Tabriz University of Medical Sciences Pharmaceutical Sciences 1735-403X 32 4 2026 10 31 Hematologic Toxicity Patterns and Prognostic Value of Complete Blood Count in Advanced HR-positive/HER2-negative Breast Cancer Treated with Palbociclib plus Letrozole: A Real-World Single-Center Study 632 639 10.34172/ps.43631 EN Adnan Mustafa Isamail https://orcid.org/0009-0001-4525-7518 Eman Saadi Saleh https://orcid.org/0000-0003-1764-0058 Ahmed Zuhair Alsammarraie https://orcid.org/0000-0002-7419-961X Journal Article 10.34172/ps.43631 2025 12 29 2026 04 15 Introduction: The prognostic value of complete blood count (CBC) parameters in patients with advanced HR-positive/HER2-negative breast cancer receiving palbociclib plus letrozole remains unclear. This study aimed to characterize hematologic toxicity patterns associated with treatment response and survival in real-world setting. Methods: This prospective single-center study included 80 women receiving palbociclib plus letrozole. CBC was collected at baseline and after each of six cycles. Toxicities were graded by CTCAE v5.0. Analyses were descriptive with repeated-measures ANOVA, and ROC curve analysis. Results: Most hematologic toxicities were grade 1-2; grade 3-4 neutropenia was uncommon and no febrile events occurred. Higher mean WBC and RBC counts during treatment were significantly associated with non-response and shorter progression-free survival (PFS). Conclusion: Palbociclib plus letrozole showed a manageable safety profile without cumulative hematologic toxicity. Higher on-treatment WBC and RBC counts are prognostic for poorer outcomes. Routine CBC monitoring may offer a low-cost tool to identify patients at risk for treatment failure.