Abstract
Without substantial intervention, the opioid crisis is projected to continue, underscoring the need to develop new treatments for opioid use disorder (OUD). One drug under development is the µ opioid receptor antagonist methocinnamox (MCAM), which appears to offer advantages over currently available medications; however, some questions remain about its potential utility, including its ability to block the effects of ultra-potent fentanyl analogs. The goal of this study was to examine its effectiveness in attenuating the abuse-related effects of the fentanyl analogs carfentanil and 3-methylfentanyl in monkeys responding for food or intravenous infusions under a choice procedure. These drugs were compared with fentanyl, heroin, methamphetamine, and cocaine. Food was preferred over saline, and there was a dose-dependent increase in responding for drug over food with no marked decrease in response rates or number of choice trials completed for any of the six drugs studied. Naltrexone (0.032 mg/kg) antagonized choice of µ opioid receptor agonists, producing rightward shifts in dose-effect curves ranging from 27-fold (carfentanil) to 71-fold (heroin). In contrast, naltrexone was less effective in attenuating choice of methamphetamine or cocaine with curves obtained in the presence of naltrexone shifted <3-fold. Daily treatment with 0.032 mg/kg MCAM also antagonized the effects of opioids, shifting curves 20-fold (fentanyl) to 72-fold (heroin) rightward; MCAM did not significantly change dose-effect curves for methamphetamine or cocaine. Thus, antagonism by MCAM is similar across a variety of µ opioid receptor agonists, including ultra-potent fentanyl analogs, further supporting its potential utility as a treatment for OUD. SIGNIFICANCE STATEMENT: Treatments for opioid use disorder (OUD) should attenuate the effects of a variety of opioids, including emerging threats like the ultra-potent fentanyl analogs. The novel µ opioid receptor antagonist MCAM is being developed to treat OUD because it provides long-lasting blockade of the reinforcing effects of heroin and fentanyl. The current study shows that MCAM attenuates the abuse-related effects of the fentanyl analogs carfentanil and 3-methylfentanyl, further supporting the utility of MCAM as a treatment for OUD.