The role of aromatase inhibitors in the treatment of metastatic breast cancer.

Tamoxifen has been the gold standard of endocrine therapy for postmenopausal patients with hormone receptor-positive breast cancer for over 20 years. The development of the third-generation aromatase inhibitors anastrozole, letrozole, and exemestane is changing the algorithm for the treatment of the disease. Recent clinical trials have shown that all three third-generation aromatase inhibitors present significant advantages over traditional progestins and aminoglutethimide therapy after tamoxifen failure in postmenopausal women. These new agents are now accepted as first choice for second-line treatment of metastatic disease. Since 2000, phase III trials with anastrozole and letrozole have shown that third-generation aromatase inhibitors are at least as effective as tamoxifen in the first-line treatment of postmenopausal women with hormone receptor-positive or -unknown metastatic breast cancer. The first-line phase III trial of letrozole versus tamoxifen which, unlike the anastrozole trials, was prospectively designed to test superiority of the aromatase inhibitor, showed that this agent was superior to tamoxifen in all assessed outcome measures. A first-line phase III trial of exemestane versus tamoxifen in postmenopausal patients with hormone receptor-positive or -unknown advanced breast cancer is ongoing. The data presented in this article suggest that aromatase inhibitors may replace tamoxifen in the first-line hormonal management of this disease in postmenopausal women.