Negative allosteric modulation of NMDA receptors with GluN2B subunit: synthesis of γ-amino alcohols embedded in a 1-benzoxepine scaffold.

Novel ligands for the ifenprodil binding site of NMDA receptors with GluN2B subunit were designed, synthesized and pharmacologically evaluated. Derived from potent negative allosteric modulators, the γ-amino alcohol pharmacophore was conformationally restricted by embedding it into a 1-benzoxepine ring. Furthermore, the phenolic OH moiety was replaced bioisosterically by a primary or secondary amino moiety as well as by amido groups. The 7- to 9-step synthesis started with 4-aminosalicylic acid comprising a Dieckmann condensation of diester 7 and aminolysis of β-keto esters 8 as key steps. Membrane fragments of mouse fibroblast cells stably transfected with GluN1-1a and GluN2B subunits of the NMDA receptor and [3H]ifenprodil were used in radioligand receptor binding studies. In the benzylpiperidine d-series, the affinity towards GluN2B subunit-containing NMDA receptors increased from benzylamine 11d to primary amine 12d to formamide 13d (K i = 278 nM). Larger acyl moieties reduced the NMDA receptor affinity. Removal of the benzylic OH moiety led to allylamines 18b and 18d with considerable GluN2B affinity. However, their σ1 affinity was even higher than their GluN2B affinity. It was concluded that the benzylic OH moiety is not essential for high GluN2B affinity, but is important for selectivity over σ1 receptors. The novel class of 1-benzoxepine-based negative allosteric modulators of the NMDA receptor revealed promising lipophilicity (e.g., 13b: log D 7.4 = 1.99). Binding at human serum albumin correlated with the lipophilicity (e.g., 13b: PPB = 81%). Most of the 1-benzoxepines showed more than 65% metabolic stability during incubation with mouse liver microsomes and NADPH for 90 min.