Inpharmacology,partial agonists are drugs that bind to and activate a givenreceptor, but have only partialefficacy at the receptor relative to afull agonist. They may also be consideredligands which display bothagonistic andantagonistic effects—when both a full agonist and partial agonist are present, the partial agonist actually acts as acompetitive antagonist,[citation needed] competing with the full agonist for receptor occupancy and producing a net decrease in the receptor activation observed with the full agonist alone.[1] 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.[2]
Some currently common drugs that have been classed as partial agonists at particular receptors includebuspirone,aripiprazole,buprenorphine,nalmefene andnorclozapine. Examples of ligands activatingperoxisome proliferator-activated receptor gamma as partial agonists arehonokiol andfalcarindiol.[3][4] Delta 9-tetrahydrocannabivarin (THCV) is a partial agonist atCB2 receptors and this activity might be implicated in ∆9-THCV-mediatedanti-inflammatory effects.[5] Additionally, Delta-9-Tetrahydrocannabinol (THC) is a partial agonist at both theCB1 and CB2 receptors, with the former being responsible for its psychoactive effects.
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