SAR studies on mGlu5 receptor positive allosteric modulators (2003- 2013)

Metabotropic glutamate receptor 5 (mGlu5) is a class C G-protein-coupled receptor which possesses a large extracellular N-terminal domain (ATD) connected to the seven-transmembrane (7-TM) domain. In contrast to the glutamate and its close analogs binding at the orthosteric site on the ATD region, allosteric modulators bind at topographically distinct sites in the 7-TM region. Activation of mGlu5 receptors at either the orthosteric or allosteric sites results in enhancement of NMDA receptor function and represents a promising opportunity for the treatment of schizophrenia. Since the disclosure of the first mGlu5 positive allosteric modulators (PAM) in 2003, there have been intense industry-wide efforts to discover and develop safe and efficacious agents capable of selectively enhancing mGlu5 receptor function at the allosteric sites. Over the past decade, tremendous progress has been made, and multiple chemical scaffolds have been identified as mGlu5 PAMs, possibly binding to different allosteric sites on the 7-TM domain. These ligands have helped gain novel insights into the biology of mGlu5 receptor allosteric activation. Here we provide a comprehensive review on the structure-activity relationship (SAR) progress on the mGlu5 PAMs reported in the primary literature and include appropriate and complementary examples from the patent literature. Important in vivo studies of select compounds from individual scaffolds are highlighted, and challenges facing the clinical development of mGlu5 receptor PAMs are discussed.