amsterdam, 1949, 244. 93. Chang, T. M. S., Macintosh, F. C., and Mason, S. G., Semipermeable aqueous microcapsules. I. Preparation and properties. Can. J. Physiol. Pharmacol., 44, 115, 1966. 94. Jenkins, A. W. and Florence, A. T., Scanning electron microscopy of nylon microcapsules, J. Pharm. Pharmacol., 25, 1973, 57. 95. Florence, A. T., Jenkins, A. W., and Loveless, A. H., Approaches to the formulation of a long-acting intramuscular injection, J. Pharm. Pharmacol., 25, 1973, 121. 96. Thies, C., Properties of injectable microcapsules containing pharmaceuticals in Microencapsulation, New Techniques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979, 143. 97. Lim, F. and Moss, R., A two-step room temperature interfacial polycondensation process for producing biologically active microcapsules, in Microencapsulation, New Techniques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979, 167. 98. Kiritani, M. and Watanabe, A., Water absorbing microcapsules for concentrating aqueous solutions, in Microencapsulation, New Techniques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979, 349. 99. Suzuki, S. and Kondo, T., Interaction of Poly (N*, N£-L-lysinediylterephthaloyl) Microcapsules with gelatin, J. Colloid Interface Sci., 77, 1980, 280. 100. Kim, S. W., Peterson, R. V., and Feijen, X., Polymeric drug delivery systems, in Drug Design, Vol. 10, Ariens, E. J., Ed., Academic Press, New York, 1980, chap. 5. 101. Gablenick, H., Ed., Annual Report Summary, NICHD Contract Summary, National Institutes of Health, Bethesda, Md., 1976. 102. Wurster, D. E., U.S. Patent 3,207,824, 1965. 103. Wurster, D. E., U.S. Patent 2,648,609, 1953. 104. Wurster, D. E., U.S. Patent 3,196,827, 1965. 105. Wurster, D. E., U.S. Patent 2,799,241, 1957. 106. Wurster, D. E., U.S. Patent 3,241,520, 1960. 107. Luzzi, L. A., Microencapsulation, J. Pharm. Sci., 59, 1970, 1367. 108. Hall, H. S. and Pondell, R. E., The Wurster process, in Controlled Release Technologies: Methods, Theory and Applications, Vol. 2, Kydonieus, A. F., Ed., CRC Press, Boca Raton, Fla., 1980, chap. 7. 109. Wurster, D, E., Coletta, V., and Rubin, H., Wurster coated aspirin.I. Film-coating techniques. J. Pharm. Sci., 53, 953, 1964. 110. Wood, J. H. and Syarto, J., Wurster coated aspirin.II. An in vitro and in vivo correlation of rate from sustained-release preparation,/. Pharm Sci., 53, 877, 1964. 111. Somerville, G. R., U.S. Patent 3,015,128, 1962. 112. Lifchez, A. S. and Scommenga, A., Diffusion of progestrogens through silastic rubber implants, Fertil. Steril., 21, 426, (1970). 113. Green, D. M., Tablets of coated aspirin microspheruiles: a new dosage form, J. New Drugs, 6, 294, 1966. 114. Cho, Y. W. and Aviado, D. M., Clinical pharmacology for pediatrics.I. Pancreatic enzyme preparations with special reference to enterically coated microspheres of pamcrelipase, J. Clin. Pharmacol., 21, 224, 1981. 115. Holschlaw, D. S., Fahl, J. C., and Kieth, H. H., Enhancement of enzyme replacement therapy in cystic fibrosis, Cystic Fibrosis Club Abstr., 20, 19, 1979. 116. Hosclaw, D. S., Long-term benefits of pH-sensitive enteric coated enzymes in CF, Proc. 8th, Int. Congr. Cystic Fibrosis, 1980, 19A, Canadian Cystic Fibrosis Foundation. 117. Weber, A. M., Caheldere, B., Roy, C. C., Fontaine, A., Dufour, O., Morin, C. L., and LaSalle, R., Effectiveness of enteric coated PANCREASE in cystic fibrosis children under 4 years old, Cystic Fibrosis Club Abstr., 20, 19, 1979. 118. Ryan, J. R. and Elliott, B. W., A multicenter trial of microencapsulated potassium chloride, Curr. Ther. Res., 29, 838, 1981. 119. Calanchi, M., “ Eurand studies on some advantages of microencapsulation” in Microencapsulation, New Techniques and Applications, Kondo, T., Ed., Techno Books, Tokyo, Japan, 1979, 357. 88. 89. 90. 91.

19 Chapter 2 RELEASE CHARACTERISTICS OF MICROCAPSULES

Joseph Robert Nixon TABLE OF CONTENTS

I.

Theoretical Considerations.........................................................................................20

II.

The Effect of In Vitro Dissolution Technique Design Factors on Release........... 25

III.

Release from Microcapsules......................................................................................26 A. Introduction and General Review.................................................................26 B. Studies on Specific Walled Microcapsules ................................................. 28 1. Gelatin-Walled Microcapsules......................................................... 28 a. Gelatin Precipitation..............................................................28 b. Complex Coacervation......................................................... 28 c. Simple Coacervation............................................................ 32 2. Cellulose-Walled Microcapsules..................... 35 3. Polymer-Walled Microcapsules........................................................ 40 4. Release From Other Microcapsules..................................................44

References................................................................................................................................47

20

Biomedical Application of Microencapsulation I. THEORETICAL CONSIDERATIONS

There are two possible release mechanisms for microencapsulated drugs; disintegration of the particle or diffusion through the microcapsule wall. This latter mechanism is considered in the present chapter. Where R, represents the rate of solvent penetration, R2 the rate of core solution, and R3 the rate of diffusion of the core solution through the microcapsule wall the overall rate of release, Rr, will be a function of all three values and can be represented by ( 1)

It is normally suggested that R3 is the rate-controlling step, but any of these factors could become the most important function. Takenaka et al.1 extended this concept to include a consideration of the static film which exists on the outside of the microcapsule and also attempted to account for the influence of pores developed when the microcapsules are compressed into tablets. In most instances microcapsules or formulations prepared from them may be considered as providing special cases of the normal drug dissolution mechanisms so that either a diffusion layer or interfacial model may apply. In the diffusion layer model dissolution occurs rapidly at the solid-liquid interface and the rate is controlled by diffusion of solute through the liquid diffusion layer. In the interfacial model it is the dissolution at a dissolving interface which is time limiting. Both these models were developed using nonmicroencapsulated drugs and, even without the complication of a microcapsule wall, more than one mechanism may apply, depending on the dissolution conditions. The normal starting point for most diffusion-based release studies is Fick’s first law of diffusion:

where dc/dt is the mass of solute which diffuses in unit time through a cross-sectional area, A, in terms of a concentration gradient dx/dl in the direction 1. D is the diffusion coefficient. In most attempts at detailed release kinetics from microcapsules it has been normal practice to regard the individual microcapsules as spheres of mean uniform diameter which do not change in size throughout the dissolution, which is assumed to take place under sink con­ ditions. Hence: (3) where A will be the area of the spherical microcapsule and C the concentration of core material. In the case of microcapsules because there is usually no change, or at most only an insignificant change, in the external surface area available for dissolution during the time of release of the contents an integrated Fick’s law equation may be written (4) where C is the original core concentration and Cc is the concentration achieved in the outer

21

solution. Provided A can be determined a plot of In (C - CJ vs. t will allow the diffusion coefficient D to be calculated. This assumes a monodisperse mierocapsule size. It has been shown that nonmicroencapsulated powders show a log-normal distribution2 and the dimi­ nution of diameter in such a powder is linear with respect to time so that the particle size distribution remains constant until the smallest particle dissolves. Once this condition is reached dissolution depends upon the diminishing number of particles. This cannot apply with microcapsules where the outer surface area remains substantially unchanged, but might be a complicating factor where a solid core inclusion is gradually dissolving within the microcapsule. However, under conditions of constant surface area the Noyes-Whitney, Nernst-Brunner, and Higuchi diffusional equations all show a first-order dependence on (C — C0) and may be exemplified by the Noyes-Whitney equation (5) where V is the volume of the dissolution media and h the thickness of the diffusion layer. When applying Fick’s laws to diffusion from microcapsules one of the problems is how far the barrier formed by the microcapsule wall will affect the rate of dissolution. The first law may be written 6

( )

where Cm is the concentration in the microcapsule, Ct the concentration in the dissolution medium, and h the thickness of the barrier. Under sink conditions this becomes (7) where Kt — AD/Vmh and Vm is the volume of the microcapsules. Integration of Equation 7 produces a first-order kinetic equation ( 8)

where C“ is the concentration in the microcapsules at time t. In this treatment no account has been taken of the partition of the core between the capsule wall and the fluid in the dissolution chamber and it is assumed to play a negligible part, but a term Kf could be included in the numerator of Equation 6 to account for this factor. It would appear that simple treatments using basic assumptions lead to a first-order kinetic concept in describing the release of microencapsulated core materials. Even so it is well to remember that, when applying any simple treatment, a number of assumptions and simpli­ fications have been made and that anomalies may be encountered. Thus when applying the Nernst-Brunner theory the apparent diffusion coefficient can be shown to be equal to D/Vh which leads to the calculation of very thick static films on the outside of the microcapsules.

22

Biomedical Applications of Microencapsulation

Nang et al.34 combined the Noyes-Whitney and Brunner equations and, by including a tortuosity and porosity coefficient, produced the expressions (9) ( 10)

where A's and As are the external surface areas of the microcapsule and static layer, e the porosity and tortuosity coefficient, h is the coating thickness, 8 the effective diffusion layer thickness, V the volume of solution, the solubility of the substance, and C the concen­ tration at time, t. It was suggested that microcapsules of the same radii would exhibit the same wall thick­ ness, but this assumption must be somewhat controversial as the hydrated wall thickness may be seen microscopically to vary in different parts of the same microcapsule. The wall thickness will also depend on the conditions under which the microcapsules were prepared. It must, however, be remembered that Luu and Carlier were dealing with thin-walled mi­ crocapsules. A more readily acceptable assumption is that the wall thickness and the Brunner layer are small and negligible in comparison with the radius of the microcapsule provided the entity is a thin-walled microcapsule containing a single liquid core. With core:wall ratios of less than 4:1 or with multicore microcapsules the assumption may not apply but where it does then the total release will be made up of components from the coating and layer so that (ID

where r is the mean radius of the microcapsules, p the density of the microcapsules, m the mass of n microcapsules, N is the agitation rate, and a and b are constants. Away from the static film it is possible to divide the effect into a contribution due to diffusion and a second due to the stirring rate and (

12)

where CGis the concentration contribution due to diffusion, which means that (13) and (14) Because of the tortuosity factor the type and thickness of the wall should control the rate of release from the microcapsules and, as might be expected, it should be possible to increase the time taken for release by increasing either the core radius or the thickness of the microcapsule wall. A somewhat different treatment of microcapsule permeability has been presented by

23 Takamura et al.,56 who studied the permeability of microcapsules to electrolyte. The equa­ tions developed depended upon the fact that within their system the electrolyte concentration remained constant and that during diffusion a uniform gradient would obtain. A plot of ln(Ct — C,/C() — Ct) vs.t was a straight line allowing the permeability of the microcapsules, P, to be calculated from (15) where S is slope, Vm the volume of the microcapsules, A the microcapsule surface area, A X the membrane thickness, D the apparent diffusion coefficient, Ct the concentration at time t, Ct the final concentration, and C, the initial concentration. Most models assume the core particle or microcapsule during dissolution will show little change in surface area, but in the case of micromilled core particles this may not represent a true picture as, while the outer shell may not vary in size, the radius and surface area of the individual encapsulated core particles may change considerably during the course of the dissolution. The overall dissolution of the core may still be controlled by the rate of passage of dissolved core material through the wall, but the rate of dissolution of the core itself would be considerably affected. Constant surface area conditions occur in the release of drugs from tableted wax matrices7 8 and under sink conditions the release pattern may be described by (16) where Q is the weight of drug released per unit surface area at time t, D is the drug diffusion coefficient, A the concentration of drug in the tablet, and Cs the solubility of drug in the release medium. In the case of a microcapsule wall this may be an heterogeneous mixture of core and covering material being so constructed as to give intergranular or intermolecular pores so that, as with the Higuchi matrix, tortuosity and porosity factors have to be included and the modified equation (17) becomes valid where e is the porosity of the matrix and t is its tortuosity. In the case of microcapsules this is a popular treatment of results and many systems give straight line relationships with t1/2 plots. However, this treatment appears to ignore the presence of a solid core and this type of kinetic treatment may only apply when a solid solution exists in the microcapsule wall. Even in the dissolution of materials from matrices it is possible that the above represents a simplification. Singh et al.9 found a lack of diffusion control; rather the release depended on the rate and extent of penetration of the solvent. These factors are difficult to quantify although, providing that the contact angle is greater than 90° the Washburn equation holds (18) In the case of solid core microcapsules a reduction in the core surface area will accompany release and, provided the rate of passage of the dissolution material out through the wall is

24

Biomedical Application of Microencapsulation

not rate limiting, this reduction in surface area might be expected to have an important bearing on the release kinetics. Dissolution under conditions of variable surface area has been described by a number of equations utilizing first-order or cube root plots. Wagner10 has reviewed many of these kinetic approaches both to in vitro and in vivo release but points out that when sink conditions exist the percentage dissolved at time t could be simply an artifact of the percentage surface area produced in that time. Dissolution could, therefore, be described by plotting the log percent dissolved against time when a straight line should result. Wagner showed that such data could be interpreted according to first-order kinetics, but that such plots were really fortuitous. Recently an attempt has been made by Dappert and Thies11to produce a theoretical equation for the kinetics of release from a single microcapsule. The model did not require concentric shell geometry as the wall is modeled as a lumped linear resistance to mass transport and is independent of conditions both inside and outside the microcapsule. This allowed the following relationship (19) where m,. is the cumulative mass of diffusent material eventually transported out of the capsule and equals m(t) as t approaches infinity. T is the capsule wall characteristic, de­ pendent on its geometry and diffusivity, Q and Ce the concentration at the internal and external regions of the wall, respectively, Q (O) is the original concentration of core in the internal wall region, and m and t are mass and time. This single microcapsule model was incorporated into a statistical model for release from normal microcapsule populations when the individual release rate is either constant or ex­ ponential. This treatment allows the individual microcapsules to differ in their release ki­ netics, but in a given population the individuals are assumed to have kinetics of the same functional form. For a constant rate ( 20)

and for exponential rate microcapsules ( 21)

where b is characteristic rate of a “constant” microcapsule, b is the average b of a population and k is characteristic rate of an exponential microcapsule. M*(t) dimensionless fraction of payload which has been released at time t. i[/ (x) is the probability density function for x, and k _ ^(f(k)) is the Laplace transformation mapping f(k) into F(t) rather than the more usual f(t)“ into F(s). This in turn allowed the following solution in the case of constant rate of release (

22)

