2-Difluoromethoxyestratriene derivatives were designed to improve potency and in vivo stability of the drug candidate 2-methoxyestradiol (2ME2). Compound evaluation in vitro against the proliferation of MCF-7 and MDA MB-231 breast cancer cells, as inhibitors of tubulin polymerisation and also steroid sulfatase (STS) both in cell lysates and in whole cells, showed promising activities. In antiproliferative assays 2-difluoromethoxyestradiol was less potent than 2ME2, but its sulfamates were often more potent than the corresponding nonfluorinated analogues. The fluorinated bis-sulfamate is a promising anti-proliferative agent in MCF-7 cells (GI50 0.28 µM) vs the known 2- methoxyestradiol-3,17-O,O-bissulfamate (STX140, GI50 0.52 µM), confirming the utility of our approach. Compounds were also evaluated in the NCI 60-cell line panel and the fluorinated bissulfamate displayed very good overall activities with a sub-micromolar average GI50. It was a very potent STS inhibitor in whole JEG-3 cells (IC50 3.7 nM) similar to STX140 (4.2 nM) and additionally interferes with tubulin assembly in vitro and colchicine binding to tubulin. An Xray study of 2-difluoromethoxy-3-benzyloxyestra-1,3,5(10)-trien-17- one examined conformational aspects of the fluorinated substituent. The known related derivative 2-difluoromethyl-3-sulfamoyloxyestrone was evaluated for STS inhibition in whole JEG-3 cells and showed an excellent IC50 of 55 pM.
