Intrinsic activity is defined as what?

• A. A measure of the rate of excretion
• B. A measure of the ability of a drug to generate an effect
• C. A measure of receptor density
• D. A measure of half-life

Answer: (B)

Intrinsic activity is about how effectively a drug activates a receptor after it binds, i.e., its ability to generate a biological response. Once the drug-receptor complex forms, intrinsic activity describes the efficacy of that complex to trigger signaling and produce an effect. Full agonists have high intrinsic activity and can produce the maximal response, partial agonists have lower intrinsic activity and thus a smaller effect even at full receptor occupancy, and antagonists have little to no intrinsic activity (and inverse agonists can even decrease baseline activity). This concept is distinct from how quickly a drug is excreted or cleared (excretion rate or half-life) and from receptor density, which can influence the overall maximal response but not the inherent efficacy of the drug itself.