25 II. THE EFFECT OF IN VITRO DISSOLUTION TECHNIQUE DESIGN FACTORS ON RELEASE

When the conditions, apparatus, and techniques used to study the in vitro release from microcapsules are reviewed, there is seen to be great variability; in many cases this precludes the easy correlation of results by different workers. Extensive reviews of dissolution testing have been published;10 12 however, it is appropriate to briefly summarize some of the factors which ought to be considered when designing the conditions under which the release of core materials from microcapsules is studied. The fact that microencapsulated material is in small discrete particles with a large surface area does not mean that the inclusion will automatically go into solution at a rapid rate. Factors such as stirring rate and the presence or absence of turbulence will influence the result. Of the many factors affecting dissolution agitation is one of the most important and at the same time one of the least understood. The existence and extent of the Nemst stationary film and whether Fickian diffusion conditions will exist can be influenced by the stirring rate. The empirical relationship (23) holds and when (3^1 and N is the agitation rate the process is diffusion controlled. In normal dissolution studies the diffusion rates depend on Tf, where x is between — 0.2 and — 0.8, but where the dissolution is interfacially controlled the viscosity would not normally play an important rate determining role. However, in the case of microcapsules, which are frequently surrounded by a viscous hydrated colloidal coating, the solution of dissoluted material has to diffuse through the viscous wall region before reaching the outer surface. Temperature effects are usually of the straightforward Arrhenious type, but here again careful control is required as certain microcapsule wall materials are soluble at slightly elevated temperatures or become excessively diffuse due to hydration. Normally for a dif­ fusion controlled reaction the 10° Arrhenious temperature coefficient approximates to 1.3, while for an interfacially controlled mechanism this value rises to approximately 2.0. With metastable polymorphic microcapsule cores the saturation concentration, Cs, will be larger than for the stable form and dissolution will be increased, but if the stable form appears in the dissolution solution then a situation may develop in which supersaturation conditions exist and the stable polymorph would be precipitated, possibly within the mi­ crocapsule wall environment, thus preventing further release. The effects of polymorphism on dissolution has been studied by Tawashi13 and Summers.14 When the core material reacts under differing pH conditions to form a soluble salt then dissolution will be rapid, while if there is no reaction then there may be no pH effect over a wide range of pH values. Where the inclusion is a weak acid salt, which is converted to the free acid under acidic dissolution conditions, this may result in the formation of an insoluble film within the outer wall and around the surface of the microcapsule preventing further release. The surface tension of dissolution media has been shown to considerably effect the rate release of many materials,913 probably due to the decrease in the interfacial tension between the surface of the dissoluting material and the solvent. Singh et al.9 showed that both matrix permeability and the rate of solvent penetration were affected and the same considerations would be true of microcapsule walls and the interspaces between compressed microcapsules. Singh et al.9 denoted the effective diffusion as Del} and in the presence of a surface active agent and provided the static film theory applied a log-log plot should have a slope of unity according to the expression24 (24)

26

Biomedical Applications of Microencapsulation

The wall thickness of microcapsules is another problem area and as will be shown later a number of equations have been derived to account for this factor or the release of core material. These only provide approximations and are only valid in the case of single core inclusions, preferably of a liquid nature. With microcapsules containing multiple cores or irregular solid cores an accurate wall thickness cannot be determined. Usually the wall is of varying thickness in different parts of a microcapsule. It must also be remembered that microcapsules collected in a solid form and subsequently hydrated during in vitro dissolution studies are unlikely to exhibit the same wall thickness to that measured from the same microcapsules in the liquid state prior to recovery. Luu3 has sliced microcapsules with a microtome knife and made direct measurements, but it is impossible to be certain to cut the capsule in the center and the method therefore gives no better results than the use of the equation by Takamura et al.5 6 (25) where w was the weight of microcapsule membrane in unit volume of dispersion assuming complete reaction of diamine and acid dichloride, r the experimentally determined percent reacted diamine, p the density of the microcapsules membrane, and Atot the total surface area of microcapsules in unit volume. For the thin-walled type of microcapsule being studied this gave an adequate average wall thickness. Whatever method is used no information appears to exist as to the effect of dissolution media on either wall thickness or changes in wall density. The hydration of coacervate films occurs rapidly so that permeation of solvent into the film would be equally rapid and result in a corresponding rapid buildup of a saturated concentration of core material within the wall so that R, of Equation 1 would become rate limiting. Dissolution rates are modified by the surface geometry of the dissoluting particle and for microcapsules this may be further aggravated by the very large surface area. When the variety of approaches and the factors that might influence the release are considered, it is obvious that no one dissolution rate test, or set of conditions, will be applicable to all microcapsule inclusions and that results obtained under specific experimental conditions might not correlate with those obtained for the same core material using a different experimental approach. No effort appears to have been made to discuss results obtained in relation to other work of a similar nature or to look closely at the effect of the experimental conditions utilized on the results obtained. III. RELEASE FROM MICROCAPSULES A. Introduction and General Review

Early studies on the release of inclusions from microcapsules were brief and usually provided little data on which the conclusions drawn had been based. Much early work was published in patent literature and, as such, was mainly concerned with preparation. Thomas15 outlined some of the factors which controlled the rate of release such as the wall-to-core ratio, the composition of the wall, the core solubility in the extracting medium, the presence of plasticizers, the microcapsule size, and the surface area. These themes were extended by Herbig,16 who pointed out that with liquid cores the capsule wall should be sufficiently impermeable to retain the core. As an example he itemized gelatin-type walled microcapsules of 500 |am containing 80 to 90c/c volatile core which were expected to lose less than \% of their contents per year at a temperature of 25° and a relative humidity of 509c. As an example of the effect of microencapsulating solid materials he quotes the lowering of reactivity of magnesium hydride with 10c/c aqueous HC1. Nonencapsulated material reacts rapidly whereas

27 cellulose nitrate walled microcapsules containing 92% magnesium hydride showed less than 10% reacted in 30 min. Bakan17also showed examples of release patterns obtained with liquid core microcapsules. Over a period of 2 years microcapsules of toluene, lemon oil, and carbon tetrachloride showed straight line relationships for log % remaining vs. time. For acetyl /?-aminophenol the release rates did not materially change when subsequently determined after a 2-year period. He suggested that for microencapsulated, water-soluble materials the release rate was a function of the wall permeability and that provided both wall and core are insensitive to pH change that changes in this function would not have any significant effect on the rate of release. He suggested that release could be quantified by the simplified first-order kinetic equation (26) where K was a constant which was inversely proportional to the wall thickness, C was the core concentration, and t was time. Bakan asserted that for a given microcapsule system, K was very reproducible. The previously quoted work gave little information as to the technique used in the prep­ aration of the microcapsules nor of the way in which the results were obtained, but Nack18 pointed out that the release of core contents is usually followed by means of some chemical change with a material adjacent to the capsule or a physical effect such as evaporation or wetting. The efficiency of release and mixing of co-reactants is important in the assay process. Calanchi19 pointed out that release may be by diffusion where the drug is soluble in water or body fluids and the wall insoluble or permeable. In this case the rate and pattern of release can be adjusted by modifying the composition and thickness of the wall material. Enzymatic digestion by pepsin or pancreatin and physicochemical dissolution of the wall may also occur. These two latter release mechanisms would depend mainly on the volume and com­ position of the GI fluids and temperature. General in vivo studies have been reported by Antonini20 and de Sabata.21 The former showed that cinnarizine in microencapsulated form was effective as a prolonged action antihistamine releasing 100% of its activity evenly over an 8-hr period, de Sabata was concerned with the release of patented microcapsule products of Eurand and does not in general give details of the wall materials, microcapsule size, or other parameters which may control the release. Thus in a study of microencapsulated vitamin C it is suggested that microencapsulated vitamin showed a maximum blood level after 8 hr as against 2 hr with raw vitamin C and that total absorption and blood levels are as good as in multiple dosing. Other studies on Eurand products were for a sustained release dosage form of 2,5, isosorbide dinitrate, quinidine, ampicillin trihydrate, and aspirin. All showed a sustained release effect when microencapsulated, the nonmicroencapsulated drug being more rapidly available, but over a period the total drug availability from area under the curve calculations of different formulations would appear to be the same. Gayot et al.22 have recently looked at the permeability of gelatin, cellulose acetate, ethylcellulose, and degalan films to glucose, uric acid, urea, and creatinine in a simple dialysis cell with a view to their applicability to microcapsules. The gelatin films were the most permeable and ethylcellulose the least permeable to water. As is to be expected an increase in membrane thickness decreased permeability to creatinine, glucose, and uric acid. However, the passage of urea across the thicker membranes was slower, but eventually the same amount of urea crossed the membrane.

28

B iom edical Application o f M icroencapsulation

B. Studies on Specific Walled Microcapsules

7. G elatin-W alled M icrocapsules Gelatin-walled microcapsules may be prepared by a number of means. Thus simple precipitation of denatured gelatin round the core, simple coacervation of gelatin using ethanol or electrolyte solution and complex coacervation using gelatin and gum acacia have all found their place in the preparation of microcapsules. a. Gelatin Precipitation Tanaka et al.23 studied the effect of formalin treatment on the hardness of gelatin micro­ capsules of sulfanilamide and riboflavin prepared by a gelatin precipitation technique. Im­ mersion of sulfonilamide microcapsules in 10% formalin for 24 hr resulted in a tenfold increase in the time required for 100% release of the drug. They found these in vitro results were mirrored in the in vivo release when administered to dogs and the blood level vs. time measured. Paradissis and Parrott24 used the above technique to encapsulate aspirin. Using the equiv­ alent of 124 mg of drug they followed the urinary excretion. In the initial 8 hr the amount excreted and the rate were less than for an equivalent dose of powdered aspirin. The constant slope obtained from 6 to 24 hr reflected a fairly constant release typical of a sustained release formulation. The release of sulfadiazine (455-mg dose) was also reported. Again a less than normal amount was excreted when microencapsulated drug was used. A steady release rate was obtained for the period 6 to 14 hr. b. Complex Coacervation The Tanaka23 technique produced a pellet rather than a true microcapsule and the most common method of producing microcapsules with gelatin walls is by the gelatin-gum acacia technique pioneered by Green.25 Some of the earliest release studies using this technique were by Luzzi and Gerraughty.26 28 In their earlier study26 the core material was a series of oils, and they found that the saponification value had no effect on release but that the amount of oil released was directly related to the acid value. Surface active agents in either the oil or water phase during preparation caused release problems due to interference with microen­ capsulation. The authors suggest that the interference resulted in the formation of an incom­ plete shell due to adsorption at the oil-water interface. Luzzi27 28 also studied the release of a solid core, pentobarbital, from this coating. As the ratio of core:wall increased from 0.66 to 3.33 to 1 the percentage released over a 2.5-hr period rose from 42.7 to 74.3%. Unlike the results reported by Herbig16 and Bakan17 which indicated a completely linear release, Luzzi found that the bulk of the previously reported increase occurred in the initial stages of dissolution. It was suggested that this indicated a greater retaining power in the case of low ratios due to “ multiple droplet” thickness of the walls or the attraction of empty capsules to filled capsules. They found that after 2.5 hr approximately 54% of pentobarbital had been released at pHs between 5.3 and 7.0 with a maximum variation of 1.5. At pH 5.0 the release was slightly higher, 57.8%. In all cases approximately half of the release occurred in the first half-hour. The variation in release with respect of pH was small enough to suggest that this factor played little part in the release of pentobarbital. A minimum release rate at 37° was found when temperature effects were studied, but unfortunately the pH is not stated and the percentage released did not appear to correlate with the “effect of pH” results quoted in the paper. The total variance of release pattern with respect to temperature was far greater than for the pH effect and in particular caused a great increase in the amount extracted during the first half-hour in gastric juice. It was also found that during the initial exposure to either gastric or intestinal fluids the percentage of pentobarbituric acid released was greater in those samples which had received the largest amount of the formaldehyde. The data, as published, appears to indicate

29 that with intestinal fluid the percentage dissoluted in the first hour followed no standard pattern and showed a minimum at 8% formalin and a maximum at 20% for release into gastric juice. The results of total dissolution over the 2.5-hr period also showed a minimum at 8%. It was suggested that at lower formaldehyde percentages the increased release may be due to incomplete denaturation of the gelatin-gum acacia wall and that at high formal­ dehyde concentrations cracking of the wall may occur due to too much denaturation. How­ ever, the technique of extracting the final microcapsules may have resulted in broken microcapsules at both high and low formaldehyde concentrations. Luzzi29 later extended the study to include the microencapsulation of waxy solids and found that the method of prep­ aration had to be modified. Nixon and Nouh,30 studying the oxidative stability of microencapsulated oils, found that while the rate of oxidation depended on the bulk droplet size rather than surface area, there was inevitably some diffusion of oil through the microcapsule wall possibly leading to an oxidizable monolayer on the outside. However, it was the permeability of the wall to oxygen and the consequent oxidation in the microcapsule interior which was important. Nixon31 was also concerned with the stability of the gelatin forming the microcapsule wall and found that acid-pretreated gelatin solution was not subject to the growth of liquifying bacteria and did not break down on storage. From this viewpoint it may be used in the manufacture of microcapsules for up to 14 days after initial preparation without any change in microcapsule release properties. Alkali-pretreated gelatins were found to allow rapid bacterial growth and produced breakdown of gelatin which affected release. Takenaka et al. 32 used the Higuchi7 model to characterize the release of sulfamethoxazole. Linear correlations were obtained up to 60 to 80% release. They found the 50% in vitro dissolution time to depend on wall thickness, suggesting that diffusion into or out of the microcapsule may be controlled by wall thickness. Compared with Luzzi and Gerraughty28 they found that an increased time of treatment with formaldehyde delayed release. The calculated apparent diffusion coefficient varied between 1.63 to 283 x 10 “9 cm2/sec and depended on the coacervation pH and the amount of formaldehyde used. The method of calculation was based on the slope of the linear plot obtained by a percent drug dissolved vs. t12 using the equation (27) where Cs is drug solubility in the external phase, A the total amount of drug present in the matrix per unit volume, D the diffusion coefficient and Sv the specific surface area (cm2/ cm3). They found an apparent increase in the tortuosity of the microcapsule wall with increasing pH, with thicker walls and with increased amounts of formaldehyde. The cal­ culated tortuosity was found to vary between 0.86 and 11.4 x 102. These workers33 also found that the size and wall thickness was a log normal distribution and that as the pH increased the size decreased while formalized microcapsules, as previously reported by Nixon,30were larger than unformalized ones. The form of the inclusion, as previously pointed out, will affect the rate of drug release and when microcapsule slurries were spray dried the crystalline sulfamethoxazole was changed into the amorphous form. If the microcapsules were dried in the conventional manner the core material remained crystalline. These workers also determined the porosity of their microcapsule wall using (28) where e is porosity, Cg weight percent gelatin, Cs weight present drug, ps density of drug, pg density of gelatin, and pm the density of the microcapsules.

