In vitro characterization of new psychoactive substances at the μ-opioid, CB1, 5HT, and 5-HT receptors-On-target receptor potency and efficacy, and off-target effects.

New psychoactive substances (NPS) appear on the recreational market on a monthly basis, with unclear toxicology, resulting in an increasing number of fatalities. Identification of drug targets and potencies is crucial for understanding and treating intoxications and for scheduling processes. In this study 60 NPS and metabolites belonging to opioids, cannabinoids and serotonergic hallucinogens classes were screened for in vitro activation of the μ-opioid, CB1, 5-HT and 5-HT receptors using the AequoZen cell system. Fentanyl and NBOMe analogues were chosen for full dose-response characterization of the μ-opioid and 5-HT receptors, respectively. Most substances activated their corresponding target receptor. The most potent μ-opioid receptor agonists were 2-fluorofentanyl (EC = 1.0 nM), carfentanil (EC = 2.7 nM) and acrylfentanyl (EC = 2.8 nM) and in total a >1500-fold difference was seen among the tested compounds. Moreover, furanylfentanyl, 4-methoxybutyrylfentanyl and valerylfentanyl acted as partial agonists of the μ-receptor. On the 5-HT receptor, bromo-dragonfly showed the highest potency (EC = 0.05 nM, 400 times more potent than LSD), followed by most NBOMe compounds with EC values ranging from 0.11 nM (for 25N-NBOMe) to 1.3 nM (for 25T4-NBOMe)). Off-target activation of the μ-opioid receptor was identified for piperazines, phenethylamines (in particular NBOMe and 2C compounds) and tryptamines. Moreover, the synthetic cannabinoid metabolite 3-carboxy indole PB-22 activated the 5-HT receptor. Bromo-dragonfly was the only compound that activated all four receptors. These results highlight the possible interplay of known and unknown NPS targets and unveil its complexity. Moreover, the detailed, quantitative information presented facilitates our further understanding of NPS toxicology.