US imaging of tumor angiogenesis with microbubbles targeted to vascular endothelial growth factor receptor type 2 in mice

Purpose: To prospectively evaluate contrast material-enhanced ultra- sonography (US) with microbubbles targeted to vascular endothelial growth factor receptor type 2 (VEGFR2) for imaging tumor angiogenesis in two murine tumor models. Materials and Methods: Animal protocols were approved by the Institutional Administrative Panel on Laboratory Animal Care. A US contrast. agent, consisting of encapsulated gaseous microbubbles, was developed specifically to bind to VEGFR2 (by using anti-VEGFR2 antibodies and biotin-streptavidin interaction) which is up-regulated on endothelial cells of tumor blood vessels. VEGFR2-targeted microbubbles (MBV), control microbubbles (MBC), and nonlabeled microbubbles (MBN) were tested for binding specificity on cells expressing VEGFR2 (mouse angiosarcoma SVR cells) and control cells (mouse skeletal rnyoblast C2C12 cells). Expression of mouse VEGFR2 in culture cells was tested with immunocytochemical and Western blot analysis. Contrast-enhanced US imaging with MBV and MBC was performed in 28 tumor-bearing nude mice (mouse angiosarcoma, n = 18; rat malignant glioma, n = 10). Differences were calculated by using analysis of variance. Results: In cell culture, adherence of MBV on SVR cells (2.1 microbubbles per SVR cell) was significantly higher than adherence of Control microbubbles (0.01-0.10 microbubble per SVR cell; P <.001) and significantly more MBV attached to SVR cells than to C2C12 cells (0.15 microbubble per C2C12 cell; P <.001). In vivo, contrast-enhanced US imaging showed significantly higher average video intensity when using MBV compared with MBc for angiosarcoma and malignant glioma tumors (P < .001). Results of immunohistochemical analysis confirmed VEGFR2 expression on vascular endothelial cells of both tumor, types. Conclusions: US imaging with contrast microbubbles targeted to VEGFR2 allows noninvasive visualization of VEGFR2 expression in tumor vessels in mice. (C) RSNA, 2008.