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First Edition published 1983 This edition published in the Taylor & Francis e-Library, 2005.

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Second Edition published 1988

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British Library Cataloguing in Publication Data

Smith and Williams' introduction to the principles of drug design and action.—3rd ed.

1. Drugs—Design 2. Pharmacology I. Smith, H.J. (Harold John), 1930—II. Williams, Hywel III. Introduction to the principles of drug design and action 615.1

ISBN 0-203-30415-2 Master e-book ISBN

ISBN 0-203-34407-3 (Adobe eReader Format) ISBN 90-5702-037-8 (hard cover)

Front cover: A model of the active site of aromatase with the substrate audiosteredione

(yellow) as described by C.H.Laughton et al. (1993) Journal of Steroid Biochemistry and Molecular Biology 44, 399-407.

CONTENTS

Preface vi

List of Contributors vii

Abbreviations ix

Chapter 1 Processes of Drug Handling by the Body

D K Luscombe and P J Nicholls 1

Chapter 2 The Design of Drug Delivery Systems

I WKellaway 32

Chapter 3 Intermolecular Forces and Molecular Modelling

R H Davies and D Timms 60

Chapter 4 Drug Chirality and its Pharmacological Consequences

A J Hutt 121

Chapter 5 Quantitative Structure-Activity Relationships and Drug Design

J C Dearden and F K C James 202

Chapter 6 From Programme Sanction to Clinical Trials: A Partial View of the Quest for Arimidex™, a Potent, Selective Inhibitor of Aromatase

P N Edwards 253

Chapter 7 Pro-Drugs

A W Lloyd and H J Smith 285

Chapter 8 Design of Enzyme Inhibitors as Drugs

A Patel, H J Smith and J Stürzebecher 316

Chapter 9 The Chemotherapy of Cancer

D E Thurston and S G M J Lobo 403

Chapter 10 Neurotransmitters, Agonists and Antagonists

R D E Sewell, R A Glennon, M Dukat, H Stark, W Schunack and P G Strange 469

Chapter 11 Design of Antimicrobial Chemotherapeutic Agents

E G M Power and A D Russell 530

Chapter 12 Recombinant DNA Technology: Monoclonal Antibodies

F JRowell and JR Furr 599

Chapter 13 Bio-inorganic Chemistry and its Pharmaceutical Applications

D M Taylor and D R Williams 620

Index

PREFACE

The second edition of Introduction to the Principles of Drug Design was published in 1988. In the intervening years considerable strides have been made in the approaches to rational drug design as the result of the flood of knowledge coming from advances made in molecular biology. This has provided a better understanding of biological systems in terms of their structural components, cellular signalling, genomic modulation etc., leading to a more informed approach to chemotherapeutic intervention in disease.

In the third edition the aims and objectives, as well as the intended reading audience, remain the same as in previous editions but all the chapters have been revised to take into account of new developments in their subject areas and three new chapters have been included. Chapter 4 dealing with Drug Chirality and its Pharmacological Consequences reviews an ongoing field of considerable importance to pharmacologists and especially industrial concerns in view of the recent requirements imposed by Regulatory Bodies regarding drug registration. Chapter 6 provides a fascinating account of the difficulties inherent in the development of a drug from the bench to the clinic and brings out the trials and tribulations encountered by the multi-disciplinary research teams involved. Chapter 10 on Neurotransmitters, Agonists and Antagonists compensates to some extent for an area neglected in previous editions, that is, the design of drugs for action on the central nervous system, and also provides an account of membrane-bound receptors perhaps overshadowed in previous editions by emphasis on enzyme and DNA related targets.

Chapter 3 on Intermolecular Forces and Molecular Modelling has required expansion and revision due to advances in the techniques relating to ligand-receptor interactions and we are indebted to Zeneca, through Dr M.T.Cox, for their generosity in meeting the considerable cost of reproducing the necessary new colour plates in the book. We also wish to thank Dr Charlie Laughton of the School of Pharmacy, Nottingham University for providing the illustration on the front cover of the book.

LIST OF CONTRIBUTORS

Chapter 1

Professor David K Luscombe and Professor Paul J Nicholls

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK

Chapter 2

Professor Ian W Kellaway

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK

Chapter 3

Dr Robin H Davies

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK Dr David Timms

Zeneca Pharmaceutical, Alderley Park, Macclesfield, Cheshire SK10 4TG, UK

Chapter 4

Dr Andrew J Hutt

School of Pharmacy, Kings College, University of London, Manresa Road, London SW3 6LX, UK

Chapter 5

Professor John C Dearden

School of Pharmacy, John Moores University, Byrom Street, Liverpool, L3 3AF, UK fDr Kenneth C James

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK

Chapter 6

Dr Philip N Edwards

Zeneca Pharmaceuticals, CAM Department, Alderley Park, Macclesfield, Cheshire SK10 4TG, UK

Chapter 7

Dr Andrew W Lloyd

Department of Pharmacy, University Moulescoombe, Brighton BN2 4GJ, UK Dr H John Smith of Brighton, Cockcroft Building,

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK

Chapter 8

Dr Anjana Patel

The Royal Pharmaceutical Society, Lambeth High Street, London SE1 7JN, UK

Dr H John Smith

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK Dr Jörg Stürzebecher

Klinikum der Universität Jena, Zentrum für vasculäre Biologie und Medizin, Institut für Biochemie und Moleckularbiologie, Nordhäuser Strasse 78, D-99089 Erfurt, Germany

Chapter 9

Professor David E Thurston and Dr Sylvia G M Lobo

School of Pharmacy and Biomedical Sciences, University of Portsmouth,

Park Building, King Henry I Street, Portsmouth PO1 2DZ, UK

Chapter 10

Dr Robert D E Sewell

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK Professor Richard A Glennon and Dr Malgorzata Dukat

Department of Medicinal Chemistry, School of Pharmacy, Medical College of Virginia,

Virginia Commonwealth University, Richmond, Virginia 23298-0540 USA Dr Holger Stark and Professor Walter Schunack

Freie Universität Berlin, Institut für Pharmazie1, Königin-Luise-Strasse 2+4, D-14195 Berlin, Germany Philip G Strange

