The neuromodulatory role of dopamine in improved reaction time by acute cardiovascular exercise.

Acute cardiovascular physical exercise improves cognitive performance, as evidenced by a reduction in reaction time (RT). However, the mechanistic understanding of how this occurs is elusive and has not been rigorously investigated in humans. Here, using positron emission tomography (PET) with [11 C]raclopride, in a multi-experiment study we investigated whether acute exercise releases endogenous dopamine (DA) in the brain. We hypothesized that acute exercise augments the brain DA system, and that RT improvement is correlated with this endogenous DA release. The PET study (Experiment 1: n = 16) demonstrated that acute physical exercise released endogenous DA, and that endogenous DA release was correlated with improvements in RT of the Go/No-Go task. Thereafter, using two electrical muscle stimulation (EMS) studies (Experiments 2 and 3: n = 18 and 22 respectively), we investigated what triggers RT improvement. The EMS studies indicated that EMS with moderate arm cranking improved RT, but RT was not improved following EMS alone or EMS combined with no load arm cranking. The novel mechanistic findings from these experiments are: (1) endogenous DA appears to be an important neuromodulator for RT improvement and (2) RT is only altered when exercise is associated with central signals from higher brain centres. Our findings explain how humans rapidly alter their behaviour using neuromodulatory systems and have significant implications for promotion of cognitive health. KEY POINTS: Acute cardiovascular exercise improves cognitive performance, as evidenced by a reduction in reaction time (RT). However, the mechanistic understanding of how this occurs is elusive and has not been rigorously investigated in humans. Using the neurochemical specificity of [11 C]raclopride positron emission tomography, we demonstrated that acute supine cycling released endogenous dopamine (DA), and that this release was correlated with improved RT. Additional electrical muscle stimulation studies demonstrated that peripherally driven muscle contractions (i.e. exercise) were insufficient to improve RT. The current study suggests that endogenous DA is an important neuromodulator for RT improvement, and that RT is only altered when exercise is associated with central signals from higher brain centres.

[18F]-THK5351 PET Imaging in Patients With Semantic Variant Primary Progressive Aphasia.

BACKGROUND: Semantic variant primary progressive aphasia (svPPA) has been associated with a variety of proteinopathies, mainly transactive response DNA-binding protein, but also with tau and ß-amyloid. Recently selective tau tracers for positron emission tomography (PET) have been developed to determine the presence of cerebral tau deposits in vivo. Here, we investigated the topographical distribution of THK5351 in svPPA patients. MATERIALS AND METHODS: Five svPPA patients, 14 Alzheimer's disease patients, and 15 age-matched normal controls underwent [F]-THK5351 PET scans, magnetic resonance imaging, and detailed neuropsychological tests. [F]-fluorodeoxyglucose PET was obtained in 3 svPPA patients, whereas the remaining 2 underwent amyloid PET using [F]-flutemetamol. Tau distribution among the 3 groups was compared using regions of interest-based and voxel-based statistical analyses. RESULTS: In svPPA patients, [F]-THK5351 retention was elevated in the anteroinferior and lateral temporal cortices compared with the normal controls group (left>right), and in the left inferior and temporal polar region compared with Alzheimer's disease patients. [F]-THK5351 retention inversely correlated with glucose metabolism, whereas regional THK retention correlated with clinical severity. [F]-flutemetamol scans were negative for ß-amyloid. CONCLUSIONS: These findings show that [F]-THK5351 retention may be detected in cortical regions correlating with svPPA pathology.

Practical microscale one-pot radiosynthesis of 18 F-labeled probes.

