Synthesis and Biological Evaluation of 3-Heterocyclyl-7,8,9,10-tetrahydro-(7,10-ethano)-1,2,4-triazolo[3,4-a]phthalazines and Analogues as Subtype-Selective Inverse Agonists for the GABAAα5 Benzodiazepine Binding Site

The identification of a novel series of 7,8,9,10-tetrahydro-(7,10-ethano)-1,2,4-triazolo[3,4-a]phthalazines as GABAAα5 inverse agonists, which have both binding and functional (efficacy) selectivity for the benzodiazepine binding site of α5- over α1-, α2-, and α3-containing GABAA receptor subtypes, is described. Binding selectivity was determined to a large part by the degree of planarity of the fused ring system whereas functional selectivity was dependent on the nature of the heterocycle at the 3-position of the triazolopyridazine ring. 3-Furan and 5-methylisoxazole were shown to be optimal for GABAAα5 functional selectvity. 3-(5-Methylisoxazol-3-yl)-6-(2-pyridyl)methyloxy-1,2,4-triazolo[3,4-a]phthalazine (43) was identified as a full inverse agonist at the GABAAα5 subtype with functional selectivity over the other GABAA receptor subtypes and good oral bioavailability.