Abstract Background: Currently, the gold standard in cancer diagnosis, prognosis, and treatment is assessing cancer cell mitosis in tumor tissue biopsies, which identifies patients with more aggressive cancer subtypes that may require more tailored treatment regimens. Circulating tumor cells (CTCs), cancer cells that detach from primary tumors, enter the bloodstream, and seed metastatic sites, are a well-known non-invasive blood biomarker which can also be used to aid in stratifying metastatic breast cancer (mBC) patients with highly aggressive subtypes. Initial pilot studies have identified a particular subtype of CTCs which are undergoing mitosis that appears to correlate with very poor survival outcomes compared to non-mitotic CTCs alone. In a multi-institutional prospective study, we isolated CTCs from the blood of n=167 mBC patients to categorize mitotic CTCs and evaluate their association with progression-free survival (PFS) and overall survival (OS) over a 3 year period. Methods: Peripheral blood samples (7.5ml) were collected from n=167 mBC patients progressing on systemic therapies and prior to new systemic therapy induction (i.e. chemotherapy n=102, hormone therapy n=13, immunotherapy n=63, targeted therapy n=62). Samples were processed using CellSieve microfilters to isolate CTCs via size exclusion and fluorescently stained for CD45, Cytokeratin, and DAPI. CTCs were imaged, enumerated, and subtyped based on the presence of ≥1 mitotic event, using previously established visual indicators. PFS and OS were assessed over 3 years by censored univariate and multivariate analyses. Results: CTCs were detected in ∼47% (n=79/167) of patients, while mitotic CTCs were identified in a subset of this group, ∼48% (n=38/79). The presence of any CTC was associated with poor outcomes (PFS HR=0.49, 95%CI=0.3-0.7, p=0.0002 and OS: HR=0.42, 95%CI=0.3-0.7, p=0.0002). However, the presence of mitotic CTCs correlated with significantly poorer outcomes compared to non-mitotic CTCs (PFS: HR=0.3, 95%CI=0.2-0.5, p0.0001 and OS: HR=0.3, 95%CI=0.2-0.6, p=0.0002). Further, patients with no CTCs had a median PFS (mPFS) of 6 months and median OS (mOS) of 22 months, patients with non-mitotic CTCs had a mPFS of 4 months and mOS of 14.5 months; patients with mitotic CTCs had a mPFS of 2.4 months and mOS of 4.4 months. Notably, 95% of patients with a mitotic CTC progressed within 6 months, regardless of systemic therapy type. Interestingly, there were 5 patients who survived longer than 1 year with a mitotic CTC all of which were receiving specific targeted therapies. Conclusion: In this study, the detection of CTCs indicated poor outcomes in patients. Additional cytological analysis of CTCs further identified CTCs undergoing mitosis which correlated to highly aggressive disease with the poorest survival rates. These results indicate that while the presence of CTCs is a prognostic indicator for survival, subtyping CTCs and identifying mitosis may correlate to therapy response. This study highlights the need for larger, more comprehensive studies to expand upon and validate these findings. Citation Format: A. B. Duffy, M. Cristofanilli, C. Reduzzi, W. V. Williams, G. Del Priore, S. Chumsri, C. Tang, T. Iwase, N. T. Ueno, D. L. Adams. Mitotic Circulating Tumor Cells Correlates with Poor Clinical Outcomes in Metastatic Breast Cancer Patients over a 3 Year Period abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-01-05.
Duffy et al. (2026) studied this question.