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Xenobiotic metabolism and disposition : the design of studies on novel compounds
 9780849361630, 084936163X

Table of contents :
Content: INTRODUCTION. The Rationale for and Timing of Drug Metabolism Studies. IN VIVO BIO-LOGICAL STUDIES. Aims. Bal-ance Studies. Whole Body Autoradiogra-phy. Surgical Tech-niques Used in Disposition and Metabolism Stud-ies. Afterword. EXTRAPOLATION OF DATA AND STUDIES IN MAN. Studies in Man with Potential Therapeutic Agents. Pharmacokinetics and Its Application to Drug Development. METABOLITE IDENTIFICA-TION AND IN VITRO STUDIES. Analytical Procedures for Metabolites: Metabolite Profiling and the Isolation and Identification of Metabolites. In Vitro Studies on Metabolism. Biochemical Studies on Induction, Inhibition, and Activation. Index.

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Xenobiotic Metabolism and Disposition: The Design of Studies on Novel Compounds

Editor

H. P. A. Illing, Ph.D. Head of Toxicology Health Hazards Assessment Unit Health and Safety Executive Bootle, Merseyside, U.K.

~CRC Press V

Taylor & Francis Group Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 1989 by Taylor & Francis Group, LLC CRC Press is an imprint ofTaylor & Francis Group, an Informa business

No claim to original U.S. Government works ISBN 13: 978-0-8493-6163-0 (hbk) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and Iet us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www. copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety ofusers. For organizations that have been granted a photocopy license by the CCC, aseparate system of payment has been arranged. Trademark Notice: Productor corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www. taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Library of Congress Card Number 88-10571 Library of Congress Cataloging-in-Publication Data

Xenobiotic metabolism and disposition. Bibliography: p. lncludes index. I. Xenobiotics - Metabolism - Research - Technique. I. llling, H. P. A. QP529.X458 1989 599'.0192 88-10571 ISBN 0-8493-6163-X

PREFACE Disposition and metabolism studies are an important part of the corpus of knowledge required to fully understand the toxicity and efficacy of a drug or pesticide and the toxicity of industrial and household chemicals. There are many books which discuss the mechanisms of toxicity and the disposition and metabolism of individual compounds and classes of compound, but few which examine the type of study that can be employed. Although studies range from the very simple to the complex, similar basic techniques apply to many of them. Hence this book is concerned with examining the range and type of such studies and the methods which can be utilized. The book owes much to my mentors over the years and to the contributors. Without their help and expertise the book would have been impossible. Views expressed are those of the authors, and they should not be taken as the collective opinion of the Health and Safety Executive or of the authors’ sponsoring organizations. The faults, errors, and ommissions (which no doubt exist) are the editor’s. H. P. A. Illing Bootle, Merseyside

THE EDITOR H. Paul A. Illing, Ph.D., F.I.Biol., F.R.S.C., Dip.R.C. Path., is Head of Toxicology in the Medical Division of the Health and Safety Executive, Merseyside, U.K. Dr. Illing studied at Queens College, Dundee (from 1967, University of Dundee) and graduated B.Sc. (St. Andrews) in 1969 and Ph.D. (Dundee) in 1972. Following a period at the University of Mainz (West Germany) as aWellcome Trust/DFG Fellow, Dr. Illing was a Scientist and Senior Scientist in Drug Metabolism and Pharmacokinetics at Hoechst Pharmaceutical Research Laboratories, Milton Keynes, U.K. from 1974 to 1982. Since 1982 he has been at the Health and Safety Executive where he was appointed to his present position in 1986. Dr. Illing is a Fellow of the Institute of Biology and of the Royal Society of Chemistry and holds the Diploma in Toxicology of the Royal College of Pathologists. He is a member of the Biochemical Society, the British Toxicology Society, and the U.K. Environmental Mutagenicity Society. Dr. Illing’s interests include methods for studying foreign compound metabolism and the role of such studies in understanding the biological effects of chemicals.

CONTRIBUTORS Diane J. Benford, B.Sc., Ph.D. Toxicology Unit Deputy Head of Toxicology Unit Robens Institute University of Surrey Guildford, Surrey, England James W. Bridges, B.Sc., Ph.D., F.R.S.C., F.I. Biol., M.I. Env. Sci., M.R.C. Path. Toxicology Unit Director and Professor of Toxicology Robens Institute University of Surrey Guildford, Surrey, England E. R. Franklin, B.Sc. Department of Drug Metabolism and Pharmacokinetics Smith Kline and French Research Ltd. Welwyn, Herts., England H. P. A. Illing, B.Sc., Ph.D., F.I.Biol., F.R.S.C., Dip. R.C.Path. Head of Toxicology Health Hazards Assessment Unit Health and Safety Executive Merseyside, England R. M. J. Ings, B.Sc., Ph.D. Head of Department Department of Pharmacokinetics and Drug Metabolism Sender Research and Development Fulmer, Bucks., England Peter Johnson, B.Sc., Ph.D., F.R.S.C. Managing Director and Vice President Preclinical Research and Development Smith Kline & French, Research Ltd. Welwyn, Herts., England

Cameron M. Macdonald, B.Sc., Ph.D. Head of Drug Development Hoechst Pharmaceutical Laboratories Milton Keynes, Bucks., England John McEwen, M.B., Ch.B., Ph.D., F.R.C.P. Medical Director Drug Development (Scotland—) Ltd. Ninewells Hospital and Medical School Dundee, Scotland Roger Metcalf, B.Sc., Ph.D. Head of Department of Drug Metabolism and Pharmacokinetics Smith Kline and French Research Ltd. Welwyn, Herts., England D. A. Ross, B.Sc., Ph.D. Head of Drug Metabolism Department of Drug Metabolism and Pharmacokinetics Smith Kline and French Research Ltd. Welwyn, Herts., England A. G. Salmon, M.A., D.Phil., M.R.S.C. Staff Toxicologist Reproductive and Cancer Hazard Assessment Section California Department of Health Services Berkeley, California Ian D. Wilson, M.Sc., Ph.D., M.R.S.C. Senior Scientist Department of Drug Metabolism Hoechst Pharmaceutical Laboratories Milton Keynes, Bucks., England

