First-line CDK4/6 inhibitor treatment of HR-positive, HER2-negative metastatic breast cancer: real-world data from Ho Chi Minh City Oncology Hospital, Vietnam.

Objective: This study aimed to characterize the tumor response rate, progression-free survival (PFS), and overall survival (OS) among patients with HR-positive, HER2-negative metastatic breast cancer receiving CDK4/6 inhibitors plus endocrine therapy at Ho Chi Minh City Oncology Hospital. Secondary objectives included identifying clinical factors associated with survival outcomes and characterizing treatment-related toxicities. Materials and methods: We conducted a retrospective cohort study enrolling 107 patients who received first-line ribociclib or palbociclib in combination with an aromatase inhibitor or fulvestrant at Ho Chi Minh City Oncology Hospital between March 1, 2021, and April 1, 2023. Results: The median follow-up duration was 29 months. The median PFS was 28.0 months. The Kaplan-Meier-estimated PFS rates at 12 and 24 months were 66.4% (95% CI, 56.6%-74.4%) and 54.1% (95% CI, 44.2%-63.0%), respectively. The median OS was not reached; corresponding OS rates at 12 and 24 months were 88.8% and 87.8%, respectively. On multivariate Cox regression analysis, ER and PR expression levels emerged as independent prognostic determinants of PFS. Grade 3-4 neutropenia was observed in 58.8% of patients; all events were managed through dose interruption or reduction, and no cases of febrile neutropenia were documented. Conclusion: CDK4/6 inhibitors (ribociclib or palbociclib) combined with endocrine therapy demonstrated favorable efficacy and tolerability in Vietnamese patients with HR-positive, HER2-negative metastatic breast cancer. Treatment outcomes and safety profiles were consistent with those reported in pivotal clinical trials and real-world studies. These findings should be interpreted in light of the retrospective single-center design, modest sample size, non-randomized treatment allocation, and immature OS data.