Association Between Body Mass Index and Clinical Outcomes of CDK4/6 Inhibitors in HR+/HER2− Metastatic Breast Cancer: A Real-World Cohort Study

Background: Body mass index (BMI) has been widely investigated as a potential prognostic factor in breast cancer; however, its clinical relevance in patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2−) metastatic breast cancer treated with CDK4/6 inhibitors remains controversial, particularly in contemporary real-world settings. This study aimed to evaluate the association between baseline BMI and clinical outcomes, including survival and treatment-related toxicity, in a real-world cohort. Methods: This single-centre retrospective observational cohort study included patients with HR+/HER2− metastatic breast cancer treated with endocrine therapy and a CDK4/6 inhibitor (palbociclib or ribociclib) in the metastatic setting between January 2018 and May 2025. Patients were categorised by baseline BMI (