Circulating blood microbiome biomarkers as liquid biopsy in breast cancer
Circulating tumor material can already be used to help in cancer management, but host–microbiota interactions may add complementary value. This study evaluated a “liquid microbiopsy”, combining cell-free microbial DNA (cf-mbDNA) with host biomarkers of microbial translocation/permeability (LBP, sCD14, I-FABP), to refine staging and prognosis in breast cancer. We analyzed 136 individuals: 58 metastatic breast cancer (MBC), 58 early breast cancer (EBC), and 20 healthy donors. Plasma cf-mbDNA was quantified by 16S rRNA qPCR with extraction/run negative controls; LBP, sCD14, I-FABP by ELISA; and CRP by immunoturbidimetry. Circulating tumor cells (CTCs) and PD-L1 on CTCs were assessed by CellSearch® and immunostaining. Diagnostic performance was assessed by area under the ROC curve. In MBC, progression-free survival (PFS) and overall survival (OS) were analyzed using Kaplan–Meier and Cox models. LBP discriminated EBC from MBC with an AUC 0.92, with a slight improvement in the 3- or 5-marker models (AUC 0.936–0.939). For EBC vs healthy donors, CRP showed modest performance (AUC 0.73), and marker combinations offered no improvement.. In MBC (median follow-up 77.5 months), median PFS and OS were 4.5 and 14.5 months, respectively. In univariate analysis, shorter PFS was associated with tumor subtype, > 3 prior treatments, > 2 metastatic sites, CTCs ≥ 5, and PD-L1–positive CTCs detection. In multivariate analysis, sCD14 and I-FABP independently predicted PFS, while sCD14 was independently associated with OS. A measurable host–microbiota–barrier axis (LBP, sCD14, I-FABP) provided stage-related and prognostic information complementary to CTC/PD-L1 status, whereas cf-mbDNA load alone showed limited discriminative value. These findings support liquid microbiopsy as a complementary extension of liquid biopsy but must be considered as exploratory, and warrant multicenter validation in larger independent cohorts with standardized pre-analytics and low-biomass controls. NCT03449264; NCT02866149.