microPET of Tumor Integrin  v 3 Expression Using 18F-Labeled PEGylated Tetrameric RGD Peptide (18F-FPRGD4)

In vivo imaging of αvβ₃ expression has important diagnostic and therapeutic applications. Multimeric cyclic RGD peptides are capable of improving the integrin αvβ₃–binding affinity due to the polyvalency effect. Here we report an example of ¹⁸F-labeled tetrameric RGD peptide for PET of αvβ₃ expression in both xenograft and spontaneous tumor models. Methods: The tetrameric RGD peptide E{E[c(RGDyK)]₂}₂ was derived with amino-3,6,9-trioxaundecanoic acid (mini-PEG; PEG is poly(ethylene glycol)) linker through the glutamate α-amino group. NH₂-mini-PEG-E{E[c(RGDyK)]₂}₂ (PRGD4) was labeled with ¹⁸F via the N-succinimidyl-4-¹⁸F-fluorobenzoate (¹⁸F-SFB) prosthetic group. The receptor-binding characteristics of the tetrameric RGD peptide tracer ¹⁸F-FPRGD4 were evaluated in vitro by a cell-binding assay and in vivo by quantitative microPET imaging studies. Results: The decay-corrected radiochemical yield for ¹⁸F-FPRGD4 was about 15%, with a total reaction time of 180 min starting from ¹⁸F-F⁻. The PEGylation had minimal effect on integrin-binding affinity of the RGD peptide. ¹⁸F-FPRGD4 has significantly higher tumor uptake compared with monomeric and dimeric RGD peptide tracer analogs. The receptor specificity of ¹⁸F-FPRGD4 in vivo was confirmed by effective blocking of the uptake in both tumors and normal organs or tissues with excess c(RGDyK). Conclusion: The tetrameric RGD peptide tracer ¹⁸F-FPRGD4 possessing high integrin-binding affinity and favorable biokinetics is a promising tracer for PET of integrin αvβ₃ expression in cancer and other angiogenesis related diseases.