In vivo    VEGF imaging with ⁸⁹Zr-bevacizumab in a human ovarian tumor xenograft model using MicroPET and MicroCT. Free

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VEGF, released by tumor cells, is an important growth factor in tumor angiogenesis as it induces tumor neovascularization. VEGF leads to paracrine effects and results in abundant presence of VEGF in the tumor micro-environment. Non-invasive measurement of VEGF in the tumor might give insight in the available target for VEGF dependent anti-angiogenic therapy. Bevacizumab is a humanized monoclonal antibody which blocks the biological pathways of VEGF induced tumor angiogenesis by binding to VEGF, most likely in the micro-environment of the tumor. For that reason, we developed radiolabeled bevacizumab for in vivo VEGF imaging using the relatively new long-lived PET isotope ⁸⁹Zirconium (⁸⁹Zr) (T_1/2 3.27d) allowing high resolution (Micro)PET imaging. Methods: Labeling, stability and in-vitro immunoreactivity studies were performed. Thereafter, nude mice were inoculated subcutaneously with VEGF producing human SKOV-3 ovarian tumor cells. After tumor size reached 7-9 mm in diameter, mice were injected with 100 μg ( ± 3.5 MBq) of ⁸⁹Zr-bevacizumab or human ⁸⁹Zr-IgG. Human ⁸⁹Zr-IgG served as an aspecific control antibody. MicroPET images, MicroCT images and biodistribution studies were obtained at 24, 72 and 168 h post injection. MicroPET and MicroCT images were fused to calculate uptake of ⁸⁹Zr-bevacizumab and Zr⁸⁹-IgG in the micro-environment of the tumor. Results: ⁸⁹Zr-bevacizumab labeling efficiency was >95%. The tracer was stable > 168 h at 37°C in serum. The immunoreactive fraction determined by ELISA was >0.50. MicroPET images showed uptake in well perfused organs after 24 h and clear tumor localization after 72 h post injection. Tumor uptake determined by quantification of MicroPET images was higher for ⁸⁹Zr-bevacizumab 168 h post injection, namely 7.38% ± 2.06 injected dose (ID)cm^-3 compared to 3.39% ± 1.16 IDcm^-3 for human ⁸⁹Zr-IgG (p=0.011). These data were confirmed by ex-vivo biodistribution studies. Uptake in other organs was mainly seen in liver and spleen. Discussion: The labeling of bevacizumab with the PET isotope ⁸⁹Zr allows imaging matching the pharmacokinetic half life of bevacizumab. (Micro)PET imaging permits not only excellent visualization, but also dynamic quantification of ⁸⁹Zr-bevacizumab in the micro-environment of the tumor. Tumor uptake of ⁸⁹Zr-bevacizumab was significantly higher compared to control ⁸⁹Zr-IgG, signifying that ⁸⁹Zr-bevacizumab could potentially be used to visualize VEGF-levels in the micro-environment of the tumor.