Anti-estrogenic activity of prenylated isoflavones from Millettia pachycarpa:: Implications for pharmacophores and unique mechanisms

Phytoestrogens containing isoflavonoids are thought to exhibit preventative effects on estrogen-responsive diseases. Chemical modifications, such as prenylation, in biosynthetic processes enhance the structural variety of isoflavonoids and prompted us to carry out a structure-activity relationship study. We determined the estrogemc/anti-estrogenic activities and estrogen receptor (ER)-binding affinities; of eight kinds of prenylated isoflavones isolated from Millettia pachycarpa (Leguminosae), and those of two kinds of non-prenylated compounds (genistein and daidzein). By comparing these compounds, the pharmacophores for estrogenic/anti-estrogenic activities were elucidated. None of the tested compounds (except genistein) were estrogenic on ligand-dependent yeast-two hybrid assay. On the other hand, 5 isoflavones showed distinct anti-estrogenic activity. Unexpectedly, the most potent antagonists, isoerysenegalensein E and 6,8-diprenylorobol, showed anti-estrogenic activity comparable to that of 4-hydroxytamoxifen, a typical ER antagonist. This suggests that genistein became an antagonist after prenylation and hydroxylation. The pharmacophores providing genistein with strong antiestrogenic activity were as follows: prenyl groups of the 6- and 8-positions on the A-ring, hydroxyl group of the 6-prenyl moiety or the B-ring (catechol form), non-cyclization of the prenyl group with the A-ring, and non-hydroxylation of the 8-prenyl group on the A-ring. The ER-binding affinities of the isoflavonoids were not sufficiently high to explain their potent antagonistic activities, thus suggesting 17,6-estradiol-noncompetitive mechanisms.