IMPORTANCE Progressive supranuclear palsy (PSP) is a 4-repeat tauopathy. Region-specific tau aggregates establish the neuropathologic diagnosis of definite PSP post mortem. Future interventional trials against tau in PSP would strongly benefit from biomarkers that support diagnosis.OBJECTIVE To investigate the potential of the novel tau radiotracer 18 F-PI-2620 as a biomarker in patients with clinically diagnosed PSP. DESIGN, SETTING, AND PARTICIPANTSIn this cross-sectional study, participants underwent dynamic 18 F-PI-2620 positron emission tomography (PET) from 0 to 60 minutes after injection at 5 different centers (3 in Germany, 1 in the US, and 1 in Australia). Patients with PSP (including those with Richardson syndrome [RS]) according to Movement Disorder Society PSP criteria were examined together with healthy controls and controls with disease. Four additionally referred individuals with PSP-RS and 2 with PSP-non-RS were excluded from final data analysis owing to incomplete dynamic PET scans.

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Progressive supranuclear palsy (PSP) is a neurodegenerative movement disorder characterized by deposition of fibrillar aggregates of 4R tau-protein in neurons and glial cells of the brain. These deposits are a key neuropathological finding, allowing a diagnosis of “definite PSP,” which is usually established post mortem. To date criteria for clinical diagnosis of PSP in vivo do not include biomarkers of tau pathology. For intervention trials, it is increasingly important to (i) establish biomarkers for an early diagnosis and (ii) to develop biomarkers that correlate with disease progression of PSP. [18F]-THK5351 is a novel PET-ligand that may afford in vivo visualization and quantification of tau-related alterations. We investigated binding characteristics of [18F]-THK5351 in patients with clinically diagnosed PSP and correlate tracer uptake with clinical findings. Eleven patients (68.4 ± 7.4 year; N = 6 female) with probable PSP according to current clinical criteria and nine healthy controls (71.7 ± 7.2 year; N = 4 female) underwent [18F]-THK5351 PET scanning. Voxel-wise statistical parametric comparison and volume-of-interest based quantification of standardized-uptake-values (SUV) were conducted using the cerebellar cortex as reference region. We correlated disease severity as measured with the help of the PSP Rating Scale (PSPRS) as well as several other clinical parameters with the individual PET findings. By voxel-wise mapping of [18F]-THK5351 uptake in the patient group we delineated typical distribution patterns that fit to known tau topology for PSP post mortem. Quantitative analysis indicated the strongest discrimination between PSP patients and healthy controls based on tracer uptake in the midbrain (+35%; p = 3.01E-7; Cohen's d: 4.0), followed by the globus pallidus, frontal cortex, and medulla oblongata. Midbrain [18F]-THK5351 uptake correlated well with clinical severity as measured by PSPRS (R = 0.66; p = 0.026). OCT and MRI delineated PSP patients from healthy controls by use of established discrimination thresholds but only OCT did as well correlate with clinical severity (R = 0.79; p = 0.024). Regional [18F]-THK5351 binding patterns correlated well with the established post mortem distribution of lesions in PSP and with clinical severity. The contribution of possible MAO-B binding to the [18F]-THK5351 signal needs to be further evaluated, but nevertheless [18F]-THK5351 PET may still serve as valuable biomarker for diagnosis of PSP.

A BS TRACT: Background: Neuroinflammation has received growing interest as a therapeutic target in neurodegenerative disorders, including 4-repeat tauopathies. Objectives: The aim of this cross-sectional study was to investigate 18 kDa translocator protein positron emission tomography (PET) as a biomarker for microglial activation in the 4-repeat tauopathies corticobasal degeneration and progressive supranuclear palsy. Methods: Specific binding of the 18 kDa translocator protein tracer 18 F-GE-180 was determined by serial PET during pharmacological depletion of microglia in a 4-repeat tau mouse model. The 18 kDa translocator protein PET was performed in 30 patients with corticobasal syndrome (68 ± 9 years, 16 women) and 14 patients with progressive supranuclear palsy (69 ± 9 years, 8 women), and 13 control subjects (70 ± 7 years, 7 women). Group

