SUBSTITUTED 3-BETA-PHENYLETHYNYL DERIVATIVES OF 3-ALPHA-HYDROXY-5-ALPHA-PREGNAN-20-ONE - REMARKABLY POTENT NEUROACTIVE STEROID MODULATORS OF GAMMA-AMINOBUTYRIC ACID(A) RECEPTORS

Neuroactive steroids are positive allosteric modulators of gamma-amino butyric acid(A) (GABA(A)) receptor complexes. Synthetic modification g enerally does not increase neuroactive steroid potency beyond that of the naturally occurring progesterone metabolite, 3 alpha-hydroxy-5 alp ha-pregnan-20-one (3 alpha,5 alpha-P). Recently, it has been shown tha t introduction of appropriately para-substituted phenylethynyl groups at the 3 beta-position of 5 beta steroids increases receptor potency. The present report presents the synthesis and pharmacological profile of an analogous series of 5 alpha steroids. The most striking feature of this series is the further enhancement of in vitro and in vivo pote ncy obtained. In particular, 3 beta-(p-acetylphenylethynyl)-3 alpha-hy droxy-5 alpha-pregnan-20-one (Co 152791) was 11-, 16- and 49-fold more potent than 3 alpha,5 alpha-P in modulating the binding of [S-35]TBPS , [H-3]flunitrazepam and [H-3]muscimol, respectively, in rat brain mem branes (Co 152791 IC50 or EC50 = 2-7.5 nM). Similarly, Co 152791 was 3 - to 20-fold more potent than 3 alpha,5 alpha-P as an inhibitor of [S- 35]TBPS binding in human recombinant receptor combinations containing alpha 1, alpha 2, alpha 3 Or alpha 5 and beta 2 gamma 2L subunits (Co 152791 IC50 1.4-5.7 nM). Co 152791 displayed low efficacy and 3 alpha, 5 alpha-P had low potency at alpha 4/6 beta 3 gamma 2L GABA(A) recepto r complexes. Interestingly, Co 152791 demonstrated remarkable potency as a potentiator of GABA-evoked currents in Xenopus oocytes expressing alpha 1 beta 2 gamma 2L receptors (EC50 0.87 nM), being 184-fold more potent than 3 alpha,5 alpha-P. High in vitro potency was also reflect ed in enhanced in vivo activity in that Co 152791 exhibited exceptiona l anticonvulsant potency, protecting mice from pentylenetetrazol-induc ed seizures at a similar to 5-fold lower dose than 3 alpha,5 alpha-P a fter i.p. administration (Co 152791 ED50 0.6 mg/kg). Moreover, Co 1527 91 was orally active (ED50 1.1 mg/kg) and exhibited a therapeutic inde x of 7 relative to rotorod impairment. The remarkable potency of Co 15 2791 as a positive allosteric modulator of GABA, receptors may be expl ained by its interaction with an auxiliary binding pocket in the neuro active steroid binding site. In addition, modification at the 3 beta-p osition probably hinders metabolism of the 3 alpha-hydroxy group contr ibuting to the exceptional anticonvulsant potency of this compound rel ative to other neuroactive steroids.