Development of 2-[18F]trifluoromethyl-L-tryptophan (2-[18F]CF3-L-Trp) for serotonergic system imaging

Objectives Serotonergic system is related with various brain dysfunctions such as depression, anxiety or epilepsy. Thus, non-invasive imaging of serotonin synthesis is very important for the study of those brain diseases. However, only a few C-11 labeled radiopharmaceuticals are available, which have limitations for clinical applications. In the present study, we synthesized 2-[18F]CF3-L-Trp using copper-catalyzed [18F]trifluoromethylation and evaluated its brain distribution by autoradiography.

Methods Precursor was prepared by 7 step synthesis including regio-selective iodination. [18F]Trifluromethyl group was introduced by copper-catalyzed coupling using methyl chlorodifluoroacetate and tetramethylenediamine at 150℃ for 15 min. Protection groups were removed by 1 N HCl at 100℃ for 10 min and purified by high-performance liquid chromatography (HPLC). Radiochemical and chiral purities were also confirmed by HPLC. Stability studies were performed in prepared medium at room temperature and in human and mouse sera at 37℃. Protein binding assay was performed in human or mouse serum at 37℃ for 10 and 60 min. For the autoradiography, 2-[18F]CF3-L-Trp was injected into normal or LiCl-treated rats. The rats were killed after 10 min and the brains were removed and kept in the container with dry ice. The brains were cut into 30 μm sections using a cryostat and radioactivity was measured by Bio-imaging Analyzer System (BAS-2500).

Results Total radiosynthesis time was about 70 min and radiochemical yield was 20~30% based on HPLC. Retention time of 2-[18F]CF3-L-Trp was consistent with 2-CF3-L-Trp standard. Radiochemical purity was over 99% and specific activity was 550-900 MBq/μmol. 2-[18F]CF3-L-Trp was stable up to 6 h in mouse and human sera at 37℃ and protein-binding was 0.26±0.03 in human serum and 0.34±0.02 in mouse serum at 60 min. In autoradiography, 2-[18F]CF3-L-trp showed the highest uptake in pineal gland (5.41% ID/g) and showed high uptake in raphe nucleus (2.36% ID/g) , mammillary body (2.05% ID/g) and hypothalamus (2.36% ID/g). Compared to the normal rat brain, overall uptake of 2-[18F]CF3-L-Trp in LiCl-treated rat brain was higher except in pineal gland.

Conclusions [18F]CF3-L-Trp was successfully synthesized by transition metal catalyzed coupling. Its distribution pattern in the brain demonstrated serotonin neuron distribution. We expect that 2-[18F]CF3-L-Trp could be used as an imaging agent representing serotonin synthesis and metabolism non-invasively

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