WHAT IS A PARTIAL AGONIST?
Two fundamental properties of a drug determine its effect on a receptor: affinity, the propensity of a drug to form a reversible complex with its receptor, and efficacy (or intrinsic activity), the ability of a drug to produce a functional response (Stephenson, 1956; Ariens & Simonis, 1964). For a given receptor, a drug can produce a maximal response (full agonist) or have no functional effect (full antagonist). Full agonists are traditionally naturally occurring compounds (e.g. dopamine) and full antagonists are chemical entities produced by pharmaceutical chemists. In reality, the majority of chemically related drugs acting at a receptor produce a spectrum of functional responses that lie between full agonism and full antagonism (Ariens, 1964). These drugs are termed partial agonists because they have a lower intrinsic activity than full agonists. Partial agonists can elicit different receptor responses depending on their environment: in the presence of agonists with higher intrinsic