In vitro functional characterization of a panel of non-fentanyl opioid new psychoactive substances

The landscape of new psychoactive substances (NPS) is constantly evolving, with new compounds entering the illicit drug market at a continuous pace. Of these, opioid NPS form a threat given their high potency and prevalence. Whereas previously, the use of fentanyl and fentanyl derivatives was the main point of attention, legislations have reacted accordingly, which may have been a driving force towards the (ab)use of alternative µ-opioid receptor (MOR) agonists. In contrast to fentanyl (analogues), details on these novel non-fentanyl opioid NPS are scarce. We investigated the biological activity of a panel of 11 'alternative', newly emerging MOR agonists (2-methyl-AP-237, AP-237, bromadol, brorphine, butorphanol, isotonitazene, mitragynine, 7-OH-mitragynine, MT-45, piperidylthiambutene, and tianeptine) using two closely related in vitro MOR activation bio-assays, monitoring either G protein (mini-Gi), or β-arrestin2 (βarr2) recruitment. Activity profiles were obtained for all tested compounds, with values for potency (EC