TABLE OF CONTENTS SECTION 1: INTRODUCTION Chapter 1 The Rationale for and Timing of Drug Metabolism Studies.............................................. 3 Peter Johnson and Roger Metcalf SECTION 2: IN VIVO BIOLOGICAL STUDIES Chapter 2 A im s.......................................................................................................................................19 H. P. A. Illing Chapter 3 Balance Studies.....................................................................................................................23 H. P. A. Illing Chapter 4 Whole Body Autoradiography.............................................................................................. 41 E. R. Franklin and D. A. Ross Chapter 5 Surgical Techniques Used in Disposition and Metabolism Studies...................................67 Cameron M. Macdonald Chapter 6 Afterword.............................................................................................................................. 83 H. P. A. Illing SECTION 3: EXTRAPOLATION OF DATA AND STUDIES IN MAN Chapter 7 Studies in Man with Potential Therapeutic A gents.............................................................89 John McEwen Chapter 8 Pharmacokinetics and Its Application to Drug Development............................................ 99 R. M. J. Ings SECTION 4: METABOLITE IDENTIFICATION AND IN VITRO STUDIES Chapter 9 Analytical Procedures for Metabolites: Metabolite Profiling and the Isolation and Identification of Metabolites........................................................................................................... 149 Ian D. Wilson Chapter 10 In Vitro Studies on Metabolism......................................................................................... 171 A. G. Salmon

Chapter 11 Biochemica1 193 Biochemical Sturlies Studies on Induction, Inhibition, and Activation Activation........................................ 193 Diane J. Benford and James Janies W. Bridges 0

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Section 1: Introduction

3 Chapter 1 THE RATIONALE FOR AND TIMING OF DRUG METABOLISM STUDIES P. Johnson and R. Metcalf TABLE OF CONTENTS I.

Introduction..................................................................................................................4 A. Definitions........................................................................................................ 4

II.

Drug Discovery........................................................................................................... 4

III.

Metabolism Studies in the Research Phase................................................................ 6

IV.

Early Preclinical and Clinical P hase..........................................................................8 A. Preclinical Studies............................................................................................8 1. Study D esign.......................................................................................8 2. Analytical M ethods............................................................................. 8 3. Balance-Excretion Studies.................................................................. 8 4. Pharmacokinetics................................................................................ 9 5. Tissue Distribution Studies..................................................................9 6. Dose Level and Frequency of Dosing................................................ 9 7. Metabolic Profiles............................................................................. 10 8. Miscellaneous Studies........................................................................ 10 B. Early Clinical Studies....................................................................................10 1. Preliminary Assessments....................................................................10 2. Pharmacokinetics...............................................................................11 3. Radiolabeled Metabolism Study........................................................ 11

V.

Late Preclinical and Clinical Phase.......................................................................... 11 A. Preclinical Studies.......................................................................................... 12 1. Long-Term Toxicology..................................................................... 12 2. Reproductive Toxicology.................................................................. 12 3. Investigative Toxicology....................................................................13 4. Miscellaneous Studies........................................................................ 13 B. Clinical Studies...............................................................................................13

VI.

Future.......................................................................................................................... 14

VII.

Conclusion

15

4

Xenobiotic Metabolism and Disposition I. INTRODUCTION

The study of the metabolism of foreign compounds in mammalian systems began as long ago as the 19th century with the discovery of pathways of conjugation and aryl and alkyl hydroxylation. During the next hundred years major advances were made in various laboratories which provided the necessary background against which industrial pharmaceutical companies built their present approaches to drug metabolism studies. Some companies were already active in this area by the 1960s, but it was not until 1968, when the thalidomide tragedy had led to a general tightening up of regulatory requirements of new drugs, that the U.S. government declared its objective of requesting metabolic data prior to licensing new drug products, a move which was followed by regulatory authorities around the world. Although regulatory authorities have still not subjected drug metabolism to the close scrutiny which they have applied to toxicology, there are now signs of an increased awareness of the value of good drug metabolism information in the assessment of a new product’s safety and efficacy. However, the licensing authorities’ regulations do vary from country to country and do change from time to time. Furthermore, the incorporation of some standard metabolism studies into the drug development process, only as a routine measure to satisfy regulatory authorities, has long been replaced by the use of such studies to contribute to the fundamental knowledge about a drug, its mechanism of action, the optimum dosage regime, and to the discovery of new chemical entities. Thus, this chapter will not consider regulatory issues in the design of drug metabolism studies, but will outline the integration of drug metabolism investigations into programs aimed at the discovery and development of novel drugs. A. Definitions In an area which attracts specialists from a wide range of disciplines (biology, pharmacy, biochemistry, pharmacology, chemistry, physics), it is not surprising that there is considerable confusion over terminology. Thus, metabolism is sometimes considered to be a subgroup within the area of pharmacokinetics, whereas to others, pharmacokinetics is the study of rates of processes within the area of metabolism. For the purposes of this chapter, drug metabolism is understood to be the sum of the processes affecting the fate of drugs in mammalian systems and pharmacokinetics will be used to refer to the rates of all metabolic processes involving foreign compounds. Drug discovery is used to describe the entire process from the initial chemical synthesis of a novel compound through to its successful introduction as a new medicine. Three major phases are identified in this process: research, early preclinical and clinical, and late preclinical and clinical. II. DRUG DISCOVERY The process of drug discovery, which may take over 10 years and cost in excess of $100 million, should be considered prior to any discussion of the timing of drug metabolism studies. The activities which are involved in the successful discovery of a novel drug are outlined in Figure 1. No discrete drug metabolism activities are shown in this figure, because, as will become apparent later in this chapter and throughout this volume, drug metabolism studies are conducted during the entire period of drug discovery, from early research studies to late clinical investigations. For a new drug to be approved for marketing it must have been shown to be a safe and effective therapy in the course of intensive testing in animals and man. In drug discovery, the research phase represents a period in which chemists and biologists (including biochemists, pharmacologists, etc.) investigate the action of a number of chemicals

-Late Clinic al

Late Precli nical

·Early Clinic al

Early Preclin ical

Resea rch

PHASE

Ph

-Selec tion

FIGURE I.

-Selec tion of Synthe tic Routes -Scale -up

Schematic diagram of major activities in drug development.

