Endocrine therapy in the treatment of metastatic breast cancer

The goals of treating patients with metastatic breast cancer are to prolong survival, slow or halt disease progression, and enhance the patient's quality of life. In patients with estrogen receptor (ER)-positive cancers that are not progressing rapidly, endocrine therapy is generally the first treatment option. If a patient initially responds to an endocrine agent and then progresses, another endocrine agent may still provide benefit. Tamoxifen has been used as first-line therapy for metastatic breast cancer for many years. Until recently, no other endocrine agent has shown superiority to tamoxifen in this setting. The nonsteroidal aromatase inhibitors, anastrozole and letrozole, have been widely accepted as second-line therapy after failure of tamoxifen; they have replaced megestrol acetate in this setting. Recently, anastrozole was shown to have at least equivalent efficacy and a superior side effect profile compared with tamoxifen for treating postmenopausal women in the first-line setting. Thus, this aromatase inhibitor has become a viable option for first-line therapy in postmenopausal women. Trials of letrozole in this setting are nearing completion. Exemestane has been shown to be an effective second-line agent and to have at least some efficacy as a third-line agent even after failure of a nonsteroidal aromatase inhibitor. Results are anxiously awaited from trials of new endocrine agents including the first member of a new class of endocrine agent, the estrogen-receptor downregulator class.