3α-Hydroxy-3β-(phenylethynyl)-5β-pregnan-20-ones:  Synthesis and Pharmacological Activity of Neuroactive Steroids with High Affinity for GABAA Receptors

Neuroactive steroids that allosterically modulate GABAA receptors have potential uses as anticonvulsants, anxiolytics, and sedative−hypnotic agents. Recently, a series of pregnanes substituted with simple alkyl groups at the 3β-position were synthesized and found to be active in vitro. The present report describes the synthesis of a series of substituted 3α-hydroxy-3β-(phenylethynyl)pregnan-20-ones and their in vitro structure−activity relationship determined by their potency for inhibition of [35S]TBPS binding in rat brain membranes. Appropriate substitution of the phenyl group results in ligands with particularly high affinity for the neuroactive steroid site on GABAA receptors (e.g., 4-acetyl 28, IC50 10 nM). The potency of selected steroids was confirmed electrophysiologically in oocytes expressing cloned human GABAA α1β2γ2L receptors (e.g., compound 28, EC50 6.6 nM). Consistent with their in vitro activity, some of the 3β-(phenylethynyl)-substituted steroids displayed anticonvulsant activity in the pentylenetetrazol (PTZ) and maximal electroshock (MES) tests following ip administration in mice. Notably, the 3β-[(4-acetylphenyl)ethynyl]-19-nor derivative 36 demonstrated an attractive anticonvulsant profile (PTZ and MES ED50 values of 2.8 and 9.2 mg/kg, respectively). A new pharmacophore for the neuroactive steroid site of GABAA receptors is proposed based upon the high affinity of certain substituted 3β-(phenylethynyl) steroids.