REGISTRA TION AND LAUNC H

Phase IV flinica l Trials

. 1 Cl" . 1 T na s 1mca ase 111

/

Large Scale Route s of Synth esis and Formu lation

l (hea~hy subject~ I

Phase I Clinic al Trials

~

Carcin ogenic ity Studie s

Chem istry Devel opmen t

""-;·m., --------

Formu lation Devel opmen t

Phase lla Clinica l Trials (patie nts) Toxicol~ ~ ogy ~

(>1 year)

Long Term

Specia l T ricolo gy

(3-6 months )

Chron ic Toxico logy

\

-Mecha nism of Action

-f7~··r··:., ~

-Acute

/

Pharm acolog y

---------

Toxico logy

SELEC TION FOR DEVE LOPM ENT

l

Series of Biolog ically Active Comp ounds

~

ies-M edici nal Chem istry Pharm acolog ical lnves tigati ons-S tructu re Activi ty Stud

ACTIV ITIES

Ul

6

Xenobiotic Metabolism and Disposition

on biological processes which may have a direct relationship to a diseased state, or may be concerned with the modification of fundamental physiological or biochemical processes remote from potential therapy. Chemical modifications are used to enhance the specificity and/or potency of molecules in a particular test or series of tests, with a view to selecting a limited number of compounds with the desired profile of activities as candidate compounds for development. The early preclinical phase is primarily concerned with the evaluation in a number of animal tests of the compound’s toxic potential together with additional studies of mode of action, preparation of commercially viable routes of synthesis, preparation of dosage forms suitable for animal studies and early studies in man, etc. The early clinical phase is designed to assess the safety and efficacy of the compound in man. Relatively small numbers of healthy subjects are used initially before the studies are extended to include patients. In the late preclinical phase, long-term, chronic toxicology studies are conducted in animals, in order to monitor the effects of repeated exposure to the compound. These studies include daily administration for 6 or 12 months, dependent on the particular regulatory authority, and daily administration to rodents for up to 2 years or longer in carcinogenicity studies. During this period manufacturing and formulation processes are developed to production scale. The late clinical phase includes large-scale patient studies. Although these three phases have been used in this chapter to categorize various aspects of drug discovery, it should be appreciated that this is done out of convenience and should not be interpreted too literally. Indeed, good development strategies require that each compound be considered on its own merits and studies scheduled accordingly, rather than on the basis of historical precedent or by following standard packages. Historically, the major industrial commitment to drug metabolism studies has been during the preclinical phase. Once a compound had been selected for development from a discovery program and early toxicology studies had been initiated, metabolism studies were conducted to investigate the absorption, distribution, metabolism, and excretion (familiarly known as ADME) of the compound in animals (usually those species used in the toxicology studies). The climax of these metabolism studies was a study involving the administration of a radiolabeled form of the compound to volunteers in order to study its ADME in man. More recently, there has been an increased involvement of drug metabolism scientists in clinical studies and this trend will continue, with greater examination of the influence of disease on metabolism. However, experience shows that, while in the research area scientists have become increasingly proficient at identifying novel compounds with attractive pharmacological properties in vitro and in animal models, too many compounds are found to be unacceptable in the course of their preclinical and clinical phases of development. Although there may be many reasons for the high failure rate in these phases, it is arguable that the selection of compounds for development, on the basis of pharmacological activity alone and in the absence of even preliminary metabolic data, has contributed to the relative lack of success. Thus, it is anticipated that, although preclinical drug metabolism studies will continue to form a major part of the industrial approach, there will be an increased involvement of drug metabolism groups in research as well as in the clinical phases. III. METABOLISM STUDIES IN THE RESEARCH PHASE In this phase of drug discovery it is probable that only limited amounts of compound are available and that no radioactive material has been prepared. The emphasis of metabolism studies must therefore be on deriving relevant data using small amounts of material, in order to assist in the selection of one or more compounds from a series for investigation in man or for further development.

7 In vitro techniques using cell cultures, microsomal, and isolated organ preparations may be used to investigate a number of properties of novel compounds, while requiring minimal material (typically < 1'-< ..... > .....

1'-


1;:::

==:!

w u

~

u... u...

u.J

c

u

z

Oxprenolol Timolol Metoorolol Acebutolol Pindolol

0·1

0

!::"

ii5

;;.;:

1V>

~!adolol

Cl

0·01

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::::======-

====-==========.

20

Sotalol Atenolol

37

TEr'i'ERATURE ( 'C> FIGURE 3. N-Octanollbuffer, pH 7, distribution coefficients of ß-blockers at different temperatures.

xNadolol

~Proctolol

x Acebutolol

x Timolol

Oxprenolo~:lprenolol 2

3

Propranolol

x-

4

Log (portition octanol: water)

FIGURE 4. Relationship between the logarithm of the partition of some ß-blocking drugs between octanol and water and their metabolism.

104

Xenobiotic Metabolism and Disposition

FIGURE 5.

Ionization o f drugs.

for compounds which are either totally ionized or unionized. Most drugs, however, are organic molecules with one or more substituents on them which confer either acidic, basic, or amphoteric properties. Drugs with either an acidic or a basic group can exist in either an unionized or ionized form (Figure 5). At most physiological pHs, therefore, more than one molecular species will be present (ionized and unionized). In such circumstances, if one just determines the ratio of drug in the organic and aqueous phases, the coefficient calculated is not that of the partition coefficient, but that of the distribution coefficient. To calculate the true partition coefficient, normally of the unionized species, a correction factor has to be made using the HendersonHasselbach equation (Equations 2 and 3). ( 2)

(3) The pKa is defined as the pH at which a compound is 50% ionized and should be determined for any new drug before any pharmacokinetic studies are undertaken. The methods by which pKa values are determined are beyond the scope of this chapter and reference should be made to Albert and Sergeant. 1 The ionized form of a drug possesses an electrostatic charge and, as such, prefers to dissolve in water. Unionized molecules have no net charge and prefer to dissolve in lipids. However, drugs have to first dissolve in the lipid layer of the membrane to enable them to get across. Thus, the degree of ionization of a drug is very important, since it is this which determines the amount of unionized, lipid-soluble drug available to dissolve in the lipid portion of a cell membrane and, hence, transported through the membrane. The law of mass action states that “ the driving force of a chemical reaction is proportional to the active masses of the reacting substances.” This applies equally to the situation with ionized and unionized forms of a drug. Thus, if a drug is a weak acid and is in acidic environment, the ionization will be suppressed and there will be more of the lipid-soluble unionized species of our drug available for transport. Similarly, for a weak base in the basic environment the ionization will be suppressed. Thus, the pH of the environment will play a very important role in determining the level of unionized drug available for transport through membranes. This information can be used predictively for determining the anticipated relative concentrations of drug on either side of a membrane (e.g., the intestinal lumen and blood; blood and different tissues; blood and urine). For these calculations Equations 4 and 5 can be used.

105 (4)

(5) Thus, transfer of drugs across cell membranes will be affected by: 1. 2. 3.

