17β-Nitro-5α-androstan-3α-ol and its 3β-methyl derivative: Neurosteroid analogs with potent anticonvulsant and anxiolytic activities

Many 17-substituted androstan-3α-ol analogs act as positive allosteric modulators of GABAA receptors and exert anticonvulsant and anxiolytic-like activity actions in animal models. The endogenous neurosteroid allopregnanolone (17β-acetyl; 1) is among the most potent of these. Here we demonstrate that 3α-hydroxy-17β-nitro-5α-androstane (2b) and its 3β-methyl analog (3α-hydroxy-3β-methyl-17β-nitro-5α-androstane; 2c) modulate GABAA receptors as assessed by [35S]t-butylbicyclo-phosphorothionate and [3H]flunitrazepam binding with potencies equivalent to or greater than 1. These compounds also had potencies equivalent to or greater than 1 in the pentylenetetrazol and 6 Hz seizure models in the mouse. Furthermore, 2b exhibited anxiolytic-like activity in the elevated zero maze. The 3β-hydroxy, 3α-desmethyl analog (2a) was devoid of activity on GABAA receptors in vitro but had moderate activity in the seizure models, possibly as a result of epimerization in vivo at the 3-position. This conclusion was supported by the lack of in vivo activity of the 3β-hydroxy, 3α-methyl analog (2d), which is not expected to undergo epimerization. We conclude that nitro can serve as a bioisostere for acetyl at the 17β-position of 5α-androstan-3α-ol, such that the nitro analog fully retains the bioactivity of the endogenous neurosteroid at GABAA receptors.