PURPOSE

Previous research evaluating adherence to tamoxifen therapy among women with early-stage breast cancer has revealed adherence estimates ranging from 25% to 96%. No previous studies have focused on adherence to adjuvant aromatase inhibitors.

METHODS

We used longitudinal claims data from three large commercial health programs to estimate adherence with anastrozole therapy among women with early-stage breast cancer. Adherence was defined as the proportion of days that patients had medication available over the observation period (ie, days covered); women with fewer than 80% of days covered were defined as nonadherent.

RESULTS

More than 12,000 women in the databases were found to have new anastrozole prescription claims during the period of study: 1,498 women were classified as having early-stage disease in one commercial health program (Plan A) data set, 1,899 women in another program (Plan B) data set, and 8,994 women in MarketScan, a commercial data set made up of several health programs. Mean adherence over the first 12 months of therapy ranged from 82% to 88% in the three data sets. Between 19% and 28% of women had fewer than 80% of days covered. For women with 36 months of continuous eligibility, the mean adherence decreased each year, ranging from 78% to 86% in year 1 to 62% to 79% in year 3 within the three data sets.

CONCLUSION

A substantial proportion of women with early-stage breast cancer may be suboptimally adherent to adjuvant anastrozole therapy. Future research should focus on the identification of patients at risk for nonadherence with oral hormonal therapy for breast cancer and the development of interventions to improve adherence.