Research School of Biosciences, The University, Canterbury CT2 7NJ, UK

Chapter 11

Dr Edward G M Power

Department of Microbiology, United Medical and Dental Schools, Guy's Hospital, London Bridge, London SE1 9RT, UK Professor A Denver Russell

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK Chapter 12

Professor Frederick J Rowell

School of Health Sciences, University of Sunderland, Pasteur Building, Sunderland SR1 3SD, UK Dr James R Furr

Welsh School of Pharmacy, University of Wales Cardiff, Cardiff CF1 3XF, UK Chapter 13

Professor David M Taylor and Professor David R Williams

Chemistry Department, University of Wales Cardiff, Cardiff CF1 3XF, UK

ABBREVIATIONS

mx

molecular connectivities

f

hydrophobic constant

n

hydrophobic substituent constant

0

degree of freedom

G

Hammett substituent constant

G*

Taft's substituent constant

M-

dipole moment

standard partial free energy

AADC

aromatic amino acid decarboxylase

7-ACA

7-aminocephalosporanic acid

ACE

angiotensin I—converting enzyme

ADEPT

antibody—directed enzyme prodrug therapy

AG

aminoglutethimide

AGP

ai—acid glycoprotein

AIDS

autoimmune deficiency syndrome

AMP

adenosine monophosphate

ANP

atrial natriuretic peptide

ATP

adenosine triphosphate

6-APA

6-aminopenicillanic acid

2-APAs

2-arylpropionic acids

APN

aminopeptidase N

Ara-A

adenosine arabinoside, vidaribine

ARI

aromatase inhibition

ATP

adenosine triphosphate

AUC

area under plasma concentration vs time curve

AZT

azidothymidine, zidovudine

BiCNU

carmustine

BHT

butylated hydroxytoluene

c-AMP

adenosine 3',5'-cyclic phosphate

5-CAT

5-carboxamidotryptamine

CCNU

Iomustine

cDNA

copy DNA

CFCs

chlorofluorocarbons

ChAT

acetyl-CoA: choline O—transferase

^max

maximum plasma concentration

CMC

critical micelle concentration

CME

1-cyano-1-methyl-ethyl group

CMV

cytomegalovirus

CNBr

cyanogen bromide

CNS

central nervous system

CoMFA

comparative molecular field analysis

COMT

catecholamine methyltransferase

CSCC

side chain cleavage enzyme

CT

charge transfer

D4T

2',3'-didehydro-3'-deoxythymidine

Da

dalton

DAG

diacylglycerol

ddC

2',3'-dideoxycytidine

ddl

2',3'-dideoxyinosine

DHFR

dihydrofolate reductase

DHT

dihydrotestosterone

DICR

dose interval concentration

DMBA

dimethylbenzanthracene

DMSO

dimethyl sulphoxide

DOX

doxorubicin

DPI

dry powder inhaler

EAQ

eudismic affinity quotient

EBV

Epstein—Barr virus

ED50

effective dose for 50% response

EDTA

ethylenediamine-N,N,N',N'-tetraacetic acid

Es

Taft's steric substituent constant

EI

enzyme inhibitor complex

EIS

enzyme inhibitor substrate complex

ER

oestrogen receptor

Fab

antibody fragment retaining antigen binding properties

FSH

follicle stimulating hormone

5-FU

5-fluorouracil

GABA

y-aminobutyric acid

GABA-T

GABA transaminase

GC

guanidine-cytosine

G-CSF

granulocyte colony-stimulating factor

GDPT

gene-directed enzyme pro-drug therapy

GI

gastro-intestinal tract

GLcNAc

N-acetylglucosamine

GM-CSF

granulocyte-macrophage colony-stimulating factor

GSH

glutathione

GTP

guanosine triphosphate

HBV hepatitis B virus

HGH human growth hormone

HGPRT hypoxanthine-guanine-phosphoribosyl transferase

HIV human immunodeficiency virus

HLE human leucocyte elastase

HMG 3-hydroxy-3-methylglutaric acid

HNE human neutrophil elastase

HPMA N-(2-hydroxypropyl) methacrylamide

HPV human papilloma virus

HSA human serum albumin

HSAB hard soft acid base

5-HT 5-hydroxytryptamine

HTLV-1 human T-cell leukaemia virus

IC50 inhibitory concentration for 50% inhibition

IFN interferon

Ig immunoglobulin

IP3 1,4,5-inositol triphosphate

IUD idoxuridine

Kj equilibrium constant for breakdown of EI

Km Michaelis constant

LDL low density lipoprotein

LH luteinising hormone

LHRH luteinising hormone—releasing hormone

MAO monoamine oxidase

MDI metered dose inhaler

MDR multidrug resistant gene

MEC minimum effective plasma concentration

MEP membrane metalloendopeptidase

MO molecular orbital

MPS mononuclear phagocytic system

Mr molecular mass

MR molecular refractivity mRNA messenger ribonucleic acid

MSC maximum safe concentration a-MSH melanocyte stimulating hormone

MurNAc N-acetylmuramic acid

NADPH nicotinamide adenine dinucleotide phosphate

NAPAP Na-naphthylsulfonylglycyl-4-amidinophenylalanine piperidide

NMDA N-methyl-D-aspartate

NMR nuclear magnetic resonance

NSAID non-steroidal anti-inflammatory drug

ODC ornithine decarboxylase

4-OHA 4-hydroxyandrostenedione

8-OH DPAT 8-hydroxy-2(dipropylamino)tetralin

OM outer membrane of bacteria

OMP proteins of outer membrane of bacteria

P partition coefficient

P450 17 17a-hydroxy: 17,20-lyase

P450arom aromatase

P450scc side chain cleavage enzyme

PAB ¿-aminobenzoate

PAPS 3'-phosphoadenosine-5'-phosphosulphate

PBD pyrrolo[2,1] [1,4-c] benzodiazepine

PBPs penicillin binding proteins

PCMB ¿-chloromercuribenzoate pD2 -log Ka: where KA is the equilibrium constant between drug and receptor pDT photodynamic therapy

