Clinical pharmacology – the basics

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  • sBasic

    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 bodys 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, Kings 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 15cience

    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. 3 2009 elsevier Ltd. all rights reserved.

  • Basic scienceResponses 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 sURGeRY 27:4 15synthesis, 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

  • Basic s

    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,...

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