@inproceedings{oai:repo.qst.go.jp:00086454,
 author = {Ohkubo, Takayuki and Masafumi, Shimojo and Maiko, Ono and Masayuki, Fujinaga and Masanao, Ogawa and Nobuki, Nengaki and Yuji, Nagai and Takafumi, Minamimoto and Makoto, Higuchi and Zhang, Ming-Rong and Ohkubo, Takayuki and Masafumi, Shimojo and Maiko, Ono and Masayuki, Fujinaga and Masanao, Ogawa and Nobuki, Nengaki and Yuji, Nagai and Takafumi, Minamimoto and Makoto, Higuchi and Zhang, Ming-Rong},
 book = {Nuclear Medicine and Biology},
 month = {Jun},
 note = {Objectives: A gene reporter imaging is a basic technology for real-time tracking of protein dynamics in the mammalian brain. PET is a powerful modality to monitor a biosynthesized molecule in living animals and humans. Recently, we have demonstrated that E.coli dihydrofolate reductase (ecDHFR) and its small antagonist trimethoprim (TMP) serve as a solid technical platform for in vivo brain reporter imaging in living animals. We used PET with [11C]TMP or [18F]fluoroethoxy-TMP to successfully visualize ecDHFR distribution in mouse and marmoset brains [1]. This study aims to synthesize and evaluate [18F]fluoro(hydroxy)propyloxy-TMP ([18F]FHP-TMP) as a novel PET gene reporter tracer for imaging of ecDHFR in the mammalian brain. 
Methods: The radiolabeling agent [18F]epifluorohydrin ([18F]EFH) was synthesized by the reaction of 1,2-epoxypropyl tosylate with [18F]F− and was purified by distillation, using an automated synthesis system developed in house [2]. Subsequently, [18F]EFH was reacted with desmethyl trimethoprim in the presence of NaOH, and the reaction mixture was purified by preparative HPLC and formulated to obtain the [18F]FHP-TMP injection. The dissociation constant (Kd) of [18F]FHP-TMP against ecDHFR was assessed by in vitro binding assay using HEK cell lysate transduced ecDHFR overexpression. Molecular dynamics (MD) simulation of FHP-TMP was also performed to determine its binding with ecDHFR. Finally, an adeno-associated virus encoding ecDHFR-EGFP was delivered to one side of the neocortex and striatum of a 3.4-year-old male marmoset to express the reporter in the telencephalon. PET scan of this marmoset was performed 45 days after virus injection. After intravenous bolus injection of [18F]FHP-TMP (113 MBq), dynamic emission scanning with 3D acquisition mode was conducted for 120 min. 
Results: At the end of synthesis, [18F]FHP-TMP was obtained with 13% radiochemical yield (no-decay corrected, based on the cyclotron-produced [18F]F-), >99% radiochemical purity and > 370 GBq/μmol of molar activity. The fully-automated synthesis time was 90 min from the end of irradiation. This radioactive product showed >95% radiochemical purity within 180 min after the formulation. MD simulation predicted that FHP-TMP sinks tightly the pocket of ecDHFR. [18F]FHP-TMP showed a moderate dissociation constant (Kd = 50.8 nM) against recombinant ecDHFR. PET scans revealed radioactivity accumulation not only in brain regions proximal to the injection site (i.e., neocortex, caudate nucleus, and putamen) but also in other spatially segregated but interconnected areas, including the thalamus and midbrain substantia nigra. Although the brain uptake of [18F]FHP-TMP was lower than that of [18F]fluoroethoxy-TMP, the radioactive signals for [18F]FHP-TMP in the cortex and putamen increased up to 2-3 times higher than that in the hippocampus at the end of the dynamic PET scan. Moreover, the distribution of the radioactive signal was consistent with the localization of transgene expression, as assessed by postmortem microscopic imaging of brain sections. 
Conclusions: We have developed a [18F]FHP-TMP as a PET gene reporter tracer for imaging of ecDHFR in the mammalian brain. Optimization for the chemical structure of [18F]FHP-TMP is in progress.},
 publisher = {Elsevier Ltd.},
 title = {Development of [18F]FHP-TMP as a PET gene reporter tracer for imaging of E.coli dihydrofolate reductase in the mammalian brain},
 year = {2022}
}