The proconvulsant effects of the GABAA α-5 subtype-selective compound RY-080 may not be α5-mediated

RY-080 (ethyl 8-ethynyl-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate) is an imidazobenzodiazepine with 40-50-fold higher affinity for the benzodiazepine binding site of alpha 5- rather than alpha 1-, alpha 2- or alpha 3-containing GABA(A) receptors. Previous data describing RY-080 as being convulsant suggests that inverse agonists selective for the alpha 5 subtype may not be suitable for clinical development. In the present study, we show that RY-080 possesses inverse agonism for the alpha 1 and alpha 5 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 (ED50) was 108 mg/kg whereas in the presence of I and 10 mg/kg RY-080, the ED(50)s were 93 and 57 mg/kg, respectively. In vivo [H-3]L-655,708 and [H-3]Ro 15-1788 binding assays showed that the subtype selectivity of RY-080 in vivo was 7-10-fold for alpha 5-relative to alpha 1- and alpha 2/alpha 3-containing receptors (respective ID50 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 alpha 5 subtype without binding to other subtypes and accordingly the proconvulsant effects of RY-080 cannot be attributed solely to the alpha 5 subtype. (c) 2006 Published by Elsevier B.V.