OBJECTIVES: Aromatase inhibitors (AIs) are the recommended standard adjuvant therapy for postmenopausal women with hormone-receptor-positive, non-metastatic breast cancer following surgery. Non-adherence to AIs can reduce their clinical effectiveness, increasing the risk of breast cancer recurrence and cancer-related mortality. This study aims to evaluate the longitudinal adherence trajectories to AIs over a 5-year treatment course and identify factors associated with non-adherence among postmenopausal women with early-stage breast cancer in the United States.

METHODS: In this retrospective cohort study, we utilized the US Surveillance, Epidemiology, and End Results (SEER)-Medicare linked database. The study population included postmenopausal women newly diagnosed with stage I to III breast cancer between 2010 and 2014 who initiated adjuvant AI therapy within 12 months of their diagnosis. Adherence was assessed monthly using the proportion of days covered (PDC) metric over five years or until a censoring event (disenrollment, death, diagnosis of a second cancer, or breast cancer recurrence). Group-based trajectory model was employed to identify distinct adherence patterns, and multinomial logistic regression was used to analyze factors associated with these adherence trajectories.

RESULTS: A total of 10,919 individuals newly diagnosed with breast cancer who initiated AI therapy were identified. The trajectory analysis revealed five distinct adherence groups: consistently high adherence (60.7%), rapid decline (10.5%), moderate decline (16.0%), slow decline (7.8%), and decline-then-increase (5.0%). Around 32% of suboptimal adherence occurred within the first two years after AI initiation. Factors linked to less optimal adherence included age over 75, having a history of emergency room visits, chemotherapy use, and depression.

CONCLUSIONS: The study revealed heterogeneous adherence patterns to AI amongst postmenopausal women with early-stage breast cancer. Recognizing these patterns is important for developing interventions that enhance adherence and ultimately improve treatment outcomes. The findings can facilitate the implementation of specific strategies to effectively support at-risk patient groups.