In vivo VEGF imaging with radiolabeled bevacizumab in a human ovarian tumor xenograft

Vascular endothelial growth factor (VEGF), released by tumor cells, is an important growth factor in tumor angiogenesis. The humanized monoclonal antibody bevacizumab blocks VEGF-induced tumor angiogenesis by binding, thereby neutralizing VEGF. Our aim was to develop radiolabeled bevacizumab for noninvasive in vivo VEGF visualization and quantification with the single gamma-emitting isotope In-111 and the PET isotope Zr-89. Methods: Labeling, stability, and binding studies were performed. Nude mice with a human SKOV-3 ovarian tumor xenograft were injected with Zr-89-bevacizumab, In-111-bevacizumab, or human Zr-89-IgG. Human Zr-89-IgG served as an aspecific control antibody. Small-animal PET and microCT studies were obtained at 24, 72, and 168 h after injection of 89Zr-bevacizumab and Zr-89-IgG (3.5 +/- 0.5 MBq, 100 +/- 6 mu g, 0.2 mL [mean +/- SD]). Small-animal PET and microCT images were fused to calculate tumor uptake and compared with ex vivo biodistribution at 168 h after injection. Zr-89- and In-111-bevacizumab ex vivo biodistribution was compared at 24, 72, and 168 h after injection 2.0 +/- 0.5 MBq each, 100 +/- 4 mu g in total, 0.2 mL). Results: Labeling efficiencies, radiochemical purity, stability, and binding properties were optimal for the radioimmunoconjugates. Small-animal PET showed uptake in well-perfused organs at 24 h and clear tumor localization from 72 h onward. Tumor uptake determined by quantification of small-animal PET images was higher for Zr-89-bevacizumab-namely, 7.38 +/- 2.06 %ID/g compared with 3.39 +/- 1.16 %ID/g (percentage injected dose per gram or human Zr-89-IgG (P = 0.011) at 168 h and equivalent to ex vivo biodistribution studies. Tracer uptake in other organs was seen primarily in liver and spleen. Zr-89- and In-111-bevacizumab biodistribution was comparable. Conclusion: Radiolabeled bevacizumab showed higher uptake compared with radiolabeled human IgG in a human SKOV-3 ovarian tumor xenograft. Noninvasive quantitative small-animal PET was similar to invasive ex vivo biodistribution. Radiolabeled bevacizumab is a new tracer for noninvasive in vivo imaging of VEGF in the tumor microenvironment.