Discovery of a Highly Selective Sigma-2 Receptor Ligand, 1-(4-(6,7-Dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-3-methyl-1H-benzo[d]imidazol-2(3H)-one (CM398), with Drug-Like Properties and Antinociceptive Effects In Vivo.

The sigma-2 receptor has been cloned and identified as Tmem97, which is a transmembrane protein involved in intracellular Ca2+ regulation and cholesterol homeostasis. Since its discovery, the sigma-2 receptor has been an extremely controversial target, and many efforts have been made to elucidate the functional role of this receptor during physiological and pathological conditions. Recently, this receptor has been proposed as a potential target to treat neuropathic pain due to the ability of sigma-2 receptor agonists to relieve mechanical hyperalgesia in mice model of chronic pain. In the present work, we developed a highly selective sigma-2 receptor ligand (sigma-1/sigma-2 selectivity ratio > 1000), 1-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-3-methyl-1H- benzo[d]imidazol-2(3H)-one (CM398), with an encouraging in vitro and in vivo pharmacological profile in rodents. In particular, radioligand binding studies demonstrated that CM398 had preferential affinity for sigma-2 receptor compared with sigma-1 receptor and at least four other neurotransmitter receptors sites, including the norepinephrine transporter. Following oral administration, CM398 showed rapid absorption and peak plasma concentration (Cmax) occurred within 10 min of dosing. Moreover, the compound showed adequate, absolute oral bioavailability of 29.0%. Finally, CM398 showed promising anti-inflammatory analgesic effects in the formalin model of inflammatory pain in mice. The results collected in this study provide more evidence that selective sigma-2 receptor ligands can be useful tools in the development of novel pain therapeutics and altogether, these data suggest that CM398 is a suitable lead candidate for further evaluation.