Penicillin G benzyl penicillin ai-PI ai-protease inhibitor

PIP2 phosphatidylinositol biphosphate pKa ionisation constant

PKC protein kinase C

PPE porcine pancreatic elastase

QSAR quantitative structure—activity relationships r correlation coefficient

Rb biological activity rv Van der Waals radius

RMM relative molecular mass

SAFIR structure-affinity relationship

SAH S-adenosylhomocysteine

SAM S-adenosylmethionine

SAR structure-activity relationship

SDAT senile dementia of the Alzheimer's type

SLPI secretory leukocyte protease inhibitor

SN1 unimolecular nucleophilic substitution

Sn2 bimolecular nucleophilic substitution

SRS slow releasing substance

; biological half-life

3TC 2'-deoxy-3'-thiathymidine

TCR therapeutic concentration ratio

TNF tumour necrosis factor tRNA transfer ribonucleic acid

TSAR tools for structure-activity relationships

UDP-GA uridine diphosphoglucuronic acid

VD volume of distribution

VDEPT virus-directed enzyme pro-drug therapy

Vmax maximum rate for enzyme reaction

Vw van der Waals volume

PROCESSES OF DRUG HANDLING BY THE BODY

DAVID K.LUSCOMBE and PAUL J.NICHOLLS

CONTENTS

1.1 INTRODUCTION 2

1.2 ABSORPTION 2

1.2.1 Transfer of drugs across cell membranes 3

1.2.2 Oral dosing 5

1.2.3 Rectal dosing 7

1.2.4 Topical application 7

1.2.5 Injections 8

1.3 DISTRIBUTION 9

1.3.1 Binding 9

1.3.2 Blood-brain barrier 11

1.3.3 Placental barrier 11

1.3.4 Partition into fat 11

1.4 METABOLISM 12

1.4.1 Phase I metabolism 13

1.4.1.1 Oxidations 13

1.4.1.2 Reductions 15

1.4.1.3 Hydrolyses 15

1.4.2 Phase II metabolism 16

1.4.2.1 Glucuronide formation 16

1.4.2.2 Sulphate formation 16

1.4.2.3 Methylation 16

1.4.2.4 Acylation 17

1.4.2.5 Glutathione conjugation 17 1.4.3 Factors affecting metabolism 17

1.4.3.1 Stereoisomerism 18

1.4.3.2 Presystemic metabolism 18

1.4.3.3 Dose-dependent metabolism 18

1.4.3.4 Inter-species variation 18

1.4.3.5 Intra-species variation 18

1.4.3.7 Inhibition of metabolism 19

1.4.3.8 Induction of metabolism 20

1.5 REMOVAL OF DRUGS FROM THE BODY 20

1.5.1 Renal elimination 21

1.5.1.1 Glomerular filtration 23

1.5.1.2 Active tubular secretion 23

1.5.1.3 Passive reabsorption across the renal tubules 24

1.5.1.4 Renal elimination in disease 24

1.5.2 Biliary elimination 25

1.5.3 Elimination in other secretions 25

1.6 SUMMARY 26 FURTHER READING 26

1.1 INTRODUCTION

To be useful as a medicine, a drug must be capable of being delivered to its site of action in a concentration large enough to initiate a pharmacological response. This concentration will depend on the amount of drug administered, the rate and extent of its absorption and its distribution in the blood stream to other parts of the body. The medicine will continue to act until the concentration of drug drops below its threshold for pharmacological activity either due to its removal (excretion) from the body in an unchanged form or after its metabolism to a more polar substance. The interrelationship between the absorption, distribution, metabolism and excretion of a drug is referred to as pharmacokinetics and describes how drugs are handled by the body. Such knowledge of a new drug is fundamental to the drug development process, to enable selection of the optimal dose, route and frequency of dosing to produce the desired clinical effect, without producing unwanted side-effects.

1.2 ABSORPTION

Whilst most medicines are taken by mouth and swallowed, other routes of administration include sublingual dosing in which the drug is placed under the tongue, rectal, inhalation, application to epithelial surfaces (skin patches), and injection either intravenously, intramuscularly or subcutaneously. With the exception of the intravenous route, in which the drug is administered directly into the bloodstream, a drug must initially be absorbed from its site of administration before it can enter the bloodstream and be distributed to its various sites of action. Clearly, the process of absorption is of fundamental importance in determining the pharmacodynamic and hence the therapeutic activity of a medicine. Delays or losses of drug during absorption may contribute to variability in drug response and may even result in a drug appearing to lack clinical effectiveness in some patients. Different formulations of the same active ingredient may lead to varying rates of absorption resulting in markedly different pharmacokinetic profiles in the same patient. Since the process of absorption involves the passage of a drug across one or more cell membranes, physico-chemical characteristics such as molecular size and shape, as well as solubility of the ionized and non-ionized forms will play an essential role in determining the overall pharmacodynamic activity of a drug. A basic knowledge of the physical and chemical principles governing the active and passive transfer of drugs across biological membranes is therefore necessary.

1.2.1 Transfer of drugs across cell membranes

Living cells are surrounded by a semipermeable membrane measuring approximately in thickness. The ease with which a drug passes across such a membrane will reflect the concentration of drug achieved in the tissues and body fluids and hence at its pharmacological site of action. In general, there are four ways by which substances are able to cross cell membranes; diffusion through the lipid component of the membrane, diffusion through aqueous channels or pores in the membrane, combination with an active carrier molecule, by pinocytosis. The commonest and most important mechanism by which drugs are transferred across biological membranes is by passive diffusion. Transfer takes place along a concentration gradient from a region of higher concentration to one of low concentration following a first-order rate reaction. The greater this concentration gradient, the greater the rate of diffusion of a drug across the cell membrane. However, the ease with which a drug passes across a membrane will depend on the characteristics of both the drug molecule and the cell membrane. The drug's partition coefficient between the lipid cell membrane and the aqueous environment is a major source of variability. Most drugs are weak acids or weak bases, existing in aqueous solution as an equilibrium mixture of non-ionized and ionized species. The non-ionized form is lipid soluble and therefore diffuses readily across cell membranes. In contrast, ionized compounds partition poorly into lipids and as a result are only slowly transported across biological membranes. In general, the higher the partition coefficient between lipid and water the more rapidly the drug is able to pass across cell membranes.

