Abstract
BACKGROUND: Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have transformed the management of hormone receptor-positive/HER2-negative (HR+/HER2-) advanced breast cancer, yet evidence for elderly or poor-performance patients remains limited. This study examined real-world outcomes of palbociclib plus endocrine therapy in Asian patients, with additional subgroup analyses by age and performance status. METHODS: We retrospectively analyzed 46 consecutive Asian patients with recurrent or de novo HR+/HER2- breast cancer treated with first-line palbociclib plus ET between April 2021 and March 2025. The primary endpoint was progression-free survival (PFS). Secondary endpoints included overall response rate (ORR), disease control rate (DCR), and safety. Subgroup analyses were performed by age (<70 vs. ≥70 years) and performance status (PS; 0-1 vs. 2-3). RESULTS: The median PFS was 26.6 months (range, 1.4-69.5). Stratified by age, median PFS was 26.9 months in patients <70 years and 26.2 months in those ≥70 years (p = 0.760). By PS, PFS was 26.9 months for PS 0-1 and 17.8 months for PS 2-3 (p = 0.099). ORR was 60.9% and DCR 93.5%; notably, all PS 2-3 patients achieved disease control. Hematologic toxicities were common, with neutropenia (80.4%) and leukopenia (86.7%) predominating, but grade ≥ 3 anemia was rare (2.2%). Elderly patients experienced anemia more frequently, while overall toxicity remained manageable. Dose reductions occurred in 47.8% without loss of efficacy. CONCLUSIONS: In routine Japanese practice, palbociclib plus ET provided prolonged PFS and high disease control consistent with pivotal trials and international real-world evidence. Importantly, elderly patients tolerated treatment well, and selected PS 2-3 patients also derived clinical benefit. These findings indicate that neither age nor PS alone should preclude the use of palbociclib in carefully monitored real-world patients.