Ultrasound Molecular Imaging of VEGFR2 in a Rat Prostate Tumor Model Using BR55

Objectives:To evaluate BR55, a new VEGFR2-specific ultrasound contrast agent, for imaging prostate tumors in an orthotopic model in the rat. Materials and Methods:Rat prostate adenocarcinoma were established by injection of G Dunning R-3327 tumor cells in one lobe of the prostate of Copenhagen rats. Imaging experiments were performed with BR55, SonoVue, and streptavidin-functionalized microbubbles coupled with an anti-vascular endothelial growth factor receptor 2 (VEGFR2) antibody using a clinical ultrasound scanner. Contrast enhancement in the tumor and healthy prostate was followed over time by intermittent imaging at low acoustic power. Signal quantification and statistical analysis were performed in the tumor and healthy tissue to compare the behavior of the 3 contrast agents. Immunohistochemistry was performed on the prostate and tumor specimen to determine the expression of VEGFR2. Results:Comparable contrast enhancement was observed in tumors at peak intensity for BR55 and SonoVue. Then, once unbound microbubbles had cleared from the circulation, a strong enhancement of the tumor was obtained with BR55, whereas no significant microbubble accumulation was detected in the healthy prostate tissue. SonoVue microbubbles were rapidly eliminated, and no significant binding was observed in the tumor. The tumor to prostate ratio calculated after signal quantification was about 20 for the 3 doses of BR55 tested. The enhancement obtained with BR55 in the tumor was not significantly different from the one observed with antibody-coupled streptavidin microbubbles. Intense staining for VEGFR2 was detected in the tumor vessels by immunohistochemistry. Conclusions:This study showed that BR55 binding to prostate tumors resulted in a strong enhancement of the lesions as early as a few minutes after contrast injection, whereas minimal nonspecific accumulation occurred in the healthy part of the gland. BR55, like Sonovue, provide information on tissue perfusion during the early vascular phase, but BR55 binding to the tumoral endothelium allows to gain additional information by highlighting the sites of active angiogenesis. The late phase enhancement of the tumor should be particularly valuable for prostate cancer detection and for biopsy guidance.

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