ONO-8590580, a Novel GABAAα5 Negative Allosteric Modulator Enhances Long-Term Potentiation and Improves Cognitive Deficits in Preclinical Models

GABAA receptors containing α5 subunits (GABAAα5) are highly expressed in the hippocampus and negatively involved in memory processing, as shown by the fact that GABAAα5–deficient mice show higher hippocampus-dependent performance than wild-type mice. Accordingly, small-molecule GABAAα5 negative allosteric modulators (NAMs) are known to enhance spatial learning and memory in rodents. Here we introduce a new, orally available GABAAα5 NAM that improves hippocampal functions. ONO-8590580 [1-(cyclopropylmethyl)-5-fluoro-4-methyl-N-[5-(1-methyl-1H-imidazol-4-yl)-2-pyridinyl]-1H-benzimidazol-6-amine] binds to the benzodiazepine binding sites on recombinant human α5–containing GABAA receptors with a Ki of 7.9 nM, and showed functionally selective GABAAα5 NAM activity for GABA-induced Cl− channel activity with a maximum 44.4% inhibition and an EC50 of 1.1 nM. In rat hippocampal slices, tetanus-induced long-term potentiation of CA1 synapse response was significantly augmented in the presence of 300 nM ONO-8590580. Orally administered ONO-8590580 (1–20 mg/kg) dose-dependently occupied hippocampal GABAAα5 in a range of 40%–90% at 1 hour after intake. In the rat passive avoidance test, ONO-8590580 (3–20 mg/kg, by mouth) significantly prevented (+)-MK-801 hydrogen maleate (MK-801)–induced memory deficit. In addition, ONO-8590580 (20 mg/kg, p.o.) was also effective in improving the cognitive deficit induced by scopolamine and MK-801 in the rat eight-arm radial maze test with equal or greater activity than 0.5 mg/kg donepezil. No anxiogenic-like or proconvulsant effect was associated with ONO-8590580 at 20 mg/kg p.o. in the elevated plus maze test or pentylenetetrazole-induced seizure test, respectively. In sum, ONO-8590580 is a novel GABAAα5 NAM that enhances hippocampal memory function without an anxiogenic or proconvulsant risk.