Basic science

Clinical pharmacology – the basicsJohn Posner

Abstractclinical Pharmacology is the study of pharmacodynamics and pharma-

cokinetics in humans. The relationship between dose, concentration

and pharmacodynamic response may be explored using biomarkers for

both desired and undesired effects of drugs. The action of drugs is due

to receptor affinity, efficacy and the concentration of drug at the site

of action. Factors affecting the concentration of drug include bioavail-

ability, determined by absorption and pre-systemic metabolism, distri-

bution, clearance and elimination. Drug interactions may occur due to

pharmacodynamic and pharmacokinetic factors, many of the latter be-

ing attributable to changes in drug metabolism. as well as contributing

to our understanding of established treatments, clinical pharmacology

plays a critical role in the development of new medicines.

Keywords affinity; biomarkers; clinical pharmacology; drug interactions;

efficacy; pharmacodynamics; pharmacokinetics; potency; receptors

Introduction

What is clinical pharmacology?Clinical Pharmacology (CP) is the study of drug activity in man. It is a very broad subject involving a variety of disciplines but perhaps it is helpful to conceptualise CP as comprising two components: the study of drug action on the body, pharmaco-dynamics (PD), and the study of the body’s effect on the drug, pharmacokinetics (PK).

Clinicians are used to thinking about drugs in terms of dose, frequency and duration of dosing, benefits and side effects. This is perfectly sensible in terms of every-day practice but makes no attempt to understand their inter-relationships. The very term ‘side effects’ tends to make us think of them as conceptually dif-ferent from the ‘main effects’ when in reality, they are all phar-macodynamics, some of which are desired and others undesired. What relates the dosage to the PD is the concentration of drug at its site/s of action and that is determined, not only by the dose but also by the way the body handles the drug, the PK.

John Posner PhD FRCP FFPM is an Independent Consultant in

Pharmaceutical Medicine and Visiting Senior Lecturer, School of

Biomedical and Health Sciences, King’s College London, UK. He is

also Director of programmes for the Diploma and Certificate in Human

Pharmacology of the Faculty of Pharmaceutical Medicine of the Royal

Colleges of Physicians of the UK. Conflicts of interest: none declared.

sURGeRY 27:4 15

In this article we discuss some of the principles underlying PD and PK and how they apply to the use of well-established thera-peutic drugs and to the evaluation of potential new medicines.

Pharmacodynamics

Concomitant drug administration Assessment of dose-concen-tration-responseBiomarkers may be used to explore the relationships between dose, concentration and response. A biomarker is defined as a characteristic that is measured and evaluated as an indicator of normal biological or pathological processes or pharmacological responses to a therapeutic intervention. Biomarkers may be func-tional e.g. cognition, blood flow or mechanistic e.g. the activity of an enzyme, hormone or gene. Caution must always be exer-cised in the interpretation of data and biomarkers must undergo extensive validation, particularly if used to investigate novel classes of drugs, when their ability to serve as surrogates of clini-cal endpoints has not been established.

To illustrate the use of biomarkers, let us first consider the actions of an inhaled anti-inflammatory drug for asthma. We can measure a bronchodilator effect by administering the drug over a range of doses to a subject with constricted airways and mea-sure airway calibre indirectly by the forced expiratory volume in 1 second (FEV1) or airways conductance (sGaw) using plethys-mography. If the drug is an antagonist of an inflammatory media-tor involved in asthma e.g. leukotrienes, we might administer a bronchoconstrictor challenge with a leukotriene agonist prior to administering the antagonist.

Another approach would be to induce an allergic inflammatory response by administering antigen and then exploring the effect on the so called early and late asthmatic responses, again using FEV1 and/or sGaw. To validate the experiment and to under-stand whether the magnitude of the effect is likely to be clinically meaningful, the study should also include a placebo control and a positive comparator such as a corticosteroid. We may also wish to perform bronchoalveolar lavage before and after treatment to examine the changes in cells and secretion of mediators such as cytokines in the airways to learn the nature and extent of its anti-inflammatory effects.

