The 16,17-double bond is needed for irreversible inhibition of human cytochrome P45017α by abiraterone (17-(3-pyridyl)androsta-5,16-dien-3β-ol) and related steroidal inhibitors

Abiraterone (17-(3-pyridyl)androsta-5,16-dien-3 beta-ol, 1) is a potent inhibitor (IC50 4 nM for hydroxylase) of human cytochrome P450(17 alpha). To assist in studies of the role of the 16,17-double bond in its mechanism of action, the novel 17 alpha-(4-pyridyl)androst-5-en-3 beta-ol (5) and 17 beta-(3-pyridyl)-16,17 alpha-epoxy-5 alpha-androst-3 beta-ol (6) were synthesized. 3 beta-Acetoxyetienic acid was converted in three steps into 5 via photolysis of the thiohydroxamic ester 8. Oxidation of an appropriate 16,17-unsaturated precursor (21) with CrO3-pyridine afforded the acetate (23) of 6. Inhibition of the enzyme by 1, the similarly potent 5,6-reduced analogue 19 (IC50 5 nM), and the 4,16-dien-3-one 26 (IC50 3 nM) and by the less potent (IC50 13 nM) 3,5,16-triene 25 is slow to occur but is enhanced by preincubation of the inhibitor with the enzyme. Inhibition following preincubation with these compounds is not lessened by dialysis for 24 h, implying irreversible binding to the enzyme. In contrast under these conditions the still potent (IC50 27 nM) 17 alpha-(4-pyridyl)androst-5-en-3 beta-ol (5) showed partial reversal after 5 h of dialysis and complete reversal of inhibition after 24 h. This behavior was also shown by the less potent 16,17-reduced 3-pyridyl compounds 3 and 24. Further, in contrast to the compounds (1, 19, 25, 26) with the 16,17-double bond, the inhibition of the enzymic reaction was not enhanced by preincubation either with 5 or with the 17 beta-pyridyl analogues 3, 4, and 24 which also lack this structural feature. The results show that the 16,17-double bond is necessary for irreversible binding of these pyridyl steroids to cytochrome P450(17 alpha). However oxidation to an epoxide is probably not involved since epoxide 6 was only a moderately potent inhibitor (IC50 260 nM).