New aromatase inhibitors for breast cancer.

Anastrozole (Arimidex-Zeneca) and letrozole (Femara-Novartis) are the first selective, oral, non-steroidal aromatase inhibitors. They are licensed for the treatment of advanced breast cancer in postmenopausal women where tamoxifen or other anti-oestrogen therapy has failed. The manufacturers of both drugs claim that their products are more effective, less toxic and better tolerated than the progestogen megestrol acetate, the standard therapy in this clinical situation. We assess these claims.