Benzimidazole-type glycine antagonists: the role of the ring nitrogen atoms

Several derivatives of 1H-benzimidazole-2-carboxylic acid (BICA, 2a) were tested in vitro in comparison to 1H-indole-2-carboxylic acid (ICA, 1e) for their ability to displace [3H]glycine from rat hippocampal membranes. Compound 2a was 8 times more potent than 1e (Ki 5.3 microM, as compared to 42 microM). However, introduction of a carboxymethyl group or a corresponding ester at position 3 had no positive effect on the potency of 2a, while this type of structural modification increased the potency of 1e significantly. Nevertheless, 1-carboxymethyl-BICA (2b) displaced [3H]glycine with similar potency as the corresponding 3-carboxymethyl-ICA 1c, indicating that also a nitrogen atom lacking a hydrogen atom can be engaged in glycine receptor interaction. N-Methylation strongly reduced the potencies of both BICA and ICA derivatives.