Based on evidence that the therapeutic properties of Cannabis preparations are not solely dependent on the presence of Δ9 -tetrahydrocannabinol (THC), pharmacological studies have been recently carried out with other plant cannabinoids (phytocannabinoids), particularly cannabidiol (CBD) and Δ9 -tetrahydrocannabivarin (THCV). Results from some of these studies have fostered the view that CBD and THCV modulate the effects of THC via direct blockade of cannabinoid type-1 (CB1 ) receptors, thus behaving like first generation CB1 inverse agonists, such as rimonabant. Here we review in vitro and ex vivo mechanistic studies of CBD and THCV, and synthesize data from these studies in a meta-analysis. Synthesized data regarding mechanisms are then used to interpret results from recent preclinical animal studies and clinical trials. The evidence indicates that CBD and THCV are not rimonabant-like in their action, and thus appear very unlikely to produce unwanted central nervous system effects. They exhibit markedly disparate pharmacological profiles particularly at CB1 receptors: CBD is a very low affinity CB1 ligand which can nevertheless affect CB1 activity in vivo in an indirect manner, whilst THCV is a high affinity CB1 ligand and potent antagonist in vitro and yet only occasionally produces effects in vivo resulting from CB1 antagonism. THCV also has high affinity for CB2 and signals as a partial agonist, a departure from both CBD and rimonabant. These cannabinoids illustrate how in vitro mechanistic studies do not always predict in vivo pharmacology, and underlie the necessity of testing compounds in vivo before drawing any conclusion on their functional activity at a given target.
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