High specific activity is often a significant requirement for radiopharmaceuticals. To achieve that with fluorine-18 (18 F)-labeled probes, it is mandatory to start from no-carrier-added fluoride and to reduce to a minimum the amount of precursor in order to decrease the presence of any pseudocarrier. In the present study, a feasible and efficient method for microscale one-pot radiosynthesis of 18 F-labeled probes is described. It allows a substantial reduction in precursor, solvent, and reagents, thus reducing also possible side reaction in the case of base-sensitive precursors. The method is based on the use of a small amount of Kryptofix 2.2.2/potassium [18 F]fluoride in MeOH (K.222/K[18 F]F-MeOH) obtained using Oasis MAX and MCX cartridges. Five methods, differing in terms of MeOH evaporation and precursor addition, for the radiosynthesis of [18 F]fallypride and [18 F]FET in ≤50-µL scale, were examined and evaluated. The method using the addition of DMSO to the K.222/K[18 F]F-MeOH solution prior to MeOH evaporation is proposed as a versatile procedure for feasible one-pot 10- to 20-µL scale radiosyntheses. This method was successfully applied also to the radiosynthesis of [18 F]FES, [18 F]FLT, and [18 F]FMISO, with radiochemical yields comparable with those reported in the literature. Purification of a crude product by an analytical HPLC column was also demonstrated.

A report of the automated radiosynthesis of the tau positron emission tomography radiopharmaceutical, [18 F]-THK-5351.

The radiotracer, [18 F]-THK-5351, is a highly selective and high-binding affinity PET imaging agent for aggregates of hyper-phosphorylated tau protein. Our report is a simplified 1-pot, 2-step radiosynthesis of [18 F]-THK-5351. This report is broadly applicable for routine clinical production and multi-center trials on account of favorable half-life of flourine-18 and the use of a commercially available radiosynthesis module, the GE TRACERlab™ FXFN . First, the O-THP protected tosyl precursor underwent nucleophilic fluorinating reaction with potassium cryptand fluoride ([18 F] fluoride (K[18 F]/K222 )) in Dimethyl sulfoxide at 110°C for 10 minutes followed by O-THP removal by using diluted hydrochloric acid (HCl) at same temperature. [18 F]-THK-5351 was purified via semi-preparative high-performance liquid chromatography and formulated by using 10% EtOH, United States Pharmacopeia (USP) in 0.9% sodium chloride for injection, USP and an uncorrected radiochemical yield of 21 ± 3.5%, with a specific activity of 153.11 ± 25.9 GBq/µmol (4138 ± 700 mCi/µmol) at the end of synthesis (63 minutes; n = 3).

Characterization of the radiolabeled metabolite of tau PET tracer 18F-THK5351.

PURPOSE: 18F-THK5351 is a novel radiotracer developed for in vivo imaging of tau pathology in the brain. For the quantitative assessment of tau deposits in the brain, it is important that the radioactive metabolite does not enter the brain and that it does not bind to tau fibrils. The purpose of the study was to identify a radiolabeled metabolite of 18F-THK5351 in blood samples from human subjects and to characterize its pharmacological properties. METHODS: Venous blood samples were collected from three human subjects after injection of 18F-THK5351 and the plasma metabolite was measured by high performance thin layer chromatography. In addition, mass spectrometry analysis and enzymatic assays were used to identify this metabolite. Mice were used to investigate the blood-brain barrier permeability of the radioactive metabolite. Furthermore, the binding ability of the metabolite to tau aggregates was evaluated using autoradiography and binding assays using human brain samples. RESULTS: About 13 % of the unmetabolized radiotracer was detectable in human plasma at 60 min following the injection of 18F-THK5351. The isolated radiometabolite of 18F-THK5351 was the sulphoconjugate of THK5351. This metabolite could be produced in vitro by incubating THK5351 with liver but not brain homogenates. The metabolite did not penetrate the blood-brain barrier in mice, and exhibited little binding to tau protein aggregates in post-mortem human brain samples. CONCLUSIONS: These results suggest that the sole metabolite detectable in plasma seems to be generated outside the brain and does not cross into the brain, which does not affect quantitative analysis of PET images.

Radiosynthesis and preliminary biological evaluation of a new (18)F-labeled triethylene glycol derivative of triphenylphosphonium.

