Synthesis, Structure-Activity Relationships and Antagonistic Effects in Human MCF-7 Breast Cancer Cells of Flexible Estrogen Receptor Modulators

Estrogen receptors are therapeutic intervention targets for diseases such as osteoporosis and breast cancer with both tamoxifen and raloxifene established as clinical estrogen receptor antagonists. We report a series of novel selective estrogen receptor modulators (SERMs) whose structures are based on a flexible core scaffold differing from the triphenylethylene of tamoxifen analogues through the insertion of a benzylic methylene group as a flexible spacer between the aryl ring C and the ethylene group. A facile synthesis of the target compounds utilises the titanium tetrachloride/zinc mediated McMurry coupling reaction. Successive introduction onto the parent scaffold of hydroxyl functional groups afforded a series of increased potency ligands for the ER - essentially exploring the predicted in vivo metabolic activiation of such aromatic SERM ligands. This second generation compound series demonstrated high antiproliferative potency against the MCF-7 human breast cancer cell line, with low cytotoxicity. High ER binding affinity (IC50 20nM) together with up to 12 fold ERα/β selectivity was also observed. In addition, the compounds displayed antiestrogenic effects at 40nM when evaluated in the Ishikawa cell line with little estrogenic stimulation. Representative ligands were shown to be pro-apoptotic in human MCF-7 cells in a FACS based assay. Comparison of the docked structure obtained for the most active compound with the X-ray crystal structure reported for the complex of ERα and 4-hydroxytamoxifen, predict that these ligands bind in an antiestrogenic manner with some differences being observed in the benzylic Ring C orientation, as expected. This work further demonstrates the tolerance of the estrogen receptor towards flexible modulators.