Abstract PS3-09-07: Adherence to adjuvant endocrine treatment in women 36-56yrs with a hormone receptor-positive breast cancer: Patient compliance and impact on prognosis

Abstract Background Adjuvant endocrine therapy (AET) is essential in treating early hormone receptor-positive (HR+) breast cancer, significantly reducing recurrence and mortality. However, side effects often lead to poor adherence, with discontinuation rates up to 50% within five years. This study assessed adherence rate (AR) and the impact of AET compliance on prognosis in women aged 36-56 treated at UZ Leuven. Methods This retrospective cross-sectional study analyzed AET adherence in all high-risk early HR+ breast cancer patients diagnosed at UZ Leuven between 01/01/2008 and 31/12/2018 and who received chemotherapy. Data were obtained from clinical records and pharmacy dispensing information from public and hospital pharmacies, provided by Belgian health insurance. AR was measured by medication possession ratio (MPR), defined as the number of pills dispensed per year divided by 365.25 days. Non-adherence and poor adherence were defined as MPR 80% and 95%. Covariate analyses evaluated clinical factors potentially influencing adherence, including age (45 vs ≥45 years), tumor grade (1-3), pathological nodal status (pN0 vs pN1), and BMI (25, 25-30, 30). Logistic regression with generalized estimating equations estimated non-adherence over time. The relationship between adherence and overall survival (OS) and recurrence-free survival (RFS) was assessed using Cox proportional hazards models, linking yearly AR to survival outcomes starting after each treatment year. Results Data from 649 patients (mean age 48) were included (Table 1). Over five years, an absolute decrease of 12% in mean AR was observed. The proportion of patients maintaining an AR 95% dropped from 90% (628) in year 1 to 44% (589) in year 5. Non- adherence (AR 80%) increased steadily over time, resulting in 40.8% of patients being poorly adherent by the fifth year. Age, tumor grade or pN status showed no statistically significant association with AR in any year. BMI groups evolved significantly different over time (p = 0.001), patients with obesity (BMI 30) had lower adherence compared to those with BMI 25-30. Poor adherence (95%) and non-adherence (80%) were not significantly predictive of RFS and OS in this cohort. Conclusion Our study showed a gradual decline in adherence over five years, consistent with global data showing 40-60% of patients patients experience poor long-term adherence. In Belgium’s universal healthcare system, where cost barriers are low, factors like side effects, treatment fatigue, and psychosocial issues likely drive non-adherence. While no significant link to survival was found, adherence remains clinically important, as breast cancer recurrence risk persists beyond five years. Longer follow-up is needed to fully assess the impact. Citation Format: M. Van Houdt, L. De Nys, A. Laenen, S. Han, I. Huys, H. Wildiers, F. Derouane, G. Floris, M. Keupers, C. Van Ongeval, Y. Van Herck, A. Smeets, I. Nevelsteen, C. Weltens, H. Janssen, J. Verhoeven, A. Baten, V. Celis, R. Prevos, C. Remmerie, H. De Boodt, A. Coessens, P. Neven. Adherence to adjuvant endocrine treatment in women 36-56yrs with a hormone receptor-positive breast cancer: Patient compliance and impact on prognosis abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-09-07.