Advances in endocrine treatments for postmenopausal women with metastatic and early breast cancer

For the past 25 years, the estrogen antagonist tamoxifen has been the hormonal treatment of choice for postmenopausal patients with hormone-sensitive metastatic and early breast cancer (EBC). However, tamoxifen is associated with certain tolerability and safety concerns. In addition, the hormonal options after progression are limited, and thus, alternative endocrine treatments have been developed. This review provides a synopsis of the newer alternatives in endocrine therapy of breast cancer: the aromatase inhibitors (AIs) and fulvestrant Faslodex®, the estrogen receptor antagonist that down-regulates estrogen and progesterone receptors and has no known agonist activity. The third-generation AIs, anastrozole and letrozole, have been shown to be as effective or more effective than megestrol acetate and tamoxifen as second- and first-line therapies for the treatment of postmenopausal patients with metastatic breast cancer, and exemestane has been approved for second-line use. Fulvestrant has been shown to be as effective as anastrozole as second-line therapy for metastatic breast cancer and has been approved in the U.S. for the treatment of postmenopausal women with hormone-receptor-positive metastatic breast cancer following progression on antiestrogen therapy. Anastrozole is the only AI with published clinical trial data and U.S. Food and Drug Administration approval for adjuvant therapy of postmenopausal women with EBC. The 'Arimidex,' Tamoxifen, Alone or in Combination (ATAC) trial, a double-blind, multicenter trial with 9,366 patients, compared tamoxifen with anastrozole, alone and in combination, as adjuvant endocrine treatment for postmenopausal patients with operable, invasive, EBC. The first analysis (at a median follow-up of 33.3 months) showed longer disease-free survival and, in general, better tolerability with anastrozole than with tamoxifen. This pattern was maintained at later analyses with a median follow-up of 47 months for efficacy and 37 months for safety and tolerability. Although longer follow-up is warranted, anastrozole appears to be a well-documented choice of endocrine adjuvant therapy for postmenopausal women with hormone-responsive breast cancer.