Abstract 3697: Proof-of-concept study to enumerate large oncosomes using the RareCyte CTC platform

Abstract

Abstract Introduction: Large oncosomes (LO) are produced by tumor cells and can be detected in blood from cancer patients. The clinical utility of enumerating and characterizing LOs along with CTCs has been demonstrated, especially in settings where CTC counts are low. The RareCyte CTC platform has the potential to capture LOs along with CTCs. We performed a pilot study to evaluate the RareCyte CTC platform’s potential to analyze LOs using established preanalytical procedures. Methods: Metastatic breast cancer patients’ (N=10) blood smears labelled with RarePlex® Enumeration panel kit were re-evaluated to identify objects with LO-like characteristics at 10x magnification, along with negative control slides (N=5). LO-like objects were annotated and re-scanned at 40x magnification using the CyteFinder® imaging instrument. After establishing the criteria for LO identification, LO enumeration was performed on a cohort of MBC patients (N=6). Sets of five blood smear slides stained with the RarePlex® Enumeration panel kit were reviewed and LO counted. Total LO counts were established by extrapolating LO counts to 7.5 mL plasma. Pearson correlation was used to determine the level of correlation between LO counts and previously obtained CTC counts. Results: The established identification criteria enabled the consistent enumeration of LOs at 10X magnification. Total LO count was 9.6 on the negative control slides. In MBC patients the mean total LO count was 306.2 (range 3.2 - 792). There was a significant positive correlation between LO and CTC count (r=0.855, p=0.3). In samples with CTC>0 (N=3), the average total LOs count was 588, about six times higher than CTC count. In CTC=0 patient samples (N=3) the mean total LO count was 24.3 (range 3.2-36). Conclusions: We report a novel protocol to quantify large oncosomes using the RareCyte CTC platform using established preanalytical workflows. The strong CK/EpCam staining in LOs and the strong correlation of LO count with CTC count indicates their tumoral origin. Due to their higher abundance, LOs combined with CTCs offer a more sensitive tool for treatment response monitoring. Due to its broad range, LO counts in patients with CTC=0 may reveal therapy response dynamics undetectable with CTC enumeration alone. Ongoing work focuses on the parallel detection and integrated evaluation of HER2 by immunofluorescent staining of CTCs and LOs, along with digital PCR based HER2 quantification using cfDNA from matched blood plasma samples. Citation Format: Eszter Papp, Jo Vandesompele, Peter B. Vermeulen, Luc Y. Dirix, Mark Kockx. Proof-of-concept study to enumerate large oncosomes using the RareCyte CTC platform [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 3697.