Fulvestrant, also known as ICI 182,780, is a drug treatment of hormone receptor positive metastatic breast cancer in post-menopausal women with disease progression following anti-estrogen therapy. It is an estrogen receptor antagonist with no agonist effects, which works both by down-regulating and by degrading the estrogen receptor.S. Kansra, S. Yamagata, L. Sneade, L. Foster & N. Ben-Jonathan, 2005. "Differential effects of estrogen receptor antagonists on pituitary lactotroph proliferation and prolactin release." Mol Cell Endocrinol. 2005 Jul 15;239(1-2):27-36. It is administered as a once-monthly injection.

Fulvestrant is marketed by AstraZeneca with the brand name Faslodex.

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