Delocalized lipophilic cations such as [(18)F]fluorobenzyltriphenylphosphonium ([(18)F]FBnTP) can accumulate in mitochondria and have been used in myocardial perfusion imaging (MPI). In this study, we established a simplified method for [(18)F]FBnTP synthesis using triphenylphosphine hydrobromide (PPh3 •HBr) without preparing an intermediate that contains benzyl bromide structure. Applying this new method, we synthesized and evaluated a novel (18)F-labeled PEGylated BnTP derivative ([(18)F]FPEGBnTP). In vitro cellular uptake study demonstrated that [(18)F]FPEGBnTP accumulated in cells in proportion to the relative intensity of mitochondrial membrane potential. Biodistribution study revealed that the heart : liver uptake ratio of [(18)F]FPEGBnTP (4.00 at 60 min) was superior to that of [(18)F]FBnTP (1.50 at 60 min). However, [(18)F]FPEGBnTP showed slow blood clearance and high radioactivity uptake in bone at 120-min post-injection. These results imply the possibility of [(18)F]FPEGBnTP being used as a MPI agent. However, there is a need of further structural optimization and flow-dependent uptake study.

[Tau imaging].

Several promising tau imaging probes have been developed by Tohoku University ([18F]THK series), National Institute of Radiological Sciences([11C]PBB3), Eli Lilly ([18F]T807), and Hoffman-La Roche ([18F]R06958948). In this paper we discussed some of the criteria required for the tau imaging probe in Alzheimer's disease. It will be certain that tau imaging plays an important role in an accurate diagnosis and as companion diagnostics.

[(18)F]THK-5117 PET for assessing neurofibrillary pathology in Alzheimer's disease.

PURPOSE: Visualization of the spatial distribution of neurofibrillary tangles would help in the diagnosis, prevention and treatment of dementia. The purpose of the study was to evaluate the clinical utility of [(18)F]THK-5117 as a highly selective tau imaging radiotracer. METHODS: We initially evaluated in vitro binding of [(3)H]THK-5117 in post-mortem brain tissues from patients with Alzheimer's disease (AD). In clinical PET studies, [(18)F]THK-5117 retention in eight patients with AD was compared with that in six healthy elderly controls. Ten subjects underwent an additional [(11)C]PiB PET scan within 2 weeks. RESULTS: In post-mortem brain samples, THK-5117 bound selectively to neurofibrillary deposits, which differed from the binding target of PiB. In clinical PET studies, [(18)F]THK-5117 binding in the temporal lobe clearly distinguished patients with AD from healthy elderly subjects. Compared with [(11)C]PiB, [(18)F]THK-5117 retention was higher in the medial temporal cortex. CONCLUSION: These findings suggest that [(18)F]THK-5117 provides regional information on neurofibrillary pathology in living subjects.

Non-invasive assessment of Alzheimer's disease neurofibrillary pathology using 18F-THK5105 PET.

Non-invasive imaging of tau pathology in the living brain would be useful for accurately diagnosing Alzheimer's disease, tracking disease progression, and evaluating the treatment efficacy of disease-specific therapeutics. In this study, we evaluated the clinical usefulness of a novel tau-imaging positron emission tomography tracer 18F-THK5105 in 16 human subjects including eight patients with Alzheimer's disease (three male and five females, 66-82 years) and eight healthy elderly controls (three male and five females, 63-76 years). All participants underwent neuropsychological examination and 3D magnetic resonance imaging, as well as both 18F-THK5105 and 11C-Pittsburgh compound B positron emission tomography scans. Standard uptake value ratios at 90-100 min and 40-70 min post-injection were calculated for 18F-THK5105 and 11C-Pittsburgh compound B, respectively, using the cerebellar cortex as the reference region. As a result, significantly higher 18F-THK5105 retention was observed in the temporal, parietal, posterior cingulate, frontal and mesial temporal cortices of patients with Alzheimer's disease compared with healthy control subjects. In patients with Alzheimer's disease, the inferior temporal cortex, which is an area known to contain high densities of neurofibrillary tangles in the Alzheimer's disease brain, showed prominent 18F-THK5105 retention. Compared with high frequency (100%) of 18F-THK5105 retention in the temporal cortex of patients with Alzheimer's disease, frontal 18F-THK5105 retention was less frequent (37.5%) and was only observed in cases with moderate-to-severe Alzheimer's disease. In contrast, 11C-Pittsburgh compound B retention was highest in the posterior cingulate cortex, followed by the ventrolateral prefrontal, anterior cingulate, and superior temporal cortices, and did not correlate with 18F-THK5105 retention in the neocortex. In healthy control subjects, 18F-THK5105 retention was ∼10% higher in the mesial temporal cortex than in the neocortex. Notably, unlike 11C-Pittsburgh compound B, 18F-THK5105 retention was significantly correlated with cognitive parameters, hippocampal and whole brain grey matter volumes, which was consistent with findings from previous post-mortem studies showing significant correlations of neurofibrillary tangle density with dementia severity or neuronal loss. From these results, 18F-THK5105 positron emission tomography is considered to be useful for the non-invasive assessment of tau pathology in the living brain. This technique would be applicable to the longitudinal evaluation of tau deposition and allow a better understanding of the pathophysiology of Alzheimer's disease.

