Synthesis and structure–activity relationships of 6-phenylaliphatic-substituted C 19 steroids having a 1,4-diene, 4,6-diene, or 1,4,6-triene structure as aromatase inhibitors

Abstract

A series of 6α- and 6β-phenylaliphatic-substituted androsta-1,4-diene-3,17-diones [ 9b-f and 10b-f; (CH 2) nPh, n = 1–5] and their 4,6-diene and 1,4,6-triene analogs ( 11b-f and 12b-f) along with their respective phenyl analogs 9a-12a were synthesized and tested as aromatase inhibitors. All of the steroids examined were very powerful competitive inhibitors of aromatase in human placental microsomes with apparent K i values ranging from 8.5 to 80 nM. The inhibitory activities of the benzyl- and phenethyl-4,6-dienes 11b and 11c ( K i, 9.0 and 10 nM) as well as the 6-phenethyl-1,4,6-triene 12c ( K i, 8.5 nM) were extremely high among them. All of the phenylaliphatic steroids, except for the 6β-phenethyl compound 10c, and the 6-phenyl-4,6-diene 11a had higher affinity for aromatase than the corresponding parent 1,4-diene, 4,6-diene, and 1,4,6-triene steroids 9g, 11g, and 12g. All of the 6α-substituted 1,4-dienes ( 9a–9g) and the 6-substituted 1,4,6-trienes ( 12a-12g) caused a time-dependent inactivation of aromatase. On the other hand, only the 6β-substituted 1,4-dienes ( 10a–10d) having no or less than four carbon atoms between the steroid nucleus and the phenyl group also caused a time-dependent inactivation of aromatase. Their inactivation rates ( k inact 0.076–0.156 min −1) were higher than the respective parent steroids, 9g and 12g. In contrast, in the 4,6-diene series, only the 6-phenpropyl steroids 11d inactivated aromatase in a time-dependent manner with 0.155 min −1 of k inact value. The inactivation was prevented by the substrate androstenedione, and no significant effect of L-cysteine on the inactivation was observed in each case. These results indicate that length and/or stereochemistry of the C-6 substituent of steroids 9–12 as well as a terminal phenyl group incorporated in the C-6 substituent play a critical role not only in tight binding to the active site of aromatase but also in the cause of a time-dependent inactivation of the enzyme.