In vivo characterization of ⁶⁸Ga-NOTA-VEGF₁₂₁ for the imaging of VEGF receptor expression in U87MG tumor xenograft models

Purpose: Vascular endothelial growth factor receptors (VEGFRs) 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 ⁶⁸Ga-1,4,7-triazacyclononane-1,4,7-triacetic acid-benzyl (NOTA)-VEGF₁₂₁ as a positron emission tomography (PET) radioligand for the in vivo imaging of VEGFR expression. Methods: ⁶⁸Ga-NOTA-VEGF₁₂₁ was prepared by conjugation of VEGF ₁₂₁ and p-SCN-NOTA, followed by radiolabeling with ⁶⁸GaCl₃ and then purification using a PD-10 column. Human aortic endothelial cell (HAEC) binding of ⁶⁸Ga-NOTA-VEGF 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 ⁶⁸Ga-NOTA-VEGF₁₂₁. The tumor tissues were then sectioned and subjected to immunostaining. Results: The decay-corrected radiochemical yield of ⁶⁸Ga-NOTA-VEGF₁₂₁ was 40 ± 4.5 % and specific activity was 243.1 ± 104.6 GBq/μmol (8.6 ± 3.7 GBq/mg). ⁶⁸Ga-NOTA-VEGF₁₂₁ was avidly taken up by HAECs in a time-dependent manner, and the uptake was blocked either by 32 % with VEGF₁₂₁ 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 VEGF₁₂₁. In biodistribution studies, tumor uptake (1.84±0.14 %ID/g at 2 h) was blocked with VEGF₁₂₁ 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. Conclusion: These results demonstrate that ⁶⁸Ga-NOTA- VEGF₁₂₁ 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.