30

B iom edical Applications o f M icroencapsulation

Nixon and Harris34 produced microcapsules of medroxyprogesterone acetate and showed that while solution of the nonmicroencapsulated hormone was rapid into organic solvents, there was a significant slowing in the release of the microencapsulated product. Microcapsules showed approximately the same percentage saturation in both water and isopropanol with respect to time. As usual with gelatin-gum acacia-walled microcapsules there was a rapid release of a small amount of imperfectly microencapsulated drug, but a log percent release vs. time plot gave a straight line relationship for dissolution of the remainder of the drug. Straight line relationships could also be obtained for Higuchi7plots. Microcapsules containing smaller concentrations of hormone appeared to release their contents most rapidly. A sur­ prising result was that while there was no difference in the in vitro release rate at temperatures of 37 and 40° the release at 25° was faster. It was suggested that greater hydration at the higher temperatures caused swelling of the coat, resulting in the blocking of pores, while the formalin treatment used in microcapsule extraction prevented complete wall solution. Nixon and Harris35 later prepared both solid and liquid cored hormonal microcapsules con­ taining norethisterone. Irrespective of core type the size distribution of the microcapsules was the same having a mean size of 25 |Jim with 75% between 15 and 50|xm. With solid cored microcapsules dissolution was rapid and saturation of the in vitro dissolution media occurred indicating that in in vivo conditions too high a concentration of drug might result which suggests that the liquid cored microcapsules might prove of more medical interest. The radioactive tracer technique employed suggested that approximately 40% of the drug was rapidly released, but that the remainder released very slowly. Capsules which had been prepared for 2.5 years showed an aging effect in that the amount initially released was greater, but the slow release of the remainder of the drug was approximately the same rate as with freshly prepared microcapsules. This slow release was maintained for up to 100 hr. Price and Palmieri36 used an oil carrier for their core materials. Comparison between different prednisone formulations showed the fastest release from powdered drug. A study of the time of hardening the microcapsules on the t5() release appeared to show a linear relationship. The effect of hardening time appeared to be on the initial lag period, probably due to the differences in the hydration of the walls. Once the initial period was passed the latter portions of the percentage released vs. t12 plots were nearly linear and parallel. Dissolution profiles for hydrocortisone were similar to those for prednisone. The initial release was lowest with an encapsulated emulsion system having a lag (or hydration) period of about 30 min. Directly microencapsulated hydrocortisone showed a somewhat greater initial availability than a microencapsulated oil system but a longer hydration time of about 90 min. The release of thiabendazole has been studied by Nixon and Hassan37 38 dissoluting their microcapsules into 21 of dissolution media under standard conditions of temperature and stirring speed and adjusting assayed samples to pH 2.2. At pH of 2.0 the dissolution was rapid and 90% of drug had been released in 45 min while at pHs of 7 and 10 there was a definite retardation and after 4 hr approximately 60 and 50% had been, respectively, released. However, even at these higher pHs the initial release was rapid and 30% at pH 7 and 25% at pH 10 had been released after 45 min. This pH dependence of release from microcapsules has been shown many times,3g42 particularly when the solubility of the drug inclusion is pH sensitive. More surprising was the fact that as the core:wall ratio increased the percentage release was lower. It is possible that with larger quantities of thiabendazole in the micro­ capsules aggregation of the core particles occurs and presents a relatively smaller surface area for dissolution. Aggregation of the individual microcapsules also occurred and this, in turn, could lead to a smaller available surface area for dissolution. Hardening of the micro­ capsule walls produced significantly slower rates of release. The effect of compressing the microcapsules into tablets was to produce a much slower rate of release,38 but there was still an initial rapid release of 35% drug in 15 min at a pressure of 5000 kg and 60% release

31 at a pressure of 500 kg. After 15 min the release rate slowed and was approximately constant. The slower initial release rate at higher compression pressures was probably due to the increased difficulty of fluid penetration between the compressed microcapsules. Without formulation additives the compresses of microcapsules began to disintegrate after 10 min. There is also the possibility that higher compression pressures might cause damage to individual microcapsules, but the release parameters do not show this. When stearic acid was added to the microcapsules the disintegration time was increased. The effect of stearic acid was to produce a slower dissolution time with no break in the release curve corresponding to disintegration of the tablet. Sodium stearate addition slowed the release to a greater extent than stearic acid and also resulted in a greater disintegration time. While there was no break in the dissolution profile corresponding to the tablet disintegration there was a gradual slowing of the dissolution rate as the drug in the system was depleted. In the later stages release appeared to be independent of the sodium stearate concentration. Starch used as a disintegrant caused a rapid breakdown of the tablet irrespective of the presence of sodium stearate. Release rates, due to rapid disintegration, were independent of starch concentration and similar to uncompressed microcapsules. Chalk, as a filler, caused little effect on the release or disintegration time for a given compression pressure. Because of the nonbinding nature of gelatin-gum acacia microcapsules it appears that the presence of additives will be necessary in tableting and their effect on dissolution appears to be complex and interrelated. Nixon and Agyilirah43 investigated the effect of colloidal proportions used in preparing the microcapsule wall, taking phenobarbitone as their core material. The presence of excess gelatin was found to result in an increasing tendency for the microcapsules to become embedded in a gel of noncoacervated gelatin on cooling while excess acacia led to aggre­ gation. Much of the excess acacia appeared to be trapped within the microcapsule wall and was leached out during dissolution to increase the rate of drug release through the now very porous wall. Release characteristics followed a normal pattern and t1/2 plots showed straight line relationships up to about 90% release once the rapid release initial portion was passed. The higher the total percentage of colloid the slower was the release rate. However, the authors found that smaller individual microcapsules occurred at these higher colloidal con­ centrations and these would have an overall larger total surface area. Microscopic examination showed these microcapsules to have thicker walls and contain fewer individual core particles. An oil suspension of indomethacin was studied by Takeda et al.44 and, as with the work of Price and Palmieri,36 was found to slow down the release of the core. The in vitro t50 values for the microencapsulated and nonmicroencapsulated drug was respectively 20.1 and 10.1 min. The in vivo release pattern of indomethacin in beagle dogs was compared with a soybean suspension of the drug and the untreated drug. The microcapsules and oily suspensions gave higher serum concentrations, possibly due to enhanced absorption from oily dispersions. A double maxima phenomenon was observed in the serum concentration curve of microencapsulated indomethacin, but its origin was not explained. Nixon4S developed a chlorothiazide microcapsule for use in conjunction with potassium chloride microcapsules prepared by Harris46 for use as a combined diuretic and potassium supplement with a view to lowering ulceration of the gut. All in vitro studies showed the usual pattern of release. At pH above 5 approximately 50% of the drug was released in 20 min, but at pH 2 up to 90% of the drug had been released in this time, so it is probable that in vivo all the drug would be released in the stomach. As these microcapsules had been formalin treated it would appear that this did not produce a really effective enteric coated microcapsule. This had been previously observed in work on simple gelatin coacervation.47 A linear relationship existed between the percentage of drug present and the quantity released after 30 min, but after 1 hr the original drug content appeared to have little effect on the total percentage of drug released. A much slower release from “ microcap. tablets” made at compression pressures of between 0.5 and 5.0 was found. Unlike the tablets of thiaben-

32

B iom edical A pplication o f M icroencapsulation

FIGURE 1. Effect of pH and compression pressure on the in vitro release of chlorothiazide from tableted microcapsules. The concentration of chlo­ rothiazide was 65.48% in a charge of one 250 mg tablet in 2 € of water at 37°. Key # , ■ . pH 2; O, A, □ , pH 9; O, • , 0.5 t of pressure; A, 3 t of pressure; and ■ 5 t of pressure.

dazole prepared by Nixon and Hassan38 the chlorothiazide microcapsule tablets did not disintegrate although the wall material was still gelatin-gum acacia. The tablets still showed a rapidinitialrelease of drug in pH 2 medium at preparative pressures up to 3 t, but at pH 9 a slowsteady release was found at all pressures (Figure 1). Less than 10% had been released in 20 min under these pH conditions at a preparative pressure of 3. In vivo studies showed that nontableted microcapsules produced no significant difference in urine production over a normal tablet indicating that drug was rapidly available from release in the stomach. Microencapsulated material in tablet form showed a urine production effect between nondosed and microcapsule dosed effect. Urine production significantly increased after 5 hr, when the tablet had probably progressed to the intestines. After 10 hr the volume of urine produced was approximately the same as for a standard tablet. Harris’s46 microcapsules of the water-soluble KC1 were prepared by precoating the crystals with a number of celluloses and waxes. A cellulose acetate phthallate coating was found to be ineffective and release was hardly held back nor was wax coating effective. The best compromise was to use a mixed ethylcellulose-wax coat prior to encapsulation. In vitro dissolution at pH 2 showed 50% release in about 10 min and a total release in 1 hr from suspended microcapsules. Preliminary in vivo results were inconclusive. Jalsenjak and Kondo48 studied the permeability of water containing gelatin-acacia micro­ capsules to sodium chloride after fractionation by sieving. They found a decrease in perme­ ability with decreasing capsule size and suggested that “ structured” water in and around the capsule wall may be the cause. c. Simple Coacervation The use of simple coacervation of gelatin has found fewer applications than complex coacervation of gelatin-gum acacia and has in consequence been studied to a lesser extent. An early study by Nixon et al.47 on formalin hardened capsules showed that they retarded the release of sulfamerazine into both acid pepsin and alkaline pancreatin solutions. More

33 pronounced effects were found in the acid pepsin with up to 20% release inside 15 min. The extent of retardation depended on formalization and on the gelatin:sulfamerazine ratio. The microcapsules were found to be still intact after 8 hr. The microcapsules were found to rupture in pancreatin solution depending on the hardening time. The release was faster from sodium sulfate coacervated microcapsules than from ethanol coacervated systems. This is possibly due to the solution of residual sodium sulfate crystals found in the wall so leaving a very porous shell. Too long a formalization time was found to commence dissolving the sulfonamide which meant a larger proportion associated with the wall and a more rapid initial rate of dissolution. These studies were extended by Nixon and Walker49 using the sodium sulfate coacervation technique. In vitro dissolution studies showed that first-order release characteristics were exhibited by all hardened microcapsules. When the gelatin concentration was constant there was no apparent effect on the dissolution rate from increased core concentration. Because of the cross-linking effect of formaldehyde 50% release times were longer than for unhardened material. The sustained-release portion of the curve was less dependent on the hardening time (Figure 2). Unhardened samples showed a smaller amount initially released, but a faster overall release rate. These unhardened microcapsules tended to dissolve and disperse at temperatures above 30° exhibiting a straight line relationship between release and time. No simple straight line relationships were shown between temperature and dissolution rate using the Higuchi plots, but Arrhenius plots from first-order kinetics gave straight line temperature/ rate relationships. In Mcllvaine buffers at pHs between 2 and 6 there was little change in dissolution rate, but once the pKa of the NH group was passed the dissolution increased linearly with respect to pH. The release kinetics could be adequately described by (29) where w°" —w is the amount of core remaining in the microcapsules, M is the intercept [(K/ Ks)CsS°], S° the surface area at the commencement of the experiment (tG), Cs is the equilibrium solubility of the core, and t the sampling time. Nixon and Matthews50 found no evidence of pores in the ethanol coacervated microcapsules as had been previously found in sodium sulfate coacervated systems47 49 and in nylon microcapsules.51 This lack of pores probably accounts for the smaller amounts of drug released rapidly from ethanol coacervated systems. Madan52 also suggests that the rate depends on the wall thickness and that fractions of gelatin producing thicker walls yield slower release. Again changes in pH could increase or decrease the release rate. In general acid pH reduced the rate and alkaline pHs increased the rate. Madan suggests the differences are most probably due to differences in the avail­ ability of salt forming linkages in each fraction or differences in the physical nature of the coacervate produced. There appears to be an initial rapid release up to 15% followed by a lag phase, but from 20 to 85% there is a straight line release. The initial rate is due to drug in the microcapsule wall as is normally found, but the lag phase, which is not normally seen, is ascribed to gradual wetting of the drug core and microcapsule walls. A plot of wall thickness, calculated from Equation 30, vs t50% release appears to be linear. (30) where W is the weight of microcapsules, Ww weight of wall material recovered, dw density of wall material determined by pycnometer, d density of phenobarbital core, and r the radius

34

Biomedical Applications of Microencapsulation

FIGURE 2 (A). Dissolution plots illustrating the effect of hardening. Temperature, 20°, pH 6. Sulfadiazine 5%; (i) Wagner plots; (ii) first-order plots; (iii) Higuchi plots. Time of hardening; O, unhardened; □ , 30; A, 120; V, 180 min. (B) Dissolution plots illustrating the effect of temper­ ature, Sulfadiazine 5%, 30 min hardening time, pH 6, (i) Wagner plots; (ii) first-order plots; (iii) Higuchi plots. Temperature; O, 10°; ■ , 15°; A, 20°; V, 25°; • 30°; □ 40°.

of solid phenobarbital particles. The above equation is modified by a later paper53 to (31) where R is the microcapsule radius. Again sodium sulfate coacervation was used to prepare

35 clofibrate microcapsules and as was to be expected thinner-walled microcapsules gave a faster release rate following t12 plots and showing deviation from zero-order kinetics. Thickerwalled microcapsules approximated to zero-order release, but deviated from t12 plots. 2.

C ellulose-W alled M icrocapsules

Cellulose-type microcapsules have been used to microencapsulate water-soluble drugs which would not normally be possible with the gelatin-gum acacia coacervate system. Although many studies of preparative techniques have been reported the amount of release data from this type of microcapsule is not as comprehensive as with gelatin-walled systems. Aspirin has been one of the drugs most frequently studied in cellulose-walled microcap­ sules. Green 54 found that separate dosing of microencapsulated aspirin produced flat rather than the saw-toothed blood level curves for ordinary aspirin when salicylate was measured suggesting a better control of therapy. A contemporary study of sustained release aspirin by Bell et al.55 compared both single and divided doses. Total salicylate blood level was monitored and the sustained release product was found to give better results. They found that thick-walled capsules with 10% of their weight as wall material gave release rates too small for sustained release aspirin. The best results occurred when the wall constituted 3% of the weight of encapsulated material. Rotstein et al.56 discussed the use of sustained release aspirin in rheumatoid and osteo arthritis, but found no difference in pain relief when compared with nonsustained forms in short-term studies. However, in long-term usage studies all patients preferred the sustained release microencapsulated aspirin. The incidence of side effects was lower with this sustained release material. With aspirin microcapsules it is usually the salicylate which is measured and Madan and Shanbhag37 have microencapsulated sodium salicylate in cellulose acetate phthallate. In vitro studies showed a controlled release into both acidic and basic media. Although nonmicroencapsulated drug dissolved in less than 2 min there was only 50% release in acid media after about 20 min and 65% release into phosphate buffer after a similar time. It took more than 2 hr to release all the drug from the acid medium. The authors suggest that the cellulose acetate phthallate may have been modified by HC1 during the microcapsule preparation so that it was no longer soluble under alkaline conditions. First-order kinetics were found in both pH conditions. The release rate was found to be reproducible with a maximum range of less than 3%. D’Onofrio et al.58 have also studied the release of ethylcellulose coated aspirin, but in this case the microcapsules were enclosed as a light liquid paraffin slurry in soft gelatin capsules. They found that, as usual, a proportion of the drug was imperfectly microencap­ sulated, in this study only 1%. In vitro release into simulated gastric juice showed essentially first-order kinetics over a period of 12 hr and varied depending on the core:wall ratio. Batch to batch release variations were fairly good and within 5% of the mean. They suggest that at low ethylcellulose concentrations, where the wall is very thin, the microcapsules may not be able to withstand the increased internal pressure resulting from the diffusion of surrounding solvent. Such microcapsules may burst or develop leaks causing a more rapid release of aspirin in the initial stages. Microcapsules of phenacetin in cellulose acetate phthallate using sodium sulfate as a coacervating agent were prepared by Merkle and Speiser,59 who found that the amount of drug encapsulated had no effect on the size distribution but influenced the release rate of the drug indicating that diffusion through the wall was the rate-controlling step. When the wall was plasticized with glycerol drug release rate was no longer diffusion controlled. Speiser60 also examined acetophenetidine in this wall system and found the rate-determining stage was diffusion through the porous wall; the rate of release depending on the thickness of the cellulose acetate phthallate wall and once again plastisizers produced a different release

