LAUSR

BACKGROUND Adherence to aromatase inhibitors (AIs) and tamoxifen has significant survival benefits for postmenopausal women diagnosed with hormone receptor-positive (HR+) breast cancer. Reduced out-of-pocket (OOP) costs and treatment-related side effects could increase therapy adherence. Given that individuals' side effect profiles could differ across AIs, generic AI entry could facilitate switching between AIs to manage side effects and improve adherence. METHODS From SEER-Medicare, we selected women first diagnosed with HR+ breast cancer at age 65+ and initiated an AI within one year of diagnosis between 1/1/2007-5/31/2008 or 6/1/2011-12/31/2012 and followed them for up to two years (N = 20,677). We estimated changes in probabilities of adherence with and without switching for Part D enrollees with and without the low-income subsidy (LIS vs. non-LIS) before and after generic entry, using linear probability models. Tests of statistical significance are two-sided. RESULTS After generic entry reduced OOP costs of AIs (larger reduction for non-LIS), percent of women who ever switched from one AI to another AI increased from 8.8% to 14.6% for non-LIS and from 7.3% to 12.5% for LIS. Adherence without switching increased by 8.0 percentage points (pp) for non-LIS (P < 0.001) but decreased by 4.9 pp (P < 0.001) for LIS. Adherence with switching increased for both non-LIS (6.4 pp, P < 0.001) and LIS (4.4 pp, P < 0.001). CONCLUSIONS Increased switching after generic entry contributed to increased adherence, suggesting switching allowed better management of treatment-related side effects. Subsidized women also experienced increased adherence with switching after generic entry suggesting that patients and physicians might not understand Part D benefit design when making decisions.