Prognostic Role of Uric Acid-to-Albumin Ratio in Patients with Metastatic Breast Cancer Treated with CDK4/6 Inhibitors

Background/Objectives: The prognostic significance of the uric acid-to-albumin ratio (UAR) in patients with hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2−) metastatic breast cancer treated with cyclin-dependent kinase 4/6 (CDK4/6) inhibitors has not been adequately investigated. This study aimed to investigate the association between baseline UAR and survival outcomes in this patient population. Methods: This retrospective study included HR-positive/HER2-negative metastatic breast cancer patients treated with ribociclib or palbociclib at Necmettin Erbakan University between May 2020 and April 2025. UAR was calculated by dividing the serum uric acid level (mg/dL) by the serum albumin level (g/dL). Based on receiver operating characteristic (ROC) analysis, the optimal cut-off value for UAR was identified as 1.0 (AUC = 0.67; sensitivity 68%; specificity 58%). Patients were subsequently classified into two groups as UAR < 1 and UAR ≥ 1. Progression-free survival (PFS) and overall survival (OS) were estimated using the Kaplan–Meier method and compared with the log-rank test. Independent prognostic factors were evaluated using Cox regression analyses. Results: A total of 118 eligible patients were included in the analysis, including 34 (28.8%) in the UAR < 1 group and 84 (71.2%) in the UAR ≥ 1 group. The proportion of postmenopausal patients was significantly higher in the UAR ≥ 1 group (p = 0.01). Kaplan–Meier analysis showed that median PFS was not reached in the UAR < 1 group, whereas it was 33.05 months in the UAR ≥ 1 group (log-rank p = 0.06). Median OS was not reached in the UAR < 1 group and was 50.7 months in the UAR ≥ 1 group (p = 0.017). Multivariate Cox regression analysis demonstrated that UAR < 1 was associated with improved PFS (HR = 0.65; 95% CI: 0.34–0.89; p = 0.04). Postmenopausal status emerged as an independent adverse prognostic factor for PFS (HR = 1.92; 95% CI: 1.10–4.05; p = 0.04). In addition, UAR < 1 was associated with a reduced risk of mortality in the OS analysis (HR = 0.61; 95% CI: 0.26–0.87; p = 0.01). Conclusions: Lower baseline UAR was associated with more favorable survival outcomes in HR-positive/HER2-negative metastatic breast cancer patients treated with CDK4/6 inhibitors. As an inexpensive and easily accessible biomarker derived from routine laboratory parameters, UAR may provide additional prognostic information for clinical risk stratification.