MicroPET imaging of brain tumor angiogenesis with 18F-labeled PEGylated RGD peptide

We have previously labeled cyclic RGD peptide c(RGDyK) with fluorine-18 through conjugation labeling via a prosthetic 4-[F-18]fluorobenzoyl moiety and applied this [F-18]FB-RGD radiotracer for alpha(v)-integrin expression imaging in different preclinical tumor models with good tumor-to-background contrast. However, the unfavorable hepatobiliary excretion and rapid tumor washout rate of this tracer limit its potential clinical applications. The aims of this study were to modify the [F-18]FB-RGD tracer by inserting a heterobifunctional poly(ethylene glycol) (PEG, M.W. = 3,400) between the F-18 radiolabel and the RGD moiety and to test this [F-18]FB-PEG-RGD tracer for brain tumor targeting and in vivo kinetics. [F-18]FB-PEG-RGD was prepared by coupling the RGD-PEG conjugate with N-succinimidyl 4-[F-18]fluorobenzoate ([F-18]SFB) under slightly basic conditions (pH=8.5). The radiochemical yield was about 20-30% based on the active ester [F-18]SFB, and specific activity was over 100 GBq/mumol. This tracer had fast blood clearance, rapid and high tumor uptake in the subcutaneous U87MG glioblastoma model (5.2 +/- 0.5%ID/g at 30 min p.i.). Moderately rapid tumor washout was observed, with the activity accumulation decreased to 2.2 +/- 0.4%ID/g at 4 h p.i. MicroPET and autoradiography imaging showed a very high tumor-to-background ratio and limited activity accumulation in the liver, kidneys and intestinal tracts. U87MG tumor implanted into the mouse forebrain was well visualized with [F-18]FB-PEG-RGD. Although uptake in the orthotopic tumor was significantly lower (P<0.01) than in the subcutaneous tumor, the maximum tumor-to-brain ratio still reached 5.0 +/- 0.6 due to low normal brain background. The results of H&E staining postmortem agreed with the anatomical information obtained from non-invasive microPET imaging. In conclusion, PEGylation suitably modifies the physiological behavior of the RGD peptide. [F-18]FB-PEG-RGD gave improved tumor retention and in vivo kinetics compared with [F-18]FB-RGD.