36

Biom edical Application o f M icroencapsulation

pattern. In the latter instance release was rapid and independent of wall thickness. Speiser suggests that a mixture of microcapsule fractions with different wall thicknesses could be used to produce an initial dose, followed by a steady sustained release dose. Salib et al.61 used cellulose acetate in the triacetate form rather than cellulose acetate phthallate as the wall material. The release rate of sulfadiazine from these microcapsules was a function of the permeability of the polymer coating, the coating thickness, and the concentration gradient existing across the membrane. Jalsenjak and Nixon et al.62 65 developed a technique of preparing ethylcellulose-walled microcapsules based on the patent by Fanger et al.66 Basically similar patterns were obtained for the release of sodium phenobarbitone from microcapsules of ethylcellulose and tablets prepared at compression pressures of 3.9 to 358.9 MPa. The tensile strength of these tablets was related to the core:wall ratio and to the microcapsule size. The rapid initial release from microcapsules accounted for up to 50% of the drug, but was slower from thicker-walled entities. It was also found that smaller microcapsules, with greater surface area, released faster than larger ones. After the initial rapid release the rate became almost constant and with the largest microcapsules was not complete in 2 hr. Tableted microcapsules released approximately 20% in the first 10 min, but subsequently release was much slower and was complete in about 7 hr when microcapsules of 1850 |xm were used to prepare the tablets. For a given core:wall ratio and microcapsule size the time required for 50% release of sodium phenobarbitone from the tablets was apparently inde­ pendent of compression pressure until this fell below 0.8 x 104 KNm 2. This low pressure produced a friable tablet which, while it did not disintegrate released its contents rapidly. The time for 50% did not increase linearly with respect to microcapsule size and, because of the thinner wall, those with a higher proportion of drug released most rapidly. Under the pH conditions which could be expected during passage through the GI tract it was found that at pHs above 3.1 there was little difference in in vitro release rates to those reported for release into distilled water, but at pH 1.3 a much slower release resulted. For 427.5- |jim-diameter microcapsules with a core:wall ratio of 2:1 it required 140 min for drug release and for the corresponding tablet prepared at 15.61 x 104 KNm 2 over 10 hr was required for complete release, no rapid release was discernible, and 50% release took approximately 3 hr. It is probable that these extended release times at low pHs are due to the conversion of the readily water-soluble sodium salt into the far less soluble phenobar­ bitone, although the experimental conditions did not allow the approach of saturation. Jalsenjak et al.67 also investigated the effect of the viscosity characteristics of the ethylcellulose used for the microcapsule wall. The t5(y7( release in vitro increased with viscosities up to 50 cP. The dissolution plots in HC1 and enzyme-free artificial gastric fluids showed good correlation because the dissolution pattern was completely determined by the low pH prevailing at the beginning of the experiment. An activity cage, which recorded spontaneous coordinate activity in rats, showed a lengthening of the therapeutic effectiveness of the encapsulated sodium phenobarbitone in comparison with the free drug. The modified Fanger et al.66 technique was also used by Agyilirah and Nixon68 to extend the earlier studies with Jalsenjak. They found that the microcapsule shape deviated more from true spheres and the degree of aggregation increased as the core.wall ratio was lowered and the plots of t50,7( release vs. original percent core material gave straight line relationships. The preparative cooling rate affects the external characteristics of the wall. Slow cooling produced smooth uniform walls; rapid cooling a mat of ethylcellulose strands. However, dissolution from the two types showed little significant difference. It has been suggested by Donbrow and Benita6y that aggregation of ethylcellulose micro­ capsules could be reduced by the use of polyisobutylene. Salicylamide microcapsules pre­ pared in the presence of polyisobutylene gave first-order kinetics. The increase in release rate in the presence of polyisobutylene concentrations paralleled the increase in salicylamide

37 content and was thought to be due to thinner ethylcellulose walls. The thin-walled micro­ capsules increasingly had empty ethylcellulose droplets present. Donbrow70 also used cast films to study the effect of polyethylene glycol on the ethylcellulose wall used in micro­ capsules, comparing the behavior of the latter to a small monolithic device where release is controlled by diffusion through the wall. They found drug release through these films to be proportional to t12 as would be expected from a diffusional matrix model. Where poly­ ethylene glycol was present the log rate constant was proportional to the percentage of polyethylene glycol in the film and was also pH dependent. Agyilirah and Nixon68 found that ethylcellulose microcapsules had clear outer walls at up to 15% poly isobutylene, but that far from reducing aggregation, an increase was found proportional to polyisobutylene. With 20% additive the microcapsules had very thin walls and at higher concentrations microencapsulation did not take place. Dissolution behavior of tablets prepared with different moisture contents, but the same core:wall ratio, followed a trend similar to that of the mean tablet strength in that the higher the moisture content the faster the release of the core material. A detailed kinetic study of release from cellulose-walled microcapsules has been made by Koishi and Kasai,71 who microencapsulated the antacid drug magnesium aluminum hydroxide hydrate using an ethylcellulose-dichloromethane - ^-hexane coacervating system. As a starting point they took the Higuchi7 equation and assumed that for microcapsules e, A, and Cs would be constant so that (32) where a = [D € (2A - e CS)CJ12. They defined the tortuosity factor, t using the radius of the coacervate drop, R, and assuming that t was dependent on the size and shape of the coacervate drops deposited on the core. This allowed t to be written (33) where b was a proportionality constant. As R could be defined in terms of the weight of ethylcellulose, W, and its density, d, Equation 32 became (34) and at a definite time t (35) /4 m i\1/6 a where C = ( — J *^ ^ 2> The Percentage of ethyl cellulose coating was found to affect the acid neutralization which decreased with the increase of ethyl cellulose in the microcapsule coat. Application of the kinetic equations developed showed a linear relation­ ship between 20 — 45% ethyl cellulose concentrations for neutralization of 0.1 and 0.05N acetic acid. Outside these concentrations it was found that the microcapsules had either network type walls with low ethyl cellulose concentrations or very thick dense walls with high ethyl cellulose concentrations. Although the previous study had used a solvent - nonsolvent system the Fanger method has produced further studies. Jalsenjak et al.72 used isoniazid microcapsules and measured the in vitro dissolution of the drug at 37°. In the case of aggregated microcapsules the authors

38

B iom edical A pplications o f M icroencapsulation

FIGURE 3. The percentage of isonizid released against (time)12 as a function of stirring speeds. Sample (B5+ BJ microcapsules A, 150, □ , 100, O, 50 r/min-1, tablets # , 50 r/min Sample (D, + D:) microcap­ sules V, 50 r/min-1, tablets 50 r/min 1

suggest that dissolution was confined to the outer surface of the aggregate, possibly due to incomplete wetting or the formation of a concentrated static film of diffused drug existing in the center of the aggregate. Zero-order release kinetics were found to describe drug dissolution (Figure 3) without any preliminary time lag and for microcapsules was complete in 5 hr. Tablets of microcapsules prepared at a compression of 100 MPa took 24 hr for complete release of the drug under the dissolution conditions. Some samples of microcapsules were prepared by a modified technique in which one third of the ethylcellulose and one third of the cyclohexane were added when the preparative system had cooled from refluxing temperature (80°) to 66°. Microcapsules prepared in this way subsequently gave tablets with the best sustained-release properties. The walls of these microcapsules appeared to remain intact during dissolution studies. Release showed a straight line relationship against t12 for up to 90% drug release. The permeability of such microcapsules was used to calculate the apparent diffusion coefficient73 using the expression (36) where the slope was obtained from a plot of the mass of drug transferred against time, N was the number of capsules, h the wall thickness, rmc microcapsule radius, rc core radius, and C2 the concentration of drug inside the microcapsule. One problem with this type of microcapsule is that it is difficult to measure the radius of the core as it is unlikely to consist of a single particle. The diffusion coefficient must therefore be used only as a rough guide. The authors assumed a constant zero-order permeation rate through the wall and found that the apparent diffusion coefficient decreased with decreasing microcapsule size. Because of the high apparent diffusion coefficients it was proposed that both a membrane and aqueous pore pathway may be operating simultaneously and that (37) where Dm and Dp are the diffusion coefficients in the membrane materials and the aqueous pores, a is the volume fraction in the pores and K is the membrane-solution partition

39 coefficient of the drug. DmK was not thought to depend on microcapsule size and the increase in Da for larger microcapsules would be given by the relationship between «> and (1 —oo). Deasy et al.74 prepared sodium salicylate microcapsules with ethylcellulose walls. They found a finer product with a slower drug release when 100 cP grade ethylcellulose was used as compared with the 10 cP grade. During dissolution the larger microcapsules ruptured into smaller ones with a swollen surface containing pores. Using a treatment similar to Powell75 the microcapsules were "sealed” with a layer of paraffin wax. While this reduced the release rate the product tended to be coarse and with 20% wax was both greasy and adhesive. With this modification the release mechanism was a complicated combination of erosion and diffusion. A certain amount of drug binding occurred. It was found that pHs over the normal physiological range had little effect on the release of sodium salicylate. Takenaka et al.76 had earlier32 studied the release of sulfamethoxazole from gelatin-acacia microcapsules and extended this work to include the same drug microencapsulated in cellulose acetate phthalate. The in vitro release was measured in media of pH 1.2, 7.5, and distilled water. As would be expected dissolution under alkaline conditions was rapid, but unex­ pectedly the initial release into acid pH was more rapid than into distilled water. In the later stages the rate in both media was almost identical and obeyed the Higuchi7 model. Tablets of these microcapsules could be caused to disintegrate by the addition of a microcrystalline filler resulting in a more rapid dissolution. Oya Alpar and Walters77 and Mesiha and Sidhom78 both used the Fanger technique to microencapsulate different antibiotics. Both found the general characteristics of microcap­ sules prepared by this technique. However, the latter suggest that a definite interaction is taking place between the antibiotic and the ethylcellulose, most probably involving the amide groups. Nakano et al.79 prepared carboxymethyl cellulose-walled microcapsules using coacervation from ethyl acetate solution with poly(lactic acid). Release parameters similar to ethylcellulose-walled microcapsules were found in vitro. Comparisons were also made of the in vivo performance of suspended and tableted microcapsules. Apparent elimination rate con­ stants were obtained from the elimination phase of log excretion rate vs. t plots using (38) Apparent absorption rate constants were obtained from plots of residuals in log excretion vs. t plots so: (39) In these expressions ka is absorption rate constant, f the fraction of dose absorbed, Dc the dose, kel the elimination rate constant, kex the urinary excretion rate constant, and dAu/dt the excretion rate. Nakano points out that the procedures used in calculating the rate constants were not really valid for sustained-release preparations and therefore the values for microcapsules only give a coarse estimate, but nevertheless will be an indication of the slower absorption from microcapsules due to a sustained-release effect. Smaller apparent excretion rate constants result from the sustained supply of the drug into the plasma due to sustained absorption. The extent of bioavailability was only slightly decreased compared with tablets, i.e., 86% compared with 91% when based on the 12-hr excretion level.

40

B iom edical A pplication o f M icroencapsulation

3. P olym er-W alled M icrocapsules

Polymerization techniques used for the preparation of microcapsules have been given in detail by Gutcho80 81 and the resulting product may be designated as either microcapsules or microcells, the latter having very thin walls. This chapter is not concerned with the latter type of product although a number of papers by Kondo and co-workers56,82 provide infor­ mation on the permeability of polymer walls to electrolytes which may be applied to diffusion from microcapsules. Shigeri and Kondo82 originally used a different equation for permeability from that later applied.5 6 Their original equation was (40) where C, and C2 were the electrolyte concentration inside and outside the microcapsule at time, t. V, and V2 were the total volume of microcapsules and the volume of dispersing medium, and A the total surface area of the microcapsules. A number of assumptions had to be made to calculate these quantities so that the permeability, P, would be better regarded as an apparent permeability. As might have been expected it was found that P decreased with time, but eventually reached a low steady value. It is suggested that this was due to the gradual modification of the membrane due to adsorption of the Tween 20, which was used in the microcapsule preparation. Takamura et al.5continuing this work on the permeability of polyphthalamide microcapsule membranes to electrolyte found a linear relationship between time and In (Ct-Cf)/(CrCf). This allowed a different permeability equation to be written as given previously (Equation 15). By this method the permeability coefficient was found to be of the order 10-8cm/sec. and to be almost temperature independent. The low permeability rate was ascribed to the formation of a stable diffusion layer. The membrane thickness AX was determined6 from (41) where w was the total weight of microcapsule membrane in a unit volume of dispersion, assuming complete reaction of the diamine and acid chloride used, r was the experimentally determined diamine reacted, d the density of the microcapsule membrane, and A the total surface area of the microcapsules in a unit volume of dispersion. Because the microcapsules were spherical entities containing a single core the above equations can be used. Besides the effect on permeability and diffusion coefficient already noted it was found that the diffusion rate of electrolyte was higher in membranes consisting of polyamide molecules with a poly methylene chain than with those having an aromatic ring, probably due to an increase in the dimensions of the interspaces of the polymer chains. By including polyvinyl pyrolidone in the aqueous phase it was found that increasing viscosity lowered the rate of entry of sodium chloride. The Kondo-type polymer microcapsules are really variations of the microcell type. A completely different approach was that by Banker et al.83-88 The microcapsules which they prepared were due to the interaction of drugs with cross-linked polymers of the 1,12dihydroxy octadecone hemiester of poly(methyl vinyl ether/maleic anhydride) type. They found good sustained release from both the granular microcapsules and tablets prepared from them. Cationic drugs such as methapyrilene HC1, chlorpromazine HC1, and atropine sulfate were used. The sustained-release effect could be potentiated by the addition of organic acid additives. As usual factors such as pH, stirring rate, and polymer type all affected the release. Anionic drugs such as phenobarbitone sodium, sodium salicylate, and chloral hydrate were also found to present sustained-release properties when prepared using a polymer gelled emulsion system.88

