Synthesis and structure-activity relationships of 6-phenylaliphatic-substituted C19 steroids having a 1,4-diene, 4,6-diene, or 1,4,6-triene structure as aromatase inhibitors.

A series of 6alpha- and 6beta-phenylaliphatic-substituted androsta-1,4-diene-3,17-diones [9b-f and 10b-f; (CH2)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 Ki values ranging from 8.5 to 80 nM. The inhibitory activities of the benzyl- and phenethyl-4,6-dienes 11b and 11c (Ki, 9.0 and 10 nM) as well as the 6-phenethyl-1,4,6-triene 12c (Ki, 8.5 nM) were extremely high among them. All of the phenylaliphatic steroids, except for the 6beta-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 6alpha-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 6beta-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.