@misc{oai:repo.qst.go.jp:00086447,
 author = {Rong, Jian and Tomoteru, Yamasaki and Zhao, Chunyu and Li, Yinlong and Shao, Yihan and Zhe Sun, Phillip and L. Collier, Thomas and Zhang, Ming-Rong and Liang, Steven and Tomoteru, Yamasaki and Zhang, Ming-Rong},
 month = {Jun},
 note = {Purpose/Background:
The orexin 2 receptor (OX2R) is a G-protein coupled receptor expressed in brain that binds orexin neuropeptides A and B. OX2R is involved in motivation, feeding behaviour, and sleep-wake regulation. Modulation of OX2R with agonists has been found to be a potential treatment towards sleep related disorders. Herein we report [11C]1 and [11C]2 as novel and potential PET ligands for imaging orexin 2 receptor and explore their applications in small animals.

Methods:
Compounds 1 and 2 were prepared according to a published patent.1 The phenolic precursors were synthesized via demethylation of compounds 1 and 2. The radiosynthesis of [11C]1 and [11C]2 were performed by 11C-methylation of phenolic precursors with [11C]CH3I and Cs2CO3 as the base in DMF at 80 oC for 5 min. The in vitro autoradiography was performed with [11C]1 and [11C]2 on Sprague Dawley rat brain slices. Dynamic PET imaging studies with [11C]1 and [11C]2  were carried out on Sprague Dawley rats with 60 min scans.

Results:
Compounds 1 (OX2: IC50 = 4 nM, OX1: IC50 > 4000 nM) and 2 (OX2: IC50 = 4 nM, OX1: IC50 > 5000 nM) are selective antagonists at the OX2.1 Compounds 1 and 2 were prepared in 2% and 20% overall yields over four steps, respectively. The phenolic precursors were synthesized via demethylation of compounds 1 and 2 in 41% and 45% yields, respectively. The radiosynthesis of PET ligands [11C]1 and [11C]2 were via 11C-methylation of phenolic precursors with [11C]CH3I and both [11C]1 and [11C]2 were obtained in more than 10% radiochemical yields (decay corrected). Both [11C]1 and [11C]2 had radiochemical purities greater than 99%, and no obvious decompositions of [11C]1 and [11C]2 were found 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 of autoradiography, EMPA (10 μM) rendered diminished radioactivity in OX2R-rich brain regions. In dynamic PET imaging studies with [11C]1, the standard uptake value (SUV) of [11C]1 in hippocampus and cerebellum reached the max value of 0.2 at 2 min and washed out rapidly, and finally reduced to 0.1 at 60 min. PET imaging with [11C]2 showed a similar result. In blocking studies of PET imaging, no significant difference in uptake in the hippocampus and cerebellum was observed for both [11C]1 and [11C]2. Pretreatment with elacridar (3 mg/kg) led to a significant increase of uptake in hippocampus and cerebellum for both [11C]1 and [11C]2.

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). In autoradiography studies, both [11C]1 and [11C]2 showed high binding specificities in hippocampus and cortex in vitro. While in vivo PET imaging with [11C]1 and [11C]2 demonstrated moderate binding affinities towards OX2R, their poor blood-brain barrier permeabilities and rapid clearance from the brain prevent further evaluation., SNMMI2022},
 title = {Development of Novel PET Ligands for Imaging Orexin 2 Receptor},
 year = {2022}
}