Pharmacological Comparative Characterization of REL-1017 (Esmethadone-HCl) and Other NMDAR Channel Blockers in Human Heterodimeric N-Methyl-D-Aspartate Receptors

Abstract

Excessive Ca2+ currents via N-methyl-D-aspartate receptors (NMDARs) have been implicated in many disorders. Uncompetitive NMDAR channel blockers are an emerging class of drugs in clinical use for major depressive disorder (MDD) and other neuropsychiatric diseases. The pharmacological characterization of uncompetitive NMDAR blockers in clinical use may improve our understanding of NMDAR function in physiology and pathology. REL-1017 (esmethadone-HCl), a novel uncompetitive NMDAR channel blocker in Phase 3 trials for the treatment of MDD, was characterized together with dextromethorphan, memantine, (±)-ketamine, and MK-801 in cell lines over-expressing NMDAR subtypes using fluorometric imaging plate reader (FLIPR), automated patch-clamp, and manual patch-clamp electrophysiology. In the absence of Mg2+, NMDAR subtypes NR1-2D were most sensitive to low, sub-μM glutamate concentrations in FLIPR experiments. FLIPR Ca2+ determination demonstrated low μM affinity of REL-1017 at NMDARs with minimal subtype preference. In automated and manual patch-clamp electrophysiological experiments, REL-1017 exhibited preference for the NR1-2D NMDAR subtype in the presence of 1 mM Mg2+ and 1 μM L-glutamate. Tau off and trapping characteristics were similar for (±)-ketamine and REL-1017. Results of radioligand binding assays in rat cortical neurons correlated with the estimated affinities obtained in FLIPR assays and in automated and manual patch-clamp assays. In silico studies of NMDARs in closed and open conformation indicate that REL-1017 has a higher preference for docking and undocking the open-channel conformation compared to ketamine. In conclusion, the pharmacological characteristics of REL-1017 at NMDARs, including relatively low affinity at the NMDAR, NR1-2D subtype preference in the presence of 1 mM Mg2+, tau off and degree of trapping similar to (±)-ketamine, and preferential docking and undocking of the open NMDAR, could all be important variables for understanding the rapid-onset antidepressant effects of REL-1017 without psychotomimetic side effects.

Esmethadone (REL-1017) and Other Uncompetitive NMDAR Channel Blockers May Improve Mood Disorders via Modulation of Synaptic Kinase-Mediated Signaling

“…The dextromethorphan-bupropion combination was recently FDA-approved for the treatment of MDD. Recent studies have improved our understanding of the interactions between NMDAR antagonists and their binding sites in the receptor channel and the potential clinical implications of the pharmacological differences among these molecules [ 20 ].…”

“…The potency of NMDAR antagonists is determined by two distinct parameters: affinity and trapping. Trapping values for ketamine and esmethadone are similar at 85.9 ± 1.9 and 86.7 ± 1.8, respectively [ 20 ]. In a similar experiment from a different study, the trapping values for ketamine and memantine were 86 ± 1 and 71 ± 4, respectively [ 29 ].…”

“…In a similar experiment from a different study, the trapping values for ketamine and memantine were 86 ± 1 and 71 ± 4, respectively [ 29 ]. Ketamine-like affinity and ketamine-like trapping may both be required for dissociative effects [ 20 , 29 ]. Ketamine-like trapping, but not ketamine-like affinity, may be required for antidepressant effects [ 18 , 20 ].…”

“…Ketamine-like affinity and ketamine-like trapping may both be required for dissociative effects [ 20 , 29 ]. Ketamine-like trapping, but not ketamine-like affinity, may be required for antidepressant effects [ 18 , 20 ]. Uncompetitive NMDAR antagonists with low affinity (e.g., esmethadone) or with low trapping (e.g., memantine) do not cause dissociative effects.…”

“…Uncompetitive NMDAR antagonists with low affinity (e.g., esmethadone) or with low trapping (e.g., memantine) do not cause dissociative effects. Memantine, with low trapping compared to ketamine, is ineffective as an antidepressant despite ketamine-like NMDAR affinity [ 20 , 30 ].…”