In vivo evaluation of a novel tau imaging tracer for Alzheimer's disease.

PURPOSE: Diagnosis of tauopathies such as Alzheimer's disease (AD) still relies on post-mortem examination of the human brain. A non-invasive method of determining brain tau burden in vivo would allow a better understanding of the pathophysiology of tauopathies. The purpose of the study was to evaluate (18)F-THK523 as a potential tau imaging tracer. METHODS: Ten healthy elderly controls, three semantic dementia (SD) and ten AD patients underwent neuropsychological examination, MRI as well as (18)F-THK523 and (11)C-Pittsburgh compound B (PIB) positron emission tomography (PET) scans. Composite memory and non-memory scores, global and hippocampal brain volume, and partial volume-corrected tissue ratios for (18)F-THK523 and (11)C-PIB were estimated for all participants. Correlational analyses were performed between global and regional (18)F-THK523, (11)C-PIB, cognition and brain volumetrics. RESULTS: (18)F-THK523 presented with fast reversible kinetics. Significantly higher (18)F-THK523 retention was observed in the temporal, parietal, orbitofrontal and hippocampi of AD patients when compared to healthy controls and SD patients. White matter retention was significantly higher than grey matter retention in all participants. The pattern of cortical (18)F-THK523 retention did not correlate with Aß distribution as assessed by (11)C-PIB and followed the known distribution of tau in the AD brain, being higher in temporal and parietal areas than in the frontal region. Unlike (11)C-PIB, hippocampal (18)F-THK523 retention was correlated with several cognitive parameters and with hippocampal atrophy. CONCLUSION: (18)F-THK523 does not bind to Aß in vivo, while following the known distribution of paired helical filaments (PHF)-tau in the brain. Significantly higher cortical (18)F-THK523 retention in AD patients as well as the association of hippocampal (18)F-THK523 retention with cognitive parameters and hippocampal volume suggests (18)F-THK523 selectively binds to tau in AD patients. Unfortunately, the very high (18)F-THK523 retention in white matter precludes simple visual inspection of the images, preventing its use in research or clinical settings.

Tau PET imaging in Alzheimer's disease.

In several neurodegenerative diseases that are collectively called tauopathies, progressive accumulation of tau in the brain is closely associated with neurodegeneration and cognitive impairment. Noninvasive detection of tau protein deposits in the brain would be useful to diagnose tauopathies as well as to track and predict disease progression. Recently, several tau PET tracers including T807, THK-5117, and PBB3 have been developed and succeeded in imaging neurofibrillary pathology in vivo. For use of tau PET as a biomarker of tau pathology in Alzheimer's disease, PET tracers should have high affinity to PHF-tau and high selectivity for tau over amyloid-ß and other protein deposits. PET tau imaging enables the longitudinal assessment of the spatial pattern of tau deposition and its relation to amyloid-ß pathology and neurodegeneration. This technology could also be applied to the pharmacological assessment of anti-tau therapy, thereby allowing preventive interventions.

Synthesis and preliminary evaluation of 2-arylhydroxyquinoline derivatives for tau imaging.

