NATIONAL HARBOR, Md. – Anastrozole remained superior to tamoxifen as an adjuvant treatment for invasive breast cancer in postmenopausal women, based on 10-year follow-up data from the Arimidex, Tamoxifen Alone or in Combination trial known as ATAC.
After 10 years, the recurrence rate in hormone receptor–positive patients randomized to 5 years of adjuvant anastrozole (Arimidex) was 19.7%, compared with 24% in those who received tamoxifen for the same period. The hazard ratio was 0.79 (P = .0002).
Previous reports from 33, 68, and 100 months into the trial showed that the aromatase inhibitor was significantly more effective at reducing recurrence than tamoxifen, said Dr. Aman Buzdar, a professor of medicine at the University of Texas M.D. Anderson Cancer Center in Houston. This analysis assessed outcomes in an intent-to-treat population of 3,125 women who received anastrozole and 3,116 women who received tamoxifen after a median follow-up of 10 years.
The effectiveness of anastrozole, compared with tamoxifen increased over time, Dr. Buzdar noted. The difference in time to relapse between anastrozole and tamoxifen increased from 2.7% at 5 years to 4.3% at 10 years. Deaths following recurrence were lower in the anastrozole patients, although the difference was not statistically significant.
During years 5-9, the risk reduction due to the carryover effects of treatment was 50% in the anastrozole group, compared with 33% in the tamoxifen group (hazard ratio 0.81, P = .03).
The rate of distant recurrence favored anastrozole over tamoxifen at 5 years (7.9% vs. 9.2%, respectively) and 10 years (15.1% vs. 17.7%, respectively) with a P value of .02. Likewise, women treated with anastrazole were less likely to develop contralateral breast cancer; the rates were 1%, compared with 1.8% in the tamoxifen group at year 5, and 3.2% vs. 4.9% at year 10 (HR 0.62, P = .003).
During the treatment period, women taking anastrozole had significantly more fractures, compared with those taking tamoxifen (375 vs. 234, respectively), but no differences in fracture rates appeared between the two groups during the posttreatment period (175 vs. 188, respectively). No significant differences in side effects were observed between the two groups after treatment was completed.
The occurrences of heart attack and stroke were similar between the two groups both during and after treatment. Endometrial cancer was more common in the tamoxifen group, compared with the anastrozole group both on and off treatment. The death rate was 24% in both groups, suggesting no additional morbidity and mortality concerns, Dr. Buzdar said.
“At 10 years of median follow-up, anastrozole is significantly superior to tamoxifen in preventing breast cancer recurrence,” said Dr. Buzdar.
The average age of the women at the 10-year follow-up analysis was 72 years. The total follow-up equaled 48,473 women-years.
AstraZeneca, which manufactures both drugs, supported the study. Dr. Buzdar had no personal financial conflicts to disclose, but one or more of the study’s co-investigators disclosed consulting and funding relationships with AstraZeneca.