The property of a drug which determines the amount of biological effect produced per unit of drug-receptor complex formed. Two agents combining with equivalent sets of receptors may not produce equal degrees of effect even if both agents are given in maximally effective doses; the agents differ in their intrinsic activities and the one producing the greater maximum effect has the greater intrinsic activity. Intrinsic activity is not the same as "potency" and may be completely independent of it. Meperidine and morphine presumably combine with the same receptors to produce analgesia, but regardless of dose, the maximum degree of analgesia produced by morphine is greater than that produced by meperidine; morphine has the greater intrinsic activity. Intrinsic activity - like affinity - depends on the chemical natures of both the drug and the receptor, but intrinsic activity and affinity apparently can vary independently with changes in the drug molecule.
In analogy to the Lineweaver-Burk plot of the reciprocal of reaction velocity of an enzymatically catalyzed reaction against the reciprocal of the substrate concentration, one may plot the reciprocal of the magnitude of the drug-induced biological effect against the reciprocal of the drug concentration . Under ideal conditions, the slope of the resulting line is a measure of affinity and the point of intersection of the line with the ordinate is a measure of intrinsic activity.
Cf.
Affinity ,
Receptor ,
Ceiling Effect .
Antagonism ,
Dose-Effect Curve
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