S32212, a Novel Serotonin Type 2C Receptor Inverse Agonist/α2-Adrenoceptor Antagonist and Potential Antidepressant: I. A Mechanistic Characterization

Although most antidepressants suppress serotonin (5-HT) and/or noradrenaline reuptake, blockade of 5-HT 2C receptors and α 2 -adrenoceptors likewise enhances monoaminergic transmission. These sites are targeted by the urea derivative N - [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-1,2-dihydro-3- H -benzo[ e ]indole-3-carboxamide (S32212). S32212 was devoid of affinity for monoamine reuptake sites, yet displayed pronounced affinity (p K i , 8.2) for constitutively active human 5-HT 2CINI (h5-HT 2CINI ) receptors, behaving as an inverse agonist in reducing basal Gα q activation, [ 3 H]inositol-phosphate production, and the spontaneous association of h5-HT 2CINI -Renilla luciferase receptors with β-arrestin2-yellow fluorescent protein. Furthermore, upon 18-h pretreatment, S32212 enhanced the plasma membrane expression of h5-HT 2CINI receptors as visualized by confocal microscopy and quantified by enzyme-linked immunosorbent assay. Its actions were prevented by the neutral antagonist 6-chloro-5-methyl- N -[6-(2-methylpyridin-3-yloxy)pyridin-3-yl]indoline-1-carboxamide (SB242,084), which also impeded the induction by long-term exposure to S32212 of otherwise absent Ca 2+ mobilization in mouse cortical neurones. In vivo, S32212 blunted the inhibitory influence of the 5-HT 2C agonist 2-(3-chlorobenzyloxy)-6-(1-piperazinyl)pyrazine (CP809,101) on ventrotegmental dopaminergic neurones. S32212 also blocked 5-HT-induced Gα q and phospholipase C activation at the h5-HT 2A and, less potently, h5-HT 2B receptors and suppressed the discriminative stimulus properties of the 5-HT 2A agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane in rats. S32212 manifested marked affinity for human α 2A - (p K i 7.2), α 2B - (p K i 8.2), and α 2C - (p K i 7.4) adrenoceptors, at which it abolished noradrenaline-induced recruitment of Gα i3 , Gα o , adenylyl cyclase, and extracellular-regulated kinase1/2. Moreover, S32212 dose-dependently abolished the discriminative stimulus effects of the α 2 -adrenoceptor agonist ( S )-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2′-(1′,2′,3′,4′-tetrahydronaphthalene)] (S18616). Finally, S32212 displayed negligible affinity for α 1A -adrenoceptors, histamine H 1 receptors, and muscarinic M 1 receptors. In conclusion, S32212 behaves as an inverse agonist at h5-HT 2C receptors and as an antagonist at human α 2 -adrenoceptors (and h5-HT 2A receptors). Its promising profile in preclinical models potentially relevant to the treatment of depression is described in J Pharmacol Exp Ther 340 :765–780, 2012.