High levels of circulating VEGFR2⁺ bone marrow-derived progenitor cells correlate with metastatic disease in patients with pediatric solid malignancies

Purpose: Pediatric solid malignancies display important angiogenic potential, and blocking tumor angiogenesis represents a new therapeutic approach for these patients. Recent studies have evidenced rare circulating cells with endothelial features contributing to tumor neovascularization and have shown the pivotal role of bone marrow-derived (BMD) progenitor cells in metastatic disease progression. We measured these cells in patients with pediatric solid malignancies as a prerequisite to clinical trials with antiangiogenic therapy. Patients and Methods: Peripheral blood was drawn from 45 patients with localized (n = 23) or metastatic (n = 22) disease, and 20 healthy subjects. Subsets of circulating vascular endothelial growth factor receptor (VEGFR)2⁺-BMD progenitor cells, defined as CD45^-CD34⁺VEGFR2 (KDR) ⁺7AAD^- and CD45^dimCD34⁺VEGFR2 ⁺7AAD^- events, were measured in progenitor-enriched fractions by flow cytometry. Mature circulating endothelial cells (CEC) were measured in whole blood as CD31⁺CD146⁺CD45 ^-7AAD^- viable events. Data were correlated with VEGF and sVEGFR2 plasma levels. Results: The CD45^-CD34⁺VEGFR2(KDR) ⁺7AAD^- subset represented <0.003% of circulating BMD progenitor cells (<0.05 cells/mL). However, the median level (range) of the CD45^dimCD34⁺VEGFR2⁺7AAD^- subset was higher in patients compared with healthy subjects, 1.5% (0%-10.3%) versus 0.3% (0%-1.6%) of circulating BMD progenitors (P < 0.0001), and differed significantly between patients with localized and metastatic disease, 0.7% (0%-8.6%) versus 2.9% (0.6%-10.3%) of circulating BMD progenitors (P < 0.001). Median CEC value was 7 cells/mL (0-152 cells/mL) and similar in all groups. Unlike VEGFR2⁺-BMD progenitors, neither CECs, VEGF, or sVEGFR2 plasma levels correlated with disease status. Conclusion: High levels of circulating VEGFR2⁺-BMD progenitor cells correlated with metastatic disease. Our study provides novel insights for angiogenesis mechanisms in pediatric solid malignancies for which antiangiogenic targeting of VEGFR2 ⁺-BMD progenitors could be of interest.