Existing and emerging endocrine therapies for breast cancer

Endocrine therapy is first-line therapy for patients with estrogen receptor-positive or progesterone receptor-positive metastatic breast cancer. Commonly used endocrine therapies are tamoxifen, megestrol acetate, and aromatase inhibitors. Although tamoxifen and megestrol acetate have a favorable therapeutic profile, there are risks associated with these agents. With tamoxifen, the partial agonist property can lead to thromboembolic events. An important adverse event of megestrol acetate is weight gain and fluid retention in some patients. The aromatase inhibitors are currently used as second-line therapy after tamoxifen failure. A recent study showed that anastrozole, an aromatase inhibitor, is as effective or even superior to tamoxifen when used as a first-line therapy. However, not all patients will respond to currently available therapies. A new class of drug, the estrogen receptor downregulators, has been developed. Fulvestrant, the first agent in this new class, not only induces the degradation of the estrogen receptor but also is an estrogen antagonist; further, its lack of agonist activity provides a better safety profile. Two phase III trials have proven that fulvestrant is at least as effective as anastrozole in postmenopausal women with advanced breast cancer. Fulvestrant is an effective and safe endocrine therapy for postmenopausal women who have failed prior endocrine therapy.