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Targeted Microbubbles for Imaging Tumor Angiogenesis: Assessment of Whole-Body Biodistribution with Dynamic Micro-PET in Mice¹

¹ From the Molecular Imaging Program at Stanford, Department of Radiology and Bio-X Program (J.K.W., Z.C., A.M.L., M.L.S., C.H.N., S.S.G.), Department of Comparative Medicine (C.D.), and Department of Bioengineering (S.S.G.), Stanford University School of Medicine, James H. Clark Center, 318 Campus Dr, East Wing, 1st Floor, Stanford, CA 94305-5427. Received November 24, 2007; revision requested January 23, 2008; revision received February 1; accepted March 18; final version accepted May 12. S.S.G. supported by the Canary Foundation; National Cancer Institute Small Animal Imaging Resource Program; National Heart, Lung, and Blood Institute grant 1 R01 HL078632; and National Cancer Institute In Vivo Cellular and Molecular Imaging Center grant CA114747 P50. J.K.W. supported by the Swiss Foundation of Medical-Biological Grants, Novartis Research Foundation, and Swiss Society of Radiology. Address correspondence to S.S.G. (e-mail: sgambhir{at}stanford.edu).

Purpose: To evaluate in vivo whole-body biodistribution of microbubbles (MBs) targeted to tumor angiogenesis–related vascular endothelial growth factor (VEGF) receptor 2 (VEGFR2) by using dynamic micro–positron emission tomography (PET) in living mice.

Materials and Methods: Animal protocols were approved by the Institutional Administrative 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-[¹⁸F]fluorobenzoate (SFB) to the anti-VEGFR2 antibodies. These MBs were then injected intravenously into nude mice (n = 4) bearing angiosarcomas, and the whole-body biodistribution 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 antibodies 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.

Results: VEGFR2-targeted MBs rapidly cleared from the blood circulation (50% blood clearance after approximately 3.5 minutes) and accumulated in the liver (mean, 33.4% injected dose [ID]/g ± 13.7 [standard deviation] at 60 minutes) and spleen (mean, 9.3% ID/g ± 6.5 at 60 minutes) on the basis of micro-PET imaging. These findings were confirmed 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. Biodistribution of the radiolabeled antibodies and free SFB differed from the distribution of the targeted MBs.

Conclusion: Dynamic micro-PET allows assessment of in vivo biodistribution of VEGFR2-targeted MBs.

© RSNA, 2008