"It is becoming increasingly evident that aromatase inhibitors are challenging and are likely to replace tamoxifen in the treatment of postmenopausal women with endocrine-dependent breast cancer. Studies like this are critical because they provide evidence to identify the aromatase inhibitor most likely to work best," said Carsten Rose, M.D., presenter and lead investigator of the study. "The data in this trial show that more women respond to Femara (letrozole) than to Arimidex (anastrozole), which is important information for physicians to consider when treating advanced breast cancer patients."

The randomized, open-label study of 713 postmenopausal women was conducted in 19 countries and compared the efficacy of the two aromatase inhibitors in women who had residual metastatic breast cancer following failure of tamoxifen therapy. The primary and secondary endpoints were time to disease progression, objective response rate, duration of objective response, overall clinical benefit, time to treatment failure, and survival. Patients were randomly allocated to daily doses of letrozole (2.5 mg) or anastrozole 1 mg.

Objective response rates were 19 percent for letrozole and 12 percent for anastrozole. Complete response rates also favored letrozole, 7 percent versus 4 percent. No statistically significant differences were seen in time to disease progression (primary endpoint) or other endpoints.

Data also showed that overall response rate in patients with primarily soft tissue disease was significantly higher for women receiving letrozole than for women receiving anastrozole (37 percent versus 19 percent). In primarily visceral disease, the overall response rates were 14 percent for letrozole and 10 percent for anastrozole. There were no statistically significant differences in any other endpoints.

Both drugs were generally well tolerated, and there were no statistically significant differences between treatment arms in reported frequencies of adverse events, including serious adverse events.