Risks and benefits of aromatase inhibitors in postmenopausal breast cancer

Aromatase inhibitors were first reported in the early 1970s and have been used to treat breast cancer since that time. Until recently, essentially the only agent available in this class was aminoglutethimide, a nonspecific inhibitor with multiple adverse effects and drug interactions. Selective and potent aromatase inhibitors are now available (formestane, exemestane, fadrozole, anastrozole and letrozole), and we review the risks and benefits of these agents in order to assist clinicians in making treatment decisions. Formestane is an injectable steroidal aromatase inhibitor with significant activity against metastatic breast cancer. It has been shown to have similar efficacy and superior tolerability compared with megestrol, and is similar to tamoxifen in the metastatic setting. Exemestane is an oral steroidal aromatase inhibitor. It has been shown to be effective third-line therapy after tamoxifen and megestrol in postmenopausal patients with metastatic breast cancer. All the nonsteroidal (imidazole/triazole) aromatase inhibitors are orally available. Fadrozole has similar activity to megestrol and tamoxifen in the setting of metastasis, but has been shown in phase II trials to inhibit cortisol and aldosterone production. Anastrozole and letrozole have similar toxicity profiles. Compared with megestrol, anastrozole improves overall survival and has superior tolerability. Letrozole is superior to megestrol and aminoglutethimide in terms of overall survival and time to progression, and is also better tolerated. Although there is a strong rationale for using these agents in the treatment of breast cancer, the information presently available is insufficient to recommend any one agent over another. Direct comparative studies are lacking, and comparing agents across studies is limited by many biases and may not be valid. Formestane is only available as an injection and exemestane is not commercially available in many countries, making these agents more difficult to recommend over the other 3 agents. Fadrozole is less potent and less selective in inhibiting aromatase than letrozole. The efficacies of fadrozole, megestrol and tamoxifen appear to be similar; however, comparative data show no advantage of fadrozole over letrozole. Anastrozole and letrozole are generally considered to be similar agents. The clinical future of the selective aromatase inhibitors is promising, and these agents may change the way postmenopausal breast cancer is treated at all stages of the disease.