Radiosynthesis and quality control testing of the tau imaging positron emission tomography tracer [18 F]PM-PBB3 for clinical applications.

Recently, we produced 11 C-labeled 2-((1E,3E)-4-(6-(methylamino)pyridin-3-yl)buta-1,3-dienyl)benzo[d]thiazol-6-ol ([11 C]PBB3) as a clinically useful positron emission tomography (PET) tracer for in vivo imaging of tau pathologies in the human brain. To overcome the limitations (i.e., rapid in vivo metabolism and short half-life) of [11 C]PBB3, we further synthesized 18 F-labeled 1-fluoro-3-((2-((1E,3E)-4-(6-(methylamino)pyridine-3-yl)buta-1,3-dien-1-yl)benzo[d]thiazol-6-yl)oxy)propan-2-ol ([18 F]PM-PBB3). [18 F]PM-PBB3 is also a useful tau PET tracer for imaging tau pathologies. In this study, we developed a routine radiosynthesis and quality control testing of [18 F]PM-PBB3 for clinical applications. [18 F]PM-PBB3 was synthesized by direct 18 F-fluorination of the tosylated derivative, followed by removal of the protecting group. [18 F]PM-PBB3 was obtained with sufficient radioactivity (25 ± 6.0% of the nondecay-corrected radiochemical yield at the end of synthesis, EOS), radiochemical purity (98 ± 0.6%), and molar activity (350 ± 94 GBq/µmol at EOS; n = 53). Moreover, [18 F]PM-PBB3 consistently retained >95% of radiochemical purity for 60 min without undergoing photoisomerization using a new UV-cutoff light (yellow light) fixed in the hot cell to monitor the synthesis. All the results of the quality control testing for the [18 F]PM-PBB3 injection complied with our in-house quality control and quality assurance specifications. We have accomplished >200 production runs of [18 F]PM-PBB3 in our facility for various research purposes.

Efficient radiosynthesis and non-clinical safety tests of the TSPO radioprobe [(18)F]FEDAC: Prerequisites for clinical application.

INTRODUCTION: [(18)F]FEDAC ([(18)F]1) has potent binding affinity and selectivity for translocator protein (18kDa, TSPO), and has been used to noninvasively visualize neuroinflammation, lung inflammation, acute liver damage, nonalcoholic fatty liver disease, and liver fibrosis. We had previously synthesized [(18)F]1 in two steps: (i) preparation of [(18)F]fluoroethyl bromide and (ii) coupling of [(18)F]fluoroethyl bromide with the appropriate precursor (2) for labeling. In this study, to clinically utilize [(18)F]1 as a PET radiopharmaceutical and to transfer the production technique of [(18)F]1 to other PET centers, we simplified its preparation by using a direct, one-step, tosyloxy-for-fluorine substitution. We also performed an acute toxicity study as a major non-clinical safety test, and determined radiometabolites using human liver microsomes. METHODS: [(18)F]1 was prepared via direct (18)F-fluorination by heating the corresponding tosylated derivative (3) with [(18)F]fluoride as its Kryptofix 222 complex in dimethyl sulfoxide at 110°C for 15min, following by HPLC purification. Non-clinical safety tests were performed for the extended single-dose toxicity study in rats, and for the in vitro metabolite analysis with human liver microsomal incubation. RESULTS: High quality batches of [(18)F]1, compatible with clinical applications, were obtained. At the end of irradiation, the decay-corrected radiochemical yield of [(18)F]1 using 1 and 5mg of precursor based on [(18)F]fluoride was 18.5±7.9% (n=10) and 52.0±5.8% (n=3), respectively. A single-dose of [(18)F]1 did not show toxicological effects for 14 days after the injection in male and female rats. In human liver microsomal incubations, [(18)F]1 was easily metabolized to [(18)F]desbenzyl-FEDAC ([(18)F]10) by CYPs (4.2% of parent compound left 60min after incubation). CONCLUSION: We successfully synthesized clinical grade batches of [(18)F]1 and verified the absence of innocuity of this radiotracer. [(18)F]1 will be used to first-in-human studies in our facility.

Identification of a major radiometabolite of [11C]PBB3.

