Novel 17β-Substituted Conformationally Constrained Neurosteroids that Modulate GABAA Receptors

The goal of this study was to develop a series of allopregnanolone analogues substituted by conformationally constrained 17β side chains to obtain additional information about the structure−activity relationship of 5α-reduced steroids to modulate GABAA receptors. Specifically, we introduced alkynyl-substituted 17β side chains in which the triple bond is either directly attached to the 17β-position or to the 21-position of the steroid skeleton. Furthermore, we investigated the effects of C22 and C20 modification. The in vitro binding affinity for the GABAA receptor of the new analogues was measured by allosteric displacement of the specific binding of [3H]4‘-ethynyl-4-n-propyl-bicycloorthobenzoate (EBOB) to GABAA receptors on synaptosomal membranes of rat cerebellum. An allosteric binding model that has been successfully applied to ionotropic glycine receptors was employed. The most active derivative is (20R)-17β-(1-hydroxy-2,3-butadienyl)-5α-androstane-3-ol (20), which possesses low nanomolar potency to modulate cerebellar GABAA receptors and is 71 times more active than the control compound allopregnanolone. Theoretical conformational analysis was employed in an attempt to correlate the in vitro results with the active conformations of the most potent of the new analogues.