In vitro CB₁ receptor activity of halogenated indazole synthetic cannabinoid receptor agonists

Synthetic cannabinoid receptor agonists (SCRAs) are a group of new psychoactive substances (NPS) that bind to and activate the cannabinoid 1 and 2 (CB₁ and CB₂) receptors. Following the introduction of SCRA analog controls in China in July 2021, new SCRAs with a bromine at the 5 position on the indazole core emerged on the recreational drug market. In this study, the in vitro CB₁ receptor activity of 19 different SCRAs with a halogenated indazole core was systematically examined using a AequoScreen^® CB₁ assay alongside four available non-halogenated analogs, ADB-BUTINACA, MDMB-BUTINACA, MDMB-4en-PINACA, and MDMB-INACA, and the newly emerged MDMB-5'Me-INACA (MDMB-5-methyl-INACA). Of SCRAs with a substitution at the 5 position on the indazole core, analogs with a fluorine had the lowest EC₅₀ values. For SCRAs with a methyl ester head moiety (MMB or MDMB), chlorinated analogs had the next lowest EC₅₀ values followed by brominated analogs; however, the opposite relationship was observed for SCRAs with an amide head moiety (AB or ADB). All halogenated compounds had similar potency to their non-halogenated analog, except the brominated tert-leucine methyl ester SCRAs, which had significantly reduced potency. Of the different head moieties, tert-leucine methyl ester SCRAs were the most potent, followed by tert-leucinamide, valinamide, and valine methyl ester. The potencies of the SCRAs with pentyl (ADB-5'Br-PINACA/ADMB-5'Br-PINACA/ADB-P-5'Br-INACA), pent-4-enyl (ADB-4en-5'Br-PINACA/ADMB-4en-5'Br-PINACA/ADB-4en-P-5'Br-INACA and MDMB-4en-5'Br-PINACA/MDMB-4en-P-5'Br-INACA), and butyl tails (ADB-5'Br-BUTINACA/ADMB-5'Br-BUTINACA/ADB-B-5'Br-INACA) were not significantly different, whereas fluorobenzyl (ADB-5'Br-FUBINACA), tail-less (ADB-5'Br-INACA), and decyl tail (ADB-5'Br-DECINACA/ADMB-5'Br-DECINACA/ADB-D-5'Br-INACA) analogs were significantly less potent.