Synthesis and in Vitro Activity of 3β-Substituted-3α-hydroxypregnan-20-ones:  Allosteric Modulators of the GABAA Receptor

Two naturally occurring metabolites of progesterone, 3α-hydroxy-5α- and 5β-pregnan-20-one (1 and 2), are potent allosteric modulators of the GABAA receptor. Their therapeutic potential as anxiolytics, anticonvulsants, and sedative/hypnotics is limited by rapid metabolism. To avoid these shortcomings, a series of 3β-substituted derivatives of 1 and 2 was prepared. Small lipophilic groups generally maintain potency in both the 5α- and 5β-series as determined by inhibition of [35S]TBPS binding. In the 5α-series, 3β-ethyl, -propyl, -trifluoromethyl and -(benzyloxy)methyl, as well as substituents of the form 3β-XCH2, where X is Cl, Br, or I or contains unsaturation, show limited efficacy in inhibiting [35S]TBPS binding. In the 5β-series, the unsubstituted parent 2 is a two-component inhibitor, whereas all of the 3β-substituted derivatives of 2 inhibit TBPS via a single class of binding sites. In addition, all of the 3-substituted 5β-sterols tested are full inhibitors of [35S]TBPS binding. Electrophysiological measurements using α1β2γ2L receptors expressed in oocytes show that 3β-methyl- and 3β-(azidomethyl)-3α-hydroxy-5α-pregnan-20-one (6 and 22, respectively) are potent full efficacy modulators and that 3α-hydroxy-3β-(trifluoromethyl)-5α-pregnan-20-one (24) is a low-efficacy modulator, confirming the results obtained from [35S]TBPS binding. These results indicate that modification of the 3β-position in 1 and 2 maintains activity at the neuroactive steroid site on the GABAA receptor. In animal studies, compound 6 (CCD 1042) is an orally active anticonvulsant, while the naturally occurring progesterone metabolites 1 and 2 are inactive when administered orally, suggesting that 3β-substitution slows metabolism of the 3-hydroxyl, resulting in orally bioavailable steroid modulators of the GABAA receptor.