4.9 : Dose-Response Relationship: Potency and Efficacy

The potency of a drug is the measure of its ability to produce a biological response and can be compared by looking at the half-maximum effective concentration or EC₅₀ values of different drugs. A lower EC₅₀ value indicates higher potency of the drug. In the dose–response curve of two antihypertensive drugs, candesartan and irbesartan, a significant difference is observed in their EC₅₀ values. A lower EC₅₀ value for candesartan indicates that it is more potent than irbesartan, as it produces the same desired effect at a lower dose. It is noteworthy that both drugs exhibit similar maximum effect or E_max values, implying that they possess equal efficacy. Efficacy depends on the number of drug–receptor complexes formed and the extent of receptor activation. Maximal efficacy is achieved when all receptors are occupied, rendering further increases in drug concentration incapable of eliciting additional responses. Full agonists have high efficacy, as they activate receptors entirely and elicit a complete response. The efficacy of partial agonists is lower, as they activate receptors to a lesser extent. Antagonists, in contrast, exhibit zero efficacy by not activating the receptors. Instead, they act as blockers and either reduce the number of available receptors for agonist binding or induce conformational changes that prevent effective agonist binding. Overall, these factors play a crucial role in determining the efficacy and potency of drugs, which, in turn, can determine their usefulness in treating various medical conditions.