Negative allosteric modulators of the GluN2B NMDA receptor with phenylethylamine structure embedded in ring-expanded and ring-contracted scaffolds.

A set of GluN2B NMDA receptor antagonists with conformationally restricted phenylethylamine substructure was prepared and pharmacologically evaluated. The phenylethylamine substructure was embedded in ring expanded 3-benzazocines 4 as well as ring-contracted tetralinamines 6 and indanamines 7. The ligands 4, 6 and 7 were synthesized by reductive alkylation of secondary amine 11, reductive amination of ketones 12 and 16 and nucleophilic substitution of nosylates 14 and 17. The moderate GluN2B affinity of 3-benzazocine 4d (Ki = 32 nM) translated into moderate cytoprotective activity (IC50 = 890 nM) and moderate ion channel inhibition (60% at 10 µM) in two-electrode voltage clamp experiments with GluN1a/GluN2B expressing oocytes. Although some of the tetralinamines 6 and indanamines 7 showed very high GluN2B affinity (e.g. Ki (7f) = 3.2 nM), they could not inhibit glutamate/glycine inducted cytotoxicity. The low cytoprotective activity of 3-benzazocines 4, tetralinamines 6 and indanamines 7 was attributed to the missing OH moiety at the benzene ring and/or in benzylic position. Docking studies showed that the novel GluN2B ligands adopt similar binding poses as Ro 25-6981 with the central H-bond interaction between the protonated amino moiety of the ligands and the carbamoyl moiety of Gln110. However, due to the lack of a second H-bond forming group, the ligands can adopt two binding poses within the ifenprodil binding pocket.

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 (7 c), 2-Cl (15 c), or 2-OBn group (22 c) show very high GluN2B affinity (Ki =1.6-3.6 nm). 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 22 c occupies a previously unrecognized subpocket, which explains its high GluN2B affinity (Ki =3.6 nm). In two-electrode voltage clamp experiments and cytoprotection assays, the high-affinity GluN2B ligands 7 c, 15 c, and 22 c could not inhibit the glutamate-/glycine-evoked current and cytotoxic effects. However, the analogous phenols 16 c ((3-phenylpropyl)amino moiety) and 16 d ((4-phenylbutyl)amino moiety) with 10-fold lower GluN2B affinity (Ki =28 and 21 nm, 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.

Benzo[7]annulene-based GluN2B selective NMDA receptor antagonists: Surprising effect of a nitro group in 2-position.

Benzo[7]annulen-7-amines 7 without further polar substituents have been reported as conformationally restricted Ro 25-6981 analogs and show unexpectedly high GluN2B affinity. Herein the corresponding 2-NO2 derivatives 8 were synthesized and pharmacologically evaluated. NO2 derivatives 8 show 5- to 10-fold higher GluN2B affinity than the unsubstituted ligands 7. Docking studies of ligands 7c and 8c reveal an important contribution of the 2-NO2-substituent in determining the binding pose and modulating the GluN2B affinity.

Synthesis, GluN2B affinity and selectivity of benzo[7]annulen-7-amines.

Due to their beneficial side effect profile, NMDA receptor antagonists interacting selectively with the allosteric ifenprodil binding site of the GluN2B subunit are of major interest for the treatment of neurological and neurodegenerative disorders. A series of benzo[7]annulen-7-amines 6 was designed by conformational restriction of ifenprodil (1). At first the benzo[7]annulen-7-one 11 was prepared in a three-step synthesis comprising of a double Knoevenagel condensation of phthalaldehyde (7) with dimethyl 3-oxoglutarate (8), hydrogenation of 9 and saponification/decarboxylation of 10. Reductive amination of the ketone 11 with primary amines and NaBH(OAc)3 led to the secondary amines 6a-d, cis-6h and trans-6i. The tertiary amines 6e-g were obtained by SN2-substitution of the nosylate 13. Although H-bond forming substituents in 2- and 5-position are missing, the amines 6 exhibit high affinity towards GluN2B containing NMDA receptors. A distance of four to five bond lengths between the basic amino moiety and the phenyl ring in the side chain appears to be optimal for high GluN2B affinity. The phenylcyclohexylamine cis-6h and the 4-benzylpiperidine 6g show the highest GluN2B affinities (Ki=2.3nM and 2.9nM, respectively). With respect to selectivity against the PCP binding site, σ1 and σ2 receptors the phenylpiperazine 6f is the most promising GluN2B antagonist.