41 A number of studies have been made using nylon-type polymers as the wall material. Luzzi et al.X9 encapsulated sodium phenobarbital by emulsion polymerization followed by spray or vacuum drying. They found that both recovery techniques produced sustainedrelease microcapsules when compared with the solution rate of the crude drug in water, pH 6.75 phosphate buffer and 0.1 N HC1, but that spray drying gave the greater sustained release. Florence and Jenkins90 studied the release of trifluoperazine and pericyazine from nylon 6-10 microcapsules using a visking bag technique into phosphate buffer at 37°. Polyphthalamide microcapsules did not appear to be well formed, nor prolong the activity of the drug when tested on dogs. The time for 50 and 100% release of the drug contents was much shorter in vitro than in vivo, but the former rates of release could be used to compare several preparations. Plots of the log drug release vs. time were usually linear. These solid polymer bead-type microcapsules should behave like plastic matrices and give linear tl/2 plots indicating a diffusion controlled mechanism for release. Doubling the amount of nylon present in the walls increased the mean volume diameter by 13%, but produced a 21 % decrease in the apparent rate constants, suggesting that increased polymer density within the microcapsule decreased the rate of release. Because doubling of nylon caused slower release an adequate biological response was not produced and therefore the preparation was less effective. However, too rapid release would result in a short duration of activity. Embonate solubility increased markedly at higher pH. The time for 50% release decreased from 144 hr at pH 4.2 to 1 hr at pH 10 with ordinary suspended preparations. With microencapsulated preparations the change was from 132 hr at pH 4.2 to 18 hr at pH 10. At the former pH there was, in general, little difference in the effect of microencapsulation due to low drug solubility. Florence and Jenkins suggest that if the drug was completely enclosed in a solid polymer wall then the pH of the external medium should have little effect on the release rate. They suggest that the initial rapid release rate found in their microcapsules was due to surface drug and that this could be removed by washing with solvents such as ethanol. Bead-type microcapsules containing a-methacrylic acid and methyl methacrylate were prepared by Khanna and Speiser,91 who found pH and ionic strength influenced the rate of chloramphenicol release. “ Beads” containing no a-methacrylic acid did not release any drug while at the opposite extreme of preparative conditions beads with only a-methacrylic acid released their drug contents at the same rate in all pHs. As expected smaller micro­ capsules, because of their larger surface area, released faster. Polystyrene bead microcapsules also gave sustained release of acetaminophen.92 The original preparation produced nonexpanded polystyrene microcapsules, but it was also found that microcapsules produced without a drug content would produce a similar sustainedrelease effect when they were subsequently expanded with a blowing agent and allowed to absorb the drug from solution. The release of drug was rapid in the first hour with up to 64% being lost. After this rapid release only a further 29% was released in the subsequent 22 hr. Tablets were also prepared, but it is difficult to compare Craswell and Becker’s results with those of other workers as they included a channeling agent in their formulation. This resulted in a very rapid initial release from their product (approximately 86% in 30 min) although only an additional 12% was released over a 12-hr period. These results suggest that there had been fracture of the polymer beads during compression so as to increase the total surface area for release. This rapid release from polystyrene microcapsules is probably due to pores and cracks in the wall as Nozawa et al.93 94 examined electron micrographs of microcapsules containing a-amylase or sodium salicylate and found evidence of holes, 1 to 10 fim diameter. The presence of these holes was confirmed by immersion of the a-amylase containing micro­ capsules in starch-KI solution. These capsules of Nozawa94 were between 100 to 400 |xm

42

B iom edical A pplications o f M icroencapsulation

diameter, and possessed a definite wall rather than the solid bead structure of the previous work.91 92 The release of sodium salicylate from both dried and water-containing entities was affected by pH, being higher from the dried microcapsules than from those containing an aqueous solution of the drug. The use of the enteric coating materials MPM06 and dialdehyde starch both prolonged the release time in acid probably by filling in the holes in the microcapsules membrane. The release rate of sodium salicylate increased with pH when enteric coating materials were present, possibly due to an increased solubility of the enteric coating materials at high pH allowing the production of larger pores. A diffusion mechanism was suggested with the typical rapid release initial portion being followed by the slower sustained-release portion and this gave a double straight line graph when log (1 —Ct/Cn) was plotted against time where Ct was the concentration of core material in the medium at time t and C* was the concentration of core in the dissolution medium at infinite time when calculated from (42) when C0 was the initial core concentration in the microcapsules, Vc the total volume of the capsules, and V0 the volume of the medium. The kinetics of release may thus be expressed by the simple equation (43) where k = Dm.A./l. A being the specific surface area of the capsules, 1 the wall thickness, and Dm the apparent diffusivity of the core material in the capsule wall. These very typical release kinetics are due to material originally associated with the wall and subsequently to the setting up of a steady diffusion gradient of drug through the wall. The method of preparation was found to affect the release insofar that the rapid stirring produced large numbers of bubbles associated with the surface and resulted in definite holes being present. No holes resulted when slow stirring was used in the preparation of the microcapsules. It was also found that the Dmvalues increased for the microcapsules containing enteric coating materials except in pH regions below 4. Many other polymer systems have been proposed and in many the permeability can be controlled by varying the cross-linked density. For instance, the ratio of polyphenyl iso­ cyanate to toluene di-isocyanate can be used to control the permeability of polyurea microcapsules95 while bead-type microcapsules of binary and ternary polyacrylates have been tested in artificial enzymeless digestive juices to determine the release of phenobarbital, indomethacin, and meprobamate.96 The release depended not only on microcapsule bead size, but on the polymer composition. In some cases the release rate correlated directly with the swelling behavior of the polymers in the digestive juice or with the specific composition of the microcapsules. Indomethacin and meprobamate were released stepwise and correlated with the changing pore characteristics which were observed by electron microscopy. Phen­ obarbital was found to be completely released with a single maximum after a delay. Bead-type microcapsules are usually large and range up to 1.25 mm. This type of system containing salicylic acid 8 parts and avicel 2 parts encapsulated in Eudragit RL polymer with a triacetin plasticizer was studied by Gurney et al.97 The in vitro bioavailability was greater with higher triacetin levels (0.9 to 9.1%). The polymer concentration (3.12 to 12.48%) and the film thickness had no effect on bioavailability. Poly(lactic acid)-type microcapsules possess potentialities as biodegradable dosage forms and Krishnan et al.98 have made use of this wall material to study the bioavailability of

43 testosterone. The drug was incorporated in the poly(lactic acid) and leached out over a period of 3 months as the microcapsules degraded in biological media. An interesting study using this wall material has been made by Thies," who reported on the effect of microencapsulated narcotic antagonists contained in d,l-poly(lactic acid). The hope was to use either naltrexone free base or naltrexone pamoate in order to provide 30day narcotic antagonism. Microcapsules containing 50% of naltrexone pamoate reduced morphine self-administration in trained monkeys for a period of 45 to 50 days. The saturation solubility of the pamoate in the extracting medium was approximately six times lower than for the free base and the driving force for naltrexone pamoate from microcapsules was significantly lower. The permeability of both these materials through solvent cast films of the d,l-poly(lactic acid) was similar.100 Because of the lower pamoate solubility, relative to the free base, it would possess a lower rate of transport so that it should be easier to prepare microcapsules of naltrexone pamoate although the authors point out that only 65% is active drug. The original naltrexone free base microcapsules possessed wall defects and dissolution of the core was complete within 3 hr, but improved preparative techniques provided a measurable morphine antagonism for 4 weeks with microcapsules containing 50% of the drug administered as a peanut oil suspension when measured by mouse tail flick test data. The permeability of polyamide microcapsules to inorganic hydroxides has been studied by Ishizaka et al.,101 who found that with increasing amounts of cyclohexane in the preparative solvent a thicker, less permeable membrane was produced. Potassium hydroxide permeated three times faster than NaOH and twice as fast as RbOH. It was concluded that ion permeation was closely related to the water structure in and around the microcapsule membrane. Urea formaldehyde resin capsules have been used by Noren et al.102 to study the rate of release of 2,3,5,6-tetrachloro-4-methyl sulfonylpyridine. The rate was proportional to both t12 and to the drug solubility. The normal porous, monolithic matrix amenable to the Higuchi7 treatment was proposed and computer analysis of the data gave a best fit to (44) This empirical equation had a correlation coefficient of 0.986. The microencapsulation of hormones in polymer microcapsules has attracted a certain amount of interest. Benagiano and Gabelnick103 detailed several attempts which have been made to microencapsulate norethisterone and discuss devices which appear to be larger than normal microcapsules and should rather be regarded as micropellets. However, the release is similar to that found in microcapsules, d, l-Poly(lactic acid) was used by Beck et al.104 to microencapsulate crystalline progesterone. The microcapsule size being between 43 and 61 |Jim. The usual microcapsule size effect on the release rate was noted. Intramuscular injection of the microcapsules bypassed the problem of gastric absorption, first pass clearance by the hepatic portal system and short biological half-life, consequently aiding potency. This increased biological activity is possibly due to the elimination of fluctuations in the progesterone blood levels. Under in vivo conditions the release was approximately 1.3 |xg of progesterone per day per milligram of microcapsules with a release duration of 30 days. The release was found to be both by diffusion and biodegradation of the wall. A combination of a gelatin and polymer coat for microcapsules has been developed by McGinity105106 using nylon-gelatin microcapsules of sulfathiazole treated with formalin. The usual in vitro dissolution profile was obtained with up to 80 to 90% core release in 15 min. There was a degree of batch to batch variation, but size fractionation of batches reduced the variations in dissolution behavior to some extent. It was found that as the proportion of drug in the microcapsules increased the release rate decreased. Linearity was found with t1/2 plots until the microcapsules were relatively exhausted, but first-order release plots also

44

B iom edical A pplication o f M icroencapsulation

gave linearity and the correlation coefficients for both these plots were close together. However, a first-order plot provided linearity for a longer period of time and probably provides the better kinetic description of the release. A number of factors affecting release were commented on. It was found that the increased cross-linking of the gelatin contained in the complex microcapsule wall by larger amounts of formalin decreased the rate of drug release. The presence of Tween 80 caused an increased release rate, as found by Kondo82 in the case of Tween 20. McGinity suggests that this is due to wetting effects rather than micelle formation. A peak effect on release was noted with 0.01 M NaCl, but no reason was put forward. A simple nylon microcapsule, with thin walls, unlike those of Luzzi89 and Florence,90 was studied by Miyawaki et al.107 These were more typical of the microcell type, but they provide release data for ammonium chloride as well as overall reaction behavior for urease immobilized in the microcapsules. The release of the ammonium chloride could be described by (45) where t is time, C and C* the concentrations of ammonium chloride in the external solution and the microcapsule, Vp the volume of the external solution, Vmc the total volume of the microcapsules, dp the mean capsule diameter, and K the permeability. If the Donnan effect is weak then C = C' and (46) where C‘ is the final concentration and C' the initial concentration of NH4C1. 4.

Release From Other M icrocapsules

Seager and Baker108 prepared microcapsules using an atomized spray of BRL50216 which was slowly evaporated in propanol containing shellac as a coating agent. Four suspensions were used containing varying quantities of NaOH to partially neutralize the acid components of the resin. The microcapsules formed were tasteless, free flowing, and consisted of an inner core of drug and resin with an outer layer of shellac. The dissolution rate from these microcapsules increased and showed correlation with the degree of preneutralization of the shellac. Feinstein and Sciarra109 developed a dexamethasone microcapsule which they fractionated by sieving. Release rate and direct total determination indicated that some measure of timed release appeared in all four samples. The pH release rate profiles indicated dependence on this factor. Colloidal silica and isopropylmyristate formulations provided the best suspending characteristics of all aerosol formulations studied. Delivery rate, pressure, and evacuation testing demonstrated that an aerosol-type microcapsule formulation was useable. An eval­ uation by means of urinary excretion levels of 17-hydroxycortico steroid using a physical admixture of drug and timed-release microcapsules was carried out using rabbits. Signifi­ cantly higher steroid levels were obtained in animals treated with the admixture after day one might have been due both to an immediate suppression of the endogenous steroid and the "spilling over" of excess dexamethasone. A significant difference was also found at the 90% confidence level among the 24-, 48-, and 72-hr steroid levels in the admixture and the microcapsules, which substantiated the delayed effect of the latter. A complex microcapsule was mentioned by Watanabe and Hayashi110 in which a triple coat was applied using a polyamide outer, polyurea middle, and epoxy resin inner wall.

45 Release from such a microcapsule was very steady with approximately 30% of the core content (unspecified) being removed in 30 min. Deng and Luzzi111 have prepared dexamethasone microspheres of silicone rubber (silastic) in some cases incorporating lactose in the microcapsule. Where lactose was present large numbers of surface crystals were apparent. This material acts as a channeling agent due to solution of the lactose increasing the porosity. On exposure to dissoluting medium of pH 7.4 microcapsules containing 30% lactose additive showed erosion of the surface crystals within 5 min while after 5 days exposure to media bulges appeared, suggesting hydration of lactose-drug pockets for which no channels existed. Very high concentrations of lactose (above 40%) appeared to produce a slower rate of drug release, but a higher initial release, which may mean a simple concentration gradient effect was operating or that the drug was in less accessible pockets. In the presence of Carbopol® as the filler a high initial drug release resulted, probably being due to the partially exposed drug or to free drug which had migrated from the polymer matrix during dispersion vulcanization. There have been two recent studies of albumin microspheres by Widder et al.112 and Sugibayashi et al.113 The former incorporated a prototype drug, doxorubicin HC1, and ultrafine magnetic particles in albumin micropheres of 1 p,m average diameter. They did not carry out detailed kinetic studies as it was found that a significant amount of the drug was still associated with the carrier after 5 min, which represented a realistic interval for carrier injection and in vivo transit time to the desired target. In vivo extrapolation from in vitro release data was not considered feasible due to a number of complex blood flow variables. Sugibayashi et al.113 used albumin microspheres to study the effect of 5-flourouracil on Ehrlich Ascites Carcinoma in mice. They used a dialysis method to study the in vitro release characteristics and found that the temperature of microcapsule preparation caused a significant difference in release characteristics. At a high preparative temperature (180°) less than 5% had dialyzed in one day, but almost 30% had been released if the preparative temperature had been 100°. After 1 day the rate of release was approximately independent of the prep­ arative temperature. Zolle et al.114 have shown that the degree of swelling decreased with increased temperature of preparation, which suggests that drug diffusion would be reduced due to decreased microcapsule porosity and increased tortuosity. It was found that the microcapsules prolonged the life of tumor-bearing mice and indications are that single doses produce a sustained release in the peritoneum with maintenance of effective drug concen­ tration for a week. However, the suppression of tumor growth disappeared after 1 week. Takenaka et al.1 have reported on the preparation of microcapsules by the spray drying of slurries of a number of systems. Salicylic acid bound with acacia was one such system. Concentration released vs. time curves had no induction period and equilibrium was reached in 2 to 10 min while semi log plots of 1-C,/CS vs. time gave straight line relationships to initial residence time where C, is the concentration in the solvent and Cs the saturated concentration. Dissolution rate decreased with increasing ratios of acacia to salicylic acid, the apparent dissolution rate constants being between 2.14 x 10-3 and 3.05 x 10-5 cm/ sec. These are smaller than free salicylic acid and indicate that the salicylic acid particles are enclosed in a permeable and soluble film. These workers developed a three film model for release, suggesting that there was an internal diffusion layer in the capsule, a diffusion layer at the interface and a stagnant layer between the particle surface and the liquid film. If the concentration in the stagnant layer is steady then (47)

46

B iom edical A pplications o f M icroencapsulation

where K, is the particle internal mass transfer coefficient, K2 the diffusional mass transfer coefficient in the liquid film, Q the concentration in the solvent, Cs the saturation concen­ tration, A the surface area, and V the volume of bulk liquid. The mass transfer coefficients were correlated with the film thickness which was calculated from (48) where I was film thickness, r2 the capsule radius, w the weight, and p the density. The subscripts f and n mean film and core. At thin film thicknesses the data deviated from K, = De/1

