Real-World, National Study of Palbociclib in HR+/HER2− Metastatic Breast Cancer: A 2.5-Year Follow-Up PALBO01/2021

<b>Background:</b> Palbociclib, when combined with endocrine therapy, represents a valuable treatment option for patients diagnosed with hormone receptor (HR) positive/human epidermal growth factor receptor 2 (HER2) negative advanced breast cancer (BC) or metastatic breast cancer (MBC). Approved in Europe following phase II/III trials, it became the first CDK4/6 inhibitor used alongside hormone therapy. Available real-world data demonstrate the strong performance of Palbociclib in unselected, heavily pretreated patient groups. Our retrospective, observational, multicenter study, conducted in six Romanian institutions during a follow-up period of 2.5 years, aimed to assess Palbociclib's safety and effectiveness in clinical practice. <b>Objectives:</b> The primary endpoints included response rate such as overall response rate (ORR), duration of response (DOR), disease control rate (DCR) and best clinical response (BCR), progression free survival (PFS) and overall survival (OS). The secondary objectives focused on treatment duration with aromatase inhibitors (AI) or fulvestrant and subsequent therapies after disease progression. Grade 3/4 adverse events were individually recorded. Exploratory analysis evaluated the potential predictive biomarkers such as Ki67, lower levels of HER2 expression (HER2-low), and histological or luminal subtype. <b>Methods:</b> Approximately 650 patients were planned for inclusion. PFS and OS were analyzed via the Kaplan-Meier method, with median times, 1- and 2-year estimates, and 95% confidence intervals reported. <b>Conclusions:</b> This study supports the integration of clinical trial evidence into real-world settings, enhancing patient selection and treatment personalization.