The fundamental principles that govern drug therapy are often overlooked by the busy clinician. This disregard frequently results in the use of particular drugs and regimens that may be less ideal for the clinical situation being managed. By convention, these principles are categorized as pharmacokinetic and pharmacodynamic. Pharmacokinetic processes include drug absorption, distribution, biotransformation (metabolism), and elimination—essentially reflecting the influence of the body on the drug administered. These principles were addressed in the preceding issue of this journal. Pharmacodynamics deals with the actual mechanisms of action and effects a drug produces on the patient and is the topic for this continuing education article.Abstract
Receptors exist in both active (Ra) and inactive (Ri) states. Drugs may interact in a variety of manners, based on their ability to bind and activate these states. Antagonists bind to receptors but have no ability to activate either receptor state. Agonists bind and also activate the receptor. Agonists that activate both states equally are essentially inactive and behave similar to antagonists. Full agonists bind the active state selectively and produce a full response. Partial agonists also have some activity at the inactive state leading to a response that is less intense than that produced by a full agonist. Inverse agonists selectively activate the inactive state causing the cellular response to proceed in a manner opposite that generated by a natural agonist. Clinically, the effect may be indistinguishable from that produced by antagonists or inactive agonists.
Dose-response curves. Drug A is more potent and has greater efficacy that Drug B. However, Drug A 5 mg and Drug B 20 mg are equipotent doses for increasing heart rate 20%. Drug C is less potent than Drugs A or B, but is more effective, having no apparent ceiling to its response.
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