Shape matters: The application of activity-based in vitro bioassays and chiral profiling to the pharmacological evaluation of synthetic cannabinoid receptor agonists in drug-infused papers seized in prisons.

Abstract

Synthetic cannabinoid receptor agonists (SCRAs) elicit many of their psychoactive effects via type-1 human cannabinoid (CB1 ) receptors. Enantiomer pairs of eight tert-leucinate or valinate indole- and indazole-3-carboxamide SCRAs were synthesized and their CB1 potency and efficacy assessed using an in vitro β-arrestin recruitment assay in a HEK239T stable cell system. A chiral high-performance liquid chromatography method with photodiode array and/or quadrupole time-of-flight-mass spectrometry detection (HPLC-PDA and HPLC-PDA-QToF-MS) was applied to 177 SCRA-infused paper samples seized in Scottish prisons between 2018 and 2020. In most samples, SCRAs were almost enantiopure (S)-enantiomer (>98% of total chromatographic peak area), although in some (n = 18), 2% to 16% of the (R)-enantiomer was detected. (S)-enantiomers are consistently more potent than (R)-enantiomers and often more efficacious. The importance of SCRA-CB1 receptor interactions in the "head" or "linked group" moiety is demonstrated, with the conformation of the "bulky" tert-leucinate group greatly affecting potency (by up to a factor of 374), significantly greater than the difference observed between valinate SCRA enantiomers. (S)-MDMB-4en-PINACA, (S)-4F-MDMB-BINACA, and (S)-5F-MDMB-PICA are currently the most prevalent SCRAs in Scottish prisons, and all have similar high potency (EC50 , 1-5 nM) and efficacy. Infused paper samples were compared using estimated intrinsic efficacy at the CB1 receptor (EIECB1 ) to evaluate samples with variable SCRA content. Given their similar potency and efficacy, any variation in CB1 receptor-mediated psychoactive effects are likely to derive from variation in dose, mode of use, pharmacokinetic differences, and individual factors affecting the user, rather than differences in the specific SCRA present.