Anticonvulsant potencies of the enantiomers of the neurosteroids androsterone and etiocholanolone exceed those of the natural forms

RATIONALE: Androsterone [(3α,5α)-3-hydroxyandrostan-17-one; 5α,3α-A] and its 5β-epimer etiocholanolone [(3α,5β)-3-hydroxyandrostan-17-one; 5β,3α-A)], the major excreted metabolites of testosterone, are neurosteroid positive modulators of GABA(A) receptors. Such neurosteroids typically show enantioselectivity in which the natural form is more potent than the corresponding unnatural enantiomer. For 5α,3α-A and 5β,3α-A, the unnatural enantiomers are more potent at GABA(A) receptors than the natural forms. OBJECTIVES: The aim of this study was to compare the anticonvulsant potencies and time courses of 5α,3α-A and 5β,3α-A with their enantiomers in mouse seizure models. METHODS: Steroids were administered intraperitoneally to male NIH Swiss mice 15 min (or up to 6 h in time course experiments) prior to administration of an electrical stimulus in the 6-Hz or maximal electroshock (MES) seizure tests or the convulsant pentylenetetrazol (PTZ). RESULTS: In the 6-Hz test, the ED(50) values of ent-5α,3α-A was 5.0 mg/kg whereas the value for 5α,3α-A was 12.1 mg/kg; the corresponding values in the PTZ seizure test were 22.8 and 51.8 mg/kg. Neurosteroid GABA(A) receptor positive allosteric modulators are generally weak in the MES test and this was confirmed in the present study. However, the atypical relative potency relationship was maintained with ED(50) values of 140 and 223 mg/kg for ent-5α,3α- A and 5α,3α-A, respectively. Similar relationships were obtained for the 5β-isomers, except that the enantioselectivity was accentuated. In the 6-Hz and PTZ tests, the ED(50) values of ent-5β,3α-A were 11.8 and 20.4 mg/kg whereas the values for 5β,3α-A were 57.6 and 109.1 mg/kg. Protective activity in the 6-Hz test of ent-5α,3α-A persisted for somewhat longer (~5 h) than for 5α,3α-A (~4 h); protection by ent-5β,3α-A also persisted longer (~3 h) than for 5β,3α-A (~2 h). CONCLUSIONS: The unnatural enantiomers of 17-keto androgen class neurosteroids have greater in vivo potency and a longer duration of action than their natural counterparts. The more prolonged duration of action of the unnatural enantiomers could reflect reduced susceptibility to metabolism. Unnatural enantiomers of androgen class neurosteroids could have therapeutic utility and may provide advantages over the corresponding natural isomers due to enhanced potency and improved pharmacokinetic characteristics