IN-VITRO AND IN-VIVO ACTIVITY OF 16,17-DEHYDRO-EPIPREGNANOLONES - 17,20-BOND TORSIONAL ENERGY ANALYSIS AND D-RING CONFORMATION

Purpose. Certain neuroactive pregnane steroids (also known as ''epalon s'') are allosteric modulators of the GABA(A) receptor and have been s hown to be potent anticonvulsants, anxiolytics, sedative/hypnotics, an d anesthetic agents. The purpose of this study was to calculate the st ructural consequences of introduction of a double bond in the 16,17-po sition and to determine if this modification would selectively reduce sedative activity, but maintain the potent anticonvulsant activity of neuroactive steroids. Methods. We have studied the biochemical and beh avioral effects of introducing a 16,17 double bond into the naturally occurring neuroactive steroids, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) and 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alph a,5 beta-P) and three synthetic neuroactive steroid derivatives, 3 alp ha-hydroxy-3 beta-methyl-5 alpha-pregnan-20-one (3 alpha,3 beta Me,5 a lpha-P), 3 alpha-hydroxy-5 alpha-androstane (3 alpha,5 alpha-A), and a lphaxalone (3 alpha,5 alpha-11-one-P). Results. The 16-ene analogs of most of these neuroactive steroids were found to be 7- and 16-fold les s potent in inhibiting [S-35]TBPS binding to GABA(A) receptors and in a similar fashion, had reduced anticonvulsant and sedative potency in proportional amounts. The exception was the androstane (3 alpha,5 alph a-A) without a 17-acetyl group, that had virtually identical IC50 and ED(50) values for the saturated and unsaturated derivatives. Calculati on of the torsional energy profile for each of the 17-acetyl side chai n conformations showed that the conformational energy minima found in the alpha,beta-unsaturated keto systems, produce an orientation of the 20-keto group that is rotated by 165 degrees when compared to the non -conjugated acetyl group (determined by X-ray crystallography and its minimum energy conformation). Conclusions. The modified orientation of the 20-keto group of neuroactive steroids containing a 16-ene, provid es an explanation for their decreased biological activity overall, but did not lead to an enhanced protective index.