The degree of adherence to oral
anticancer agents may influence treatment efficacy.
Tamoxifen and
toremifene usage data from two International
Breast Cancer Study Group (IBCSG) studies were used to evaluate the impact of treatment adherence on the efficacy of these two
selective estrogen receptor modulators (
SERMs). Between 1993 and 1999, IBCSG Trial 13-93 randomized premenopausal women with node-positive disease to
tamoxifen or no endocrine
therapy and IBCSG Trial 14-93 compared
tamoxifen and
toremifene in postmenopausal women with node-positive disease. 690 women with
estrogen-receptor positive enrolled in these two trials were alive and disease-free 4 years after the start of
SERM. The median follow up for this analysis was 9.0 years. Using a Kaplan-Meier landmark analysis at 4 years, the 609 women completing at least 4 years of
SERM had improved 10-year disease-free survival (DFS) (71 %) compared with the 81 women taking less than 4 years (64 %), but these differences did not reach statistical significance [DFS hazard ratio (<4 year/4+ year) 1.31, 95 % CI 0.86-1.98), p value = 0.20]. Women who completed less than 4 years of
SERM treatment (12 %) appeared not to have achieved the full benefit from their
therapy. These results suggest that more effort should be made to educate women regarding the benefits of a full course of treatment. This is especially important in light of the results of the recently reported ATLAS and aTTom trials which suggest a benefit of 10 years of
tamoxifen.