The ratio of non-ionized to ionized drug when in aqueous solution is pH-dependent and can be calculated from the general form of the Henderson-Hasselbach equation:

where pKa is the dissociation constant. For drugs that are weak acids, the acid form is in the non-ionized form whilst for drugs that are weak bases, the base form is non-ionized. Thus, a solution of the weak acid aspirin (pKa 3.5) in the stomach at pH 1 will have greater than 99% of the drug in the non-ionized form and consequently is lipid soluble and will be rapidly absorbed into the bloodstream. Likewise, other weak acidic drugs will be absorbed in the stomach because they exist largely in their non-ionized form at low pH values. In contrast, most basic drugs are so highly ionized in the acid content of the stomach that absorption is negligible whilst in the near neutral fluids of the small intestine the absorption of weak basic drugs such as codeine (pKa8) is rapid. Nevertheless, it should be pointed out that the absorption of all orally administered drugs, weak acids as well as weak bases, probably takes place more rapidly in the small intestine than in the stomach. This is because the gastric mucosa has a relatively small surface area and its covering of protective mucus provides a poor site for absorption compared with the large surface area provided within the small intestine. Consequently, whilst only 0.1% of aspirin is in its non-ionized form at pH 7.0, aspirin is well absorbed from the small intestine following oral dosing. Strong organic acids and bases such as sulphonic acid derivatives and quaternary ammonium bases, are ionized over a wide range of pH values resulting in low lipid solubility and in consequence, such drugs are poorly absorbed from the gastrointestinal tract when administered orally. The pKa values for a number of acidic and basic drugs are illustrated in Figure 1.1.

Figure 1.1 pKa values of some acidic and basic drugs.

Whilst most drugs cross cell membranes by passive diffusion, some drugs such as methotrexate and 5-flourouracil are carried by an active transport mechanism which requires the expenditure of metabolic energy. The carrier is a membrane component capable of forming a complex with the drug to be transported. The complex moves across the membrane releasing the drug on the other side. Not surprisingly, carrier-aided transport systems can be saturated, thus limiting the rate of transport. This is in contrast to the process of passive diffusion across lipid membranes, or passage through pores, where the amount of drug conveyed increases proportionally with an increase in concentration. Active transport processes take place in the gastrointestinal tract (e.g. amino acids), in the renal tubules, and across membranes dividing extracellular from intracellular compartments at the blood-brain and placental barriers.

Water-soluble substances such as alcohol are able to readily diffuse through the aqueous channels or pores in cell membranes providing their molecular weights are not greater than 100-200 Da. Since most drugs fall within the molecular weight range 2001000 Da, diffusion through these aqueous pores is unimportant for almost all substances with the exception of water, alcohol and other small polar molecules. Drug molecules can also be transported across cell membranes by an active uptake process similar to phagocytosis called pinocytosis. This involves the invagination of part of the cell membrane and the trapping of drops of extracellular fluid containing solute molecules which are thus carried through the membrane in the resulting vacuoles. Whilst this mechanism appears important in the absorption of some large molecules such as insulin which crosses the blood brain barrier by this process, pinocytosis is of little importance in the transport of small molecules across biological membranes except possibly in the case of oral vaccines. However, this process may become important if liposomes are used as a means of targeting a drug at a specific site of action since they may be taken up selectively by cells capable of pinocytosis.

1.2.2 Oral dosing

The most common route of drug administration is by swallowing. This provides a convenient, relatively safe and economical method of dosing which, subject to the drug being presented in a palatable and suitable form, is the route preferred by most patients. Normally, about 75% of a drug given orally will be absorbed in 1 to 3 hours after dosing. To be effective a drug must be stable in the acid of the stomach fluids and not cause irritation of the gastrointestinal mucosa which might induce nausea and vomiting. It should not pass too rapidly through the stomach or interact with other drugs being administered concurrently. Whether the drug is formulated as a tablet, capsule or liquid preparation the most important site for drug absorption is the small intestine because it offers a far greater epithelial surface area for drug absorption than other parts of the gastrointestinal tract. Apart from the above, many other factors influence the rate and extent of drug absorption such as the physico-chemical properties of the drug, particle size, its concentration at the absorption site and splanchnic blood flow. In fact, the intestine has an excellent blood supply which ensures that any absorbed drug is rapidly transported into the bloodstream as soon as it passes through the intestinal membrane, maintaining a concentration gradient across the membrane. For highly lipid-soluble drugs, or those that pass freely through the aqueous-filled pores, passage across a membrane may be so rapid that equilibrium is established between the drug in the bloodstream and that at the site of absorption by the time the blood is removed from the membrane. In such cases, the rate-limiting step controlling drug absorption is blood flow and not transportation across the intestinal cell membranes.

Drug absorption following oral dosing is generally favoured by an empty stomach. Food will effectively reduce the concentration of drug in the gastrointestinal tract which will limit its rate of absorption although not the total amount of drug absorbed.

Furthermore, gastric emptying will be delayed slowing the onset of action of drugs such as antibiotics, analgesics and sedatives. In particular, gastric emptying is slowed by fats and fatty acids in the diet, and bulky or viscous foods. Some disorders will also slow gastric emptying, for example, mental depression, migraine, gastric ulcers and hypothyroidism whilst many drugs including propantheline, imipramine and the antacid aluminium hydroxide will all produce the same effect. In contrast, factors which promote gastric emptying will result in an increased rate of absorption of nearly all drugs. Such factors include fasting or hunger, alkaline buffer solutions, diseases such as hyperthyroidism and the anti-emetic agent, metoclopramide. Generally, the gastric emptying of liquids is much faster than that of solid food or solid dosage forms. It is for this reason, that tablets and capsules should be taken orally with at least half a glassful of water. In contrast, drugs known to irritate the gastric mucosa, for example, anti-inflammatory agents, should be taken immediately after a meal, even though this may decrease its rate of absorption, as the likelihood of induced nausea will be diminished.

The term bioavailability is used to describe the proportion of orally administered drug that passes unchanged into the bloodstream. It is particularly useful because it takes into account absorption and any local metabolic degradation that takes place in the stomach and small intestine. Bioavailability is also influenced by gastrointestinal motility, gastric pH, drug solubility, the presence or absence of food in the gastrointestinal tract and the formulation of the dosage form administered (particularly when a drug is prepared by different manufacturers). Benzylpenicillin, the only naturally occurring penicillin in clinical use, is destroyed by gastric acid and therefore has to be administered by injection. Ampicillin in contrast is acid stable and in consequence can be given orally. However, its bioavailability is variable and absorption is incomplete. In an attempt to improve absorption following oral dosing, lipophilic esters have been prepared with some success. Whilst esters of penicillins are inactive in vivo, once absorbed they are hydrolysed to release the active penicillin (see Section 7.4.1). As a result, a number of these so-called 'prodrugs' have been successfully developed. This prodrug approach to increasing bioavailability has also been used with angiotensin-converting enzyme (ACE) inhibitors (see Section 7.4.1).

