Fulvestrant: a new type of estrogen receptor antagonist for the treatment of advanced breast cancer

Postmenopausal women with hormone receptor- positive tumors are candidates for endocrine treatment. Current treatment options include the selective estrogen receptor modulators (e.g., tamoxifen and toremifene), which inhibit estrogen receptor signaling, and the aromatase inhibitors (e.g., anastrozole and letrozole), which prevent the conversion of androgens into estrogen in postmenopausal women. As most patients eventually become resistant to endocrine agents, there is a need for new treatments that are effective, well tolerated and lack cross-resistance with currently available therapies. This review describes the development of fulvestrant (Faslodex), a new type of endocrine agent, which is an estrogen receptor antagonist with no agonist effects. Phase III clinical trials have found that fulvestrant is as effective and well tolerated as anastrozole for treating postmenopausal patients with advanced breast cancer who have progressed on one prior endocrine therapy. In addition, fulvestrant has first-line efficacy similar to that of tamoxifen in patients with estrogen receptor-positive and/or progesterone receptor-positive breast cancer. Moreover, in a compassionate-use program, it has become clear that fulvestrant is not cross-resistant with other therapies. Therefore, fulvestrant is a versatile new treatment option for postmenopausal women with advanced breast cancer who have progressed on prior endocrine therapy.