In vitro pharmacological activity of twenty-eight synthetic cannabinoid receptor agonists at the type 1 and 2 cannabinoid receptors

Synthetic cannabinoid receptor agonists (SCRAs) are a class of novel psychoactive substances whose prevalence in illegal markets continues to grow. Δ⁹-tetrahydrocannabinol (THC) is the primary intoxicating compound present in cannabis and is well-known to behave as a partial agonist at both the type 1 and 2 cannabinoid receptors (CB1R, CB2R). Unlike THC, the SCRAs characterized to date generally behave as CB1R and/or CB2R full agonists. The high potency and full agonism of these ligands are thought to drive the toxicity of SCRAs, including psychoses, emesis, and tachycardia. In this study, twenty-eight compounds (including the reference ligands CP55,940 and THC) were evaluated for binding affinity, Gi protein-dependent inhibition of cAMP, and βarrestin2 recruitment in Chinese hamster ovary (CHO-K1) cells stably expressing either receptor. Radioligand binding results demonstrate a general lack of selectivity between cannabinoid receptor subtypes. In signaling assays, most compounds displayed the anticipated full agonism with low nanomolar potency characteristic of SCRAs. Many compounds displayed bias for the inhibition of cAMP over the recruitment of βarrestin2, and this was especially true at CB2R, where several compounds were inactive in the βarrestin2 recruitment assay. However, no clear structure-activity relationship emerged among the tested SCRAs that could account for their selectivity, potency, efficacy, or bias, although potential patterns are discussed herein. Overall, our data support growing evidence that the cannabinoid receptors accommodate a diverse range of ligands, and that compound function may be dictated by factors that are not yet well characterized, such as binding kinetics.