Once absorbed from the gastrointestinal tract, an orally administered drug will enter the portal blood circulation and pass immediately to the liver before entering the systemic circulation and delivery to its site(s) of action. On passing through the liver, the drug may be partially or completely metabolized by hepatic microsomal enzymes to less active metabolites or be excreted in the bile from where it passes into the small intestine. These processes may result in a marked reduction in the amount of unchanged (active) drug that is available to exert a pharmacological effect, a phenomenon known as first-pass metabolism. Oral dosing is clearly inappropriate for drugs such as lignocaine which undergo an extensive first-pass effect. Despite rapid absorption from the gastrointestinal tract, lignocaine is so extensively degraded by the hepatic microsomal enzyme system on its initial passage through the liver that the remaining lignocaine level in the peripheral blood circulation is inadequate to exert its therapeutic effect.

For drugs which are absorbed through the mucosa of the buccal cavity when placed under the tongue and allowed to dissolve, first-pass metabolism can be avoided. This sublingual route of administration is not often encountered, but since the drug does not have to enter the stomach or intestines to exert its effect, absorption is generally more rapid than after swallowing and the drug is likely to be effective at a lower dose. Hydrolytic enzymes in the intestinal mucosa inactivate glycerol trinitrate and is the reason for this anti-angina drug being administered sublingually rather than being swallowed. This route offers the patient the opportunity to terminate the therapeutic effect once relief has been achieved simply by spitting out the dosage form from under the tongue. Drugs with an unpleasant taste cannot be administered sublingually and neither can high molecular weight substances which are only poorly absorbed through the buccal cavity mucosa.

1.2.3 Rectal dosing

Some drugs cause nausea and vomiting when given orally and these may be formulated as suppositories or enemas and given rectally to be absorbed in the rectum. Drugs administered rectally are not subject to first-pass metabolism in the liver, however, absorption is generally irregular, unpredictable and incomplete. Apart from being used in the treatment of constipation or to evacuate the bowels before surgery, this route is generally avoided.

1.2.4 Topical application

The application of drugs to the skin or mucous membranes, such as the conjunctiva, nasopharynx or vagina, is used principally for local effects. However, in the past decade a number of drugs have been successfully formulated as self-adhesive skin patches. When placed on the skin, these patches slowly release drug which passes across the skin and into the systemic blood circulation to produce a generalised effect in the body. For example, glyceryl trinitrate when formulated as a transdermal patch will slowly and continuously release drug into the bloodstream providing prophylactic treatment for angina over a 24 h period. The patch is replaced daily, using a different area of the body on each occasion. The same drug has also been formulated as an ointment which can be applied to the chest, abdomen or thigh without rubbing in, being secured with a dressing. This provides short-term prophylactic cover for angina, being repeated every 3-4 hours as required. Self-adhesive nicotine patches are widely available for smokers who wish to give up the habit. They are applied on waking to dry, non-hairy skin on the hip, chest or upper arm, being removed before retiring. The siting of the replacement patch should be on an unused area, used areas being avoided for several days. Nasal sprays containing nicotine are also available for people wishing to stop cigarette smoking. Nicotine is delivered into each nostril as required up to a maximum of two sprays an hour for 16 hours a day. The strong opioid analgesic, fentanyl, has been introduced recently in a transdermal drug delivery system as a self-adhesive patch which provides pain relief for up to 72 hours before needing to be replaced. Women requiring hormone replacement therapy are now offered oestrogen formulated either as a self-adhesive skin patch or a gel preparation in addition to tablet dosage forms. The anti-motion sickness drug hyoscine hydrobromide is likewise available in a self-adhesive patch dosage form being placed on a hairless area of skin behind an ear some 5-6 hours before travelling as protection against motion sickness.

1.2.5 Injections

The most common method of introducing a drug directly into the bloodstream is to inject it intravenously. This route is particularly useful when a rapid therapeutic response is required since absorption is circumvented. It is used for the induction of anaesthesia, relief from some epileptic seizures and for administering antibiotics such as benzylpenicillin which is inactivated by gastric acid if given orally. On intravenous dosing, the drug is rapidly removed from the injection site, being diluted in the venous blood as it is carried initially to the heart and then to other tissues. Since the total circulation time in humans is of the order of 15s, the onset of drug action is almost immediate. Drugs delivered by the intravenous route may be administered either as a single rapid injection lasting only 1-2 minutes, known as a 'bolus' injection, or as a slow infusion lasting an hour or longer. This latter choice is preferred when a sustained level of drug is required in the bloodstream over a relatively long period (e.g. antibiotics for life-threatening infections in hospitalized patients). It is also useful for administering large drug volumes and for diluting otherwise irritant substances. This route is not suitable for water-insoluble drugs and suffers the disadvantage that the dosage form must be sterile. Furthermore, intravenous administration must be by trained personnel and great care is needed to ensure that overdosage is avoided, since the rapidity of drug action may not permit the reversal of any drug-induced toxicity. Due to stability problems most antibiotics such as cloxacillin, flucloxacillin, amoxycillin for intravenous use are provided as a dry sterile powder (i.e. sodium salt) to be reconstituted with water for injection before use.

Drugs may also be administered by intramuscular injection enabling the exact quantity of drug to be delivered to a localized site such as the deltoid muscle of the arm, the vastus lateralis of the thigh or the gluteus maximus of the buttocks. From these muscular sites the drug must be absorbed before passing into the general circulation. Factors which influence absorption from these sites include the vascularity of the injection site, the degree of ionization and lipid solubility of the drug, volume of injection, and osmolarity. The intramuscular route is often used in patients who are unable to swallow oral medication, for drugs which are poorly absorbed from the gastrointestinal tract, or for drugs which undergo extensive first-pass metabolism. For example, 4-hydroxyandrostenedione is a potent mechanism-based enzyme inactivator of aromatase used to lower oestrogen levels in post-menopausal women with breast cancer. This has to be administered as an intramuscular injection to avoid extensive first-pass metabolism to the inactive glucuronidated conjugate which takes place following oral dosing. Drugs administered by intramuscular injection generally exert their pharmacological effect more rapidly than after oral dosing. However, intramuscular injections tend to be painful and are not generally favoured by patients. Nevertheless, a number of penicillins, such as cloxacillin, flucloxacillin and ampicillin may be given intramuscularly as an alternative to oral dosing.

