TABLE 2

Experimental measures of drug action: general

Term Suggested Usage Notes
The relationship between concentration and effect: Hill equation In the following, drug action is expressed in terms of the effect, E, produced when an agonist, A, is applied at a concentration [A]. The relationship between E and [A] can often be described empirically by the Hill equation, which has the form: where Emax is the maximal action of A, nH is the Hill coefficient and [A]50 is the concentration that produces an effect that is 50% of Emax. [A]50 in the Hill equation is sometimes denoted by K, and Emax by α. The choice between [A]50 and K will depend on the directness of the measurement. The former is appropriate if an indirect action, such as the contraction of an intact smooth muscle preparation, is observed. However, in a ligand binding experiment, K would be preferable, although whether the value of K corresponds to a single, microscopic, equilibrium dissociation constant (even if nH is unity) will depend on the circumstances (see Section IV. A.). The Hill equation and the logistic equation are closely related but not identical (see Section IV. C.).
Potency An expression of the activity of a drug, in terms of the concentration or amount needed to produce a defined effect; an imprecise term that should always be further defined (see EC50, IC50, etc.). Drug potency depends on both receptor (affinity, efficacy) and tissue (receptor numbers, drug accessibility) parameters. The term is sometimes, incorrectly, used to refer to the maximum effect attainable.