Investigation in the CB1 and CB2 receptor binding profile and intrinsic activity of (−) and (+)-enantiomers of some naturally occurring phytocannabinoids or synthetic derivatives

Abstract

Cannabidiol (CBD) and cannabidivarin (CBDV) have shown promising clinical efficacy for the management of epilepsy, and beneficial effects have been demonstrated for CBD in a diversity of other pathologies (pain, schizophrenia, Tourette syndrome, anxiety). However, the mechanism(s) involved are still largely elusive, as are the molecular target(s) involved. CBD and CBDV do not orthosterically bind the cannabinoid type-1 (CB₁) and type-2 (CB₂) receptors, showing only modest allosteric modulation of both end-points. CBD and CBDV are biosynthesized as optically highly pure (−)-enantiomers, and most bioactivity data refer to these forms. (+)-CBD and related analogues [(+)-cannabidiolic acid (CBDA), its esters, and (+)-CBDV] can be obtained by chemical synthesis, and we present evidence that the (+)- and (−)-enantiomers of CBD, CBDV and of a selection of derivatives of CBDA have distinct binding profiles and functional activity at the CB₁/CB₂ receptors. Thus, with the single exception of the methyl ester of CBDA, all the (+)-enantiomers showed higher affinities than the (−)-isomers for both receptors, in particular for the CB₂ receptors. The affinity of the (+)-enantiomers for both CB₁ and CB₂ receptors showed a marked dependence on the nature of the alkyl residue on the aromatic ring and the esterification pattern of CBDA. Potency was rarely in the low nM value for CB₁, but generally so for CB₂. Enantiomers showing low nM activity were further investigated for their intrinsic activity using GTPγS binding assays. This proved that (+)-CBD, (+)-CBDV and the methyl ester of (+)-CBDA are agonists at the CB₂ receptor, with the β-hydroxyethyl ester of (+)-CBDA being an inverse agonist, and its β-hydroxypentyl ester behaving as an agonist at CB₁ and an inverse agonist at CB₂. Finally, we assayed in vitro the anti-inflammatory and neuroprotective properties of three compounds [(+)-CBD, (+)-CBDV and (+)-CBDA methyl ester] strongly activating CB₂, showing their ability to reduce the production of proinflammatory factors and protecting neurons against their toxicity. Remarkably, these benefits were eliminated by the selective blockade of the CB₂ receptor, highlighting its role as a (+)-CBD target. In summary, our data show that remarkable differences between (−)- and (+)-enantiomers of CBD, CBDV and related compounds exist in terms of CB₁/CB₂ receptor binding profile and intrinsic activity. The observation that the natural (−)-enantiomers do not bind CB₂ receptors suggests that their effects are associated with different targets.