To take a very different example, we might consider a drug intended to produce benefit in schizophrenia. In this case, we cannot gain direct access to the site of action either for drug administration or to measure a response. However, all such anti-psychotics act as antagonists at the Dopamine-2 (D2) receptor and it is now possible to quantify D2 receptor occupancy in the striatum of the human brain using positron emission tomography (PET scanning). Occupancy is measured after administration of drug in a range of doses by displacement of an administered radioligand 11C-raclopride, which binds specifically to the D2 receptor. Most antipsychotics require D2 receptor occupancy of approximately 70% to elicit an adequate response. Less than 50% occupancy will usually result in failure to produce benefit and greater than 80% occupancy is liable to be associated with adverse effects without additional benefit.

Adverse effects may also be the prime focus of our studies with biomarkers. The unwanted sedative effects of a drug may be quantified using carefully performed tests of cognition and psychomotor performance in healthy volunteers or patients.

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Responses can be related to plasma concentrations and the PK/PD relationship modelled. Studies must be performed under double blind conditions, usually employing a randomised, crossover design with more than one dose level of the test drug, a positive control and a placebo.

Thus, biomarkers help us gain greater understanding of mechanism of action and may be used to establish the dose-con-centration-response relationships for both desired and undesired effects, thereby evaluating the ratio of benefit:risk in the relevant dose-range. The importance of understanding dose-response cannot be over-emphasised. It is salutary to consider how many effective and well-tolerated drugs in use today, were associated with severe toxicity when they were introduced. Failure to estab-lish dose-response at the outset has led to prescribing of grossly excessive doses. Examples include captopril for hypertension and cardiac failure, zidovudine for HIV disease and haloperidol for psychosis.

Affinity, efficacy and potencyUnderpinning the assessment of all PD responses is the basic principle of a drug binding to a receptor which triggers a series of cellular events. These, in turn, result in a measureable response. The affinity of a drug is a measure of its ability to bind to a particular receptor. This can be quantified by binding and dis-placement of radioactive ligands to cloned receptors in vitro. The relative specificity or selectivity of a drug for certain receptors can thereby be characterised.

However, affinity does not tell us about function. Efficacy is the ability of a drug to elicit a response, once bound to its target and in terms of the dose-response curve it defines the maximum response. This can be studied in vitro and in vivo in animals and humans. Efficacy should not be confused with potency, which refers to the amount of drug required to produce a response of a certain magnitude. Thus, the ED50 is the dose at which 50% of the maximal effect is produced and the EC50 and IC50 are the concentrations producing 50% of maximum effect or inhibition respectively.

ReceptorsThere are a variety of different types of receptors for drugs, mostly situated in the cell membrane, but they generally fall into one of a few types. They may be ion channels such as voltage-gated sodium-ion channels blocked by local anaesthetics, cal-cium-ion channels modulated by dihydropyridine vasodilators (calcium antagonists) and the GABAA-gated chloride channel, a target for benzodiazepines. For muscarinic activity of acetylcho-line, the membrane receptors are G-protein coupled, as are those for dopamine, catecholamines, 5-HT, opiates, purines and many other drugs, as well as endogenous chemicals and hormones. Such receptors may also be coupled to ion channels, which open when the receptor is occupied by an agonist. Enzymes also serve as targets for drugs e.g. cyclo-oxygenases for non-steroidal anti-inflammatory drugs (NSAIDs), HMG-CoA reductase for statins, angiotensin converting enzyme for the ACE inhibitors and tyro-sine kinase for insulin and drugs for certain cancers acting on epidermal growth factor receptors. There are also some important receptors sited in the cell nucleus with a DNA-binding domain for the action of hormones such as corticosteroids, thyroxine and cholecalciferol (Vitamin D3) and enzymes involved in DNA

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synthesis, targeted by some cytotoxic agents. Finally, there are transporter molecules, such as those for noradrenaline and 5-HT reuptake targeted by tricyclic antidepressants.

Pharmacokinetics

Drug absorption and dispositionAs stated in our introduction, many factors other than dose determine the concentration of drug at its sites of action. These sites are rarely accessible, therefore to understand how the body is handling the drug (PK), concentrations are usually, but not exclusively, assayed in plasma. Since most drugs are adminis-tered orally or parenterally and are transported to the tissues via the blood stream, plasma drug concentrations usually serve as a reasonable surrogate for concentrations at the active site. The brain is an important exception, as entry of many molecules is prevented by the blood-brain barrier.

