Are all aromatase inhibitors the same? A review of the current evidence.

Third-generation aromatase inhibitors (AIs)--letrozole, anastrozole, and exemestane--are challenging tamoxifen as the standard endocrine therapy for postmenopausal women with hormone receptor-positive breast cancer. AIs suppress estrogen levels by inhibiting aromatase, the enzyme that catalyzes the final step of estrogen biosynthesis. Studies have shown that AIs are highly effective and safe in the treatment of advanced disease, and more recently, AIs have shown promise in the neoadjuvant, adjuvant, and extended adjuvant settings. However, all AIs are not equal. In direct comparisons with anastrozole, letrozole has demonstrated superior estrogen suppression and clinical response in patients with advanced metastatic breast cancer. In addition, letrozole is the only AI to demonstrate consistent superiority over tamoxifen in the neoadjuvant and first-line advanced breast cancer settings. This publication summarizes the available evidence for the efficacy of all 3 agents throughout the breast cancer continuum.