Alzheimer's disease (AD) is the most common cause of dementia. Senile plaques, consisting of ß-amyloid, and neurofibrillary tangles (NFTs), composed of tau protein, are representative pathological hallmarks of AD. It is believed that the accumulation of NFTs precedes the onset of clinical symptoms of AD and correlates with the progression of memory dysfunction. Thus, the use of noninvasive detection techniques including radiolabeled probes and positron emission tomography (PET) will facilitate early diagnosis or staging of AD. In this study, we synthesized and evaluated novel hydroxylated 2-arylquinoline derivatives as tau imaging PET probes. The binding affinities of compounds for tau were evaluated by fluorescent staining of the AD hippocampal section and a competitive binding assay using [(18) F]THK-523. THK-951 showed high binding affinity for tau pathology in an AD brain section and K18Δ280K fibrils (Ki = 20.7 nM); thus, we radiosynthesized a (11) C-labeled THK-951 and further studied its potential as a tau PET probe. The [(11) C]THK-951 demonstrated excellent kinetics in a normal mouse brain (3.23% ID/g at 2 min postinjection and 0.15% ID/g at 30 min postinjection) and showed the labeling of NFTs in an AD brain section by autoradiography assay. These findings indicate the availability of [(11) C]THK-951 for in vivo PET imaging of tau pathology in AD.

Amyloid accumulation in cases of suspected comorbid cerebral amyloid angiopathy and isolated cortical venous thrombosis.

BACKGROUND AND AIM: The differentiation of isolated cortical venous thrombosis (ICVT) from cerebral amyloid angiopathy (CAA) can be difficult because both diseases share similar neurological symptoms and imaging findings. N-methyl-11C-2-(4'-methylaminophenyl)-6-hydroxybenzo-thiazole (11C-PiB) positron emission tomography (PET) functions as a diagnostic modality for CAA by detecting amyloid deposition. The present prospective study evaluated amyloid deposition using 11C-PiB-PET in consecutive patients with suspected ICVT. METHOD: This study was a prospective observational study. Patients who attended or were hospitalized between May 2019 and March 2020 were included in the analysis. Consecutive patients who met the criteria for suspicion of ICVT were enrolled in the study, and the clinical course, symptoms, imaging findings (including magnetic resonance imaging), and the 11C-PiB-PET findings of each case were analyzed. RESULTS: The study cohort included four patients (64-82 years of age, all women). In one younger patient, 11C-PiB-PET afforded no findings suggestive of CAA, whereas the remaining three patients exhibited 11C-PiB-PET findings suggestive of CAA. CONCLUSION: Although 11C-PiB-PET would be a reasonable modality for distinguishing ICVT from CAA, especially in younger patients, it might be difficult to differentiate ICVT from CAA in elderly patients because of the potential deposition of amyloid. CLINICAL TRIAL REGISTRATION: URL: https://www.umin.ac.jp/ctr/ Unique identifier: UMIN 000037101.

Comparison of the binding characteristics of [18F]THK-523 and other amyloid imaging tracers to Alzheimer's disease pathology.

PURPOSE: Extensive deposition of senile plaques and neurofibrillary tangles in the brain is a pathological hallmark of Alzheimer's disease (AD). Although several PET imaging agents have been developed for in vivo detection of senile plaques, no PET probe is currently available for selective detection of neurofibrillary tangles in the living human brain. Recently, [(18)F]THK-523 was developed as a potential in vivo imaging probe for tau pathology. The purpose of this study was to compare the binding properties of [(18)F]THK-523 and other amyloid imaging agents, including PiB, BF-227 and FDDNP, to synthetic protein fibrils and human brain tissue. METHODS: In vitro radioligand binding assays were conducted using synthetic amyloid ß(42) and K18ΔK280-tau fibrils. Nonspecific binding was determined by the addition of unlabelled compounds at a concentration of 2 µM. To examine radioligand binding to neuropathological lesions, in vitro autoradiography was conducted using sections of AD brain. RESULTS: [(18)F]THK-523 showed higher affinity for tau fibrils than for Aß fibrils, whereas the other probes showed a higher affinity for Aß fibrils. The autoradiographic analysis indicated that [(18)F]THK-523 accumulated in the regions containing a high density of tau protein deposits. Conversely, PiB and BF-227 accumulated in the regions containing a high density of Aß plaques. CONCLUSION: These findings suggest that the unique binding profile of [(18)F]THK-523 can be used to identify tau deposits in AD brain.