The fate of a drug after absorption is called ‘disposition’. Disposition comprises, distribution, metabolism and elimina-tion. Thus absorption and disposition are commonly sum-marised by the acronym ‘ADME’. However, this is not entirely satisfactory since it suggests that the processes are separate and occur consecutively when, in fact, they are all inter-related and many occur simultaneously. In reality, we need to con-sider the concepts: bioavailability, distribution, clearance and elimination.When we administer a drug topically to the skin, eye, airways or lower bowel, the drug will reach the target tissue, provided the formulation has appropriate physical characteristics such as solubility, particle size etc. However, to achieve adequate con-centrations at the target sites of action after oral administration we rely on: • adequate absorption from the gut, usually the upper small

intestine • avoidance of metabolism in the intestinal mucosa • portal blood flow through the liver • avoidance of metabolism in the liver and • passage through the systemic circulation and distribution to

sites of action before too much is cleared by metabolism in the liver, glomerular filtration and/or tubular secretion in the kidney.

BioavailabilityBioavailability is the proportion of an administered dose that reaches the systemic circulation. It is determined by the extent of both absorption and pre-systemic or ‘first-pass’ metabolism in the intestine and liver. For example, the antiepileptic drug lamotrigine is virtually completely absorbed and does not undergo significant first-pass metabolism. In this case, the bio-availability (100%) is highly predictable. On the other hand bio-availability of the bisphosphonate alendronate is <1% and very variable due to very poor and erratic absorption. The bioavail-ability of oral morphine is also highly variable ranging from 10% to 50% due to a combination of moderate absorption (25–50%) and extensive first-pass metabolism (≥50%). Owing to the lag time for absorption and variability in dosage requirements, oral administration is not recommended for relief of acute pain such as post-operatively or during a heart attack. Similarly, glyceryl trinitrate is usually administered sublingually for relief of angina

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to obtain rapid and reliable absorption with avoidance of first-pass metabolism.

Since drug administered i.v. is 100% bioavailable, it is possible to calculate the absolute bioavailability of an oral formulation by calculating the ratio of the dose-normalised area under curve (AUC) of plasma concentrations administered by the oral and i.v. routes on separate occasions. The comparative bioavailability of a new oral formulation relative to a reference formulation can be studied by comparing the AUCs after administration of each. Generic drug manufacturers are required to demonstrate that their formulations are ‘bioequivalent’ to a reference formulation in terms of the total exposure (AUC) and the peak concentration (Cmax) reached falling within defined limits.

Apart from the formulation itself, many factors may affect bio-availability. For example, ingestion of food at the time of taking the drug may reduce absorption of a poorly soluble drug. On the other hand, fatty food may increase absorption of a lipid soluble drug and reduce the extent of first pass metabolism, thereby increasing bioavailability.

DistributionOnce drug molecules have started to enter the systemic circu-lation, they will be distributed to different tissues and may be bound, to a greater or lesser extent, to plasma proteins or enter red cells. After i.v. administration, the apparent volume of distri-bution (V) can be obtained directly by extrapolating the declining concentrations on a log scale back to the time of dosing to give the concentration at time zero (C0) i.e. before distribution since

V = Dose/C0

It should be appreciated that V is not a real volume but is simply a parameter relating the concentration of drug in plasma to the total amount in the body, wherever it may be. If the drug is lipophylic, it distributes rapidly to fat and tends to accumulate there. The plasma concentration will fall rapidly and it will seem as though the drug has been distributed in a large volume. On the other hand, if drug is mainly bound to plasma protein and stays in the vascular compartment, the concentration of bound and unbound drug together will be high and the V will be small.

The magnitude of V determines how much a loading dose should be and the rate of distribution affects the onset of drug action. For example, the anaesthetic thiopentone distributes to lipid tissue including the brain, rapidly and for that reason is suit-able for induction of anaesthesia. By contrast, digoxin distributes to tissues very slowly and will not exert its effect on the heart for several hours; therefore, there is no point in giving an i.v. loading dose of digoxin; several oral doses are preferred. For therapeutic monitoring of digoxin concentrations, blood should be sampled many hours after dosing, when distribution is complete.

Clearance and eliminationOf critical importance for the duration of drug action is the rate at which it is cleared. Clearance (CL) is a measure of the effi-ciency with which a substance is removed from the systemic circulation. Unlike distribution, clearance is not reversible; by definition, once the drug is cleared, it cannot return to the circu-lation. However, drug-related material may still be present in the body i.e. it has not necessarily been eliminated.