INTRODUCTION: [(11)C]PBB3 is a clinically used positron emission tomography (PET) probe for in vivo imaging of tau pathology in the brain. Our previous study showed that [(11)C]PBB3 was rapidly decomposed to a polar radiometabolite in the plasma of mice. For the pharmacokinetic evaluation of [(11)C]PBB3 it is important to elucidate the characteristics of radiometabolites. In this study, we identified the chemical structure of a major radiometabolite of [(11)C]PBB3 and proposed the metabolic pathway of [(11)C]PBB3. METHODS: Carrier-added [(11)C]PBB3 was injected into a mouse for in vivo metabolite analysis. The chemical structure of a major radiometabolite was identified using LC-MS. Mouse and human liver microsomes and liver S9 samples were incubated with [(11)C]PBB3 in vitro. In silico prediction software was used to assist in the determination of the metabolite and metabolic pathway of [(11)C]PBB3. RESULTS: In vivo analysis showed that the molecular weight of a major radiometabolite of [(11)C]PBB3, which was called as [(11)C]M2, was m/z 390 [M+H(+)]. In vitro analysis assisted by in silico prediction showed that [(11)C]M2, which was not generated by cytochrome P450 enzymes (CYPs), was generated by sulfated conjugation mediated by a sulfotransferase. CONCLUSION: The major radiometabolite, [(11)C]M2, was identified as a sulfated conjugate of [(11)C]PBB3. [(11)C]PBB3 was metabolized mainly by a sulfotransferase and subsidiarily by CYPs.

Radiosynthesis, photoisomerization, biodistribution, and metabolite analysis of 11C-PBB3 as a clinically useful PET probe for imaging of tau pathology.

UNLABELLED: 2-((1E,3E)-4-(6-((11)C-methylamino)pyridin-3-yl)buta-1,3-dienyl)benzo[d]thiazol-6-ol ((11)C-PBB3) is a clinically useful PET probe that we developed for in vivo imaging of tau pathology in the human brain. To ensure the availability of this probe among multiple PET facilities, in the present study we established protocols for the radiosynthesis and quality control of (11)C-PBB3 and for the characterization of its photoisomerization, biodistribution, and metabolism. METHODS: (11)C-PBB3 was synthesized by reaction of the tert-butyldimethylsilyl desmethyl precursor ( 1: ) with (11)C-methyl iodide using potassium hydroxide as a base, followed by deprotection. Photoisomerization of (11)C-PBB3 under fluorescent light was determined. The biodistribution and metabolite analysis of (11)C-PBB3 was determined in mice using the dissection method. RESULTS: (11)C-PBB3 was synthesized with 15.4% ± 2.8% radiochemical yield (decay-corrected, n = 50) based on the cyclotron-produced (11)C-CO2 and showed an averaged synthesis time of 35 min from the end of bombardment. The radiochemical purity and specific activity of (11)C-PBB3 were 98.0% ± 2.3% and 180.2 ± 44.3 GBq/µmol, respectively, at the end of synthesis (n = 50). (11)C-PBB3 showed rapid photoisomerization, and its radiochemical purity decreased to approximately 50% at 10 min after exposure to fluorescent light. After the fluorescent light was switched off, (11)C-PBB3 retained more than 95% radiochemical purity over 60 min. A suitable brain uptake (1.92% injected dose/g tissue) of radioactivity was observed at 1 min after the probe injection, which was followed by rapid washout from the brain tissue. More than 70% of total radioactivity in the mouse brain homogenate at 5 min after injection represented the unchanged (11)C-PBB3, despite its rapid metabolism in the plasma. CONCLUSION: (11)C-PBB3 was produced with sufficient radioactivity and high quality, demonstrating its clinical utility. The present results of radiosynthesis, photoisomerization, biodistribution, and metabolite analysis could be helpful for the reliable production and application of (11)C-PBB3 in diverse PET facilities.

Simple and effective method for producing [11C]phosgene using an environmental CCl4 gas detection tube.

INTRODUCTION: Carbon-11-labeled phosgene is an important labeling precursor for PET molecular probes. Despite the usefulness of [(11)C]phosgene, some difficulties, especially in the formation of [(11)C]phosgene process from [(11)C]CCl(4), hamper its use. The present article shows a simple preparation method for [(11)C]phosgene. METHOD: [(11)C]CCl(4) was obtained using the conventional method by passing a mixture of [(11)C]CH(4) and Cl(2) through a heated quartz tube. The [(11)C]CCl(4) was transformed to [(11)C]phosgene simply by passing through a pretreatment tube of a Kitagawa gas detection system for the working-environmental CCl(4) concentration measurement at room temperature with a flow rate of 50 ml/min. RESULT: This tube successfully transformed [(11)C]CCl(4) to [(11)C]phosgene at room temperature. [(11)C]Phosgene was obtained at nearly 80% radiochemical yield (EOB) in a short synthesis time with high reproducibility. CONCLUSION: A high yield and reliable [(11)C]phosgene production method using a gas detector tube system for working-environmental CCl(4) concentration measurement was developed.