Comparative Pharmacological Study of Common NMDA Receptor Open Channel Blockers Regarding Their Affinity and Functional Activity toward GluN2A and GluN2B NMDA Receptors

Because only a few studies have investigated the affinity and functional activity of NMDA receptor open channel blockers under the same assay conditions, a comparative study of common open channel blockers is of major interest. The pharmacological activities of MK-801, phencyclidine (PCP), dexoxadrol, etoxadrol, (S)- and (R)-ketamine, dextromethorphan, memantine, and amantadine were analyzed under uniform assay conditions. Affinity toward the PCP and ifenprodil binding sites was recorded in radioligand binding assays. GluN2A and GluN2B subtype-specific cytoprotective activity was determined in lactate dehydrogenase (LDH) assays. The data were correlated with published IC50 values obtained in two-electrode voltage clamp experiments. A high correlation was found between PCP affinity, ion flux inhibition, and cytoprotective activity. The channel blockers were classified into four groups showing high, moderate, low, and very low potency. Some of the open channel blockers display unexpected subtype selectivity. The comparative study allows the characterization of open channel blockers from their receptor ligand interaction via inhibition of ion flux up to overall cytoprotective activity. The subtype preference of some open channel blockers will stimulate the development of novel subtype-selective open channel blockers with decreased side effect potential.