The subcutaneous route of injection is only suitable for small dose volumes and few drugs are currently administered by this route with the notable exception of insulin. Drugs given by this route spread out through the loose connective tissue of the subcutaneous layer. Since the skin is rich in sensory nerves, subcutaneous injections are more painful than intramuscular injections. In general, a subcutaneous injection results in faster absorption than a corresponding intramuscular injection, although the difference is not marked and of little clinical significance. Absorption is influenced by the same factors that determine the rate of drug absorption from intramuscular sites. However, the blood supply to subcutaneous layers may be poorer than in muscle tissues and in consequence absorption may appear slower. In both instances, the local action of an injected drug can be prolonged by decreasing its rate of removal from the site of injection. The action of subcutaneously administered drugs can also be sustained if drugs are injected as solutions in oil, the drug diffusing out only slowly from the vehicle. Oestradiol may be administered in the form of a solid pellet which is implanted under the skin, the active hormone slowly dissolving in the tissue fluid before diffusing through the capillary walls and into the bloodstream. Such implants have the benefit of remaining effective for 4 to 8 months.

1.3 DISTRIBUTION

Once absorbed into the bloodstream most drugs are distributed throughout body fluids and tissues with relative ease. The pattern of distribution depends on the drug's permeability, lipid solubility and capacity to bind to macromolecules (largely proteins). The apparent volume of distribution (VD) is a useful term to describe a drug's pattern of distribution. It represents the volume in which the drug appears to be dissolved in a body fluid (i.e. compartment) and is a proportionality constant relating drug concentration to the total amount of drug in the body. A drug such as heparin whose distribution is largely restricted to the plasma compartment has a small volume of distribution (i.e. 0.05 litre kg-1) whilst nortriptyline (22-27 litre kg-1) has a large volume of distribution which in fact is in excess of total body water (about 0.6 litre kg-1). This indicates that nortriptyline is not only widely distributed throughout total body water but is being accumulated or stored in extravascular sites. Generally, weak bases have a large VD value owing to their lipid solubility and as a result will be present in low concentrations (i.e. ng ml-1) in plasma (eg. diazepam, morphine, imipramine). The reverse situation applies to weakly acidic drugs which will tend to exhibit high (i.e. pg ml-1) plasma concentrations (eg. aspirin, sulphamethoxazole).

For those drugs with a molecular weight of less than 600 Da, and which are being transported as free drug in solution in plasma water, transfer from blood vessels out into interstitial fluid is rapid. This is because capillary walls generally behave like a leaky sieve. The lining endothelial cells of the capillary have junctions with each other that are not continuous (i.e. loose), and allow free passage of such relatively small molecules across the capillary wall. This is important for polar compounds, however, both this route and diffusion through the actual capillary wall are also available pathways for lipid-soluble compounds.

1.3.1 Binding

An important factor in the distribution of drugs is their binding to macromolecules such as plasma proteins. Such binding is generally a reversible process. The extent to which a drug is bound to plasma proteins will influence its pharmacological profile. This is because it is only that fraction of a drug which is in solution in the plasma (unbound) that is free to cross cell membranes and interact with receptors thus effecting a pharmacological response. Drug-protein binding complexes have such high molecular weights that they will not cross cell membranes and are in effect pharmacologically inactive (i.e. the drug is protein bound). In plasma, the main protein for drug binding is albumin while the binding forces involved may be ionic, van de Waals', hydrogen and/or hydrophobic bonds. Since binding is mostly reversible, there is an equilibrium in plasma between bound and unbound drug, this interaction following the Law of Mass Action. Plasma drug binding depends on the association constant of the drug, the number of binding sites and the concentration of both drug and plasma protein. As the plasma concentration of drug gradually increases following absorption, the fraction of drug in its free form rises slowly at first, but as the protein binding sites become saturated this fraction rises sharply. In practice, the fraction of free drug in the plasma is essentially constant over the range of therapeutic concentrations for most drugs. Saturation is most likely to occur with drugs which have high association constants and are administered in high doses, such as sulphonamides.

The extent to which drugs are bound to plasma proteins, particularly albumin, is variable depending on their physico-chemical characteristics. Examples of drugs which bind to albumin are presented in Table 1.1. Since protein-binding sites are non-specific, one drug can displace another thereby increasing the proportion of free (unbound/active) drug to diffuse from the plasma to its site of action. This is only clinically important if the drug is firstly, extensively bound (greater than 90%) and secondly, is not widely distributed throughout the body (i.e. warfarin VD=0.05 litre kg-1). Hence, the pharmacological activity of warfarin is markedly increased when administered concurrently with

Table 1.1 Some drugs that bind to plasma albumin.

Drug

Binding

Diazepam

95-99%

Diclofenac

Warfarin

Amitriptyline

90-95%

Chlorpromazine

Imipramine

Nortriptyline

Tolbutamide

90-95%

Valproic acid

Phenytoin

90%

Hydralazine

80-90%

Sulphadimidine

60-80%

Aspirin

45-60%

Lignocaine

Sulphadiazine

sulphonamides due to displacement of the former by the latter drug. This leads to higher free (unbound) warfarin concentrations in the plasma which potentiates its anticoagulant effect which could lead to fatal haemorrhage. It is important to view the degree of binding to plasma proteins in relation to a drug's apparent volume of distribution. Thus, while nortriptyline is 93% bound under steady state conditions, the drug concentration in plasma is less than 1% of the total amount of drug in the body and any displacement by another drug will be clinically insignificant.

