Methocinnamox (MCAM) is a Selective, Long Acting Antagonist at Mu Opioid Receptors In Vitro
Authors: Zamora Joshua C. , Kotipalli Varun , Jennings Elaine M. , Disney Alex , Husbands Stephen , Winger Gail , Clarke William P. , Woods James H. , Berg Kelly A. 
Published: June 2021

The occurrence of opioid overdose is reaching epidemic proportions; an estimated 115 Americans die daily from an opioid overdose. The current pharmacological treatment relies exclusively on naloxone (Narcan), a competitive antagonist at mu opioid receptors, that can reverse the respiratory depression and sedation produced by opioid agonists like heroin and fentanyl. However, naloxone's effectiveness is limited by its short half‐life. By contrast, methocinnamox (MCAM) has been shown to be a long‐acting mu opioid receptor antagonist and as such, may be more effective for reversing opioid overdose. However, rigorous characterization of the pharmacological properties of MCAM has not yet been done. Here we characterized the antagonist properties of MCAM using HEK cell lines that stably express either human mu, delta or kappa opioid receptors along with a bioluminescent cAMP sensor. Opioid receptor agonist concentration response curves were obtained in response to either DAMGO (mu agonist), DPDPE (delta agonist) or U50488 (kappa agonist) for inhibition of forskolin‐stimulated cellular cAMP levels.We first compared MCAM to that of naloxone at mu receptors. Incubation with the mu receptor agonist DAMGO inhibited forskolin‐stimulated cAMP levels by 40% ± 2 % with an EC50 of 10 nM. Pretreatment with MCAM (10 nM, 10 × Ki) for 15 min shifted the concentration response curve (CRC) to DAMGO to the right 1000‐fold and suppressed the maximal response by 50%. The magnitude of MCAM's antagonist effect increased following a 2h pretreatment. Pretreatment with naloxone (NLX, 100 nM, 10 × Ki) for 15 min also antagonized DAMGO‐mediated inhibition of stimulated cAMP levels. However, in contrast to MCAM, the shift in the DAMGO CRC by NLX was fully surmountable and there was no further antagonist effect with 2h of NLX pretreatment. Furthermore, MCAM‐mediated antagonism following a 2h pretreatment was unchanged after a rigorous washout period whereas antagonism produced by NLX was fully reversible. At delta opioid receptors, pretreatment with MCAM (20 nM, 10 × Ki at delta) for either 15 min or 2h shifted the DPDPE CRC ~10‐fold to the right with no change in the maximal response. Similarly, the CRC for the kappa agonist, U50488, was shifted ~10‐fold to the right with no change in the maximal response following pretreatment for either 15 min or 2h with MCAM (50 nM, 10 × Ki at kappa). Furthermore, antagonism produced by MCAM was fully reversible at both delta‐ and kappa opioid receptors.The time‐dependent nature of antagonism, the depression of the maximal mu agonist response, the lack of reversibility following wash‐out, and the lack of long‐term antagonist effects at either delta or kappa opioid receptors suggests that MCAM is a selective, long acting (perhaps irreversible) mu opioid receptor antagonist.Support or Funding InformationSupported by the ARTT Center of Excellence, UT Health San Antonio.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.