The lipid solubility of the drug, particularly of the unionized form The pKa(s) of our drug molecule The pH of the physiological environment

Ideally, the first two should be determined for any candidate drug about to undergo development before any pharmacokinetic analysis is undertaken, so that this information can be used predictively. B. Absorption Drugs usually elicit their action systemically. In most cases drugs are administered extravascularly and have to be absorbed into the systemic blood circulation before they can become effective. Thus, failure of a drug to be absorbed can result in ineffective therapy. A great many different sites have been explored for the administration of drugs, including the buccal cavity, skin, lung, eye, vagina, muscle, and gastrointestinal tract (both following oral and rectal administration). The route of administration must, however, take into account the ease of dosing in order to ensure patient compliance. Normally, the oral route is the most convenient and, not surprisingly, is the most popular route for administering drugs. Oral dosing may be unsuitable because of the properties of the drug or the disease; hence, it may be necessary to use other sites. A good example of this is disodium cromoglycate, a drug which is highly effective in the treatment of asthma when given by inhalation, but is totally ineffective when given orally, due to nonabsorption. Some drugs, such as nitroglycerine, can be absorbed dermally. However, this latter route of administration is really only applicable to those compounds which are given in very small doses. With all the possible different sites of extravascular administration, the definition for the term “ absorption” needs to be general. Such a definition is “ the net transfer of a drug from the site of its administration to the site of measurement.” As so many drugs are administered orally, most of the discussion in this section will refer to absorption from the gastrointestinal tract. However, the principles of absorption are common whenever a drug is not administered directly into blood. Basically, the absorption of the drug from its site of administration can be considered as overcoming a series of barriers designed to prevent a foreign substance from entering the body. In many cases these barriers can be a source of frustration for the effective use of a drug, but in others they can be usefully exploited to the benefit of the patient. Before considering these various limiting factors of drug absorption, let us examine the relevant physicochemical and biological aspects of the process in more detail. 1. Factors Affecting Absorption: Lipid Solubility, Ionization, etc. A drug must first cross a biological, lipid membrane for it to be absorbed, no matter what the site of administration. To do this the compound must be in solution. The ease with which the dissolved material will cross the membrane depends principally on its lipid solubility, since the compound has to dissolve in the lipid layer of the membrane.

106

Xenobiotic Metabolism and Disposition

X

Polar Drug

Lipophillic Prodrug

Absorptton

Inside Body FIGURE 6.

Absorption

Metabolism

Active Polar Drug

Prodrugs.

Table 1 EXAMPLES OF VARIOUS PRODRUGS Vitamins

Antibiolies

Vitamin A acetates, palmitates, etc. Thiamine Nicotinamide

Oxytetracycline magnesium chelate Pivampicillin, etc. Hetacillin

Riboflavin species Vitamin D species Pro-thiamines

Chloramphenicol palmitate (taste masking) Chloramphenicol succinate, etc. Clindamycin esters Erythromycin esters (taste masking) Triacetyloleandomycin Steroid phosphates (for solubility)

B"

Steroids Steroid acetonides, esters, etc. Conjugated estrogens, estrogens of different degrees of saturation Mestranol Depot progestagens

Others Ara-C derivatives Aspirin Derivatives of cardiac glycosides Clofibrate Chloral hydrate Codeine Cyclophosphamide Dipivalyl adrenaline Dyphylline Fluorouracil Levodopa Phenacetin Phenylbutazone Sulphanilamide

The intrinsic Iipid solubility of the drug is, thus, important to the absorption process. However, most drugs are organic molecules with acidic or basic substituents on them so that they can exist in either ionized or unionized forms. Also, since the unionized species contains no net electrostatic charge, it is this, and not the ionized species, which is Iipid soluble. One would therefore predict that the compounds which exist largely in their unionized form would be preferentially absorbed. Polar, water-soluble compounds, such as the penicillins and cephalosporins, because of their low Iipid solubility, are poorly absorbed. Consequently, considerable effort has been put into designing derivatives of these antibiotics which are more Iipid soluble, but unfortunately, the antimicrobial activity resides with the polar ß-lactam nucleus. To overcome the conflict between the polar nature of the active moiety and the resultant problems of absorption, derivatives have tobe made. Usually these are esters of known antimicrobial agents, which more readily cross the gastrointestinal tract because of their higher Iipid solubility and, once absorbed, are enzymatically hydrolyzed, via nonspecific esterases, to the original antibiotic (Figure 6). This approach to drug design is known asthat of the "prodrug" and has been successfully used for a number of clinicaluseful agents (Table 1). The pH of the various areas of the gastrointestinal tract (Table 2) could have a profound effect on the overall absorption of most drugs. It is the pH of the environment that will be responsible for the degree of ionization of these compounds. For instance, if the drug is a base, it will exist largely in its ionized forms in the acidic environment of the stomach and the duodenum and would be expected tobe poorly absorbed from these sites. The converse would be true for an acidic drug. Thus, for acidic drugs such as the nonsteroidal antiin-

107 Table 2 pH VALUES AND AMOUNTS OF VARIOUS FLUIDS SECRETED IN SECTIONS OF THE GASTROINTESTINAL TRACT pH Mouth Stomach Duodenum Jejunum Ileum Colon

6.2— 7.4 1— 3 5.5— 7 6.5— 7 6— 8 6.5— 8

Volume of fluid (1/day) 3— 5 6 10 0.5— 1.5

flammatory agents and some of the diuretics, it could be postulated that absorption would be best from the stomach. This forms the basis of the pH-partition hypothesis, in which the degree of absorption can be predicted from the known pH of the various areas of the gastrointestinal tract and the pKa of your drug. While this can provide a useful “ rule of thumb” , biological systems are not so simple, and other factors must be taken into consideration. 2. Factors Affecting Absorption: Surface Area Probably one of the most important of these factors is the effective surface area available for absorption. Although the stomach has folds, it is relatively smooth providing only a limited surface area for drug absorption. Consequently, the absorption of drugs from the stomach, even the acidic drugs which will exist almost entirely in their lipid-soluble, unionized forms, tends to be minimal. Most absorption of orally administered drugs takes place in the duodenum and jejunum, where the surface area is increased, as there are folds in the surface (mucosal convolutions, folds of kercking, and villi) as well as in the cell membranes of the villi to form microvilli (Figure 7). The result of this is an effective increase in the surface area available for absorption of approximately 600-fold. Since most of the absorption of orally administered drugs actually occurs in the upper intestinal tract, the rate of gastric emptying will play an important role in the extent and rate of drug absorption. This has been demonstrated by examining the effects of drugs which either increase or decrease stomach emptying (metoclopramide and propantheline, respectively), on the absorption of the drugs (Figure 8 ). Food can also delay gastric emptying and slow absorption of some drugs. It might be predicted, therefore, that taking drugs with food would always impair absorption. However, food can have quite an unpredictable effect on the absorption of orally administered drugs and this has been reviewed by Welling15 and Melander.7 Some of the reasons for this apparent unpredictability will be discussed later when we consider the dissolution of solid dosage forms, but in any drug development program it is essential to determine the effect of food on the pharmacokinetics of a new drug. Only then can a recommendation be made as to whether the drug should be administered before or after meals. Another important factor that should be considered if food is likely to be coadministered is a chemical reaction with a constituent of the food. An example is the reaction between calcium in milk and the tetracycline groups of antibiotics which yields an insoluble and unabsorbable complex. This complexation also occurs with aluminum ions, thus, these drugs should never be administered with either milk or other medicines, such as the antacids, containing calcium and aluminum ions.

