Fulvestrant (ICI-182780) is an estrogen receptor (ER) antagonist with an IC50 of 9.4 nM in a cell-free assay. It shows potent inhibitory effect in the growth of MCF-7 human breast cancer cells, with an IC50 of 0.29 nM. [1] Unlike tamoxifen, Fulvestrant (ICI-182780) is devoid of agonist activity and serves as a ??pure?? antioestrogen. [1]

Binding of Fulvestrant to the ER induces a rapid loss of ER protein from breast carcinoma cells. [2]
Fulvestrant (ICI-182780) is approved by FDA for the treatment of ER?-positive metastatic breast cancer in postmenopausal women with disease progression following anti-estrogen therapy. It has been also subjected in a Phase III clinical trial in patients with HER2 negative postmenopausal metastatic breast cancer. [3]"