Radiolabeling of RGD peptide and preliminary biological evaluation in mice bearing U87MG tumors
2-[18F]Fluoroethyl azide ([18F]FEA) and terminal alkynyl modified propioloyl RGDfK were selected in this study. [18F]FEA was prepared by nucleophilic radiofluorination of 2-azidoethyl 4-toluenesulfonate with radiochemical yield of 71 ± 4% (n = 5, decay-corrected). We assessed the various conditions of the CuAAC reaction between [18F]FEA and propioloyl RGDfK, which included peptide concentration, reaction time, temperature and catalyst dosage. The 18F-labeled-RGD peptide ([18F]F-RGDfK) could be obtained in 60 min by a two-step radiochemical synthesis route, with total radiochemical yield of 60 ± 2% (n = 3, decay-corrected) through click chemistry. [18F]F-RGDfK showed high stability in phosphate buffered saline and new-born calf serum. Micro-PET imaging at 1 h post injection of [18F]F-RGDfK showed medium concentration of radioactivity in tumors while much decreased concentration in tumors in the blocking group. These results showed that [18F]F-RGDfK obtained by click chemistry maintained the affinity and specificity of the RGDfK peptide to integrin avb3. This study provided useful information for peptide radiofluorination by using click chemistry.