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Clearance of a drug by the kidney is the volume of blood cleared of that drug by the kidney per unit of time; it is measured in units of mL/min or L/h, per kg body weight. The antidiabetic drug metformin is cleared almost exclusively by the kidney and eliminated in urine. If glomerular filtration is reduced due to aging or glomerulonephritis, renal drug clearance is reduced and the dose may need to be reduced accordingly.

Drugs may also undergo clearance by biotransformation to metabolites. Examples of drugs undergoing oxidative metabo-lism by cytochrome P450 enzymes in the liver include the antidia-betic sulphonylureas e.g glipizide, the coumarin anticoagulant warfarin and the benzodiazepine sedatives e.g midazolam. Bio-transformation per se does not eliminate the drug from the body, since metabolites may continue to circulate and be distributed in the tissues; they may indeed be pharmacologically active.

Elimination of drug in the form of metabolites may occur by excretion in urine or via the biliary tract with excretion in faeces. Many oxidative metabolites undergo further biotransformation to form conjugates such as glucuronides, before being excreted. Drugs can also be eliminated in expired air and breast milk, the latter being of importance to the suckling child. For first order elimination, which is applicable to most drugs, the rate of elimi-nation is the product of total CL (sum of metabolic, renal etc) and plasma concentration:

Elimination rate (mg/h) = CL (L/h) * Plasma concentration (mg/L)

From this simple equation, we can see that if clearance of a drug in an individual is constant, as it will be, if blood flow and renal and liver function are not changing, the elimination rate will be directly proportional to the plasma concentration. Fur-thermore, at steady state, when the rates of input (drug admin-istration) and elimination are equal, the plasma concentration is inversely proportional to CL.

The plasma elimination half-life (t1/2) is the time taken for the amount of drug in the body and the plasma concentration to fall by 50%. The t1/2 predicts the time taken to eliminate most of the drug after stopping dosing at steady state since the plasma concentration will fall by 50% after one half-life, 75% after two, 87.5% after three, 93.75% after four and ca.97% after five half-lives. Similarly, it will take about five half-lives for a drug to reach steady state after the start of repeat dosing.

The slope of the line of falling log concentration against time represents the fractional rate of drug removal, called the elimina-tion rate constant k. The t1/2 can be measured from this slope and is determined by both CL and V as follows:

t1/2 = the natural logarithm of 2/k or 0.693/k

now CL = k * V

therefore t1/2 = 0.693 * V/CL

The t1/2 of different drugs may range from a few minutes to days or weeks; this should be taken into account when considering the frequency of dosing and the effect of factors such as age, renal and liver disease which affect clearance and hence half-life.

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Drug interactions

Concomitant drug administrationPolypharmacy is a common problem, particularly in certain groups of patients and the elderly, who may be taking a number of medicines for different disorders. In addition to the problems of compliance, concomitant administration of several drugs can lead to a variety of drug-drug interactions (DDIs). It is impos-sible to keep in one’s head all the possibilities for even clini-cally important interactions and there are formularies, reference books and computer databases to assist us. However, there are some pharmacological mechanisms which underlie the major-ity of DDIs that can help make sense of all this and make likely interactions of drugs predictable. Again, the division into PD and PK is helpful.

PharmacodynamicTwo drugs exerting PD effects at the same site are likely to interact, irrespective of the actual mechanism of action or structure of the molecules. For example, any centrally acting drug in combination with alcohol can result in life threatening impairment of driving ability. Similarly, two nephrotoxic drugs such as aminoglycoside and cephalosporin antibiotics are more likely to result in toxicity than either drug alone. PD effects of concomitant medication may also be antagonistic such as the effect of corticosteroids and oral hypoglycaemic agents on blood glucose or NSAIDs and antihypertensives on blood pressure.

PharmacokineticPharmacokinetic interactions between drugs may be mediated at the level of absorption by means of chelation, effects on the rate of gastric emptying and neutralisation of gastric acid. However, most important PK interactions are due to inter-ference with metabolism. The synthesis and activity of cyto-chrome P450 enzymes in the liver and gut may be increased by enzyme inducing agents such the antiepileptic drug car-bamazepine, the antibiotic rifampicin or the herbal St John’s Wort. Chronic alcohol ingestion is not only an inducer of its own metabolism but also that of many drugs. Increased clearance caused by enzyme induction may result in inad-equate anticoagulation with warfarin, failure of protection by oral contraception or inadequate immunosuppression with cyclosporin.

