In vivo characterization of ^sup 68^Ga-NOTA-VEGF^sub 121^ for the imaging of VEGF receptor expression in U87MG tumor xenograft models

Vascular endothelial growth factor receptors (VEGFR) are associated with tumor growth and induction of tumor angiogenesis and are known to be overexpressed in various human tumors. In the present study, we prepared and evaluated ^sup 68^Ga-1,4,7-triazacyclononane-1,4,7-triacetic acid-benzyl (NOTA)-VEGF^sub 121^ as a positron emission tomography (PET) radioligand for the in vivo imaging of VEGFR expression. ^sup 68^Ga-NOTA-VEGF^sub 121^ was prepared by conjugation of VEGF^sub 121^ and p-SCN-NOTA, followed by radiolabeling with ^sup 68^GaCl^sub 3^ and then purification using a PD-10 column. Human aortic endothelial cell (HAEC) binding of ^sup 68^Ga-NOTA-VEGF^sub 121^ was measured as a function of time. MicroPET and biodistribution studies of U87MG tumor xenografted mice were performed at 1, 2, and 4 h after injection of ^sup 68^Ga-NOTA-VEGF^sub 121^. The tumor tissues were then sectioned and subjected to immunostaining. The decay-corrected radiochemical yield of ^sup 68^Ga-NOTA-VEGF^sub 121^ was 40±4.5 % and specific activity was 243.1±104.6 GBq/[mu]mol (8.6±3.7 GBq/mg). ^sup 68^Ga-NOTA-VEGF^sub 121^ was avidly taken up by HAECs in a time-dependent manner, and the uptake was blocked either by 32 % with VEGF^sub 121^ or by 49 % with VEGFR2 antibody at 4 h post-incubation. In microPET images of U87MG tumor xenografted mice, radioactivity was accumulated in tumors (2.73±0.32 %ID/g at 2 h), and the uptake was blocked by 40 % in the presence of VEGF^sub 121^. In biodistribution studies, tumor uptake (1.84±0.14 %ID/g at 2 h) was blocked with VEGF^sub 121^ at a similar level (52 %) to that of microPET images. Immunostaining analysis of U87MG tumor tissues obtained after the microPET imaging showed high levels of VEGFR2 expression. These results demonstrate that ^sup 68^Ga-NOTA-VEGF^sub 121^ has potential for the in vivo imaging of VEGFR expression. In addition, our results also suggest that the in vivo characteristics of radiolabeled VEGF depend on the properties of the radioisotope and the chelator used.[PUBLICATION ABSTRACT]