Positron Emission Tomography Imaging of Vascular Endothelial Growth Factor Receptor Expression with 61Cu-Labeled Lysine-Tagged VEGF121

Overexpression of vascular endothelial growth factor (VEGF) and VEGF receptors (VEGFRs) indicates poor prognosis for cancer patients in a variety of clinical studies. Our goal is to develop a tracer for positron emission tomography (PET) imaging of VEGFR expression using recombinant human VEGF(121) with three lysine residues fused to the N-terminus (denoted as K-3-VEGF(121)), which can facilitate radiolabeling without affecting its VEGFR binding affinity. K-3-VEGF(121) was conjugated with 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) and labeled with Cu-61 (t(1/2): 3.3 h; 62% beta(+)). The IC50 value of NOTA-K-3-VEGF(121) for VEGFR-2 was comparable to that of K-3-VEGF(121) (1.50 and 0.65 nM, respectively) based on a cell binding assay. Cu-61 labeling was achieved with good yield (55 +/- 10%) and specific activity (4.2 GBq/mg). Serial PET imaging showed that the 4T1 tumor uptake of Cu-61-NOTA-K-3-VEGF(121) was 3.4 +/- 0.5, 4.9 +/- 1.0, 5.2 +/- 1.0, and 4.8 +/- 0.8%ID/g (n = 4) at 0.5, 2, 4, and 8 h postinjection, respectively, which was consistent with biodistribution data measured by gamma counting. Blocking experiments and ex vivo histology confirmed the VEGFR specificity of Cu-61-NOTA-K-3-VEGF(121). Extrapolated human dosimetry calculation showed that liver was the organ with the highest radiation dose. The use of Cu-61 as the radiolabel is desirable for small proteins such as K-3-VEGF(121), which has a much higher beta(+) branching ratio than the commonly used Cu-64 (62% vs 17%), thereby offering stronger signal intensity and lower tracer dose for PET imaging.