1.3.2 Blood-brain barrier

Penetration of drugs from the blood-stream into the brain and cerebrospinal fluid is restricted by a specialised protective lipid membrane, the blood-brain barrier. Whilst highly lipid-soluble compounds reach the brain rapidly following dosing, more polar compounds penetrate at a much slower rate and highly polar drugs will not cross into the brain under normal circumstances. As a general rule, the rate of passage of a drug into the brain is determined by its degree of ionization in the plasma and its lipid solubility. Thus, penicillin which is highly ionized will be excluded from the brain unless very large doses are administered. However, the permeability of the blood-brain barrier can be increased by infections which lead to meningeal or encephalic inflammation. This is the reason why penicillin is used in the treatment of meningococcal meningitis.

1.3.3 Placental barrier

Foetal blood is separated from maternal blood by a cellular barrier the thickness of which is greater in early pregnancy (25 pm) than in the later stages (2 pm). Although specific transport systems for endogenous materials are present in the placenta and may provide a method for transporting some drugs such as methyldopa and 5-fluorouracil, it appears that most drugs cross the placenta by passive diffusion. Thus, penetration is rapid with lipid-soluble non-ionized drugs but slow with very polar compounds. However, some degree of foetal exposure is likely to occur with most drugs and so caution is required with drug administration during pregnancy. Some drugs such as the sulphonamides readily cross the placental barrier and may reach concentrations in the foetal blood circulation high enough to be antibacterial and lead to toxicity.

1.3.4 Partition into fat

Lipid-soluble drugs may achieve high concentrations in adipose tissue, being stored by physical solution in the neutral fat. Since fat is normally 15 per cent of body weight (in grossly obese subjects it can be as high as 50 per cent), it can serve as an important reservoir for such drugs. It also has a role in terminating the effects of highly lipid-soluble compounds by acting as an acceptor of the drug during a redistribution phase. Thus, after intravenous injection, thiopentone enters the brain rapidly, but also leaves it rapidly because of falling plasma levels and this terminates its pharmacodynamic action. It then slowly redistributes into fatty tissues where as much as 70 per cent of the drug may be found 3 h after administration.

1.4 METABOLISM

Most drugs, prior to removal from the body, are subjected to biotransformation (metabolism). The enzymic reactions leading to such changes are classified as Phase I reactions (asynthetic changes) and Phase II reactions (conjugations). As the original compound is chemically altered by these means, metabolism may be considered as a drug elimination mechanism although the problem of excreting the metabolites remains. In most instances, the metabolites have a markedly different partition character from the parent compound, in that lipophilicity is decreased. Such products tend to be easily excreted, as they are not readily reabsorbed from the renal tubular fluid. Drug metabolites also often have a smaller apparent volume of distribution than their precursors.

Metabolism influences the biological activity of a drug in a number of ways (see Table 1.2). In many instances, pharmacological activity is reduced or lost by metabolism and for such drugs this may be an important determinant of duration of action and even intensity of effect. Occasionally a drug may be transformed into a metabolite possessing a pharmacological effect of comparable intensity (see Chapter 7).

The benzodiazepines are a good example of this phenomenon. Thus the major metabolite of diazepam, N-desmethyldiazepam, has a similar pharmacological potency and long half life (t0.5). Minor metabolites of diazepam are temazepam and oxazepam which are also active. For a relatively small number of drugs (prodrugs), biologically

Table 1.2 Examples of effect of drug metabolism on pharmacological activity.

Effect_Drug_Metabolic reaction

Deactivation

Table 1.2 Examples of effect of drug metabolism on pharmacological activity.

Effect_Drug_Metabolic reaction

Deactivation

Drug metabolite

Aminoglutethimide Conjugation (with

less active than

Amphetamine

acetic acid)

parent molecule

Barbiturates

Oxidation

or inactive

Chloramphenicol

Oxidation

Procaine

Conjugation (with

Tolbutamide

glucuronic acid)

Hydrolysis

Oxidation

Trans-activation

Drug metabolite

Diazepam

Oxidation (to

possessing

Phenylbutazone

nordiazepam)

equivalent

Propranolol

Oxidation (to

activity to parent Procainamide

oxyphenylbutazone)

molecule

Oxidation (to 4-

hydroxypropranolol)

Conjugation (to N-

acetyl procainamide)

Activation

Activation

Reduction (to trichloroethanol) Oxidation (to nordiazepam) Hydrolysis (to chloramphenicol) Oxidation (to cycloguanyl) Reduction (to sulphanilamide)

Oxidation (to malaoxon) Oxidation (to formaldehyde and formic acid) Oxidation (to an electrophilic imido-quinone)_

inactive per se, metabolic activation is a prerequisite for therapeutic utility (see Chapter 7), e.g. the popular angiotensin-converting enzyme inhibitor, enalapril, is hydrolysed in vivo to its active form enalaprilat.

An interesting example of the application of this principle to achieve selectivity of pharmacological action is the anti-epileptic drug vigabatrin (y-vinyl-GABA) which is a substrate for the neuronal GABA-ketoglutarate transaminase responsible for inactivating the inhibitory neurotransmitter GABA. The resultant metabolite of vigabatrin is an irreversible inhibitor of the transaminase and this action leads to increased levels of GABA in the brain. Finally, a growing list of drugs and other xenobiotic compounds is metabolized to intermediates that may subsequently react with tissue macromolecules leading to toxic effects. For example, it is considered that the occurrence of haemorrhagic cystitis in bone marrow transplant patients receiving cyclophosphamide is related to the drug's metabolism to the toxic compound, acrolein.

The main site of drug metabolism is the liver, followed by the gastointestinal tract. However, metabolism also occurs in the kidney, lung, skin and blood but, quantitatively, these sites are less important.

