Intrinsic Activity
The maximum response a drug can produce relative to a reference full agonist at the same receptor. A full agonist has an intrinsic activity of 1, a partial agonist has an intrinsic activity between 0 and 1, and an antagonist has an intrinsic activity of 0.
Technical Context
Intrinsic activity (α) was defined by Ariens as the proportional maximum response: α = 1 for full agonists, 0 < α < 1 for partial agonists, α = 0 for antagonists. The concept was later refined by Stephenson's efficacy theory, which separated receptor occupancy from stimulus production. In modern pharmacology, intrinsic efficacy (Furchgott's concept) accounts for receptor reserve — tissues with many spare receptors can achieve maximum response at submaximal receptor occupancy, meaning even partial agonists may produce full responses in receptor-rich tissues. These concepts are relevant to understanding why peptide drugs may produce different responses in different tissues expressing the same receptor.