IBIS-II update shows no difference in anastrozole, tamoxifen efficacy in DCIS
medwireNews: Long-term results from the IBIS-II trial show a comparable risk for breast cancer recurrence with anastrozole and tamoxifen among postmenopausal women with hormone receptor (HR)-positive ductal carcinoma in situ (DCIS).
However, there were “very clear differences in terms of adverse events” between the two agents, reported Ivana Sestak (Queen Mary University of London, UK) at the 2020 San Antonio Breast Cancer Symposium.
The study included 2980 postmenopausal women, aged an average of 60 years, who were randomly assigned to receive anastrozole 1 mg/day or tamoxifen 20 mg/day for 5 years, starting within 6 months of excision surgery for HR-positive DCIS.
After a median follow-up of 11.6 years, the cumulative incidence of breast cancer recurrence was 8.5% in the anastrozole group and 9.7% in the tamoxifen group, translating into a nonsignificant risk reduction of 11% with the aromatase inhibitor.
These findings were consistent with those from the first 5 years of follow-up when patients were on active treatment. Specifically, the rates of breast cancer recurrence were 2.6% and 3.0% among anastrozole- and tamoxifen-treated patients, respectively, giving a nonsignificant 16% reduced risk for recurrence with anastrozole.
When stratified by type of breast cancer recurrence, anastrozole treatment was associated with a significant 44% reduction in the risk for HR-positive disease relative to tamoxifen during the active treatment period, with significance not extending to the post-treatment period.
And the results were similar for HER2-negative disease, the risk for which was a significant 60% lower with anastrozole than tamoxifen, during the active treatment period.
There were no significant differences, however, in the rates of HR-negative, invasive, or DCIS recurrences, or in all-cause mortality rates during or after treatment among patients who took anastrozole or tamoxifen.
Reporting on toxicities, Sestak noted that fractures were a significant 34% more likely among patients treated with anastrozole versus tamoxifen, and the risk for cerebrovascular accident or stroke and transient ischemic attack were also a significant threefold higher with anastrozole.
By contrast, tamoxifen was associated with a significant 83% and 87% increased risk for endometrial and ovarian cancer, respectively, compared with anastrozole.
Sestak therefore concluded that “an improved understanding of adverse event profiles will help patients with [HR]-positive [DCIS] to make an informed decision regarding their treatment.”
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