OBJECTIVES: Breast cancer (BC) is one of the leading causes of cancer death among American women. Oral endocrine therapy (OET) including tamoxifen and aromatase inhibitors (AIs) reduces recurrence risk and is associated with better disease-free and overall survival in hormone receptor-positive (HR+) BC patients. However, poor adherence of up to 50-60% contributes to recurrences in about one-third of patients. This study aimed to evaluate longitudinal patterns of adherence to OET using a group-based trajectory model (GBTM).

METHODS: A retrospective study was conducted at the Harris Health System in Houston, Texas. Monthly adherence to OET was measured (October 2019-December 2020) using the proportion of days covered (PDC). The monthly PDC was modeled as a longitudinal response in a logistic GBTM to provide distinct patterns of adherence. The final trajectory model was estimated using 2–5 adherence groups considering the Bayesian information criteria, clinical relevance, and a 5% minimum membership requirement. Patient characteristics were compared between trajectory groups using chi-square and ANOVA tests.

RESULTS: A total of 496 patients were included in the analysis. Patients were predominantly Hispanic/Latino (62.5%) and <65 years old (82.6%). Considering GBTM selection criteria, the 4-group model was selected and categorized patients into 4 distinct adherence trajectories: 1. Low adherence with gradual decline trajectory (14.6%), 2. Rapid decline trajectory (11.5%), 3. Constant high adherence (41.4%), and 4. Constant PDC at ~0.6 (32.6%). Patients in group 1 had significantly higher mean years on therapy compared to group 3 (3.1 vs 2.17, p-value=0.01). Moreover, more patients in group 3 had hypertension (45.5%, p-value=0.01) and hyperlipidemia (41.3%, p-value=0.01) compared to others.

CONCLUSIONS: Despite the significant benefits of OET in BC survival, ~60% of patients presented nonadherent trajectories over 15 months of follow-up. Identifying various factors associated with non-adherent trajectories can guide the development of tailored interventions and enhance adherence to OET.