The proconvulsant effects of the GABA A α5 subtype-selective compound RY-080 may not be α5-mediated

Abstract

RY-080 (ethyl 8-ethynyl-5,6-dihydro-5-methyl-6-oxo-4 H-imidazo[1,5- a][1,4]benzodiazepine-3-carboxylate) is an imidazobenzodiazepine with 40–50-fold higher affinity for the benzodiazepine binding site of α5- rather than α1-, α2- or α3-containing GABA A receptors. Previous data describing RY-080 as being convulsant suggests that inverse agonists selective for the α5 subtype may not be suitable for clinical development. In the present study, we show that RY-080 possesses inverse agonism for the α1 and α5 subtypes of human recombinant GABA A receptors and whilst not convulsant it was proconvulsant. Hence, with pentylenetetrazole alone, the dose predicted to give tonic convulsions in 50% of the mice (ED 50) was 108 mg/kg whereas in the presence of 1 and 10 mg/kg RY-080, the ED 50s were 93 and 57 mg/kg, respectively. In vivo [ 3H]L-655,708 and [ 3H]Ro 15–1788 binding assays showed that the subtype selectivity of RY-080 in vivo was 7–10-fold for α5-relative to α1- and α2/α3-containing receptors (respective ID 50 values of 0.93, 9.7 and 6.2 mg/kg) and is therefore much lower than seen in vitro. Consequently, it is not possible to define a dose of RY-080 which gives high occupancy of the α5 subtype without binding to other subtypes and accordingly the proconvulsant effects of RY-080 cannot be attributed solely to the α5 subtype.