Pharmacological Characterization and Modeling of the Binding Sites of Novel 1,3-Bis(pyridinylethynyl)benzenes as Metabotropic Glutamate Receptor 5-Selective Negative Allosteric Modulators

Metabotropic glutamate receptor subtype 5 (mGluR5) is a potential drug target in neurological and psychiatric disorders, and subtype-selective allosteric modulators have attracted much attention as potential drug candidates. In this study the binding sites of three novel MPEP-derived negative allosteric modulators, 2-, 3- and 4-BisPEB, have been characterized. 2-, 3-, and 4-BisPEB are 1,3-bis(pyridinylethynyl)-benzenes and differ only by the position of the nitrogen atom in the pyridine rings. Despite their high structural similarity, 2-BisPEB (1,3-bis(pyridin-2-ylethynyl)-benzene, nitrogen atom in ortho position), with an IC50-value in the nanomolar range, is significantly more potent than the 3- and 4-pyridyl analogues. Mutational analysis, directed by a previously published mGluR5 homology model, was used to determine key residues for the ligand-receptor interactions that may explain the potency differences of 2-, 3-, and 4-BisPEB. Residues I651, P655, Y659, N747, W785, F788, Y792, S809, and A810 were found to have critical roles for the activity of one or more of the three BisPEBs and the reference compound MPEP. The mutational data suggest that the higher potency of 2-BisPEB is due to hydrogen bonding to S809 as the S809A mutation made 2-BisPEB equipotent to 3- and 4-BisPEB (IC50: 1-2.5 μM). The potency of MPEP was also greatly affected by S809A (52-fold) suggesting that a S809-mediated hydrogen bond is also a key interaction between MPEP and mGluR5. Potential binding modes of 2-, 3- and 4- BisPEB obtained by molecular docking to the mGluR5 homology model provides a structural context for the reported major mutational effects.