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
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.
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.
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 exclusive