Conversely, drug clearance may be inhibited by an enzyme inhibitor or by competition with another substrate metabolised by the same enzymes resulting in severe toxicity. Some potent inhibitors of an important cytochrome P450 isozyme, known as CYP3A4, include the antifungal agent ketoconazole, the antibi-otic clarithromycin and the HIV antivirals ritonavir and saquin-vavir.

PK drug interactions may be studied using probe com-pounds as substrates or examining the PK of one or both drugs alone and in combination. Such studies need to be care-fully designed taking many factors into account including the respective half-lives of the drugs and the time course of effect, since enzyme inhibition can be manifest after a single dose while induction usually takes two or three weeks to develop fully.

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Adverse reactions

All drugs have undesired as well as desired effects and the accept-able benefit:risk ratio depends on the indication and the nature of the adverse reactions. Adverse reactions are often simply exag-gerated PD effects due to excessive concentrations. These may result from reduced clearance attributable to a drug interaction, liver or renal disease or a genetic variant of a polymorphic metabolic enzyme (poor metaboliser). Adverse reactions may be mediated by the same receptor mediating effects at different sites in the body or different receptors, since many drugs are not highly selective for one type of receptor.

The effectiveness of a drug may be greatly impaired by poor tolerability. For example, an anticholinergic drug prescribed for Overactive Bladder may be of little benefit because the dose is limited by undesired effects on saliva production (dry mouth) and bowel transit (constipation). The low therapeutic index (ratio of toxic:therapeutic dose) not only limits the dose that is tolerable but is also liable to lead to poor compliance with the prescribed dosing regimen.

Other reactions may cause a drug to be unsafe, either directly e.g. opiate causing respiratory depression or indirectly e.g. seda-tive in the elderly causing falls with resultant injury. Adverse reactions are a common cause of hospital admissions with sig-nificant morbidity and mortality.

Much of the effort in development of new medicines is devoted to establishing the benefit:risk ratio. This is feasible for very common (>1 in 10) and common (1 in 100 to 1 in 10) adverse reactions but it should be recognised that medicines are usually licensed on the basis of experience limited to a few thou-sand patients at most. The chances of detecting an uncommon (1 in 1000 to 1 in 100), rare (1in 10,000 to 1 in 1000) or very rare (<1 in 10,000) reaction are diminishingly small and if they are very serious reactions e.g. angioneurotic oedema, neutropaenia, sclerosing peritonitis, may lead to withdrawal of the drug from the market, depending on the indication for which the drug is licensed.

Evaluation of new drugs

Starting with the first administration to humans, CP plays a criti-cal role in exploring the tolerability, PD and PK of each new molecular entity in early drug development. The aim is to learn as much about the molecule as possible and thereby to establish whether the drug has the profile needed to justify commitment of massive resources required to obtain a licence to market it, a process taking an average of 10 years.

The first studies in humans require careful selection and administration of a range of single doses based on the results of in vitro and animal in vivo primary and safety pharmacology studies, PK and toxicology. Safety is paramount but from the PK and PD data generated from single, and subsequently mul-tiple doses, it is usually possible to establish a dose range which achieves plasma concentrations of therapeutic interest. These doses will be used in subsequent studies to focus on benefit risk and answering the specific questions which must be answered about that specific molecule. If the data do not support the mini-mum acceptable profile, development should be discontinued at the earliest opportunity. If, on the other hand, the data are

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encouraging, they can be used to optimise the design of subse-quent clinical trials.

For drugs selected for full development, CP will support the programme with PK studies of bioavailability, interactions, effects of age and disease and frequently detailed PD studies to charac-terise the desired and undesired actions of the drug. CP studies comprise a substantial proportion of the Marketing Authorisation Application and the data comprising the Summary of Product Characteristics. ◆

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Griffin JP, O’Grady J. The textbook of pharmaceutical medicine, chapters

4 and 5, 5th edn. Blackwell: BMJ Books, 2006.

Ritter JM, Lewis LD, Mant TGK, Ferro a. A textbook of clinical

pharmacology and therapeutics, 5th edn. Hodder arnold,

2008.

Tozer Tn, Rowland M. introduction to pharmacokinetics and

pharmacodynamics. The quantitative basis of drug therapy.

Lippincott, Williams & Wilkins, 2006.

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