In an international study of the two leading aromatase inhibitors, data demonstrate that 50% more women with advanced breast cancer respond to letrozole(Drug information on letrozole) (Femara) than to anastrozole(Drug information on anastrozole) (Arimidex); ie, more women treated with letrozole achieved at least a 50% reduction in the size of their tumor. Data from this multicenter, international study were presented at the 38th annual meeting of the American Society of Clinical Oncology (ASCO). The study enrolled 713 postmenopausal women with estrogen-receptor (ER)-positive and/or progesterone(Drug information on progesterone)-receptor (PR)-positive or ER/PR status unknown breast cancer who were receiving second-line treatment for advanced disease.
"It is becoming increasingly evident that aromatase inhibitors are challenging and are likely to replace tamoxifen(Drug information on tamoxifen) in the treatment of postmenopausal women with endocrine-dependent breast cancer," said Carsten Rose, md, director, department of oncology, Universitetkliniken, Onkologiska Klinik, Lund, Sweden, and lead investigator in the study. "Studies like this are critical because they provide evidence to identify the aromatase inhibitor most likely to work best. The data in this trial show that more women respond to Femara than to Arimidex, which is important information for physicians to consider when treating advanced breast cancer patients."
The investigation was conducted in 19 countries and compared the efficacy of letrozole vs anastrozole in women with metastatic breast cancer following failure of antiestrogen therapy (eg, tamoxifen). The primary and secondary end points were time to disease progression, objective response rate, duration of objective response, overall clinical benefit, time to treatment failure, and survival. Patients were randomized to receive letrozole at 2.5 mg once daily or anastrozole at 1 mg once daily.
The data show that, based on objective response rate, 50% more women responded to letrozole than to anastrozole (19% vs 12%, P =.013). The complete response rate was 7% for letrozole vs 4% for anastrozole. No statistically significant differences were seen in time to disease progression (primary end point) or other end points.
The data also show that the overall response rate in patients with soft-tissue disease (ie, soft-tissue dominant, no bone or visceral involvement) was two times higher for women receiving letrozole than for women receiving anastrozole (37% vs 19%). In viscera-dominant disease (visceral involvement with or without bone and soft-tissue involvement), the overall response rate to letrozole was also higher than to anastrozole (14% vs 10%). There were no statistically significant differences in any other end points.
Both letrozole and anastrozole were generally well tolerated, and the investigators found no statistically significant differences between arms in the frequency of adverse events.