(49)

where e was the film porosity, probably due to increasing porosity. Takenaka et al.1also reported on the dissolution behavior of spray-dried microencapsulated sulfisomidine in both acid and alkaline media. The dissolution behavior varied depending on whether the feeding slurry was aqueous or an ammoniacal solution, on the type of solvent used in the dissolution test, and on the type of binder present in the feeding liquid. Some release rates were faster than the original sulfisomidine which would require a dissolution mechanism which could easily transport solute from matrix to solvent in addition to the normal diffusion process. If the binder used was hydrophilic it is easily dissolved and leaves pores so that agitation could produce an increased capillary flow which produces hydrodynamic transport in addition to diffusion. A rate equation based on this model was proposed (50) where Kh was the mass transfer coefficient for capillary dissolution. Raghunathan et al.115 have coated an ion-exchange drug complex with a polymeric film to produce diffusion controlled sustained-release microcapsules. Direct application of the film was ineffective as the outer coating peeled in the dissolution medium due to the swelling of the ion-exchange particles. Pretreatment of the ion-exchange particle with materials such as polyethylene glycol was essential. With a divinylbenzenesulfonic acid resin complex with phenylpropanolamine as a model, mixtures of ethylcellulose coated and uncoated complexes were prepared. These mixtures gave varying drug release patterns showing rank-order cor­ relation with plasma concentration profiles obtained in bioavailability studies with suspension dosage forms. As a final example the work of Rupprecht et al.116117 may be cited. Almost all materials for the preparation of microcapsule walls are organic based, but these workers have produced an inorganic silica bead system which produced controlled release by adsorption of drugs onto the beads from aqueous solutions. Adsorption and subsequent desorption depended on the porosity and type of interaction between the solvated drug molecule and the surface silanol group of the silica. The rate of desorption of codeine from silica beads shows that substantial retardation could take place with microporous silica having diameters within the range 1 to 2 nm. Chemically bound hydrophobic groups at the silica surface could also be used as a means of controlling retardation of codeine. These matrix beads had disadvantages due to the uneven course of drug desorption and the authors developed a silica microcapsule system where liquid or solid cores were covered with a porous layer of silica. They suggested that the presence of pores, coupled with drugs of high solubility should yield zero-order release kinetics because diffusion through the wall was expected to be the rate-controlling

47 step. When silica microcapsules of this type were coated with a layer of microporous poly(ethoxy siloxane) or poly(phenyl ethoxy siloxane) the release of the microcapsule core was considerably slowed down.

REFERENCES

1. Takenaka, H., Kawashima, Y., Saitoh, M., and Ishibashi, R., Microencapsulation of some medicaments by a spray drying technique and dissolution rate of encapsulated particles, in Microencapsulation, New Techniques and Application,Kondo, T., Ed., Techno Books, Inc.. Tokyo, 1979, 35. 2. Carstensen, J. T. and Musa, M. N., Dissolution rate patterns of log-normally distributed powders, J. Pharm. Sci., 61, 223, 1972. 3. Luu Si-Nang, Carlier, P. F., Delort, P., Gazzola, J., and Lafont, D., Determination of coating thickness of microcapsules and influence upon diffusion, J. Pharm, Sci., 62, 452, 1973. 4. Luu Si-Nang, and Carlier, P. F., Some physical chemical aspects of diffusion from microcapsules, in Microencapsulation, Nixon, J.R., Ed., Marcel Dekker, New York, 1976, 185. 5. Takamura, K., Koishi, M., and Kondo, T., Studies on microcapsules, IX. Permeability of polyphthalamide microcapsule membranes to electrolyte, KolloidZ. Z. Polym., 248, 929, 1971. 6. Takamura, K., Koishi, M., and Kondo, T., Microcapsules. XIV. Effects of membrane materials and viscosity of aqueous phase on permeability of polyamide microcapsules towards electrolytes, J. Pharm. Sci.. 62, 610, 1973. 7. Higuchi, T., Mechanism of sustained-action medication: theoretical analysis of rate of release of solid drugs dispersed in solid matrices, J. Pharm. Sci., 52, 1145, 1963. 8. Schwartz, J. B., Simonelli, A. P., and Higuchi, W. I., Drug release from wax matrices.!. Analysis of data with first-order kinetics and with the diffusion-controlled model, J. Pharm. Sci., 57, 274, 1968. 9. Singh, P., Desai, S. J., Simonelli, A. P., and Higuchi, W. I., Role of wetting on the rate of drug release from inert matrices, J. Pharm. Sci., 57, 217, 1968. 10. Wagner, J. G., Biopharmaceutics and Relevant Pharmacokinetics, Drug Intelligence Publishing, Wash­ ington, D.C., 1971. 11. Dappert, T. and Thies, C., Statistical models for controlled release microcapsules: rationale and theory, J. Memhr. Sci., 4, 99, 1978. 12. Chien, Y. W., Methods to achieve sustained drug delivery. The physical approach: implants, in Sustained and Controlled Release Drug Delivery Systems, Robinson, J. R., Ed., Marcel Dekker, New York, 1978, 211 .

13. Tawashi, R. and Speiser, P., Accurate dosage of potent drugs in tablets, Pharm. Acta Helv., 39, 734, 1964. 14. Summers, M. P., An Investigation of the Effects of Crystal Form on the Compression Characteristics of Pharmaceutical Materials, Ph.D. thesis, London, 1972. 15. Thomas, M. J., Some N.C.R. encapsulation techniques and applications, in Proc. 53rd Mid Year Meet. Chem. Specialities Assoc., Washington, D.C., 1967, 118. 16. Herbig, J. A., Microencapsulation, in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 13, 2nd ed., Standen, A., Ed., John Wiley & Sons, New York, 1967, 436. 17. Bakan, J. A., Microencapsulation as applied to pharmaceutical products, presented at Eastern Regional I.P.T. section, Academy of Pharmaceutical Sciences, Philadelphia, Pa., October 4, 1968. 18. Nack, H., Microencapsulation techniques, applications and problems, J. Soc. Cosmet. Chem., 21, 85, 1970. 19. Calanchi, M., New dosage forms, in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976. 93. 20. Antonini, R., Pharmacological basis of drugs with prolonged action. Rev. Bras. Clin. Ter., 5, 197, 1976. 21. de Sabata, V., Bioavailability from microencapsulated drugs, in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976, 143. 22. Gayot, A., Merle, C., Traisnel, M., and Guyot, J. C., Studies of diffusion across microcapsules, Lab. Pharm. Probl. Tech., 26, 92S, 1978. 23. Tanaka, N., Takino, S., and Utsumi, I., A new oral gelatinized sustained-release dosage from, J. Pharm. Sci., 52, 664, 1963.

48

Biomedical Application of Microencapsulation 24. Paradissis, G. N. and Parrott, E. L., Gelatin encapsulation of pharmaceuticals, J. Clin. Pharm., 8, 54. 1968. 25. Green, B. K. and Schleicher, L., U.S. Patent 2,730,456, 1956. 26. Luzzi, L. A. and Gerraughty, R. J., Effects of selected variables on the extractability of oils from coacervate capsules, J. Pharm. Sci., 53, 429, 1964. 27. Luzzi, L. A. and Gerraughty, R.J., Effects of selected variables on the microencapsulation of solids. J. Pharm. Sci., 56, 634, 1967. 28. Luzzi, L. A. and Gerraughty, R. J., Effects of additives and formulation techniques on controlled release of drugs from microcapsules, J. Pharm. Sci., 56, 1174, 1967. 29. Madan, P. L., Luzzi, L. A., and Price, J. C., Factors influencing microencapsulation of a waxy solid by complex coacervation, J. Pharm. Sci., 61, 1586, 1972. 30. Nixon, J. R. and Nouh, A., The effect of microcapsule size on the oxidative decomposition of core material, J. Pharm. Pharmacol., 30, 533, 1978. 31. Bloomfield, S. F., Mounajed, R., and Nixon, J. R., The effect of bacterial contamination on the release of core material from gelatin-acacia coacervate microcapsules, Acta Pharm. Technol. Suppl., 7, 189, 1979. 32. Takenaka, H., Kawashima, Y., and Lin, S. Y., The effects of wall thickness and amount of hardening agent on the release characteristics of sulfamethoxazole microcapsules prepared by gelatin-acacia complex coacervation, Chem. Pharm. Bull., 27, 3054, 1979. 33. Takenaka, H., Kawashima, Y., and Lin, S. Y., Micromeritic properties of sulfamethoxazole microcap­ sules prepared by gelatin-acacia coacervation, J. Pharm. Sci., 69, 513, 1980. 34. Nixon, J. R. and Harris, M. S., Preparation and in vitro release of a microencapsulated hormone, in Microencapsulation, New Techniques and Application, Kondo, T., Ed., Techno Books, Inc., Tokyo. 1979. 93. 35. Nixon, J. R. and Harris, M. S., In vitro release of norethisterone from gelatin-gum acacia coacervate microcapsules, in Proc. 2nd Int. Conf. Pharm. Technol., Vol. 3, Paris, June 3 to 5, 1980, 1. 36. Price, J. C. and Palmieri, A., Microencapsulation of drugs suspended in oil: preparation and evaluation of prednisone and hydrocortisone microcapsules, in Microencapsulation, New Techniques and Application, Kondo, T., Ed., Techno Books, Inc., Tokyo, 1979, 119. 37. Nixon, J. R. and Hassan, M. A. M., The effect of pH on the release characteristics of thiabendazole microcapsules, in Proc. 2nd Int. Conf. Pharm. Technol., Vol. 2, Paris, June 3 to 5, 1980, 37. 38. Nixon, J. R. and Hassan, A. M., The effect of tableting on the dissolution behaviour of thiabendazole microcapsules, J. Pharm. Pharmacol., 32, 857, 1980. 39. Nogami, H., Nagai, T., and Suzuki, A., Studies on powdered preparations. XVII. Dissolution rate of sulfonamides by rotating disc method, Chem. Pharm. Bull., 14, 329, 1966. sulfonamides by rotating disc method, Chem. Pharm. Bull., 14, 329, 1966. 40. Bates, T. R., Young, J. M., Wu, C. M., and Rosenberg, H. A., pH-dependent dissolution rate of nitrofurantoin from commercial suspensions, tablets and capsules, J. Pharm. Sci., 63, 643, 1974. 41. Mounajed, R., The Effect of Bacterial Contamination of Gelatin on the Release of Core Content from Gelatin-Acacia Walled Microcapsules, Associateship thesis, Chelsea College, London University. 1977. 42. Agyilirah, G. A., A Study of Phenobarbitone Microcapsules, M.Sc. dissertation. Chelsea College, London University. 1978. 43. Nixon, J. R. and Agyilirah, G. A., The influence of colloidal proportions on the release of phenobarbitone from microcapsules, Int. J. Pharm., 6, 277, 1980. 44. Takeda, Y., Nambu, N., and Nogai, T., Microencapsulation and bioavailability in beagle dogs of indomethacin, Chem. Pharm. Bull., 29, 264, 1981. 45. Nixon, J. R., In vitro and in vivo release of microencapsulated chlorothiazide, J. Pharm. Sci., 70, 376. 1981. 46. Harris, M. S., Preparation and release characteristics of potassium chloride microcapsules, J. Pharm. Sci., 70, 391, 1981. 47. Nixon, J. R., Khalil, S. A. H., and Carless, J. E., Gelatin coacervate microcapsules containing sulphamerazine: their preparation and the in vitro release of the drug, J. Pharm. Pharmacol., 20. 528. 1968. 48. Jalsenjak, I. and Kondo, T., Effect of capsule size on permeability of gelatin-acacia microcapsules towards sodium chloride, J. Pharm. Sci., 70, 456, 1981. 49. Nixon, J. R. and Walker, S. E., The in vitro evaluation of gelatin coacervate microcapsules. J. Pharm. Pharmacol., 23(Suppl.). 147S, 1971. 50. Nixon, J. R. and Matthews, B. R., Surface characteristics of gelatin microcapsules by scanning electron microscopy, J. Pharm. Pharmacol., 26, 383, 1974. 51. Jenkins, A. W. and Florence, A. T., Scanning electron microscopy of nylon microcapsules. J. Pharm. Pharmacol., 25(Suppl.), 57P, 1973.

49 52. Madan, P. L., Polyhydric fractionation of gelating and suitability of gelatin fractions for microencapsulation, in Microencapsulation, New Techniques and Application, Kondo, T., Ed., Techno Books, Inc., Tokyo. 1979, 11. 53. Madan, P. L., Clofibrate microcapsules.il. Effect of wall thickness on release characteristics, J. Pharm. Sci., 70, 430, 1981. 54. Green, D. M., Tablets of coated aspirin microspherules — a new dosage form, J. New Drugs, 6. 294, 1966. 55. Bell, S. A., Berdick, M., and Holliday, W. M., Drug blood levels as indices in evaluation of a sustainedrelease aspirin, J. New Drugs, 6, 284, 1966. 56. Rotstein, J., Estrin, I., Cunningham, C., Gilbert, M., Jordan, A., Lamstein, J., Safrin, M., Wimer, E., and Silson, J., The use of a sustained-release aspirin preparation in the management of rheumatoid arthritis and osteoarthritis, J. Clin. Pharm., 7, 97, 1967. 57. Madan, P. L. and Shanbhag, S. R., Cellulose acetate phthalate microcapsules. I. Method of preparation, J. Pharm. Pharmacol., 30, 65, 1978. 58. D’Onofrio, G. P., Oppenheim, R. C., and Bateman, N. E., Encapsulated microcapsules, Int. J. Pharm., 2. 91, 1979. 59. Merkle, H. P. and Speiser, P., Preparation and in vitro evaluation of cellulose acetate phthalate coacervate microcapsules, J. Pharm. Sci., 62, 1444, 1973. 60. Speiser, P., Microencapsulation by coacervation. spray encapsulation and nanoencapsulation, in Microen­ capsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976, 1. 61. Salib, N. N., El-Menshawy, M. E., and Ismail, A. A., Preparation and in vitro evaluation of potentially long acting cellulose acetate microcapsules, Pharm. Ind., 39, 1278, 1978. 62. Jalasenjak, I., Nicolaidou, C. F., and Nixon, J. R., The in vitro dissolution of phenobarbitone sodium from ethyl cellulose microcapsules, J. Pharm. Pharmacol., 28, 912, 1976. 63. Jalsenjak, I., Nicolaidou, C. F., and Nixon, J. R., Dissolution from tablets prepared using ethyl cellulose microcapsules, J. Pharm. Pharmacol., 29, 169, 1977. 64. Nixon, J. R., Jalsenjak, I., Nicolaidou, C. F., and Harris, M. S., Release of drugs from suspended and tabletted microcapsules, in Proc. 1st Int. Conf. Pharm. Technol, Paris, May 31 to June 2, 1977, Vol. 4, 167. 65. Nixon, J. R. Jalsenjak, I., Nicolaidou, C. F., and Harris M., Release of drugs from suspended and tabletted Microcapsules Drug Dev. Ind. Pharm., 4, 117, 1978. 66. Fanger, G. O. Miller, E., and McNiff, R. G„ U S Patent 3,531,418, 1970 67. Jalsenjak, I., Vidmar, V., and Stivic, I., In vivo and in vitro evaluation of ethyl cellulose microcapsules of sodium phenobarbitone, Acta Pharm. Jugosl., 27, 191, 1977. 68. Agyilirah, G. A. and Nixon, J. R., Preparation, tabletting and release characteristics of ethyl cellulose walled microcapsules of phenobarbitone sodium, Acta Pharm. Tech., 26, 251, 1980. 69. Donbrow, M. and Benita, S., The effect of polysiobutylene on the coacervation of ethyl cellulose and the formation of microcapsules, J. Pharm. Pharmacol., 29(Suppl.), 4P, 1977. 70. Samuelov, Y., Donbrow, M., and Friedman, M., Sustained release of drugs from ethylcellulose-polyethylene glycol films and kinetics of drug release, J. Pharm. Sci., 68, 325, 1979. 71. Koishi, M. and Kasai, S., Preparation of ethyl cellulose coacervate microcapsules containing magnesium aluminum hydroxide hydrate and their application to acid-neutralization, in Microencapsulation, New Tech­ niques and Application, Kondo, T., Ed., Techno Books, Inc., Tokyo, 1979, 291. 72. Jalsenjak, I., Nixon, J. R., Senjkovic, R., and Stivic, I., Sustained-release dosage forms of microen­ capsulated isoniazid, J. Pharm. Pharmacol., 32, 678, 1980. 73. Senjkovic, R. and Jalsenjak, I., Apparent diffusion coefficient of sodium phenobarbitone in ethyl cellulose microcapsules: effects of capsule size, J. Pharm. Pharmacol., 33, 279, 1981. 74. Deasy, P. B., Brophy, M. R., Ecanow, B., and Joy, M. M., Effect of ethyl-cellulose grade and sealant treatments on the production and in vitro release of microencapsulated sodium salicylate, J. Pharm. Phar­ macol., 32, 15, 1980. 75. Powell, T. C., U.S. Patent 3,623,997, 1971. 76. Takenaka, H., Kawashima, Y., and Lin, S. Y., Preparation of enteric-coated microcapsules for tabletting by spray-drying technique and in vitro simulation of drug release from the tablet in GI tract, J. Pharm. Sci., 69, 1388, 1980. 77. Oya Alpar, H. and Walters, V., The prolongation of the in vitro dissolution of a soluble drug (phenethicillin potassium) by microencapsulation with ethyl cellulose, J. Pharm. Pharmacol., 33, 419, 1981. 78. Mesiha, M. S. and Sidhom, M. B., The interaction of oxytetracycline hydrochloride with ethyl cellulose in microcapsules, J. Pharm. Pharmacol., 32(Suppl.), 26P, 1980. 79. Nakano, M., Itoh, M., Juni, K., Sekikawa, H., and Arita, T., Sustained urinary excretion of sulfamethizole following oral administration of enteric coated microcapsules in humans, Int. J. Pharm., 4, 291, 1980.

