Discovery of a new class of orthosteric antagonists with nanomolar potency at extrasynaptic GABAA receptors

Brain GABA Α receptors are ionotropic receptors belonging to the class of Cys-loop receptors and are important drug targets for the treatment of anxiety and sleep disorders. By screening a compound library (2,112 compounds) at recombinant human α 4 β 1 δ GABA Α receptors heterologously expressed in a HEK cell line, we identified a scaffold of spirocyclic compounds with nanomolar antagonist activity at GABA Α receptors. The initial screening hit 2027 (IC 50 of 1.03 μM) was used for analogue search resulting in 018 (IC 50 of 0.088 μM). 018 was most potent at α 3,4,5 -subunit containing receptors, thus showing preference for forebrain-expressed extrasynaptic receptors. Schild analysis of 018 at recombinant human α 4 β 1 δ receptors and displacement of [ 3 H]muscimol binding in rat cortical homogenate independently confirmed a competitive profile. The antagonist profile of 018 was further validated by whole-cell patch-clamp electrophysiology, where kinetic studies revealed a slow dissociation rate and a shallow hill slope was observed. Membrane permeability studies showed that 2027 and 018 do not cross membranes, thus making the compounds less attractive for studying central GABA Α receptors effects, but conversely more attractive as tool compounds in relation to emerging peripheral GABA Α receptor-mediated effects of GABA e.g. in the immune system.