(3-Cyano-5-fluorophenyl)biaryl negative allosteric modulators of mGlu₅: Discovery of a new tool compound with activity in the OSS mouse model of addiction

Glutamate is the major excitatory transmitter in the mammalian central nervous system (CNS), exerting its effects through both ionotropic and metabotropic glutamate receptors. The metabotropic glutamate receptors (mGlus) belong to family C of the G-protein-coupled receptors (GPCRs). The eight mGlus identified to date are classified into three groups based on their structure, preferred signal transduction mechanisms, and pharmacology (group I: mGlu₁ and mGlu₅; group II: mGlu₂ and mGlu₃; group III: mGlu₄, mGlu₆, mGlu₇, and mGlu₈). Noncompetitive antagonists, also known as negative allosteric modulators (NAMs), of mGlu₅ offer potential therapeutic applications in diseases such as pain, anxiety, gastresophageal reflux disease (GERD), Parkinson's disease (PD), fragile X syndrome, and addiction. The development of structure-activity relationships (SAR) in a (3-cyano-5-fluorophenyl)biaryl series using our functional cell-based assay is described in this communication. Further characterization of a selected compound, 3-fluoro- 5-(2-methylbenzo[d]thiazol-5-yl)benzonitrile, in additional cell based assays as well as in vitro assays designed to measure its metabolic stability and protein binding indicated its potential utility as an in vivo tool. Subsequent evaluation of the same compound in a pharmacokinetic study using intraperitoneal dosing in mice showed good exposure in both plasma and brain samples. The compound was efficacious in a mouse marble burying model of anxiety, an assay known to be sensitive to mGlu5 antagonists. A new operant model of addiction termed operant sensation seeking (OSS) was chosen as a second behavioral assay. The compound also proved efficacious in the OSS model and constitutes the first reported example of efficacy with a small molecule mGlu5 NAM in this novel assay.