Abstract

Neuroactive steroids are positive allosteric modulators of γ-aminobutyric acid_A (GABA_A) receptor complexes. Synthetic modification generally does not increase neuroactive steroid potency beyond that of the naturally occurring progesterone metabolite, 3α-hydroxy-5α-pregnan-20-one (3α,5α-P). Recently, it has been shown that introduction of appropriately para-substituted phenylethynyl groups at the 3β-position of 5β steroids increases receptor potency. The present report presents the synthesis and pharmacological profile of an analogous series of 5α steroids. The most striking feature of this series is the further enhancement of in vitro andin vivo potency obtained. In particular, 3β-(p-acetylphenylethynyl)-3α-hydroxy-5α-pregnan-20-one (Co 152791) was 11-, 16- and 49-fold more potent than 3α,5α-P in modulating the binding of [³⁵S]TBPS, [³H]flunitrazepam and [³H]muscimol, respectively, in rat brain membranes (Co 152791 IC₅₀ or EC₅₀ = 2–7.5 nM). Similarly, Co 152791 was 3- to 20-fold more potent than 3α,5α-P as an inhibitor of [³⁵S]TBPS binding in human recombinant receptor combinations containing α1, α2, α3 or α5 and β2γ2L subunits (Co 152791 IC₅₀1.4–5.7 nM). Co 152791 displayed low efficacy and 3α,5α-P had low potency at α4/6β3γ2L GABA_A receptor complexes. Interestingly, Co 152791 demonstrated remarkable potency as a potentiator of GABA-evoked currents in Xenopus oocytes expressing α1β2γ2L receptors (EC₅₀ 0.87 nM), being 184-fold more potent than 3α,5α-P. High in vitropotency was also reflected in enhanced in vivo activity in that Co 152791 exhibited exceptional anticonvulsant potency, protecting mice from pentylenetetrazol-induced seizures at a ∼5-fold lower dose than 3α,5α-P after i.p. administration (Co 152791 ED₅₀ 0.6 mg/kg). Moreover, Co 152791 was orally active (ED₅₀ 1.1 mg/kg) and exhibited a therapeutic index of 7 relative to rotorod impairment. The remarkable potency of Co 152791 as a positive allosteric modulator of GABA_A receptors may be explained by its interaction with an auxiliary binding pocket in the neuroactive steroid binding site. In addition, modification at the 3β-position probably hinders metabolism of the 3α-hydroxy group contributing to the exceptional anticonvulsant potency of this compound relative to other neuroactive steroids.