Proteomic Analysis of Urinary Exosomal S100 Proteins Identifies S100A6 as a Non-Invasive Biomarker for Small Cell Lung Cancer in Male Patients

Background: Small cell lung cancer (SCLC) has a poor prognosis, creating a need for non-invasive diagnostic biomarkers. Urinary exosomes, enriched with tumor-derived biomolecules, offer a non-invasive avenue for biomarker discovery. This study investigates the expression profiles of S100A proteins in urinary exosomes for SCLC detection. Methods: Urinary exosomes were isolated from 26 male SCLC patients and 26 male healthy controls. The cohort was randomly divided into discovery and validation sets of 13 patients and 13 controls each. Proteomic profiling using data-independent acquisition mass spectrometry identified differentially expressed S100A proteins. Age- and sex-specific patterns were examined in 210 healthy individuals. Key findings were validated by parallel reaction monitoring. Results: SCLC exosomes showed significant downregulation of S100A2/A6/A8/A9/A11/A16 (fold change >1.5, p < 0.05) and specific upregulation of S100A7 (p < 0.001). S100A6 demonstrated superior diagnostic performance with area under the curve (AUC) = 0.98 (95% CI: 0.94–1.00) in the discovery cohort and maintained AUC = 0.86 (95% CI: 0.71–1.00) in the validation cohort. Protein interaction analysis uncovered potential interactions among S100A members, particularly between S100A8/A16 (r = 0.86, p < 0.001). Age-sex stratification revealed significant population heterogeneity in the abundance of S100A2/A7/A8/A9/A16 (p < 0.001), whereas S100A6 abundance remained stable across subgroups. Conclusion: Urinary exosomal S100 proteins, particularly the stable and high-performing S100A6, serve as promising non-invasive biomarkers for male patients with SCLC. The identified population-specific abundance patterns provide a basis for personalized diagnostics.