ChEBI: Fulvestrant is a 3-hydroxy steroid that is 17beta-estradiol in which the 7alpha hydrogen has been replaced by a nonyl group in which one of the hydrogens of the terminal methyl has been replaced by a (4,4,5,5,5-pentafluoropentyl)sulfinyl group. An estrogen receptor antagonist, it is used in the treatment of breast cancer. It has a role as an antineoplastic agent, an estrogen receptor antagonist and an estrogen antagonist. It is a 3-hydroxy steroid, a 17beta-hydroxy steroid, an organofluorine compound and a sulfoxide. It derives from a hydride of an estrane.NCI Thesaurus (NCIt): Fulvestrant is a synthetic estrogen receptor antagonist. Unlike tamoxifen (which has partial agonist effects) and the aromatase inhibitors (which reduce the estrogen available to tumor cells), fulvestrant binds competitively to estrogen receptors in breast cancer cells, resulting in estrogen receptor deformation and decreased estrogen binding. In vitro studies indicate that fulvestrant reversibly inhibits the growth of tamoxifen-resistant, estrogen-sensitive, human breast cancer cell lines. (NCI04)