Abstract 1968: Comparable detection of large tumor-derived vesicles (oncosomes) by known CTC platforms: Potential new analyte for biomarker monitoring?

Abstract Background: Monitoring tumors and detecting biomarkers through blood analysis remains challenging. Large oncosomes (LOs) are tumor-derived extracellular vesicles that offer a promising avenue for understanding tumor biology and biomarker status, potentially surpassing circulating tumor cells (CTCs) in accuracy. Validated LO detection methods are required to unlock LOs analytical potential. We evaluated two LO detection methods using known CTC platforms, the FDA-approved CellSearch, and RareCyte to compare their performance. In addition, we assessed the potential of LOs for HER2 detection in metastatic breast cancer (MBC) patients. Methods: LO counts were analyzed in parallel blood samples from 96 MBC patients using either the CellSearch/ACCESS tdEV analysis package, or the RareCyte method with in-house LO analysis. Overall survival (OS) was defined as the time from baseline blood draw to death or last follow-up. HER2 scores from primary tumor biopsies were retrieved from patient charts. HER2 overexpression in LOs and CTCs was determined using the RarePlex HER2/ER CTC panel kit (n=9). Correlations between LO and CTC counts were assessed using Spearman’s Rho test, and comparability of LO counts between platforms was evaluated with the Wilcoxon Signed Ranks test. Cutoffs for favorable vs. unfavorable LO counts were determined using an online tool. Survival curves for these groups were compared using the log-rank test. Correlation of HER2+ analytes with tissue biopsy HER2 scores was also analyzed using Spearman’s Rho test. Results: A strong positive correlation was observed between LO counts from the two platforms (r = 0.49, p < 0.001). LO counts also correlated with their respective CTC counts (CellSearch: r = 0.88, p < 0.001; RareCyte: r = 0.93, p = 0.001). Higher LO counts were associated with worse survival outcomes (CellSearch: r = 0.35, p < 0.001; RareCyte: r = 0.39, p = 0.006). Patients with unfavorable LO counts had significantly shorter survival compared to patients with favourable LO counts (CellSearch: HR = 3.5, p < 0.001; RareCyte: HR = 3.6, p = 0.02). RareCyte detected significantly more LOs than CellSearch (median LO count: 132 vs. 41, p = 0.01). HER2 signal was detected in 5/9 patients with CTCs and in 8/9 patients with LOs. Notably, the proportion of HER2+ LOs positively correlated with tissue biopsy HER2 score (r=0.799, p=0.01). In contrast, the proportion of HER2+ CTCs did not correlate with tissue biopsy. Conclusions: While RareCyte captured more LOs than CellSearch, LO counts were highly concordant across platforms, validating both methods. The association of LO counts with overall survival suggests that LOs reflect tumor progression, and may complement CTC-based monitoring. Furthermore, the correlation between LO HER2 status and tumor biopsy HER2 scores underscores the potential of LO analysis for biomarker monitoring in metastatic breast cancer. Citation Format: Eszter Papp, Pieter Mestdagh, Luc Y. Dirix, Peter Vermeulen, Mark Kockx. Comparable detection of large tumor-derived vesicles (oncosomes) by known CTC platforms: Potential new analyte for biomarker monitoring [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1968.