This revealed that the fluorosulfate derivative of Fulvestrantpossesses significantly enhanced activity to down-regulate estrogenreceptor (ER) expression in the ER+ breast cancer cell line MCF-7(IC50 = 5.5 nM), whereas the fluorosulfate derivative ofCombretastatin A4 shows a large increase in potency in the drug-resistantcolon cancer cell line HT-29 (IC50 = 70 nM) compared tothe phenol precursors of fluorosulfated products (IC50 =4947 nM). This evidence concerns the gene ESR1 and breast cancer.