Synthetic Cannabinoid Hydroxypentyl Metabolites Retain Efficacy at Human Cannabinoid Receptors

Synthetic cannabinoids (SCs) are novel psychoactive substances that are easily acquired, widely abused as a substitute for cannabis, and associated with cardiotoxicity and seizures. Although the structural bases of these compounds are scaffolds with known affinity and efficacy at the human cannabinoid type-1 receptor (hCB₁), upon ingestion or inhalation they can be metabolized to multiple chemical entities of unknown pharmacological activity. A large proportion of these metabolites are hydroxylated on the pentyl chain, a key substituent that determines receptor affinity and selectivity. Thus, the pharmacology of SC metabolites may be an important component in understanding the in vivo effects of SCs. We examined nine SCs (AB-PINACA, 5F-AB-PINACA, ADB/MDMB-PINACA, 5F-ADB, 5F-CUMYL-PINACA, AMB-PINACA, 5F-AMB, APINACA, and 5F-APINACA) and their hydroxypentyl (either 4-OH or 5-OH) metabolites in [³H]CP55,940 receptor binding and the [³⁵S]GTPγS functional assay to determine the extent to which these metabolites retain activity at cannabinoid receptors. All of the SCs tested exhibited high affinity (<10 nM) and efficacy for hCB₁ and hCB₂ The majority of the hydroxypentyl metabolites retained full efficacy at hCB₁ and hCB₂, albeit with reduced affinity and potency, and exhibited greater binding selectivity for hCB₂ These data suggest that phase I metabolites may be contributing to the in vivo pharmacology and toxicology of abused SCs. Considering this and previous reports demonstrating that metabolites retain efficacy at the hCB₁ receptor, the full pharmacokinetic profiles of the parent compounds and their metabolites need to be considered in terms of the pharmacological effects and time course associated with these drugs.