Design, Synthesis, Pharmacological Evaluation and Docking Studies of GluN2B-Selective NMDA Receptor Antagonists with a Benzo[7]annulen-7-amine Scaffold

Antagonists that selectively target GluN2B-subunit-containing N-methyl-d-aspartate (NMDA) receptors are of major interest for the treatment of various neurological disorders. In this study, relationships between variously substituted benzo[7]annulen-7-amines and their GluN2B affinity were investigated. 2-Nitro-5,6,8,9-tetrahydrobenzo[7]annulen-7-one (8) represents the central building block for the introduction of various substituents at the 2-position and various 7-amino moieties. N-(3-Phenylpropyl)-6,7,8,9-tetrahydro-5H-benzo[7]annulen-7-amines with a 2-NO2 (7c), 2-Cl (15c), or 2-OBn group (22c) show very high GluN2B affinity (K-i=1.6-3.6nm). Docking studies revealed the same binding poses for benzo[7]annulen-7-amines and ifenprodil at the interface of GluN1b and GluN2B subunits. The large 2-OBn moiety of 22c occupies a previously unrecognized subpocket, which explains its high GluN2B affinity (K-i=3.6nm). In two-electrode voltage clamp experiments and cytoprotection assays, the high-affinity GluN2B ligands 7c, 15c, and 22c could not inhibit the glutamate-/glycine-evoked current and cytotoxic effects. However, the analogous phenols 16c ((3-phenylpropyl)amino moiety) and 16d ((4-phenylbutyl)amino moiety) with 10-fold lower GluN2B affinity (K-i=28 and 21nm, respectively) showed promising inhibition of glutamate-/glycine-evoked effects in both assays. The presence of a phenolic hydroxy group seems to be essential for inducing conformational changes of the receptor protein, which finally results in closure of the ion conduction pathway.