1 We investigated the ability of N-benzyl-N-ethyl-2-(7,8-dihydro-7-methyl-8-oxo-2-phenyl-9H-purin-9-yl)acetamide (AC-5216), a novel mitochondrial benzodiazepine receptor (MBR) ligand, to produce anti-anxiety and antidepressant-like effects in various animal models. 2 AC-5216 showed high affinity for MBRs prepared from rat whole brain (K i 0.297 nM), rat glioma cells (IC 50 3.04 nM) and human glioma cells (IC 50 2.73 nM), but only negligible affinity for the other main receptors including central benzodiazepine receptors. 3 AC-5216 produced anti-anxiety effects in the Vogel-type conflict test in rats, and in the light/dark box and social interaction tests in mice at 0.1-3, 0.003-0.01 and 0.01-0.3 mg kg À1 , p.o., respectively. These effects of AC-5216 were antagonized by PK11195, an MBR antagonist. In the forced swimming test in rats, AC-5216 (3-30 mg kg À1 , p.o.) reduced the immobility time, and this effect was blocked by PK11195. 4 AC-5216 had no myorelaxant effects, did not affect the memory or prolong hexobarbitoneinduced sleep in mice, even at doses as high as 1000 mg kg À1 , p.o. Although it did slightly prolong the ethanol-induced sleep time at 1000 mg kg À1 , AC-5216 (1-100 mg kg À1 , p.o.) produced no distinct change in the rat electroencephalogram. 5 These results indicate that AC-5216 produces anti-anxiety and antidepressant-like effects that are mediated by MBR, but does not cause the side effects normally associated with conventional benzodiazepines. Hence, AC-5216 shows potential for the treatment of stress-related disorders including anxiety and depression.

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In contrast to the dose-occupancy relationship in the treatment of schizophrenia, the minimal effective level of dopamine receptor 2 (D2R) blockade for antipsychotics in the treatment of bipolar depression is unknown. Lower doses aimed at relieving extrapyramidal side effects are balanced against retaining D2R benefit on emergent cycling, mixed, manic, anxiety and/or psychotic symptoms. Dose-reductions intended to lower D2R blockade however could also decrease concomitant serotonin receptor antagonism and its potential benefit on depressive symptoms. Here we uncoupled the potential antidepressant activity in amisulpride, driven by 5-HT7 receptor (5-HT7R) antagonism, from the D2R-mediated antipsychotic activity by discovering that each enantiomer favors a different receptor. Aramisulpride was more-potent at 5-HT7R relative to esamisulpride (Ki 47 vs 1,900 nM, respectively), while esamisulpride was more potent at D2R (4.0 vs 140 nM). We hypothesized that a non-racemic ratio might achieve greater 5-HT7R-mediated antidepressant effects at a lower level of D2R blockade. The doseoccupancy relationship of esamisulpride at D2R was determined by PET imaging in human volunteers. Separately the dose-relationship of aramisulpride was established in humans using suppression of rapid eye movement (REM) sleep as a marker of 5-HT7R antagonism. These results led to the discovery of an 85:15 ratio of aramisulpride to esamisulpride (SEP-4199) that maximizes the potential for antidepressant benefit of aramisulpride via 5-HT7R and reduces esamisulpride to minimize D2R-related extrapyramidal side effects while still retaining D2Rmediated effects predicted to provide benefit in bipolar depression. SEP-4199 was recently evaluated in a proof of concept trial for the treatment of bipolar depression (NCT03543410).

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