The μ-opioid receptor (OPRM1) is the primary site of action for morphine and other opioids associated with the treatment of pain. The occupancy of δ-opioid receptors (OPRD1) by antagonists has been shown to enhance OPRM1 binding and signaling activity. Because heteromerization of OPRM1 and OPRD1 is thought to be a potential mechanism by which this occurs, compounds are being sought that selectively activate OPRM1-OPRD1 heterodimerization while alleviating the unwanted effects associated with prolonged opiate use. ML-335 is the first identified agonist for OPRM1-OPRD1 heterodimerization (EC50 = 403 nM) that demonstrates selectivity over the individual OPRM1 and OPRD1 receptors and the serotonin HT5A receptor (EC50s = 14.9, 1.1, and >40 μM, respectively).
