First real-world experience with 1L pembrolizumab and chemotherapy treatment for advanced triple-negative breast cancer in Poland: safety analysis and first survival outcomes from a multicenter cohort

Background Pembrolizumab combined with chemotherapy has improved clinical outcomes in PD-L1-positive metastatic triple-negative breast cancer (mTNBC) in KEYNOTE-355 trial. However, real-world studies confirming these data are lacking. We aimed to evaluate the efficacy and safety of pembrolizumab with chemotherapy in a real-world cohort of Polish patients with PD-L1-positive (CPS ≥ 10), previously untreated mTNBC. Methods This retrospective study included 89 female patients, who initiated this regimen between September 2022 and February 2025 at thirteen Polish oncology centers. Patients received pembrolizumab combined with paclitaxel or carboplatin/gemcitabine as per Polish reimbursement criteria. Primary endpoints included overall survival (OS), progression-free survival (PFS), and safety, assessing immune-related adverse events (irAEs) and chemotherapy-related toxicities (trAEs) according to CTCAE v5.0 criteria. Results The median follow-up was 10.1 months (IQR 5.5–14.8). OS data were immature (n = 15 events). Median PFS was 9.3 months (95% CI: 6.6–14.7). A higher number of metastatic sites was significantly associated with shorter PFS (HR = 1.43, 95% CI: 1.09–1.88, p = 0.01), while other clinical and treatment parameters showed no significant associations. irAEs occurred in 37.1% (n = 33) of patients, one-third were grade 3/4. Neither irAE occurrence (p = 0.55) nor corticosteroid use (p = 0.67) significantly impacted PFS. Conclusion In this real-world cohort of first-line pembrolizumab plus chemotherapy for PD-L1-positive mTNBC, treatment achieved meaningful efficacy and an acceptable safety profile, with median PFS broadly consistent with clinical trial results. OS data remain immature. Clinical parameters showed limited prognostic value, suggesting a need for integrating molecular biomarkers and tumor biology into treatment selection and prognostication to optimize patient outcomes.