DEFINITION
A partial agonist is a ligand that binds and activates a receptor but cannot elicit the maximum response the system is capable of, even at saturating concentrations. Its intrinsic efficacy lies between 0 (a pure antagonist) and 1 (a full agonist).
In systems with a full agonist already present, a partial agonist can behave as a functional antagonist: it occupies the receptor without producing the full signal and thereby lowers the overall response. This behaviour is exploited in clinical pharmacology to buffer against both under- and over-activation of a receptor.
WHY IT MATTERS
Some research peptides display partial-agonist behaviour at one of several receptors they engage — a nuance that shapes selectivity profiles and mechanism-of-action reporting.
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