Evaluation of D-isomers of O-18F-fluoromethyl, O-18F-fluoroethyl and O-18F-fluoropropyl tyrosine as tumour imaging agents in mice.
The aim of this study was to evaluate the properties of the D-amino acid isomers O-(18)F-fluoromethyl tyrosine ((18)F-FMT), O-(18)F-fluoroethyl tyrosine ((18)F-FET) and O-(18)F-fluoropropyl tyrosine ((18)F-FPT) as tumour-detecting agents with PET in comparison with the corresponding L-isomers. L- or D-(18)F-FMT, (18)F-FET or (18)F-FPT, prepared by (18)F-fluoromethylation, (18)F-fluoroethylation or (18)F-fluoropropylation of L- and D-tyrosine, was intravenously injected into BALB/cA Jcl-nu mice bearing HeLa tumour cells. At 5, 15, 30 and 60 min post intravenous administration, the uptake of each compound in normal abdominal organs and xenotransplanted HeLa cells was determined using the tissue dissection method. Metabolic stability analyses of these compounds in the plasma were performed with the thin-layer chromatography method. In the plasma fraction, although L- and D-isomers of (18)F-FMT, (18)F-FET and (18)F-FPT provided comparable metabolic stability, D-isomers of these labelled compounds revealed a faster elimination rate than their L-isomers, with a higher peak uptake in the blood and kidney 5 min post administration. Compared with natural amino acid ligands, such as L-(11)C-methionine, the uptake of L-isomers of these labelled compounds was relatively low and stable in the abdominal organs, while D-isomers revealed much lower and faster clearance rates compared with the corresponding L-isomers. Among the abdominal organs, the pancreas showed relatively high uptake of all the labelled compounds used here, and the uptake of D-isomers was much lower than that of the L-isomers. Although tumour uptake levels of D-isomers of (18)F-FMT, (18)F-FET and (18)F-FPT were almost 95%, 43% and 39% of the uptake levels of each of the L-isomers 60 min post administration, the tumour-to-blood ratios of these D-isomers were 181%, 137% and 101% of the ratios of the corresponding L-isomers. D-isomers of (18)F-FMT and (18)F-FET indicated improved tumour-to-liver ratios compared with the corresponding L-isomers, and D-(18)F-FPT showed the highest tumour-to-pancreas ratio among all the other compounds assayed here. These results suggest that D-isomers of (18)F-fluoroalkyl tyrosine analogues are potential tracers for tumour imaging with PET.