Radiosynthesis and quality control testing of two <sup>18</sup>F-labeled radiopharmaceuticals for α-synuclein imaging in clinical investigations.

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Background18F-Labeled (E)-1-fluoro-3-((2-(4-(5-(methylamino)pyrazin-2-yl)but-1-en-3-yn-1-yl)benzo[d]thiazol-6-yl)oxy)propan-2-ol ([18F]SPAL-T-06) and (E)-1-fluoro-3-((2-(4-(6-(methylamino)pyridine-3-yl)but-1-en-3-yn-1-yl)benzo[d]thiazol-6-yl)oxy)propan-2-ol ([18F]C05-05) are positron emission tomography (PET) imaging agents for α-synuclein pathologies. Recently, we demonstrated the utility of PET imaging with [18F]SPAL-T-06 for α-synuclein pathologies in patients with multiple system atrophy and with [18F]C05-05 for α-synuclein pathologies in Parkinson's disease and related disorders. In this study, to establish [18F]SPAL-T-06 and [18F]C05-05 as PET radiopharmaceuticals for clinical investigations, we developed routine radiosynthetic procedures using an 18F-labeling synthesizer and performed quality control (QC) testing of these two PET radiopharmaceuticals.Results[18F]SPAL-T-06 and [18F]C05-05 were synthesized by direct 18F-fluorination of their respective tetrahydropyranyl-protected tosylated precursors, followed by deprotection, using an 18F-labeling synthesizer. The non-decay-corrected radiochemical yields of [18F]SPAL-T-06 and [18F]C05-05 from 18F- at the end of synthesis (EOS) were 11 ± 4.6% (n = 19) and 20 ± 6.8% (n = 37), respectively. The radiochemical purities of both PET radiopharmaceuticals exceeded 99%. The molar activities of each PET radiopharmaceutical at EOS exceeded 68 GBq/µmol, and the total synthesis times were within 100 min after irradiation. In addition, [18F]SPAL-T-06 and [18F]C05-05 were efficiently separated from byproducts using a semi-preparative high-performance liquid chromatography system. All QC results for the two PET radiopharmaceuticals complied with in-house QC and quality assurance specifications. The stabilities of the two PET radiopharmaceuticals were evaluated under standard fluorescent light in a general laboratory environment and under UV-cutoff light in a natural light-shielded environment. After 10 min of exposure to the standard fluorescent light, the radiochemical purity of each tracer decreased to approximately 86%. In contrast, the radiochemical purity of each tracer exceeded 99% after 60 min under UV-cutoff light in a natural light-shielded environment.ConclusionsWe successfully prepared two PET radiopharmaceuticals, [18F]SPAL-T-06 and [18F]C05-05, for clinical investigation. The radiosynthesis and QC findings provide a practical basis for establishing standardized production procedures and facilitating technology transfer to other PET centers. In particularly, all post-deprotection radiosynthesis steps and QC testing for [18F]SPAL-T-06 and [18F]C05-05 should be performed under UV-cutoff light in a natural light-shielded environment to prevent photoisomerization.

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