Partial Agonist
A molecule that binds to and activates a receptor but produces only a partial response compared to a full agonist, even at full receptor occupancy. Partial agonists can functionally reduce overall receptor signalling when a full agonist is also present.
Technical Context
Partial agonists have lower intrinsic efficacy than full agonists — they cannot produce the maximum possible receptor response even when all receptors are occupied. In the presence of a full agonist, a partial agonist competes for receptor binding while producing a weaker signal, effectively reducing overall response (functional antagonism). This dual agonist/antagonist behaviour can be therapeutically useful when moderate, controlled receptor activation is desired. The concept of partial agonism is quantified by the intrinsic activity (alpha value) — full agonists have alpha = 1, partial agonists have 0 < alpha < 1, and antagonists have alpha = 0.