Serum proteomic profiling reveals metastasis-associated molecular signatures and early systemic remodeling in triple-negative breast cancer

Background and Objectives: Triple-negative breast cancer (TNBC) is an aggressive subtype with high risk of distant metastasis and limited biomarkers for early detection of recurrence. Clinical surveillance relies on imaging, which often identifies metastasis only after systemic progression. This study evaluated whether serum proteomic profiling can identify molecular changes associated with metastatic transition in TNBC. Methods: Serum samples were collected from 29 patients with TNBC and 10 healthy donors. Among TNBC cases, 12 and 9 samples were obtained at the time of initial diagnosis from patients who never developed distant recurrence (NR-DIG) and those who developed distant recurrence (DR-DIG), respectively. Eight samples were collected at recurrence (DR-REC). All samples were analyzed using the SomaScan® 11K proteomic platform. Differential protein expression (|log₂[Fold Change]| > 0.35, False Discovery Rate (FDR) < 0.10), pathway enrichment, and immune deconvolution analyses were performed. Results: Comparison between DR-REC and DR-DIG identified 299 differentially expressed proteins (DEPs; 260 upregulated, 39 downregulated). Inflammation, coagulation, angiogenesis, and extracellular matrix remodeling were significantly enriched by the DEPs. Unsupervised hierarchical clustering distinguished DR-REC from DR-DIG, and one serum sample clinically classified as DR-DIG clustered with DR-REC, suggesting that metastasis-associated proteomic changes may precede clinical detection. Immune deconvolution revealed increased monocyte and neutrophil signatures in DR-REC compared with NR-DIG and DR-DIG, indicating systemic immune remodeling during metastatic progression. Conclusions: Serum proteomic profiling can detect molecular alterations associated with metastatic transition in TNBC, even before radiographic evidence of recurrence. These findings support the feasibility of a proteomics-based liquid biopsy as a minimally invasive approach for early detection and monitoring of metastatic progression in TNBC.