General Pharmacology: TERMINOLOGY - 2

DOSE: total concentration or amount of a drug present in the body. Or, ita lso can denote the amount of drug administered to a pt (as in “repetitive-dosing”)… the first definition is the one used in this discussion.

ASSOCIATION CONSTANT: also called the drug-receptor affinity constant or equilibrium binding constant, K_eq. Defined in eq.2 (handout). Tells how strongly a drug binds to a receptor.

DISSOCIATION CONSTANT: K_d = 1/K_eq. K_d is the value of [D] at which 50% of the receptors are occupied by the drug.

RECEPTOR OCCUPATION CURVE: gives the praction (or percent) of receptors occupied as a function of the amount of drug present.

DOSE-RESPONSE CURVE: gives the pharmacologic response of a subject as a function of the amount of drug (or dose) present.

ED₅₀: dose that produces a half-maximal resoonse. For a receptor mechanism, ED₅₀ = K_d.

The following two are VERY IMPORTANT TERMS… both are measures of strength, but in different aspects

POTENCY: measure of the dose required to get a given response. The larger the dose you need to get a response, the weaker (less potent) the drug is. Potency is inversely proportional to the required dose to produce some response. Potency is measured on the x-axis of the dose-response curve. (see Brody fig. 3-4, for illustrative graph of two different drugs with different potencies).

EFFICACY: measure of the response that the drug produces (on the y-axis). If two drugs can occupy the same receptor but one has a higher maximum response (“ceiling of activity”) they may have the same K_d, but one can have a higher maximum. Even with the same binding one can produce a bigger response. They could have the same receptor occupation curves, but could have different dose-response curves because one drug is more efficacious than the other (maybe it opens the channel wider, or something like that). Efficacy response shouldn’t be measured by %, but some sort of clinical response.

Category: Pharmacology Notes