108

Xenobiotic Metabolism and Disposition

Structure

]:)

Area of simple tylinder

lncreose in surfoce area (relative to tylinder)

3,300

Folds of kerckring (valvulae conniventes)

Villi

Microvilli

Surface area (cm 2 )

3

10,000

30

100,000

600

2,000,000

FIGURE 7. Anatomical features influencing the mucosal surface area of the small intestine. (From Wilson, Intestinal Absorption, Saunders, Philadelphia, 1962. With permission.)

PLASMA CONC.

PLASMA CONC.

TIME FIGURE 8.

TIME

Effect of drugs which decrease or increase gastric emptying on the absorption of paracetamol.

109 0.2

.

e.. u

c::

.. ..

e

0,

D

Q. Q.

D

'C N

~

D 'C

c:: D (i)

Total intestinal blood flow-rate (mllmin/g) FIGURE 9. The relationship between intestinal blood flow and rate of appearance of a compound in mesenteric blood. (From Winne, D., J. Pharmacokin. Biopharm., 6, 55, 1978. With permission.)

3. Factars Affecting Absorption: Blood Flow Other physiological processes, such as the blood flow, also affect absorption from the intestinal tract. Most drugs are absorbed by passive diffusion, with the drug moving from an area of high to low concentration, and it is essential to maintain the concentration gradient if the absorption process is to continue, i.e., sink conditions must be maintained. This is normally achieved as the blood removes the drug more rapidly than it is absorbed. If, however, the physicochemical properties of the drug allow it to freely cross the intestinal wall, the rate-limiting step can become the speed with which the compound is removed from the site of absorption. Forthose compounds which tend to be more polar (e.g., ribitol and erythritol), penetration through the membrane (permeability) is rate limiting and the absorption of these compounds tends tobe independent of blood flow (Figure 9). Absorption ofthose compounds which rapidly cross the membrane, such as water, methanol, and ethanol, is blood flow dependent (Figure 9). 4. Other Factars That May Affect Absorption The previous discussion has highlighted three important factors, Iipid solubility and ionization, surface area, and blood flow, all of which can affect drug absorption. However, there are some other factors which can play an important role in the overall effectiveness of extravascular administered drugs. These are

1. 2. 3. 4. 5. 6.

Drug stability Gastrointestinal transit time Gut microflora Gut wall metabolism Presysternic hepatic metabolism Drug formulation

110

Xenobiotic Metabolism and Disposition

Stability of some drugs, especially in the highly acidic conditions of the stomach, should be taken into consideration. Examples are penicillin G, erythromycin, and digoxin, all of which can undergo acidic hydrolysis. Since food delays stomach emptying, it would be inappropriate to administer these drugs with food, since this would increase the residence time of the drug in the acidic conditions, resulting in a lower availability. Instability problems should, however, be known at a relatively early stage in the drug’s development, since instability can be determined using relatively simple in vitro techniques. Also, even if the compound is unstable in the stomach, enteric coated formulations can be used to protect the drug by delaying its release until the formulation reaches the intestine. Although gastric emptying has been considered, the intestinal motility could have an effect on the overall efficiency of absorption. If the gastrointestinal transit time is less than the time required for a drug to be fully absorbed, unabsorbed drug will be lost in the feces. Some drugs may only be absorbed at specific regions of the intestinal tract and the quicker these drugs move through these regions, the less chance there is for them to be absorbed. This is an important consideration if one is contemplating animal models for bioavailability testing, since the gastrointestinal transit time can vary greatly between animal species. This and the many other problems of choosing in vivo animal models for bioavailability testing have recently been reviewed.4 Another possible cause of drug loss before absorption through the intestinal wall is metabolism by the gut microflora. There are many types of bacteria and fungi which constitute the gut microflora, such as Escherichia coli, Clostridia, Streptococci, Lactobaccilli, Bacteroides, and yeasts. They are able to undertake a variety of enzymic reactions such as hydrolysis, dehydroxylation, o-demethylation, and reduction of nitro and azo groups. This loss tends to be less of a problem in man where the upper portion of the gastrointestinal tract is sterile, but it can be important when animal models are being used, especially for those species which indulge in coprophagy. Once the drug has moved from the lumen of the intestinal tract, it can still be effectively lost by presystemic metabolism, either by the gut wall or by the liver. The human gut wall has been shown to metabolize a number of compounds, including morphine, isoproterol, estradiol, and 17-P-estradiol, before they reach the systemic circulation. Most of these reactions involve the formation of sulfate or glucuronide conjugates, and it has also been reported that some sulfur-containing compounds can undergo oxidation of the sulfur atom. Gut wall metabolism can be determined by a variety of in vitro, in situ, or in vivo techniques, and some in vitro and in situ animal models are described in Chapter 5 which should be consulted if such methods are being contemplated. An in vivo technique for estimating presystemic metabolism, which is often used in animals (usually rat), is to administer the drug via different routes and to compare the areas under the curve for the routes when the plasma level of the drug is plotted against time. For example, if the drug is administered orally and intraportally, any decrease in the area under the curve of the orally administered drug in comparison to that administered intraportally must be due to either loss within the gut lumen (instability or gut microflora) or the gut wall. Probably one of the most common and most well-documented physiological reasons for loss of drug before it reaches the system circulation is metabolism by the liver on the first pass of the drug through the liver. This occurs in varying degrees for a great many of the drugs in clinical use and results in a reduced and variable amount of the drug reaching the systemic circulation. First-pass metabolism by the liver is generally an undesirable phenomenon because it makes prediction of the amount of the drug reaching the systemic circulation very difficult. The difficulties can be further compounded in cases of drug-drug interactions, and disease states such as hepatic or renal failure. In cases of extensive hepatic first-pass metabolism, only a very small proportion of the drug escapes the liver. A slight reduction

