量研学術機関リポジトリ「QST-Repository」は、国立研究開発法人 量子科学技術研究開発機構に所属する職員等が生み出した学術成果（学会誌発表論文、学会発表、研究開発報告書、特許等）を集積しインターネット上で広く公開するサービスです。 Welcome to QST-Repository where we accumulates and discloses the academic research results(Journal Publications, Conference presentation, Research and Development Report, Patent, etc.) of the members of National Institutes for Quantum Science and Technology.
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Objectives: The orexin network mainly relies on two G protein-coupled receptors, orexin 1 receptor (OX1R) and orexin 2 receptor (OX2R), and two neuropeptides, orexin-A (a 33 amino acid peptide) and orexin-B (a 28 amino acid peptide). Orexin-A is a non-selective neuropeptide binding both OX1R and OX2R with similar affinities, while orexin-B has a much higher selectivity towards OX2R against OX1R (> 10 fold). It has been found that OX2R plays a key role in motivation, arousal and sleep-wake regulation. Several OX2R ligands have been developed for OX2R PET imaging, but highly specific PET probes are still lacking. Herein we report [11C]1 and [11C]2 as novel and potential PET ligands for orexin 2 receptor imaging.
Methods: Compounds 1 and 2 were synthesized according to patent literature WO2004033418. The phenolic precursors were prepared from demethylation of compounds 1 and 2. The radiosynthesis of [11C]1 and [11C]2 were conducted by 11C-methylation of phenolic precursors precursors with [11C]CH3I and Cs2CO3 as the base in DMF at 80 oC for 5 min. The in vitro autoradiography studies on Sprague Dawley rat brain slices was performed.
Results: Compounds 1 was prepared starting from 2-methoxyaniline, which was treated with 3-methylpyridine-2-sulfonyl chloride to obtain N-(2-methoxyphenyl)-3-methylpyridine-2-sulfonamide in 21% yield. The resulting compound was coupled with methyl 2-bromoacetate in 45% yield and then hydrolyzed with NaOH to form carboxylic acid, which was coupled to N-benzylethanamine in the presence of HATU providing compound 1 in 2% overall yield over four steps. Compounds 2 was synthesized in a similar pathway and achieved in 20% overall yield over four steps. The phenolic precursors were prepared from demethylation of compounds 1 and 2 in 41% and 45% yields, respectively. The PET ligands [11C]1 and [11C]2 were prepared via reactions with [11C]CH3I and both achieved in more than 10% radiochemical yields (decay corrected). The radiochemical purities of these tracers were greater than 99%, and both [11C]1 and [11C]2 were stable in saline in 90 min. In in vitro autoradiography studies with [11C]1 and [11C]2, high radioactivity accumulation was observed in the hippocampus, and cortex. In blocking studies, EMPA (10 μM) rendered diminished radioactivity in OX2R-rich brain regions.
Conclusion: We have developed two novel PET ligand [11C]1 and [11C]2 for imaging OX2R. The PET ligands [11C]1 and [11C]2 were both prepared in more than 10% radiochemical yield (decay corrected). High radioactivity accumulation was observed in the hippocampus, and cortex in in vitro autoradiography studies with [11C]1 and [11C]2. These tracers will be evaluated in the PET imaging study for OX2R in rodents and the results will be reported in due course.
Synthesis and Evaluation of Novel PET Ligands for Imaging Orexin2 receptor (OX2R)