1.4.1 Phase I metabolism The Phase I reactions are oxidation, reduction and hydrolysis.

Metabolite is responsible for (pro-) drug activity

Chloral hydrate Chlorazepate Palmitic ester of chloramphenicol Proguanil Prontosil red

Toxification

Drug metabolite possessing toxic effects

Malathion

Methanol

Paracetamol

1.4.1.1 Oxidations

Many of the oxidation reactions, such as aliphatic and aromatic hydroxylation, epoxidation, dealkylation, deamination, N-oxidation and S-oxidation (see Table 1.3), are catalysed by enzymes (mixed function oxidases) bound to the endoplasmic reticulum. This latter is a branching tubular system within cells that is also involved in protein synthesis and lipid metabolism. When a tissue such as liver is homogenized, this reticulum fragments into rounded bodies (microsomes) sedimenting at 10-100 S. Many metabolic oxidations have been studied using this microsomal enzyme fraction. The terminal oxygen transferase of the system is cytochrome P450. This is coupled to the flavoprotein enzyme, cytochrome P450-reductase, and linked to NADPH as a source of electrons. Under the influence of cytochrome P450, an oxygen atom from molecular oxygen is transferred to a drug molecule (DH^-DOH). The remaining oxygen atom combines with two protons to yield a molecule of water. Thus the enzyme is characterised as a mixed function oxidase. Cytochrome P450 is so named because its reduced carbon monoxide-ligand spectrum has a maximum absorption at 450 nm. It is now known that cytochrome P450 and its reductase both exist in multiple forms and the cytochrome P450 variants appear to possess overlapping substrate specificities.

The differences between the cytochrome P450 isozymes are due to modified sequences of the amino acids in the protein of this haemoprotein. A general nomenclature for the isoforms based on structural homology has been agreed. Thus P450 proteins from all sources with a 40% or greater sequence identity are included in the same family (designated by an Arabic numeral). Those isoforms with greater than 55% identity are then included in the same sub-family (designated by a capital letter). The individual genes (and gene products) are then arbitrarily assigned a number. An example is the major phenobarbitone-inducible cytochrome (see 1.4.3.8) in rabbit liver microsomes. This was originally called form 2 or P450LM2. With the present system, this enzyme has

Table 1.3 Some microsomal oxidations.

been assigned to family 2 and sub-family B; the gene and the enzyme are designated CYP2B4. The enzyme may also be styled P450 2B4.

In addition to an ability to bind to cytochrome P450, requirements of a substrate for metabolism by this system include: a molecular size above 150 p (below this size, compounds are normally capable of ready excretion), a sufficient degree of lipophilicity to enter the endoplasmic reticulum and the appropriate chemical substituents. Chemical reactivity at sites on a molecule influences the site of oxidative enzyme attack. Thus, with nitrobenzene, the main oxidative metabolite is 3-hydroxynitrobenzene, while with aniline, 2- and 4-hydroxyaniline are the major ring-oxidized products.

While most interest in the cytochrome P450 system is focused on drug metabolism, it must be recognised that several of the isozymes are responsible for the biotransformation of endogenous compounds such as steroid hormones, leukotrienes, prostaglandins, vitamins and free fatty acids.

A non-cytochrome P450-dependent microsomal flavoprotein oxidase has been described in liver that effects sulphoxidation of nucleophilic sulphur compounds (e.g. methimazole), hydroxylamine formation from secondary amines (e.g. desipramine, nortriptyline) and amine oxide formation from tertiary amines (e.g. brompheniramine, guanethidine).

Oxidations are also carried out by non-microsomal enzymes such as alcohol and aldehyde dehydrogenases and monoamine and diamine oxidases. Although the oxidations are less varied than those of the microsomal enzymes, they are important pathways for several naturally occurring compounds as well as drugs.

1.4.1.2 Reductions

Only a small number of drugs is metabolized by reduction, the reductases being located at both microsomal and non-microsomal sites. Some reductases are also found in the micro-organisms of the gut. Aromatic azo and nitro compounds are reduced by microsomal flavoprotein enzymes. The nitro-reductase converts the substrate (e.g. chloramphenicol, nitrazepam) to the corresponding amine by the following sequential reactions: *rNO: —> ArNO —> ArNHQH -> ArNH^ Azo-reductase effects a reductive cleavage of its substrate by the following sequence: Ar'N = NAr" Ar"NH-NHAr" -> Ar'NH, + (e g prontosil red^sulphanilamide+1,2,4 triaminobenzene). There is a marked azo-reductase activity in the gut microflora. A hepatic microsomal enzyme, requiring NADPH and oxygen, is responsible for replacing halogen with hydrogen in aliphatic halogenated compounds such as halothane, methoxyflurane and carbon tetrachloride (e.g. CCl4^CHCl3). Examples of reductions carried out by non-microsomal enzymes are the transformation in the blood of disulphiram ((C2H5)2 NCSS-SSCN(C2H5)2) into diethyldithiocarbamate ((C2H5)2NCSSH) and the reduction of chloral hydrate to trichloroethanol by alcohol dehydrogenase.

1.4.1.3 Hydrolyses

Drugs containing an ester group may be hydrolysed by esterases which have both microsomal and non-microsomal locations. The former tend to be more concentrated in the liver. Such an enzyme is responsible for the hydrolysis of pethidine. The non-microsomal esterases occur in blood and some tissues; procaine is metabolized by a plasma esterase. The esterases also hydrolyse amides (e.g. procainamide), though more slowly than the corresponding esters. Epoxide hydrases, present in the microsomal fraction of many tissues, convert epoxides to the corresponding dihydrodiols. This is an important detoxifying reaction for reactive electrophilic epoxides formed as a result of metabolism. A (minor) epoxide metabolite of phenytoin is possibly associated with a higher than normal incidence of neonatal cleft palate when phenytoin is administered to pregnant women. It is likely that women at risk are those in whom there is a relative deficiency of the epoxide hydrase(s) responsible for the inactivation of the toxic metabolite.

1.4.2 Phase II metabolism

Phase II metabolism involves the coupling of a drug or its metabolites with various endogenous components. The reaction, which is carried out by a transferase enzyme, requires that either the endogenous or the exogenous component is activated prior to conjugation. Although generally considered to be detoxication pathways, conjugation reactions may result in "metabolic activation". An example of this is where the 6-glucuronide of morphine acts as a carrier molecule allowing its ready passage across the blood-brain barrier. In the brain, the conjugate is cleaved by a hydrolase releasing the active molecule, morphine. Under the influence of tissue deacetylases, the N-acetyl conjugate of isoniazid may give rise to the hepatotoxin, N-acetylhydrazine.

1.4.2.1 Glucuronide formation

Probably the most common conjugation pathway is that of glucuronide formation. The combination with glucuronic acid occurs with compounds possessing a functional group with a reactive proton, usually attached to a hetero-atom (e.g. hydroxyl, carboxyl, amino and sulphydryl). Th

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