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 (EC₅₀) ranging from 1.89 nM (bromadol) to > 3 µM (AP-237 and tianeptine). Bromadol, brorphine, isotonitazene, piperidylthiambutene, and tianeptine had the highest efficacy (E_max) values, exceeding that of the reference compound hydromorphone ≥ 1.3-fold (βarr2 assay) and > 2.6-fold (mini-Gi assay). Information on the recruitment of two distinct signaling molecules additionally enabled evaluation of biased agonism; none of the evaluated opioids being significantly biased. Taken together, this study is the first to systematically investigate the in vitro biological activity of a diverse panel of emerging non-fentanyl opioid NPS at MOR. Given the known danger of (fatal) intoxications with many opioid NPS, it is important to continuously monitor and characterize newly emerging compounds.