Targeted Microbubbles for Imaging Tumor Angiogenesis: assessment of whole body biodistribution with dynamic micro-PET in mice.

Purpose: Materials and Methods: Results: Conclusion: To evaluate in vivo whole-body biodistribution of micro- bubbles (MBs) targeted to tumor angiogenesis–related vascular endothelial growth factor (VEGF) receptor 2 (VEGFR2) by using dynamic micro–positron emission to- mography (PET) in living mice. Animal protocols were approved by the Institutional Ad- ministrative Panel on Laboratory Animal Care. Lipid-shell perfluorocarbon-filled MBs, targeted to VEGFR2 via anti- VEGFR2 antibodies, were radiolabeled by conjugating the radiofluorination agent N-succinimidyl-4-[18 F]fluoroben- zoate (SFB) to the anti-VEGFR2 antibodies. These MBs were then injected intravenously into nude mice (n ? 4) bearing angiosarcomas, and the whole-body biodistribu- tion of these probes was assessed for 60 minutes by using dynamic micro-PET. Results were compared with ex vivo gamma counting (n ? 6) and immunofluorescence staining (n ? 6). Control studies in angiosarcoma-bearing mice were performed with injection of the radiolabeled antibod- ies alone (n ? 3) or free SFB (n ? 3). A mixed-effects regression of MB accumulation on fixed effects of time and tissue type (tumor or muscle) and random effect of animal was performed. VEGFR2-targeted MBs rapidly cleared from the blood cir- culation (50% blood clearance after approximately 3.5 minutes) and accumulated in the liver (mean, 33.4% in- jected dose [ID]/g ? 13.7 [standard deviation] at 60 min- utes) and spleen (mean, 9.3% ID/g?6.5 at 60 minutes) on the basis of micro-PET imaging. These findings were con- firmed with ex vivo gamma counting. Uptake of targeted MBs was significantly higher (P < .0001) in tumor than in adjacent skeletal muscle tissue. Immunofluorescence staining demonstrated accumulation of the targeted MBs within hepatic Kupffer cells and splenic macrophages. Bio- distribution of the radiolabeled antibodies and free SFB differed from the distribution of the targeted MBs. Dynamic micro-PET allows assessment of in vivo biodistri- bution of VEGFR2-targeted MBs.