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

Cannabinoid (CB) receptors type 1 (CB₁) and type 2 (CB₂) are G_i 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 G_i 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, E_max 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, E_max 57 %). These findings provide new lead structures for the development of allosteric modulators of CB receptors, which constitute important drug targets.