Synthesis, Structure Elucidation, and Biochemical Evaluation of 7α- and 7β-Arylaliphatic-Substituted Androst-4-ene-3,17-diones as Inhibitors of Aromatase

The inhibition of aromatase, the cytochrome P450 enzyme complex responsible for the conversion of androgens to estrogens, may be useful for the endocrine treatment of breast cancer. Previously, several 7α-thio-substituted androstenediones have been shown to be potent inhibitors of aromatase. Recent research has focused on producing a more metabolically stable aromatase inhibitor by replacing the carbon-sulfur bond at the 7α-position with a carbon-carbon bond. The new inhibitors, 7α-arylaliphatic-substituted androst-4-ene-3,17-diones (2-4), have alkyl chains of varying length between the steroid and the aryl ring at the 7α-position. The desired targets were synthesized via a 1,6-conjugate addition of the appropriate cuprate to 17β-(tert-butyldimethylsiloxy)androsta-4,6-dien-3-one (7). The synthesis also resulted in the formation of the 7β-substituted diastereomers (10-11 and 13) as minor products. Initial assignments of the 7α-phenethyl and 7β-phenethyl diastereomers were made using highfield 1-D and 2-D NMR studies. The assignment of the diastereomers was confirmed using X-ray crystallography. These compounds were all good inhibitors of aromatase in vitro when assayed using microsomes isolated from human placenta. The 7α-substituted androst-4-ene-3,17-diones (2-4) were effective inhibitors with apparent K_is of 13-19 nM. The corresponding 17β-hydroxy analogs (8 and 14) and the 7β-substituted androstenediones (13 and 16) were less effective inhibitors with apparent K_is of 36-44 nM. Thus, a new series of 7α-arylaliphatic-substituted androst-4-ene-3,17-diones has been synthesized, and the compounds are potent competitive inhibitors of aromatase.