Evaluation of O-[18F]fluoromethyl-d-tyrosine as a radiotracer for tumor imaging with positron emission tomography

O-[18F]Fluoromethyl-d-tyrosine (d-[18F]FMT) has been reported as a potential tumor-detecting agent for positron emission tomography (PET). However, the reason why d-[18F]FMT is better than l-[18F]FMT is unclear. To clarify this point, we examined the mechanism of their transport and their suitability for tumor detection. The stereo-selective uptake and release of enantiomerically pure d- and l-[18F]FMT by rat C6 glioma cells and human cervix adenocarcinoma HeLa cells were examined. The results of a competitive inhibition study using various amino acids and a selective inhibitor for transport system L suggested that d-[18F]FMT, as well as l-[18F]FMT, was transported via system L, the large neutral amino acid transporter, possibly via LAT1. The in vivo distribution of both [18F]FMT and [18F]FDG in tumor-bearing mice and rats was imaged with a high-resolution small-animal PET system. In vivo PET imaging of d-[18F]FMT in mouse xenograft and rat allograft tumor models showed high contrast with a low background, especially in the abdominal and brain region. The results of our in vitro and in vivo studies indicate that l-[18F]FMT and d-[18F]FMT are specifically taken up by tumor cells via system L. d-[18F]FMT, however, provides a better tumor-to-background contrast with a tumor/background (contralateral region) ratio of 2.741 vs. 1.878 with the l-isomer, whose difference appears to be caused by a difference in the influence of extracellular amino acids on the uptake and excretion of these two isomers in various organs. Therefore, d-[18F]FMT would be a more powerful tool as a tumor-detecting agent for PET, especially for the imaging of a brain cancer and an abdominal cancer.