Abstract PS1-01-08: Adherence and Influencing Factors of CDK4/6 Inhibitors Plus Endocrine Therapy in HR+/HER2- Early Breast Cancer: A Real-World Retrospective Analysis

Abstract Background: The disease-free survival benefits of combining abemaciclib or ribociclib with endocrine therapy (ET) in patients with hormone receptor-positive, HER2-negative (HR+/HER2-) early breast cancer (EBC) have been demonstrated in the monarchE and NATALEE trials. However, treatment adherence in real-world settings remains underexplored. Non-adherence to CDK4/6 inhibitors has been associated with increased risk of disease recurrence, yet real-world patterns, patient compliance, and effective intervention strategies remain poorly defined. Given the potential impact of adherence on treatment efficacy and patient outcomes, this study aimed to assess real-world adherence rates, identify influencing factors, and explore the impact of adverse events (AEs) and quality of life (QoL) on adherence to CDK4/6 inhibitors plus ET in patients with HR+/HER2- EBC. Methods: This single-center retrospective cohort study included patients with HR+/HER2- EBC treated with abemaciclib or ribociclib plus ET between December 2022 and February 2025. The starting dose of abemaciclib was 150 mg twice daily, and ribociclib was 400 mg once daily, with dose adjustments permitted for toxicity. Patient-reported outcomes (PROs) were used to evaluate adherence, which was calculated using the proportion of days covered (PDC). Patients were categorized as adherent (PDC ≥ 0.8) or non-adherent (PDC 0.8). AEs were documented by type and timing, and QoL was assessed using the EORTC QLQ-C30 questionnaire. Statistical analyses included rank-sum and chi-square tests, with significance set at P 0.05. Results: A total of 136 patients (median age 53 years, range 26-83) were included. Of these, 113 (83.1%) received abemaciclib and 23 (16.9%) received ribociclib. Most patients were stage II (64.0%), with the remainder at stage III (36.0%). After a median follow-up of 23.5 months (range 5-30), 124 patients (91.2%) were adherent, while 12 (8.8%) were non-adherent. AEs led to dose reduction or interruption in 25 patients (18.4%) and discontinuation in 10 patients (7.3%). Severe AEs (grade ≥3 diarrhea, leukopenia) accounted for 9 discontinuations; one case was due to cerebral infarction. AEs occurred in 123 patients (90.4%), most commonly diarrhea (67.5%), leukopenia (6.5%), and fatigue (4.9%). The majority of AEs (84.6%) occurred within the first month of treatment. Diarrhea incidence was significantly higher in the abemaciclib group (P 0.01). Patients treated with ribociclib reported significantly better global health scores (82.62 ± 13.67 vs. 73.01 ± 18.91, P = 0.04) and marginally better physical functioning (P = 0.05). No significant differences were observed in other QoL domains. Conclusion: Adherence to CDK4/6 inhibitors combined with ET was high among patients with HR+/HER2- early breast cancer. However, adverse events—particularly diarrhea—remained the leading cause of treatment discontinuation. Abemaciclib was associated with a higher incidence of diarrhea, while ribociclib appeared to offer a better QoL profile. These findings highlight the importance of early recognition and proactive management of treatment-related AEs, especially gastrointestinal toxicity, to improve adherence and optimize therapeutic outcomes. Citation Format: W. Wang, X. Shen, Y. Wu, X. Wang, S. Li, J. Qu. Adherence and Influencing Factors of CDK4/6 Inhibitors Plus Endocrine Therapy in HR+/HER2- Early Breast Cancer: A Real-World Retrospective Analysis 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 PS1-01-08.