Abstract 2303: Identifying and Stratifying Circulating Tumor Cell Cluster Subtypes in Metastatic Breast Cancer Patients and Evaluating Their Clinical Outcomes

Abstract Background: Circulating tumor cell clusters (CTCCs) are aggregated groups of tumor cells that detached from primary tumors and circulate the bloodstream. However, while Circulating Tumor Cells (CTCs) are a well studied phenomenon, CTCCs remain relatively unexplored and ill-defined, with only a few descriptive investigations evaluating their clinical utility. Adding to CTCC complexity is that various subtypes exist, including homotypic clusters consisting of a homogenous population of tumor cells and heterotypic CTCCs made of CTCs with immune/stromal white blood cells (WBCs). Furthermore, CTCs can undergo Epithelial-Mesenchymal Transition (EMT), a process characterized by loss of epithelial traits and upregulation of mesenchymal traits, and can form clustered EMTs (CEMTs). Moreover, CTCs can fuse with macrophages forming Tumor Macrophage Hybrid Cells, aka Cancer-Associated Macrophage Like cells (CAMLs) when in circulation. We enumerated all these types of cells and complex aggregates in the peripheral blood of patients with metastatic breast cancer (mBC). Methods: We enumerated the 6 populations from patient samples enrolled in IRB aproved prospective pilot studies. Whole peripheral blood (7.5mL) was filtered and stained with cytokeratin (CK) & CD45/CD14 to identify CTCs. CTCs were defined as an intact DAPI nuclei and strong filamentous CK. Homotypic CTCCs were defined as ≥2 CTCs attached together. Heterotypic CTCCs were defined as ≥1 CTC attached to ≥1 WBC. EMTs were defined as having DAPI nuclei and weak non-filamentous CK. CEMTS were defined as ≥2 EMTs. CAMLs were defined as having an enlarged polynucleated DAPI, and positive for CD45/CD14 or non-filamentous CK. Median progression free survival (mPFS) and median overall survival (mOS) over 24 months (m) were evaluated. Results: We evaluated the blood of 79 mBC patients. Single CTCs were found in 57% of patients (n=34/79), homotypic CTCCs 15% (n=12/79), heterotypic CTCCs 66% (n=27/79), EMTs 56% (n=44/79), CEMTs 23% (n=18/79) and CAMLs 97% (n=77/79). Over 24 ms, patierns with detectable heterotypic CTCCs & homotypic CTCCs had the shortest response times at mPFS=2.8 & mOS=5.8 ms, or mPFS=2.1 & mOS=4.3 ms, respectively. This was followed by CTCs mPFS=3.7 & mOS=13.7 ms, EMTs mPFS=4.3 & mOS=14.4 ms, CAMLs mPFS=4.3 & mOS=14.4 ms, and CEMT mPFS=4.7 & mOS=13.2 ms. Additionally, CTCCs were rare in HER2+ patients at 4.4% (n=1/23), but no associations to other hormone statuses were found. Conclusion: CTCCs appear to represent an array of subtypes with different biological and clinical meanings. Here we enumerated CTCC subtypes in the blood of mBC patients and stratified the various types to clinical outcomes. These data suggest CTCCs are clinically important and further understanding of their biology in the context of tumor pathogenesis is needed. Citation Format: Daniel L. Adams, Carolina Reduzzi, Aravind Aryasomayajula, R. Katherine Alpaugh, Saranya Chumsri, Cha-Mei Tang, Giuseppe Del Priore, William V. Williams, Massimo Cristofanilli. Identifying and stratifying circulating tumor cell cluster subtypes in metastatic breast cancer patients and evaluating their clinical outcomes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2303.