Discovery and characterization of quinoxaline and quinoline carboxamides as allosteric cannabinoid receptor modulators.

Cannabinoid (CB) receptors type 1 (CB1) and type 2 (CB2) are Gi protein-coupled receptors that play important roles in brain function and immune regulation. While orthosteric CB1 and CB2 receptor agonists and antagonists have been widely studied, only few allosteric modulators are known. Screening of a proprietary compound library led to the discovery of quinoxaline-6-carboxamides as novel allosteric modulators of CB receptors. N-(4-Ethylphenyl)quinoxaline-6-carboxamide (3j, PSB-18579) acted as a negative allosteric CB1 receptor modulator as determined in β-arrestin-2 recruitment assays (IC50 6.78 µM), but had no effect on agonist-induced Gi protein dissociation, while it enhanced agonist radioligand binding to the receptor. Docking studies predicted its interaction with an allosteric site previously elucidated by X-ray crystallography. In contrast, the related N-(2-(benzyloxy)phenyl)quinoline-6-carboxamide (4a, PSB-21644) behaved as a potent partial CB2 receptor agonist, showing nanomolar potency in β-arrestin-2 recruitment assays (EC50 0.0371 µM, Emax 65 %). Schild plot analysis suggested an allosteric interaction of 4a with the human CB2 receptor. Similar potency and efficacy were observed in cyclic adenosine monophosphate (cAMP) accumulation assays (EC50 0.0765 µM, Emax 57 %). These findings provide new lead structures for the development of allosteric modulators of CB receptors, which constitute important drug targets.