A Novel Near-Infrared Fluorescence Probe THK-565 Enables In Vivo Detection of Amyloid Deposits in Alzheimer's Disease Mouse Model.

PURPOSE: Noninvasive imaging of protein aggregates in the brain is critical for the early diagnosis, disease monitoring, and evaluation of the effectiveness of novel therapies for Alzheimer's disease (AD). Near-infrared fluorescence (NIRF) imaging with specific probes is a promising technique for the in vivo detection of protein deposits without radiation exposure. Comprehensive screening of fluorescent compounds identified a novel compound, THK-565, for the in vivo imaging of amyloid-ß (Aß) deposits in the mouse brain. This study assessed whether THK-565 could detect amyloid-ß deposits in vivo in the AD mouse model. PROCEDURES: The fluorescent properties of THK-565 were evaluated in the presence and absence of Aß fibrils. APP knock-in (APP-KI) mice were used as an animal model of AD. In vivo NIRF images were acquired after the intravenous administration of THK-565 and THK-265 in mice. The binding selectivity of THK-565 to Aß was evaluated using brain slices obtained from these mouse models. RESULTS: The fluorescence intensity of the THK-565 solution substantially increased by mixing with Aß fibrils. The maximum emission wavelength of the complex of THK-565 and Aß fibrils was 704 nm, which was within the optical window range. THK-565 selectively bound to amyloid deposits in brain sections of APP-KI mice After the intravenous administration of THK-565, the fluorescence signal in the head of APP-KI mice was significantly higher than that of wild-type mice and higher than that after administration of THK-265. Ex vivo analysis confirmed that the THK-565 signal corresponded to Aß immunostaining in the brain sections of these mice. CONCLUSIONS: A novel NIRF probe, THK-565, enabled the in vivo detection of Aß deposits in the brains of the AD mouse model, suggesting that NIRF imaging with THK-565 could non-invasively assess disease-specific pathology in AD.

Evaluation of 18F labeled glial fibrillary acidic protein binding nanobody and its brain shuttle peptide fusion proteins using a neuroinflammation rat model.

Astrogliosis is a crucial feature of neuroinflammation and is characterized by the significant upregulation of glial fibrillary acidic protein (GFAP) expression. Hence, visualizing GFAP in the living brain of patients with damaged central nervous system using positron emission tomography (PET) is of great importance, and it is expected to depict neuroinflammation more directly than existing neuroinflammation imaging markers. However, no PET radiotracers for GFAP are currently available. Therefore, neuroimaging with antibody-like affinity proteins could be a viable strategy for visualizing imaging targets that small molecules rarely recognize, such as GFAP, while we need to overcome the challenges of slow clearance and low brain permeability. The E9 nanobody, a small-affinity protein with high affinity and selectivity for GFAP, was utilized in this study. E9 was engineered by fusing a brain shuttle peptide that facilitates blood-brain barrier permeation via two different types of linker domains: E9-GS-ApoE (EGA) and E9-EAK-ApoE (EEA). E9, EGA and EEA were radiolabeled with fluorine-18 using cell-free protein radiosynthesis. In vitro autoradiography showed that all radiolabeled proteins exhibited a significant difference in neuroinflammation in the brain sections created from a rat model constructed by injecting lipopolysaccharide (LPS) into the unilateral striatum of wildtype rats, and an excess competitor displaced their binding. However, exploratory in vivo PET imaging and ex vivo biodistribution studies in the rat model failed to distinguish neuroinflammatory lesions within 3 h of 18F-EEA intravenous injection. This study contributes to a better understanding of the characteristics of small-affinity proteins fused with a brain shuttle peptide for further research into the use of protein molecules as PET tracers for imaging neuropathology.

In vivo [18F]THK-5351 imaging detected reactive astrogliosis in argyrophilic grain disease with comorbid pathology: A clinicopathological study.