I ll in the first-pass effect may result in the patient being exposed to a disproportionately large amount of the drug with resultant toxicity and side effects. Since hepatic metabolism is so important to drug development, it will be discussed in further detail in the elimination section. The term bioavailability is defined as the rate and extent intact drug reaches systemic blood or plasma and it is quite possible, because of first pass metabolism, to have good absorption of drug-related products, but a negligible bioavailability. C. Distribution The distribution of a drug throughout the body can best be defined as the reversible transfer of a drug to and from the site of measurement. To put it more simply, when we are studying drug distribution, we are measuring where a drug goes within the body, how much gets there, and how long it stays there. Although it is possible to measure the distribution of a drug in the tissues of animals, it is not normally possible in man, where measurements can only be made using either blood or plasma. In pharmacokinetics, therefore, parameters have to be derived which can indicate what, in general terms, is happening to the drug in tissues. More often than not, especially when studying the distribution of drugs, radiolabeled compounds are used. These measurements are nonspecific and, in general, should not be used for deriving pharmacokinetic parameters. Nevertheless, some useful preliminary information can still be obtained from radiolabeled studies. 1. Factors Influencing Distribution: pH For a drug to distribute throughout the body, it must first cross cell membranes. In the majority of cases, this will be by passive diffusion. The drug will move from an area of high concentration to an area of low concentration until the concentration of the drug on both sides of the membrane is equal. This process requires no energy, there is no saturation, and there is no competition. It is worth reiterating that the degree of ionization of a drug, which has a pKa around physiological pH, will profoundly influence how it distributes. Since this is determined by the pH of the environment as well as the pKa of the drug, the pH of the various tissues can be critical. The pH of tissues tends to be slightly more acidic than that of plasma. Thus, for weakly basic drugs there will be more existing in its lipid-soluble, unionized form in plasma which can then distribute through the cell membrane into tissue. However, once in tissue, it will become ionized and, since the ionized species is polar, will not be able to distribute back into plasma. The converse will be true for weakly acidic compounds. As a rule of thumb, therefore, basic drugs tend to distribute out of plasma into tissues more than acidic drugs. This has also been shown experimentally by inducing acidosis and observing that acidic compounds move out of plasma into tissues. The reverse has been exploited clinically, since with phenobarbitone overdosing, alkalosis can be induced with sodium bicarbonate, and since it is an acidic drug, it would be withdrawn from the brain, its site of action, into plasma. Unlike capilliary membranes, those from tissues tend to have fewer aqueous pores. A good example of a tissue with very few aqueous pores is brain. To get to the brain, a drug has to cross a lipid membrane and to do so must first dissolve in the lipid. If it does not, it is said to have failed to cross the “ blood-brain barrier” . 2. Factors Influencing Distribution: Rate-Limiting Processes There are two types of rate-limiting processes that occur with drug distribution. The first of these is perfusion rate limitation. If the compound is a low molecular weight, lipophilic drug, the physicochemical properties of our drug will allow it to move freely across membranes. The process limiting drug distribution will depend on how quickly the drug can be

112

Xenobiotic Metabolism and Disposition Table 3 SOME PARAMETERS FOR DIFFERENT BODY ORGANS

Organ Adrenal glands Blood Bone Brain Fat Heart Kidneys Liver Portal Arterial Lungs Muscle Skin Thyroid gland Total body

Percent of body volume 0.03 7 16 2 10 0.5 0.4 2.3

0.7 42 18 0.03 100

Percent water

Blood flow (ml/min)



25 5000 250 700 200 200 1100 1350 1050 (300) 5000 750 300 50

60

5000



83 22 75 10 79 83 68

79 76 72

delivered to the tissue, i.e., the rate of perfusion of tissue with blood. Blood flow can vary quite considerably in different tissues (Table 3), with tissues such as lungs, liver, and kidneys being extremely well perfused; other tissues such as fat, heart muscle, and skin are not so well perfused. In general, with drugs that are perfusion rate limited, well-perfused tissues will take up drugs more rapidly than poorly perfused tissues. Equilibrium, on the other hand, takes longer to achieve in poorly perfused tissues and/or if the amount to be partitioned into tissues is very large. In the opposite extreme, very polar compounds have difficulty diffusing across the lipid membrane, especially if the membrane has relatively few aqueous pores in it. In this case, the diffusion across the membrane becomes a rate-limiting process, hence the term “ diffusion rate limitation” . A good example of this is the equilibration of drugs in cerebral spinal fluid, since the brain has relatively few aqueous pores in its membranes. Drugs must, therefore, cross this membrane by diffusion across the lipid barrier. For very polar acidic compounds such as 5-sulfo-salicylic acid and sulfo-guanidine, the diffusion across the cerebral spinal membrane is very slow. For nonpolar lipid-soluble compounds such as thiopental, aniline, aminopyrine, and antipyrine, the diffusion across the membranes is very much more rapid. From this it can be concluded that compounds such as 5-sulfo-salicylic acid and sulfoguanidine are diffusion rate limited. Another good example of diffusion rate limitation is penicillin, which has difficulty in crossing into the brain since penicillin is a polar compound. Muscle tissue, however, contains many more aqueous pores than the brain and, thus, the drug is able to cross this membrane more easily resulting in relatively high levels of the drug in this tissue. It should be stressed that equilibrium can be reached with distribution by passive diffusion; it occurs when the concentrations of unbound drug in the tissue and plasma are the same. The final level is independent of the rate-limiting processes which have just been discussed. 3. Factors Influencing Distribution: Binding to Macromolecules Our considerations so far have assumed that all the drug is free and available for distribution. This is rarely the case, since most drugs bind to varying degrees to macromolecules within the cell. Emphasis is often put on the binding of drugs to plasma proteins, largely

