Abiraterone (17-(3-pyridyl)androsta-5,16-dien-3beta-ol, 1) is a potent inhibitor 
(IC50 4 nM for hydroxylase) of human cytochrome P45017alpha. To assist in 
studies of the role of the 16,17-double bond in its mechanism of action, the 
novel 17alpha-(4-pyridyl)androst-5-en-3beta-ol (5) and 
17beta-(3-pyridyl)-16,17alpha-epoxy-5alpha-androst-3beta-ol (6) were 
synthesized. 3beta-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) 
17alpha-(4-pyridyl)androst-5-en-3beta-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 17beta-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 
P45017alpha. However oxidation to an epoxide is probably not involved since 
epoxide 6 was only a moderately potent inhibitor (IC50 260 nM).