Temporal trends in adherence to adjuvant endocrine therapy across age groups in hormone receptor–positive breast cancer.

634 Background: Adjuvant endocrine therapy (ET) reduces recurrence and mortality in hormone receptor–positive breast cancer, yet adherence remains suboptimal. Adherence varies by age, with younger patients at higher risk for non-adherence. Whether increased awareness has improved adherence over time is unclear. We evaluated temporal trends in ET adherence. Methods: We conducted a retrospective cohort study using the MarketScan claims database. Eligible patients were 18–100 years old with non-metastatic breast cancer (2009–2022), treated with surgery within 6 months of diagnosis, and began adjuvant ET (aromatase inhibitor AI or tamoxifen) within 1 year of diagnosis. The analysis included patients insured 1 year prior to and 1 year following ET initiation. ET was assessed using proportion of days covered (PDC) and defined as adherent (PDC ≥0.80) vs non-adherent for the year after ET initiation. Trends in adherence were evaluated using the Cochran–Armitage trend test and logistic regression with an age–calendar year interaction. Age was categorized as 10 concurrent medications) were associated with lower odds of adherence, while receipt of 90-day prescription supplies, chemotherapy (neoadjuvant/adjuvant), and CDK4/6 inhibitors were associated with higher odds of adherence. Conclusions: Young women remain at high risk for ET non-adherence. While tamoxifen adherence did not improve over time, AI adherence increased over time in younger patients. This likely reflects heightened awareness, better supportive care, and clinician–patient discussions emphasizing AI adherence in young, high-risk populations. In contrast, despite similar increases in awareness, AI adherence has not improved in postmenopausal patients. These results highlight a persistent gap in ET adherence and the need for targeted interventions across age groups. Factors associated with adherence to adjuvant endocrine therapy. Covariate Adjusted OR 95% CI Age <50 vs ≥50 years 0.85 0.81–0.89 Polypharmacy 0.80 0.78–0.84 90-day prescription supply 2.80 2.71–2.89 Neoadjuvant chemotherapy 1.30 1.19–1.43 Adjuvant chemotherapy 1.29 1.23–1.34 CDK4/6 inhibitor use 1.55 1.15–2.08