113 because this is one of the easiest binding interactions to measure. However, tissue binding is just as important and a major factor in drug distribution as the ratio of binding to plasma and tissue macromolecules. In plasma acidic drugs tend to bind mainly to albumin, whereas basic drugs bind to oxacid glycoproteins. This latter group of proteins are known as stress proteins and tend to be more abundant under stressful situations such as disease. Basic drugs can also bind to lipoproteins and some compounds, mainly endogenous, bind to globulins. It should be emphasized that this classification is very general and exceptions can be found. It is beyond the scope of this chapter to discuss methods for measuring protein binding in detail. Personal experience has indicated that the most generally useful technique is equilibrium dialysis. Another good method is ultracentrifugation performed at 37°C, although it does take a heavy toll on expensive ultracentrifuges. Ultrafiltration is very popular, but it can suffer from several disadvantages, the most common of these being the binding of the drug to the filter. Gel filtration is a method which is often quoted, but it is generally considered rather poor for the type of studies used in pharmacokinetics. There are numerous reviews of protein binding techniques and these should be consulted for further details.9 4. Determination of Distribution The determination of the distribution of drugs and metabolites is important when interpreting toxicological data, and for determining organ radiation doses for dosimetry which must be included in any application for administering a radioactive material to man. Practically, measuring distribution can be quite difficult, and the various techniques are discussed in detail in different chapters of this book. Whole-body radiography and liquid scintillation counting of isolated tissues (quantitative tissue distribution studies) are nonspecific, and this limitation must be borne in mind when attempting to relate these data with those from pharmacokinetic studies. 5. Volume of Distribution There are three “ volumes” of distribution terms which are commonly used in any pharmacokinetic analysis. These are initial volume of distribution, volume of distribution based on area, and volume of distribution at steady-state. There are other terms which are used less frequently such as volume of the central compartment and volume of distribution extrapolated. The calculations of these various volumes of distribution terms are shown in Equations 6 to 10. Initial distribution volume

( 6)

where Cpo is the concentration at zero time. Volume of distribution based on area

(7)

where \ z is the exponent of the terminal phase.

Volume of distribution at steady state for a bolus injection

(8)

where Q are the exponential coefficients and X, is the exponential constant to the ith term.

114

Xenobiotic Metabolism and Disposition Volume of central compartment

(9)

where Q is the exponential coefficient to the ith term. Volume of distribution extrapolate

( 10)

where Cz is the exponential coefficient for the terminal phase. In conceptual terms, the initial distribution volume, as the name suggests, is the volume in which the drug instantaneously distributes into after a bolus injection. This is equivalent to the volume of the central compartment if one is using a compartmental analysis. The volume of distribution based on area is the term that relates the drug concentration in plasma or blood to the total amount of drug in the body during the terminal exponential phase, once all distribution is complete. This is perhaps the most frequently calculated volume term and approximates, for many drugs, to what is probably the most useful of all the volume terms, the volume of distribution at steady state. The volume of distribution based on area suffers from the disadvantage that it is dependent on the elimination process, whereas the volume of distribution at steady state is the volume term in which a drug would appear to distribute if the drug existed throughout that volume at the same concentration as measured in plasma or blood. The advantage of volume of distribution at steady state over that of volume of distribution based on area is that it is independent of elimination. The difference can be illustrated with cephamandole, which is almost exclusively eliminated by the renal route. There is a marked difference in the clearance of the drug between normal subjects and subjects with complete renal failure. There is also a significant difference between the two groups of patients for volume of distribution based on area, because the elimination is different, but there is no significant difference in the volume of distribution at steady state. If one was to use a volume of distribution based on area, then one would be drawn to the erroneous conclusion that uremia was having an effect on the distribution of the drug. When elimination is only from the central compartment, the initial volume of distribution is less than the volume of distribution at steady state, which is less than the volume of distribution based on area. If elimination occurs from the tissue compartment, the volume of distribution at steady state tends to be greater than that based on area. The volume of distribution extrapolated, which is also a measure of the volume once all the drug has distributed, is calculated from the dose divided by the intercept of the terminal exponential line. This volume term tends to be greater than that of volume of distribution based on area, but is probably the least accurate volume term that can be determined. It is therefore not used very much. One question which is often asked is “ does the volume of distribution relate to a physiological volume?” The answer is usually no. In the human, if the volume of distribution is compared to some physiological volumes such as the volume of plasma or extravascular water and total body water, it is quite conceivable that the volume of distribution term might approach one of these volumes. If the volume of distribution is very low, such as approaching the 3 1 of plasma, it would indicate that the drug does not distribute readily and most of the drug stays within the plasma. However, if the volume of distribution is similar to that of extracellular or total body water, it does not necessarily mean that the drug distributes into these spaces, since only the unbound drug distributes and, in most cases, the calculated volume of distribution term relates to total drug (i.e., both unbound and bound drug).

115 50,000

I

t'

-"'

0 r-

Q)

...,1-


50

5

FIGURE 10. The variation of volume of distribution, plotted on logarithmic scale, between different drugs in man.

It was stated in Section II. A that the ratio of binding to macromolecules in plasma and tissue is a major determinant in drug distribution. This is reflected in the volume of distribution as shown in Equation ll. Valurne of distribution (V)

( II)

where VP and VT are the physical volumes of plasma and tissue , respectively , f" is the fraction unbound in plasma, and fuT is the fraction unbound in tissue. All this equation is saying is that the calculated volume of distribution is a function of the physical volumes of plasma (VP) and the tissues (VT) and the ratio of unbound drug in plasma (f.) and tissue (fuT). It can be seen that for those drugs which have a high affinity for tissues, the amount of unbound drug in tissues will be very small and, hence, the ratio of unbound drug in plasma to tissue will be !arge, making the final volume of distribution !arge. The range of volumes of distribution for different drugs can be very wide, and, as is seen in Figure lO, the volumes of distribution can be considerably !arger than the body volume of man (e.g., quinidine and chloroquine). High volumes of distribution drugs tend to be lipophilic basic compounds which have a high affinity for tissue proteins. At the other end of the spectrum, some acidic compounds, such as tolbutamide and warfarin , have

