Abstract 3771: Monitoring tumor dynamics through circulating cancer stem cells in colorectal cancer patients

Abstract Background: Colorectal cancer (CRC) remains one of the most commonly diagnosed and lethal malignancies worldwide. Tumor recurrence and metastasis are major determinants of patient survival. Circulating cancer stem cells (cCSCs), a rare subpopulation of tumor cells present in peripheral blood, are believed to drive tumor growth, metastasis, and therapy resistance. While tumorsphere culture has been used to identify cancer stem cells from primary tumors or cell lines, we have established an effective method for detecting and characterizing circulating cancer stem cells in the blood of CRC patients. Methods: Metastatic and non-metastatic CRC patients were included in this study. For the detection of circulating cancer stem cells, we used a functional assay for tumorsphere formation (stemtrac®). Immunofluorescence and qRT-PCR were employed to assess surface marker expression and pluripotency-associated genes. Additionally, we report a case of a 40-year-old man with metastatic KRAS-positive CRC, who was longitudinally monitored using both circulating epithelial tumor cell (CETCs/CTCs) and cCSC analyses in correlation with clinical and imaging findings. Results: Patients with metastatic disease exhibited a significantly higher number of tumorspheres compared with non-metastatic patients (median: 48 vs. 15 spheres per 100 µl blood), suggesting a correlation between tumorsphere count and disease stage. Tumorspheres showed high expression of EpCAM and CD133, elevated ALDH1 activity, and upregulation of pluripotency genes such as SOX2, OCT4, and NANOG. No sphere formation was observed in healthy controls (n = 50). In the case study, fluctuations in CETCs/CTCs and cCSC levels reflected treatment response and disease activity, with sustained increases preceding clinical or radiological evidence of progression. Conclusion: This study demonstrates that tumor stem cells can be detected in peripheral blood from both metastatic and non-metastatic CRC patients. The number of tumorspheres derived from circulating cancer stem cells serves as an independent indicator of metastatic potential. Serial monitoring of CETCs/CTCs and cCSCs provides a non-invasive and dynamic tool for evaluating treatment efficacy and identifying early disease progression. A deeper understanding of the biology of circulating cancer stem cells may facilitate the development of more effective and personalized therapeutic strategies for colorectal cancer. Citation Format: Monika Pizon, Dorothea Schott, Ulrich Pachmann, Katharina Pachmann. Monitoring tumor dynamics through circulating cancer stem cells in colorectal cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3771.