In an international study of the two leading aromatase inhibitors, data
demonstrate that 50% more women with advanced breast cancer respond to letrozole
(Femara) than to 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-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 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.