Fulvestrant is a competitive estrogen receptor (ER) antagonist with an affinity comparable to that of estradiol. Fulvestrant blocks the trophic effects of estrogens without exhibiting partial agonist (estrogen-like) activity. The mechanism of action is associated with negative regulation of estrogen receptor protein levels. Clinical studies in postmenopausal women with primary breast cancer have shown that fulvestrant significantly reduces ER protein levels in ER-positive tumors compared with placebo. A significant decrease in progesterone receptor expression was also observed, consistent with the lack of effects typical of estrogen agonists. Fulvestrant 500 mg has also been shown to be more effective than fulvestrant 250 mg in suppressing ER and the proliferation marker Ki67 in breast tumors in the neoadjuvant setting in postmenopausal women.