Automated Synthesis of18F-Fluoropropoxytryptophan for Amino Acid Transporter System Imaging
Objective. This study was to develop a cGMP grade of [18F]fluoropropoxytryptophan (18F-FTP) to assess tryptophan transporters using an automated synthesizer.Methods. Tosylpropoxytryptophan (Ts-TP) was reacted with K18F/kryptofix complex. After column purification, solvent evaporation, and hydrolysis, the identity and purity of the product were validated by radio-TLC (1M-ammonium acetate : methanol = 4 : 1) and HPLC (C-18 column, methanol : water = 7 : 3) analyses.In vitrocellular uptake of18F-FTP and18F-FDG was performed in human prostate cancer cells. PET imaging studies were performed with18F-FTP and18F-FDG in prostate and small cell lung tumor-bearing mice (3.7 MBq/mouse, iv).Results. Radio-TLC and HPLC analyses of18F-FTP showed that the Rf and Rt values were 0.9 and 9 min, respectively. Radiochemical purity was >99%. The radiochemical yield was 37.7% (EOS 90 min, decay corrected). Cellular uptake of18F-FTP and18F-FDG showed enhanced uptake as a function of incubation time. PET imaging studies showed that18F-FTP had less tumor uptake than18F-FDG in prostate cancer model. However,18F-FTP had more uptake than18F-FDG in small cell lung cancer model.Conclusion.18F-FTP could be synthesized with high radiochemical yield. Assessment of upregulated transporters activity by18F-FTP may provide potential applications in differential diagnosis and prediction of early treatment response.