Tumor-Derived Extracellular Vesicles as Complementary Prognostic Factors to Circulating Tumor Cells in Metastatic Breast Cancer

PURPOSE Circulating tumor cells (CTCs) are strongly prognostic for overall survival (OS) in metastatic breast cancer although additional prognostic biomarkers are needed. We evaluated the complementary prognostic value of tumor‐derived extracellular vesicles (tdEVs) next to CTCs. METHODS We applied the open-source ACCEPT software to archived CellSearch images from the prospective clinical trial SWOG0500 to enumerate CTCs and tumor-derived extracellular vesicles (tdEVs) before and after one cycle of chemotherapy. RESULTS CTCs enumerated by ACCEPT were strongly correlated with classical ocular enumeration (correlation r = 0.98). OS was worse with elevated tdEVs (median OS for high/medium/low groups: 17.1 v 29.0 v 43.3 months; P < .0001). In patients with longer OS by CTC counts (< 5 CTC/7.5 mL blood), elevated tdEV levels were independently associated with poorer OS (multivariable analysis P < .001). OS was also longer for patients with low tdEVs after one cycle of chemotherapy (median OS for high/medium/low group: 10.8 v 17.8 v 26.7; P < .0001). CONCLUSION This study highlights the complementary prognostic significance of tdEVs in metastatic breast cancer before and after one cycle of chemotherapy.