Quantification of in vivo reactive astrogliosis, which represents neural inflammation and remodeling in the brain, is an emerging methodology for the evaluation of patients with neurodegenerative diseases. [18F]THK-5351 is a positron emission tomography (PET) tracer for monoamine oxidase B (MAO-B), a molecular marker of reactive astrogliosis. We performed in vivo [18F]THK-5351 PET in a patient who at autopsy was found to have argyrophilic grain disease (AGD) with comorbid pathology to visualize reactive astrogliosis for the first time. We aimed to validate an imaging-pathology correlation using [18F]THK-5351 PET and the autopsy brain. The patient, a 78-year-old man, was pathologically diagnosed with AGD combined with limbic-predominant age-related transactive response DNA-binding protein of 43 kDa encephalopathy and Lewy body disease without Alzheimer disease-related neuropathological changes. Reactive astrogliosis in the postmortem brain was abundant in the inferior temporal gyrus, insular gyrus, entorhinal cortex, and ambient gyrus where premortem [18F]THK-5351 signals were high. We found a proportional correlation between the amount of reactive astrogliosis in the postmortem brain and the in vivo [18F]THK-5351 standardized uptake value ratio (r = 0.8535, p = 0.0004). These results indicated that reactive astrogliosis in AGD with comorbid pathology could be identified and quantified by in vivo MAO-B imaging.

Preclinical Characterization of the Tau PET Tracer [18F]SNFT-1: Comparison of Tau PET Tracers.

Tau PET tracers are expected to be sufficiently sensitive to track the progression of age-related tau pathology in the medial temporal cortex. The tau PET tracer N-(4-[18F]fluoro-5-methylpyridin-2-yl)-7-aminoimidazo[1,2-a]pyridine ([18F]SNFT-1) has been successfully developed by optimizing imidazo[1,2-a]pyridine derivatives. We characterized the binding properties of [18F]SNFT-1 using a head-to-head comparison with other reported 18F-labeled tau tracers. Methods: The binding affinity of SNFT-1 to tau, amyloid, and monoamine oxidase A and B was compared with that of the second-generation tau tracers MK-6240, PM-PBB3, PI-2620, RO6958948, JNJ-64326067, and flortaucipir. In vitro binding properties of 18F-labeled tau tracers were evaluated through the autoradiography of frozen human brain tissues from patients with diverse neurodegenerative disease spectra. Pharmacokinetics, metabolism, and radiation dosimetry were assessed in normal mice after intravenous administration of [18F]SNFT-1. Results: In vitro binding assays demonstrated that [18F]SNFT-1 possesses high selectivity and high affinity for tau aggregates in Alzheimer disease (AD) brains. Autoradiographic analysis of tau deposits in medial temporal brain sections from patients with AD showed a higher signal-to-background ratio for [18F]SNFT-1 than for the other tau PET tracers and no significant binding with non-AD tau, α-synuclein, transactiviation response DNA-binding protein-43, and transmembrane protein 106B aggregates in human brain sections. Furthermore, [18F]SNFT-1 did not bind significantly to various receptors, ion channels, or transporters. [18F]SNFT-1 showed a high initial brain uptake and rapid washout from the brains of normal mice without radiolabeled metabolites. Conclusion: These preclinical data suggest that [18F]SNFT-1 is a promising and selective tau radiotracer candidate that allows the quantitative monitoring of age-related accumulation of tau aggregates in the human brain.

Rapid biochemical synthesis of (11)C-labeled single chain variable fragment antibody for immuno-PET by cell-free protein synthesis.

Immuno-PET is a promising approach for improved cancer diagnosis, by taking advantage of the high specificity of antibodies. Here, we present a novel cell-free protein synthesis method for preparing a positron emitter labeled-antibody. Functional anti-human EGFRvIII single chain Fv, MR1-1, was successfully labeled with carbon-11 (half-life=20.4 min) in 5 min (36% yield) by the direct incorporation of the clinical PET tracer, l-[(11)C]methionine. The product [(11)C]MR1-1 was easily and rapidly isolated with high radiochemical purity (>95%) from the reaction solution, by affinity purification. This method would be widely applicable to the preparation of radiolabeled antibodies for PET imaging.