Targeted microbubbles for imaging tumor angiogenesis: assessment of whole-body biodistribution with dynamic micro-PET in mice.
OBJECTIVE
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.


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-[(18)F]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.


CONCLUSIONS
Dynamic micro-PET allows assessment of in vivo biodistribution of VEGFR2-targeted MBs.
Willmann
Jürgen K
JK
Molecular Imaging Program at Stanford, Department of Radiology and Bio-X Program, Stanford University School of Medicine, James H. Clark Center, 318 Campus Dr, East Wing, 1st Floor, Stanford, CA 94305-5427, USA.
Cheng
Zhen
Z
Davis
Corrine
C
Lutz
Amelie M
AM
Schipper
Meike L
ML
Nielsen
Carsten H
CH
Gambhir
Sanjiv S
SS
D000818:Animals; D000906:Antibodies; D001565:Benzoates / Q000494:pharmacology; D005455:Fluorescent Antibody Technique; D005466:Fluorocarbons; D006394:Hemangiosarcoma / Q000000981:diagnostic imaging; D008099:Liver / Q000000981:diagnostic imaging; D051379:Mice; D008819:Mice, Nude; D045423:Microbubbles; D009374:Neoplasms, Experimental / Q000098:blood supply*; D009389:Neovascularization, Pathologic / Q000000981:diagnostic imaging*; D049268:Positron-Emission Tomography / Q000379:methods*; D013154:Spleen / Q000000981:diagnostic imaging; D013388:Succinimides / Q000494:pharmacology; D014018:Tissue Distribution; D040301:Vascular Endothelial Growth Factor Receptor-2 / Q000032:analysis* / Q000276:immunology
D000906:Antibodies; D001565:Benzoates; D005466:Fluorocarbons; C092711:N-succinimidyl 4-(fluoromethyl)benzoate; D013388:Succinimides; D040301:Vascular Endothelial Growth Factor Receptor-2