What is the mechanism of action of Fulvestrant?

Fulvestrant: is an estrogen receptor antagonist that downregulates the estrogen receptor and has no agonist effects. Fulvestrant competitively inhibits binding of oestradiol to the estrogen receptor (ER), with a binding affinity that is 89% that of oestradiol. Reduces the number of estrogen receptor. Good to know: It is approved for the treatment of metastatic breast cancer in postmenopausal women who have failed antiestrogen therapy. Given once in a month as a intramusclar injection. Ref: Goodman & Gillman's Pharmacological Basis of Therapeutics' 12th Edition, Page 1759, 1760.