50

Biomedical Applications of Microencapsulation

80. Gutcho, M. H., Microcapsules and Microencapsulation Techniques, Noyes Data Corporation, Park Ridge, N.J., 1976. 81. Gutcho, M. H., Microcapsules and Other Capsules, Advances Since 1975, Noyes Data Corporation, Park Ridge, N.J., 1979. 82. Shigeri, Y. and Kondo, T., Studies on microcapsules. III. Permeability of polyurethane microcapsule membranes, Chem. Pharm. Bull., 17, 1073, 1969. 83. Willis, C. R., Jr. and Banker, G. S., Polymer-drug interacted systems in the physicochemical design of pharmaceutical dosage forms I. drug salts with PVM/MA and with PVM/MA hemi-ester, J. Pharm. Sci., 57, 1598, 1968. 84. Goodman, H. and Banker, G. S., Molecular-scale drug entrapment as a precise method for controlled drug release. I. Entrapment of cationic drugs by polymeric flocculation, J. Pharm. Sci., 59, 1131, 1970. 85. Rhodes, C. T., Wai, K., and Banker, G. S., Molecular scale drug entrapment as a precise method of controlled drug release.II. Facilitated drug entrapment to polymeric colloidal dispersions, J. Pharm. Sci., 59, 1578, 1970. 86. Rhodes, C. T., Wai, K., and Banker, G. S., Molecular scale drug entrapment as a precise method of controlled drug release.III. in vitro and in vivo studies of drug release, J. Pharm. Sci., 59, 1581, 1970. 87. Larsen, A. B. and Banker, G. S., Attainment of highly uniform solid drug dispersions employing molecular scale drug entrapment in polymeric latices, J. Pharm. Sci. 65, 838, 1976. 88. Boylan, J. C. and Banker, G. S., Molecular-scale drug entrapment as a precise method of controlled drug release.IV. Entrapment of anionic drugs by polymeric gelatin, J. Pharm. Sci., 62, 1177, 1973. 89. Luzzi, L. A., Zoglio, M. A., and Maudling, H. V., Preparation and evaluation of the prolonged release properties of nylon microcapsules, J. Pharm. Sci., 59, 338, 1970. 90. Florence, A. T. and Jenkins, A. W., In vitro assessment of microencapsulated drug systems as sustained release dosage forms, in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976, 39. 91. Khanna, S. C. and Speiser, P., In vitro release of chloramphenicol from polymenr beads of ^-methacrylic acid and methylmethacrylate, J. Pharm. Sci., 59, 1398, 1970. 92. Croswell, R. W. and Becker, C. H., Suspension polymerization for preparation of timed-release dosage forms, J. Pharm. Sci., 63, 440, 1974. 93. Nozawa, Y., Higashide, F., and Kanamoto, T., Drug release from and properties of polystyrene micro­ capsules, J. Appl. Polym. Sci., 20, 3197, 1976. 94. Nozawa, Y., Higashide, F., and Ushikawa, T., Drug dissolution from microcapsules containing entenc coating materials, in Microencapsulation, New Techniques and Application, Kondo, T., Ed., Techno Books, Inc., Tokyo, 1979, 79. 95. Scher, A. B., Microencapsulated pesticides, in Controlled Release Pesticides, Scher, A. B., Ed., Symp. Series 53, American Chemical Society, Washington, 1977. 96. Dittegen, M., Kala, H., and Schmollack, W., Study on in vitro drug release from polyacrylate-based bead polymerizates, Pharmazie, 33, 64, 1978. 97. Gurney, R., Guitard, P., Buri, P., and Sucker, H., Realization and theoretical development of controlled release drug forms using methacrylate films. III. Preparation and characterization of controlled release drug forms, Pharm. Acta HeIv., 52, 182, 1977. 98. Krishnan, M., Sriram, N., Jain, R. K., Jaitley, R., and Singh, P., Controlled release of bioactive compounds with special reference to agriculture, Pop. Plast., 23, 28, 1978. 99. Thies, C., Properties of injectable microcapsules containing pharmaceuticals, in Microencapsulation, new techniques and application, Kondo, T., Ed., Techno Books, Inc., Tokyo, 1979, 143. 100. Mason, N., Thies, C., and Cicero, T. J., In vivo and in vitro evaluation of a microencapsulated narcotic antagonist, J. Pharm. Sci., 65, 847, 1976. 101. Ishizaka, T., Koishi, M., and Kondo, T., Permeability of polyamide microcapsules towards ions and the effect of water structure, J. Membr. Sci., 5, 283, 1979. 102. Noren, G. K., Korpi, G. K., and England, G. J., Release rate characteristics of microencapsulated 2, 3, 5, 6-tetrachloro-4-methysulfonylpyridine, J. Appl. Polym. Sci., 24, 2369, 1979. 103. Benagiano, G. and Gabelnick, H. L., Biodegradable systems for the sustained release of fertility-regulating agents, J. Steroid Biochem., 11,449. 1979. 104. Beck, L. R., Cowsar, D. R., Lewis, D. H., Cosgrove, R. J., Jr., Riddle, C. T., Lowry, S. L., and Epperly, T., A new long-acting injectable microcapsule system for the administration of progesterone, Fertil. Steril., 31, 545, 1979. 105. McGinity, J. W., Combs, A. B., and Martin, A. N., Improved method for microencapsulation of soluble pharmaceuticals, J. Pharm. Sci., 64, 889, 1975. 106. McGinity, J. W., Combs, A. B., and Martin, A. N., In vitro release characteristics of sodium sulfathiazole from formalin-treated nylon gelatin microcapsules, in Microencapsulation, New Techniques and Applica­ tion, Kondo, T., Ed., Techno Books, Inc., Tokyo, 1979, 57.

51 107. Miyawaki, O., Nakamura, J., and Yano, T., Permeability and molecular sieving characteristics of nylon microcapsule membrane, Agric. Biol. Chem., 44, 2865, 1980. 108. Seager, H. and Baker, P., The preparation of controlled release particles in the sub-sieve size range, J. Pharm. Pharmacol., 24(Suppl.), 123P, 1972. 109. Feinstein, W. and Sciarra, J. J., Development and evaluation of a dexamethasone timed-release aerosole formulation, J. Pharm. Sci. 64, 408, 1975. 110. Watanabe, A. and Hayashi, T., Microencapsulation techniques of Fuji Photo Film Co. Ltd., and their application, in Microencapsulation, Nixon, J. R., Ed., Marcel Dekker, New York, 1976, 13. 111. Deng, C. and Luzzi, L. A., Silastic microspheres-preparation and evaluation in Microencapsulation, New Techniques and Application, Kondo, T., Ed., Techno Books, Inc., Tokyo, 1979, 149. 112. Widder, K., Flouret, G., and Senyei, A., Magnetic microspheres: synthesis of a novel parenteral drug carrier, J. Pharm. Sci., 68, 79, 1979. 113. Sugibayashi, K., Akimoto, M., Morimoto, Y., Nadai, T., and Kato, Y., Drug-carrier property of albumin microspheres in chemotherapy. III. Effect of microsphere-entrapped 5-fluorouracil on ehrlich ascites carcinoma in mice, J. Pharm. Dyn., 2, 350, 1979. 114. Zolle, I., Rhodes, B. A., and Wagner, H. N., Jr., Preparation of metabolizable radioactive human serum albumin microspheres for studies of the circulation, Int. J. Appl. Radiat. Isotop., 21, 155, 1970. 115. Raghunathan, Y., Amsel, L., Hinsbark, O., and Bryant, W., Sustained release drug delivery system. I. Coated ion exchange resin system for phenylpropanolamine and other drugs, J. Pharm. Sci., 70, 379, 1981. 116. Rupprecht, H., Unger, K., Kramer, H., and Kricher, W., Controlled release of silica embedded drugs, in Microencapsulation, New Techniques and Application, Kondo, T., Ed., Techno Books, Inc., Tokyo, 1979, 107. 117. Rupprecht, H. and Biersack, M. J., Einfluss der porenstruktur von gerustsubstanzen auf die verfugbarkeit von arzneistoffen, Pharm. Ind., 36, 260, 1974.

53 Chapter 3 BIODEGRADABLE MICROSPHERES FOR PARENTERAL ADMINISTRATION Curt Thies and Marie-Christine Bissery TABLE OF CONTENTS

I.

Introduction............................................................................................................... 54

II.

General Considerations............................................................................................. 54 A. Structure of Drug-Loaded Microspheres....................................................... 54 B. Fate of Parenterally Administered Microspheres........................................ 54 1. Flow Behavior and Physiological Effects.........................................54 2. Clearance from the Circulatory System........................................... 56 3. Transfer to Lymph Nodes..................................................................57

III.

Formation of Microspheres.......................................................................................58 A. Synthetic Polymers.........................................................................................58 1. Lactide/Glycolide Microspheres........................................................58 a. Preparation............................................................................. 58 b. Characterization..................................................................... 59 c. Degradation Mechanism........................................................60 2. Poly (Alkyl a-Cyanoacrylate) Microspheres.....................................61 a. Preparation............................................................................. 61 b. Degradation Mechanism........................................................62 3. Polyacrylamide Microspheres........................................................... 63 a. Preparation............................................................................. 63 b. Characterization..................................................................... 63 c. Degradation Mechanism........................................................63 B. Natural Polymers............................................................................................63 1. General................................................................................................63 2. Protein Microspheres..........................................................................65 a. Uncross-Linked Microspheres...............................................65 b. Chemically Cross-Linked Microspheres.............................. 66 c. Thermally Denatured Microspheres...................................... 67 1. Preparation...................................................................67 2. Characterization..........................................................69 3. Degradation................................................................. 69 3. Polysaccharide Microspheres.............................................................70 a. Starch Microspheres...............................................................70 b. Dextran-Based Microspheres................................................ 71

References................................................................................................................................72

54

B iom edical A pplications o f M icroencapsulation I. INTRODUCTION

A number of parenterally administered drug delivery systems based on microspheres are being developed. The microspheres involved range in diameter from well below 1 \im to well over 100 |xm. Oppenheim has recently reviewed the status of small microspheres (nanoparticles) as drug carriers.1This chapter is concerned primarily with somewhat larger microspheres designed to be administered subcutaneously (s.c.), intramuscularly (i.m.), intravenously (i.v.) or intra-arterially (i.a.). The discussion is organized around the materials used to prepare microspheres and the preparation processes. Materials that are biodegradable within a reasonable time period are emphasized. Before discussing the formation of drug-loaded microspheres, it is appropriate to comment briefly on the many possible structures that such particles may have as well as their fate following parenteral administration. II. GENERAL CONSIDERATIONS

A. Structure of Drug-Loaded Microspheres The structure of drug-loaded microspheres varies greatly. This, in turn, has a big effect on drug release. Accordingly, an effort should be made to characterize the type of structure that specific microspheres have. Figure 1 contains schematic diagrams of four hypothetical structures. Structure A is a continuous drug phase surrounded by a continuous barrier membrane or shell. Structures of this type have traditionally been called microcapsules. Structure B represents another shell structure, but now the drug phase is subdivided into numerous domains scattered uniformly throughout the interior of the microspheres. If the outer shell or membrane remains intact for a finite time under in vivo conditions, both structures are capable of acting as membrane reservoir devices and providing a constant rate of drug release. Structure C represents a microsphere that is essentially a polymer matrix throughout which drug particles are uniformly dispersed. In this case, no drug-free shell of finite thickness separates the drug particles from the environment in which the microspheres are placed. Finally, Structure D represents a case where the drug is either dissolved or molecularly dispersed in the carrier material from which the microsphere is prepared. Structures C and D are not reservoir devices and hence, neither will give constant drug release over a prolonged period. Drug/polymer interactions can significantly affect drug release rates from both struc­ tures, although they are likely to play a more significant role in Structure D. Undoubtedly a number of structures other than those shown in Figure 1 could exist. Which structure most closely represents the actual structure of drug-loaded microspheres prepared by various workers remains to be determined. B. Fate of Parenterally Administered Microspheres Drug-loaded microspheres often are designed to be injected into the circulatory system. Accordingly, their arterial flow behavior, clearance from the circulatory system, and dis­ tribution in the body are important factors that must be defined. The following discussion is concerned with results of experimental studies carried out to shed light on these questions. l.F lo w Behavior and P hysiological Effects

The flow behavior of rigid microspheres through a medium size artery of rabbits has been characterized by Phibbs and Dong.2 They injected 5 x 104 to 5 x 106 microspheres of 7.5to 80-|xm diameter into the ascending aorta or left ventricle of a rabbit. Immediately after injection, the femoral artery was rapidly frozen with dry ice. The radial distribution of

55

FIGURE 1. Schematic diagrams of four hypothetical microsphere struc­ tures. (A) Continuous drug core surrounded by a continuous shell; (B) discrete drug core regions surrounded by a continuous shell; (C) discrete drug core regions dispersed in a matrix; (D) drug is either dissolved or molecularly dispersed in a carrier.

microspheres in this frozen artery was measured microscopically. The ratio of microsphere diameter to artery diameter ranged from 0.0032 to 0.103, but 75% of the 469 microspheres counted had a ratio of less than or equal to 0.025. The radial distribution of 7.5- to 10-|xm microspheres was found to approach that of erythrocytes.2 Erythrocytes are distributed uniformly in an artery, except for a small con­ centration decrease in the region adjacent to the artery wall.3 Larger microspheres tend to be concentrated in the central region of an artery thereby giving rise to a nonuniform radial distribution. The larger the microsphere, the more pronounced this effect is. Schroeder et al.4 characterized the physiological effects caused by administering various sizes of rigid microspheres to beagle dogs. The polystyrene-divinyl benzene microspheres (mean diameter in |xm: 3.7, 8.4, 15.9, 24.7) were injected into the right femoral vein. The doses administered ranged from 1 x 105 to 2.4 x 109 microspheres. Arterial blood pH and blood gases (partial pressures of oxygen and carbon dioxide in the arterial blood) were evaluated in order to detect possible lodging of particles in the lungs. Arterial blood pressure, pulse rate, and electrocardiograms (ECG) were recorded in order to determine if the heart functions in a different way because of the obstructed capillary bed. White blood cell (WBC) counts were made in order to evaluate the inflammatory response caused by microsphere embolization.

