Abstract 1065: Mitosis in circulating tumor cells correlates with highly aggressive disease in metastatic breast cancer

Abstract Background: Circulating tumor cells (CTCs) are a well-known non-invasive blood biomarker which can stratify metastatic breast cancer (mBC) patients, especially those with highly aggressive subtypes. Initial pilot studies have characterized a distinct subtype of CTCs undergoing mitosis, whose presence correlates with worse survival outcomes than CTCs alone. However, their prognostic value and their influence in different therapeutic regimes remains unknown. In a multi-institutional prospective study, we isolated CTCs from n=138 mBC patients to categorize mitotic CTCs, evaluate their association with progression-free survival (PFS) & overall survival (OS) over 2 years, and how their response rates effect different therapeutic strategies. Methods: Peripheral blood samples (7.5ml) were collected from n=138 mBC patients progressing on systemic therapies but prior to new systemic therapies (i.e. chemotherapy n=48, hormone therapy n=17, immunotherapy n=64, targeted therapy n=22). CellSieve microfilters isolated CTCs via size exclusion and cells fluorescently stained for CD45, Cytokeratin, and DAPI. CTCs were imaged, enumerated, and subtyped based on the presence of ≥1 mitotic CTC, using previously established visual indicators. PFS and OS were assessed over 2 years by censored univariate and multivariate analyses. Therapy efficacy was assessed by comparing median PFS (mPFS) and median OS (mOS) in the therapy categories. Results: Conclusion: In this study, patients with CTCs had poorer outcomes than patients with no CTCs, while patients with mitotic CTCs had the worst outcomes. Further, patients with no CTCs or non-mitotic CTCs showed similar PFS & OS in some treatment types, while patients with no CTCs treated with hormone therapy had better PFS and patients with mitotic CTCs had better OS if treated with targeted therapy. This study highlights the need for larger more comprehensive studies to expand on these prognostic findings and correlations to therapy types. Citation Format: Alexis B. Duffy, Massimo Cristofanilli, Carolina Reduzzi, Wlliam V. Williams, Giuseppe Del Priore, Saranya Chumsri, Cha-Mei Tang, Toshiaki Iwase, Naoto Ueno, Daniel L. Adams. Mitosis in circulating tumor cells correlates with highly aggressive disease in metastatic breast cancer [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 1065.