Abstract PS1-09-01: Analysis of Prognostic Factors in HR+/HER2- Metastatic Breast Cancer with Long-Term Response to CDK4/6 Inhibitors

Abstract Introduction: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) combined with endocrine therapy are the standard of care for hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) metastatic breast cancer (MBC). Although most patients eventually develop resistance, a subset achieves durable responses lasting at least 24 months with a single CDK4/6i. Identifying prognostic factors associated with this long-term benefit is essential for guiding clinical practice. Methods: We conducted a retrospective cross-sectional study of HR+/HER2- MBC patients treated with CDK4/6i plus endocrine therapy at Ho Chi Minh City Oncology Hospital between January 2021 and July 2023. Eligible patients were those who received continuous treatment with one CDK4/6i for ≥24 months. Progression-free survival (PFS) was estimated using Kaplan-Meier methods, and prognostic factors were evaluated by univariate and multivariate Cox proportional hazards regression models. Results: Of 236 patients treated with CDK4/6i, 80 met inclusion criteria by maintaining therapy with a single CDK4/6i for ≥24 months (30 with palbociclib 36.1%, 50 with ribociclib 63.9%). Median follow-up was 31.4 months (range, 24.08-46.16). The median PFS for the cohort was not reached. Subgroup analysis demonstrated that patients achieving partial or complete response (PR/CR) did not reach median PFS, whereas those with stable disease (SD) had a median PFS of 37.65 months (95% CI, 34.11-41.19; p = 0.024). In multivariate analysis, both treatment response (PR/CR vs SD) and Ki-67 30% were significantly associated with PFS. Conclusion: In HR+/HER2- MBC patients achieving long-term response (≥24 months) to a single CDK4/6i plus endocrine therapy, treatment response and high Ki-67 index (30%) emerged as significant prognostic factors for PFS. These findings highlight the prognostic value of clinical response and proliferative activity in predicting durable benefit. Citation Format: T. N. Nguyen, D. T. H. Phan, T. B. Diep, H. D. Vo, P. H. M. Vo, Q. H. Nguyen, T. T. T. Nguyen, H. N. Vo. Analysis of Prognostic Factors in HR+/HER2- Metastatic Breast Cancer with Long-Term Response to CDK4/6 Inhibitors 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 PS1-09-01.