Carfentanil stabilizes µ opioid receptor conformations that are ultra-efficient in inhibiting cAMP, resistant to naloxone or nalmefene but sensitive to naltrexone

The highly potent opioid carfentanil (CAR) represents a growing health risk. CAR acts via G_i/o-coupled µ opioid receptors (µOR) and exhibits ultra-high toxicity. So far, no clear association between pharmacodynamics and toxicity of CAR has been described. We created a HEK-293 cell line stably expressing the µOR and, determined ligand binding affinity (K_i) and potency (EC₅₀) of CAR, fentanyl, remifentanil, morphine or the endogenous ligand endomorphin-1. We found that µOR bind CAR with ~ 10-times higher affinity than fentanyl or remifentanil and with ~ 70-times higher affinity than morphine. Potency of CAR to inhibit cAMP was ~ 85-times higher compared to the fentanyl's and ~ 620 higher compared to morphine. Thus, CAR's toxicity rather associates with receptor potency than affinity. When receptor occupancy at EC₅₀-values was calculated, it appeared that CAR is ~ 8-times more efficient to inhibit cAMP in comparison to morphine, fentanyl or remifentanil. Hence, we postulate that CAR stabilizes µOR conformations that are ultra-efficient in inhibiting cAMP. The OR antagonists naloxone and nalmefene are used as antidotes against opioid intoxication. Both antagonists revealed 10 to 100-times higher IC₅₀-values against CAR-mediated cAMP inhibition compared to the other opioids, indicating that µOR conformations stabilized by CAR are rather resistant towards clinically used antidotes. Of note, when the long acting OR antagonist naltrexone was tested, it exhibited a ~ 65-times higher potency to inhibit CAR but not fentanyl compared to naloxone. Our data highlight the unique nature of CAR's interactions with µOR and provide first pharmacodynamic indication that naltrexone might be a superior antidote.