Agonist‐Dependent Modulation by the Long‐Acting Mu Opioid Receptor Antagonist, Methocinnamox (MCAM)

Opioid overdose is a leading cause of death in the US. Naloxone (NLX), a mu opioid receptor (MOR) competitive antagonist, is the only treatment currently available for reversing opioid overdose. Unfortunately, NLX’s effects are limited by a short duration of action and ease of surmountability by opioid agonists. Methocinnamox ( MCAM) is a novel MOR antagonist that has a long duration of action in vivo and holds promise as a better treatment for overdose and perhaps opioid use disorder. In this study, we compared the antagonist properties of MCAM to that of NLX and beta‐funaltrexamine (β‐FNA; an irreversible MOR antagonist). In HEK cells that express human MOR, DAMGO inhibited forskolin‐stimulated cAMP levels in a concentration dependent manner with an EC50 of 10 nM and a maximal inhibition of 40%. Pretreatment (15 min or 2 hrs) with NLX (100 nM, 10 x Ki) shifted the DAMGO concentration response curve (CRC) to the right, 100‐fold, in a fully surmountable manner that was independent of pretreatment time and fully reversed following NLX washout. Pretreatment with β‐FNA (10 nM) reduced the DAMGO maximal response in a time‐dependent, non‐surmountable and irreversible manner with no effect on the potency of DAMGO. By contrast to both β‐FNA and NLX, pretreatment with MCAM (10 nM) reduced the maximal response in a time‐dependent, non‐surmountable, non‐washable, and irreversible manner and shifted the DAMGO CRC to the right 1000‐fold. We hypothesized that this rightward shift in the DAMGO CRC may be due to allosteric properties of MCAM at MOR. A hallmark of allosterism is ligand dependence, so we next tested effects of MCAM, β‐FNA and NLX on a different mu opioid agonist, fentanyl. As expected, pretreatment with β‐FNA reduced the maximal response to fentanyl in a non‐surmountable manner without affecting potency, whereas NLX pretreatment shifted the CRC of fentanyl to the right (decreased the potency) in a manner that was fully surmountable and independent of pretreatment time. By contrast, pretreatment with MCAM reduced the maximal response to fentanyl in a non‐surmountable manner and shifted the CRC to the left (increased potency). Altogether, these data suggest that MCAM may be an irreversible orthosteric antagonist and an allosteric modulator at MOR. Support or Funding Information Supported by NIH/NIDA RO1 grant DA048214