Abstract 2300: Blood microbiome, circulating tumor cells, and survival in Black and White women with metastatic breast cancer

Abstract Background: Metastatic breast cancer (MBC) is a devastating disease, and recent evidence suggests that the human microbiome may play a critical role in cancer metastasis. However, the role of the blood microbiome in MBC prognosis has not been explored. Methods: This study analyzed sixteen blood samples collected from women diagnosed with MBC who participated in the hospital-based cohort at Thomas Jefferson University. Plasma DNA samples were extracted from these samples collected before initiating new lines of therapy. We utilized 2bRAD sequencing for Microbiome (2bRAD-M), a reduced sequencing technology capable of decoding the blood microbiome (bacteria, fungi, archaea, and viruses) with high precision and resolution. Circulating tumor cells (CTCs) were enriched from blood samples using CellSearch®. To investigate blood microbiome differences by CTC levels and race, both conventional statistical analysis and machine learning are performed to examine the variations in blood microbial diversity, taxonomic structures, and functional pathways. Cox proportional hazard regression models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for associations between microbial diversity index and MBC overall survival. Results: We identified 265 microbial species in the plasma samples. Lower blood microbial alpha diversity was significantly correlated with higher CTC counts (R = -0.66, p = 0.01), a marker associated with poorer breast cancer outcomes. Random forest analysis identified fifteen key species (e.g., Sphingopyxis_sp001468225, Malassezia restricta) that can differentiate between CTC groups, with an area under the curve of 0.67 to 0.75. Pathway analysis revealed the bacterial Type VI Secretion System (T6SS) as the top pathway associated with differential microbes across CTC levels; women with a CTC count >5 showed over 1000-fold higher abundance of the T6SS pathway. For survival prediction, patients with low microbial alpha diversity (≤ median of 2.04) exhibited a nearly threefold increased risk of death (HR = 2.62, 95% CI=0.74 -17.26), with even higher risk after adjusting for tumor ER status (HR = 4.43), age at diagnosis (HR = 4.86), and CTC count (HR = 6.83). These findings indicate a strong negative association between blood microbial diversity and key prognostic factors in MBC. Lastly, we also observed that Black MBC patients had a lower microbial richness than White participants (median 27 vs. 42), and had a significantly lower abundance of Klebsiella pneumoniae than White women (p<0.001). Conclusion: Our compelling pilot findings suggest that 1) blood samples may serve as a novel sample type for microbiome assessment in a diverse cohort of women with MBC, and 2) blood microbiome features differ by CTC levels and race and may significantly influence MBC survival. Further studies with larger cohorts are warranted as the next step. Citation Format: Tengteng Wang, Prachi Trevedi, Mouadh Barbirou, Maysa Abu-Khalaf, Zheng Sun, Yang-Yu Liu, Mridula George, Elisa V. Bandera, Hushan Yang. Blood microbiome, circulating tumor cells, and survival in Black and White women with metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2300.