116

Xenobiotic Metabolism and Disposition

volumes of distribution not much larger than the plasma volume. These compounds tend to be ionized at physiological pH and extensively plasma protein bound, so that distribution is diffusion rate limited. D. Elimination Elimination has been defined as the irreversible transfer of the drug from the site of measurement and this can include metabolism, renal excretion, biliary excretion, elimination through the lungs, the sweat, milk, etc. Elimination differs from distribution in that the latter was defined as the reversible transfer of a drug to and from the site of measurement. Elimination includes metabolism. This is one of the main reasons why, when performing pharmacokinetic analyses, only one chemical species should be measured. Unfortunately, total radioactivity measurements are invariably nonspecific and are not normally suitable for pharmacokinetic analysis. If you cannot differentiate between the drug and its metabolites, then you cannot properly follow the elimination of a drug from the body, especially as each metabolite will also be undergoing its own elimination. This is exemplified by a radioactive plasma profile following an i.v. bolus injection of a radiolabeled compound which is sometimes seen. It is the profile that is quite common where there is an initial rapid decrease, followed by an increase to levels higher than the apparent zero time value, so that the profile looks more like that of an oral one rather than i.v. This type of profile is usually due to a rapid uptake of the unchanged drug by tissues, followed by metabolism of the drug and distribution of the metabolites into plasma. Because the metabolites tend to be more polar, their volume of distribution tends to be less so that the concentration in plasma tends to be higher than that of the intact drug. Propranolol is a good example of this, where the intact drug levels fall rapidly but the metabolite levels rise. If the total radioactivity profile of propranolol was used to predict what was happening to intact drug, completely the wrong conclusions would be obtained. 1. Renal Excretion Before examining the renal route of elimination it is first necessary to look at a single structural unit of the kidney, the nephron (Figure 11 ). As can be seen, there are four distinct regions: at the top there is the Bowmans capsule or the glomerulus, which is followed by the proximal tubule, then the distal tubule, and finally the collecting duct. The overall excretion of drugs by the renal route occurs by several processes, each of which occurs in different areas of the nephron. All unbound low molecular weight compounds undergo glomerular filtration in the Bowmans capsule. This produces an ultrafiltrate of the blood plasma which contains drug and metabolites in approximately the same concentration as the free drug in blood. The rate at which this occurs in man is approximately 125 ml min -1 and this is termed the glomerular filtration rate (GFR). It is often determined by measuring the creatinine or inulin elimination as clearance values (that is the volume of plasma or blood completely cleared of creatinine or inulin per unit time). The glomerular filtration rate decreases in the diseased kidney, and often creatinine clearance or inulin clearance is used as a measure of reduced kidney function. Elimination of those drugs that are extensively eliminated via the kidneys often correlates with the creatinine clearance when renal impairment occurs. Glomerular filtration occurs with all unbound low molecular weight compounds, but with some compounds, there is also active secretion of the drug from plasma into the tubular lumen. This occurs mainly along the proximal tubule. Furthermore, as it is an active process, compounds can ultimately be transferred into the tubular fluid against very high concentration gradients. There is a separate mechanism for acids and bases, but within each of these broad groups the process is relatively nonspecific, with competition between related compounds.

117

Bawman's Capsule - -f l !Giomerular filtra tion 1

Proximal tubule (Active secretionl

Distal tubule (Passive absorption and excreti on 1

--

FIGURE II . Stylized drawing of a kidney nephron.

A very well-known example of the competition between the tubular secretion of compounds within the same group is that between the ß-lactam antibiotics (such as the cephalosporins or penicillins) and probenecid. When such a competition occurs, the elimination of the antibiotic is effectively reduced. Consequently, probenecid is used experimentally to demonstrate renal tubular secretion of acidic drugs and is used clinically to inhibit tubularsecretion of some antibiotics so as to increase their duration of action. Using cefotaxime as an example, when probenecid is given as an i. v. infusion there is an immediate increase in the steadystate Ievels of cephalosporin. Unlike some other cephalosporins, cefotaxime is also metabolized and each of the metabolites is also eliminated by active tubular secretion, since, when probenecid is given, the steady state of these products also increases . The competition between probenecid and the ß-lactam antibiotics is an example of the acidic group compounds. An example of the basic compounds is that between digoxin and quinidine. This competition is potentially quite dangerous, since digoxin has a very narrow therapeutic window, and if coadministered with quinidine the Ievels of digoxin can increase to toxic Ievels.

118

Xenobiotic Metabolism and Disposition

Unlike the glomerular filtration which is dependent on the degree of plasma protein binding, renal tubular secretion is independent, since the drug tends to be stripped from the protein. A final, but very important factor when considering overall renal excretion of the drug is the reabsorption of the drug which takes place by passive diffusion throughout the length of the nephron. The ability of a drug to be reabsorbed will depend on its ability to cross this lipid membrane and, hence, its polarity and degree of ionization at the pH of tubular contents. As polar compounds do not readily dissolve in lipid, they have difficulty in crossing back from tubular contents into plasma. These compounds, therefore, tend to be extensively renally excreted. The pH of urine is an important variable when considering the renal excretion of weakly acidic or basic drugs. If the urine is acid and a drug is a base with a pKa of approximately 6 to 1 2 , the compound will be mainly ionized and will not be able to cross back from the tubular fluid to the plasma. Hence, the net renal excretion of these types of compounds will increase. More reabsorption occurs with bases when the urine is made basic and this will result in a decreased overall renal excretion of the compound. The opposite is true for acid compounds with a pKa between 3 and 7.5. Urinary pH, therefore, plays a very important part in the renal excretion of some drugs, but how many people measure it routinely? Sometimes the control of urinary pH is used clinically in treating overdose of the patients to aid drug elimination. The urinary pH, which is on average about 6 , can be made acid with ammonium chloride or alkaline with sodium bicarbonate. By controlling urinary pH in this way, experiments have been performed on the elimination of drugs such as phenobarbitone, amphetamines, and nicotine. In fact, sodium bicarbonate is used in treating overdose with phenobaritone, since this significantly increases the drug’s elimination. For those who smoke, it is also interesting to note that the pharmacokinetics of nicotine are affected by urinary pH. If your urine is alkaline, this tends to suppress the ionization of nicotine, so that it is more easily transferred from the renal tubular contents back into plasma and its excretion is diminished. For those with an acid urine, however, the ionization is increased so the excretion of nicotine also increases. This has been shown to have a significant effect on the halflife of nicotine as shown in Figure 12. A final consideration when examining the overall renal excretion of drugs is the effect of urine flow, since this is related to the rate of reabsorption of some drugs. Urinary excretion of drugs can, thus, be summarized: net renal excretion = rate of glomular filtration 4- rate of active tubular secretion — rate of passive reabsorption. Each of these processes should be thought of in relation to each particular drug. 2. Biliary Excretion Unlike renal excretion, this is an extremely complex and less well-understood process. Uptake of acidic compounds into liver may be facilitated by a selective binding onto proteins in the liver, one of which is there exclusively. Moreover, after pretreatment with phenobarbitone, its concentration increases with a concomitant increase in the uptake of organic acids in the liver. Drugs which are biliary excreted are secreted into bile against a concentration gradient; consequently, this requires an active process for which there is competition between compounds. There are several factors which affect the degree of biliary excretion of drugs. These include molecular weight and polarity. A strongly polar group aids biliary excretion. For all compounds with the molecular weight of less than 300 there is very little biliary excretion. Also, there is an upper molecular weight threshold, and very high molecular weight compounds, such as proteins, are poorly biliary excreted. Perhaps what is more important, when considering biliary excretion in relation to drug development, is a species difference in molecular weight threshold with compounds with a molecular weight between

119 100

'"e" Ö'

5

''l.~~

z

0

H

E-