Partial agonist

In pharmacology, partial agonists are drugs that bind to and activate a given receptor, but have only partial efficency at the receptor relative to a full agonist. They may also be considered ligands which display both agonistic and antagonistic effects—when both a full agonist and partial agonist are present, the partial agonist actually acts as a competitive antagonist,^[1] competing with the full agonist for receptor occupancy and producing a net decrease in the receptor activation observed with the full agonist alone.^[2] Clinically, partial agonists can be used to activate receptors to give a desired submaximal response when inadequate amounts of the endogenous ligand are present, or they can reduce the overstimulation of receptors when excess amounts of the endogenous ligand are present.^[3]

Some currently common drugs that have been classed as partial agonists at particular receptors include buspirone, aripiprazole, buprenorphine, clobazam, nordazepam, nalmefene and norclozapine. Examples of ligands activating peroxisome proliferator-activated receptor gamma as partial agonists are honokiol and falcarindiol.^[4][5] Delta 9-tetrahydrocannabivarin (THCV) is a partial agonist at CB2 receptors and this activity might be implicated in ∆9-THCV-mediated anti-inflammatory effects.^[6] Additionally, Delta-9-Tetrahydrocannabinol (THC) is a partial agonist at both the CB1 and CB2 receptors, with the former being responsible for its psychoactive effects.