56

B iom edical A pplication o f M icroencapsulation

The dose of injected microspheres had more of an effect on the above parameters than did their size over the size range studied. Even at the highest doses administered, the only important change observed was an increase in WBC count. The highest increase in WBC count occurred with a dose of 1.3 to 3.0 x 108 8.4-p.m microspheres. However, no cor­ relation between increased WBC count and size or amount of injected microspheres was established. Changes in ECG were observed, the response increasing with increasing dose of injected microspheres. Slight changes in the PR and QT intervals as well as in the QRS and ST segments were attributed to ventricular strain. Cardiac output was disturbed due to blood vessel blockage. Recently, Kanke et al.5 showed that 3.4-|xm-diameter ,4lC-labeled mi­ crospheres injected i.v. induce severe hypotension associated with a secondary elevation in pulmonary pressure. This might be due to a neurohumoral reflex response to the severe drop in blood pressure caused by the injected microspheres. No acute hemodynamic change occurred when 7.4- or 11.6-p-m-diameter microspheres were administered at the dose levels studied. Kanke et al.5 attributed these differences in hemodynamic effect to differences in distribution kinetics of the 3.4|xm microspheres, since they readily pass through the lungs and remain in the systemic circulation longer than larger particles. 2.

Clearance from the C irculatory System

How fast microspheres are removed from the circulatory system and where they concentrate after being removed are determined by such factors as size, material of manufacture, man­ ufacturing process, and route of administration. Kanke et al.5 and De Luca et al.6 report that the removal of particles from the circulatory system seems to be due to a simple filtering effect of the pulmonary capillary beds. They injected 3.4-, 5.4-, 7.4-, and 11.6-|xm polystyrene-divinylbenzene microspheres i.v. or i.a. into beagle dogs. All sizes of microspheres are essentially completely cleared from the circulation within 10 min after i.v. or i.a. injection. The number of 5.4- and 7.4-|xm microspheres in the blood stream increases significantly between 10 and 30 min after i.v. or i.a. injection. This increase may be due to the opening of arteriovenous shunts which allows initially entrapped microspheres to reenter the blood stream. The 3.4- and 11.6-p.m microspheres have some tendency to show this effect, but it is small. The 7.4- and 11.6fjim diameter microspheres are largely retained in the lungs while the 3.4- and 5.4-|xm microspheres pass through. This observation implies that microspheres passing freely through the lungs must be smaller than a critical diameter which falls between 5 and 7 p,m for rigid, nondeformable particles. The 3.4- and 5.4-|xm microspheres become concentrated in the liver and spleen due to phagocytosis by cells of the reticuloendothelial system. The above observations were made with cross-linked, hydrophobic polystyrene-divinyl­ benzene microspheres that are not biodegradable. Sjoholm and Edman7 carried out similar studies in mice and rats with 14C-labeled cross-linked polyacrylamide microspheres of 0.25to 0.30-jxm diameter. Such spheres are more hydrophilic in nature than polystyrene-based microspheres and biodegrade in vivo slowly. One hour after i.v. injection in the tail vein, the microspheres were found in a variety of locations including the liver, spleen, lungs, kidneys, heart, and bone marrow; six hr after injection, over 75% of the injected microspheres were found in the liver and spleen; 2 weeks after injection, 81% of the injected microspheres were found there. Thus, a redistribution of the particles occurred as the time after injection increased. 10-(xm microspheres injected i.p. were also concentrated in the spleen and liver 2 weeks after injection. Wagner et al.8 observed that nonbiodegradable microspheres will remain intact in the body and be encysted by the surrounding tissue. In contrast, biodegradable microspheres will be metabolized and eliminated once they are trapped. Sugibayashi et al.9 report that 0.4- to 1.0-|xm bovine serum albumin (BSA) microspheres

57 containing 5-fluorouracil injected i.v. into mice accumulate mainly in the liver. How the biodegradable protein microsphere is made affects its in vivo distribution.10 For example, BSA microspheres formed at relatively low thermal denaturation temperatures are trapped in the lungs whereas BSA microspheres formed at high temperatures gather in the liver. The difference in behavior is attributed to the high water swellability of the particles formed at low temperature. Because they swell so much, their size is great enough to enable them to be trapped in the lungs. Histological studies have shown that less than lO-pm microspheres do not cause major tissue damage. Such particles are found in vascular channels (vascular endothelium cells, monocytoid phagocytes), Kiipffer cells, and in the RES cells of the sinusoids located in the liver, spleen, and bone marrow.5 Larger spheres, especially those used for embolization, may cause an inflammatory reaction, infarction, and necrosis. However, most authors in­ dicate microspheres have almost no toxicity.6 81112 Microspheres made from BSA,1011 carnauba wax,13 and cross-linked polyacrylamide7 are reported to be nonantigenic. Lote14 examined possible thrombogenic effects caused by degradable starch microspheres. Such microspheres injected in the superior mesenteric artery of cats caused no consumption of blood platelets, fibrogen, or Factor VIII. In vitro experiments showed that aggregation of human platelets was not influenced by exposure to the microspheres and retention of platelets in a column packed with starch microspheres was minimal. Thus, Lote found no evidence of thrombotic hazards associated with starch microspheres. Two anaphylactic reactions caused by injected microspheres have been reported. The first occurred with polystyrene-divinylbenzene5 microspheres while the second occurred with human serum albumin (HSA) microspheres.15This type of reaction would occur if vasoactive amines are liberated as the microspheres pass through the vascular network.3 3. Transfer to Lymph Nodes Although most microspheres are designed to be injected into the circulatory system, injection into tissue as a fat emulsion is being explored as a means of concentrating antitumor agents in regional lymph nodes. Initial work involved the injection into tissue of water-in­ oil (W/O) or water-in-oil-in-water (W/O/W) emulsions containing water-soluble chemo­ therapy agents.16 Such emulsions are poorly absorbed by the vascular system, but eventually are absorbed from lymph capillaries and transferred to the regional lymph nodes. This approach to targeted delivery utilizes the fact that lipids and chylomicrons are accumulated by the lymphatic system. Several studies with injected emulsions have been reported.1619 More recently, attention has focused on injectable oil emulsions containing drug-loaded microspheres.20 23 These emulsions are more effective than W/O or W/O/W emulsions. With the rat as the test animal, microsphere-oil dispersions containing 5-fluorouracil were ad­ ministered by intragastric and i.m. injection.21 The dispersions provided enhanced delivery of 5-fluorouracil into the lymphatic systems of the rat. W/O emulsions showed a similar effect, but not as great as that caused by the microsphere-oil dispersions. Intragastric admim istration was superior to i.m. injection. This was attributed to a rich supply of stomach wall lymph vessels. A microsphere-oil dispersion containing bleomycin was injected in the sub-serosal space of the appendix22 and caused increased drug retention at the injection site. It also caused enhanced and sustained delivery of bleomycin into the regional lymph node. A dispersion formed with a mixture of medium chain triglycerides as the oil phase gave better results than a dispersion formed with sesame oil. The mixture of medium chain triglycerides had a lower viscosity than sesame oil. The enhanced lymphatic transfer of drugs caused by microsphere-oil dispersions is at­ tributed to the presence of surfactants dissolved in the oil phase.23 The surfactants facilitate

58

Biomedical Applications of Microencapsulation

conversion of the microsphere-oil dispersion into a multiple emulsion after injection.22 The microspheres are surrounded by oil droplets that act as lymphotropic carriers. III. FORMATION OF MICROSPHERES

A. Synthetic Polymers 7. Lactide/Glycolide Microspheres a. Preparation Poly(dl-lactide) and lactide/glycolide copolymers rich in lactide have been fabricated into biodegradable microspheres by a solvent evaporation process24 25 and a coacervation proc­ ess.26 Beck et al.24 prepare progesterone-loaded microspheres from a methylene chloride solution of progesterone and poly(dl-lactide) emulsified in excess 5 wt % aqueous poly(vinyl alcohol). The poly(vinyl alcohol) is 88% hydrolyzed and acts as an emulsifier. Once the desired emulsion droplet size is formed, the agitation rate is reduced and evaporation of methylene chloride from the system is begun. Evaporation can be done at atmospheric or reduced pressure. The temperature of evaporation may also be varied. Once a significant portion of the methylene chloride has been evaporated, agitation is stopped, the partially solidified microspheres are allowed to settle, and the aqueous poly(vinyl alcohol) solution is replaced with water. The system is then promptly resuspended by agitation and the remaining methylene chloride evaporated. After solvent evaporation is complete, the mi­ crospheres are isolated and dried. If the poly(vinyl alcohol) emulsifier is not removed from the system before methylene chloride evaporation is complete, free progesterone crystals form in the aqueous phase and on the surface of the microspheres.27 This occurs even if the progesterone loading level in the microsphere is only 13%. Other sparsely water-soluble, crystalline drugs that dissolve completely in methylene chloride behave similarly.27ab The poly(vinyl alcohol) assists nucleation of drug crystals outside the microspheres which is undesirable. Since the poly(vinyl alcohol) must be removed from the system prior to complete methylene chloride evaporation, it is necessary to determine how much solvent must be evaporated in order to keep the microspheres from coalescing during the removal process. If too much solvent is evaporated, free drug crystals form outside the microspheres. Thus, there is a critical time period during which poly(vinyl alcohol) removal must be carried out.27b This period must be determined for each polymer/drug/solvent combination. Parameters like polymer molecular weight, initial polymer concentration, and agitation rate affect the critical time period during which poly (vinyl alcohol) removal may be safely accomplished. Although methylene chloride is a good solvent for forming microspheres by the solvent evaporation procedure, other solvents or solvent mixtures can be used. For example, a mixture of chloroform and acetone (proportions of each solvent not specified) was used to form norethisterone-loaded poly(dl-lactide) microspheres.25 Evaporation was carried out at 6°C for 10 hr before the system was allowed to warm to 25°C. After 6 hr agitation at 25°C, the microspheres were isolated, washed, and dried. The poly(vinyl alcohol) suspending medium was not removed after the methylene chloride had partially evaporated, because the norethisterone incorporated in the microspheres is essentially insoluble in water and the chloroform/acetone solvent mixture used to dissolve the poly(dl-lactide). The norethisterone remained suspended in the poly(dl-lactide)-rich phase throughout the evaporation process. The solvent-evaporation procedure for forming microspheres is conceptually simple. How­ ever, a number of variables affect the product obtained. These include the organic solvent(s) being evaporated, temperature of solvent evaporation, volume of organic phase per unit volume of aqueous phase, nature and amount of emulsifier dissolved in the aqueous phase, polymer structure and molecular weight, and solubility of the drug being incorporated into the microsphere.

59 Success of the solvent-evaporation procedure depends upon retaining a preponderance of the drug in the phase containing the biodegradable polymer (e.g., poly(dl-lactide)). This can be a problem, especially if high drug loadings are desired. For drugs completely soluble in the solvent being evaporated, efforts to prepare microspheres with drug loadings exceeding 30 to 40 wt % often give free drug crystals. Finally, it is appropriate to consider the structure of microspheres formed by solvent evaporation. If the drug being incorporated in a microsphere is insoluble in the casting solvent and is micronized, it should be distributed as discrete particles throughout the final microsphere. If the drug being incorporated is completely soluble in the casting solvent, it could crystallize as the solvent evaporates and thereby form discrete drug-rich domains scattered throughout the microsphere. However, the drug could remain molecularly dispersed throughout the microsphere and not crystallize. Progesterone-loaded poly (dl-lactide) mi­ crospheres formed by evaporation of methylene chloride are an example of this latter case. Differential thermal analyses of such microspheres show no transition at the 131°C melting point of progesterone.27"1 The progesterone has not crystallized, but remains molecularly dispersed in the microspheres. The progesterone apparently does not act as a plasticizer, because it does not alter Tg of the poly (dl-lactide). Low temperature phase-separation is another means of forming drug-loaded poly(dl-lactide) microspheres.26 The polymer is dissolved in a solvent, drug particles are dispersed in this solution, and a polymer incompatible with poly(dl-lactide) (e.g., polybutadiene) is added to the system. The poly (dl-lactide) is phase separated and thereby concentrated in a dispersed phase that spontaneously engulfs the drug particles. Figure 2 is a photomicrograph that shows embryo microspheres formed in this manner. The drug is micronized naltrexone pamoate, a narcotic antagonist. The liquid polymer-rich phase surrounding the drug particles contains poly (dl-lactide). The poly(dl-lactide)-rich phase shown in Figure 2 is desolvated and solidified by adding excess nonsolvent to the system. Nonsolvent addition is done carefully at a low temperature in order to minimize aggregation. The microspheres are then isolated and dried. The phase-separation approach to forming biodegradable drug-loaded microspheres is limited to situations where the drug being encapsulated can be kept insoluble in the organic solvent used to dissolve the polymer from which the microcapsules are formed. Drugs that have measurable solubility in a range of organic solvents are difficult to handle by this process. Furthermore, biodegradable polyesters like poly(L +-lactide) and lactide/glycolide copolymers are solubilized only under relatively strong solvent conditions that favor solu­ bilization of many drugs. Because poly(dl-lactide) is solubilized under moderate solvent conditions, it is the preferred biodegradable polymer for the phase-separation encapsulation process. Finally, it should be noted that small amounts of the phase-inducing agent used in phaseseparation encapsulation processes may be entrapped in the microspheres that are isolated. If so, the phase-inducing agent should be biodegradable like the polymer from which the microsphere is formed. If a nonbiodegradable phase-inducing agent is used, it may have to be completely removed from the microspheres prior to injection. b. Characterization Good yields of