Purpose Second-generation tau radiotracers for use with positron emission tomography (PET) have been developed for visualization of tau deposits in vivo. For several β-amyloid and first-generation tau-PET radiotracers, it has been shown that earlyphase images can be used as a surrogate of neuronal injury. Therefore, we investigated the performance of early acquisitions of the novel tau-PET radiotracer [ 18 F]PI-2620 as a potential substitute for [ 18 F]fluorodeoxyglucose ([ 18 F]FDG). Methods Twenty-six subjects were referred with suspected tauopathies or overlapping parkinsonian syndromes (Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, multi-system atrophy, Parkinson's disease, multi-system atrophy, Parkinson's disease, frontotemporal dementia) and received a dynamic [ 18 F]PI-2620 tau-PET (0-60 min p.i.) and static [ 18 F]FDG-PET (30-50 min p.i.). Regional standardized uptake value ratios of early-phase images (single frame SUVr) and the blood flow estimate (R 1) of [ 18 F]PI-2620-PET were correlated with corresponding quantification of [ 18 F]FDG-PET (global mean/cerebellar normalization). Reduced tracer uptake in cortical target regions was also interpreted visually using 3-dimensional stereotactic surface projections by three more and three less experienced readers. Spearman rank correlation coefficients were calculated between early-phase [ 18 F]PI-2620 tau-PETand [ 18 F]FDG-PET images for all cortical regions and frequencies of disagreement between images were compared for both more and less experienced readers. Results Highest agreement with [ 18 F]FDG-PET quantification was reached for [ 18 F]PI-2620-PET acquisition from 0.5 to 2.5 min p.i. for global mean (lowest R = 0.69) and cerebellar scaling (lowest R = 0.63). Correlation coefficients (summed 0.5-2.5 min SUVr & R 1) displayed strong agreement in all cortical target regions for global mean (R SUVr 0.76, R R1 = 0.77) and cerebellar normalization (R SUVr 0.68, R R1 = 0.68). Visual interpretation revealed high regional correlations between early-phase tau-PET and [ 18 F]FDG-PET. There were no relevant differences between more and less experienced readers. Conclusion Early-phase imaging of [ 18 F]PI-2620 can serve as a surrogate biomarker for neuronal injury. Dynamic imaging or a dual time-point protocol for tau-PET imaging could supersede additional [ 18 F]FDG-PET imaging by indexing both the distribution of tau and the extent of neuronal injury.

Objectives: In recent years several 18F-labeled amyloid PET (Aβ-PET) tracers have been developed and have obtained clinical approval. There is evidence that Aβ-PET perfusion can provide surrogate information about neuronal injury in neurodegenerative diseases when compared to conventional blood flow and glucose metabolism assessment. However, this paradigm has not yet been tested in neurodegenerative disorders with cortical and subcortical affection. Therefore, we investigated the performance of early acquisition 18F-flutemetamol Aβ-PET in comparison to 18F-fluorodeoxyglucose (FDG)-PET in corticobasal syndrome (CBS).Methods: Subjects with clinically possible or probable CBS were recruited within the prospective Activity of Cerebral Networks, Amyloid and Microglia in Aging and Alzheimer’s Disease (ActiGliA) observational study and all CBS cases with an available FDG-PET prior to Aβ-PET were selected. Aβ-PET was acquired 0–10 min p.i. (early-phase) and 90–110 min p.i. (late-phase) whereas FDG-PET was recorded statically from 30 to 50 min p.i. Semiquantitative regional values and asymmetry indices (AI) were compared between early-phase Aβ-PET and FDG-PET. Visual assessments of hypoperfusion and hypometabolism were compared between both methods. Late-phase Aβ-PET was evaluated visually for assessment of Aβ-positivity.Results: Among 20 evaluated patients with CBS, 5 were Aβ-positive. Early-phase Aβ-PET and FDG-PET SUVr correlated highly in cortical (mean R = 0.86, range 0.77–0.92) and subcortical brain regions (mean R = 0.84, range 0.79–0.90). Strong asymmetry was observed in FDG-PET for the motor cortex (mean |AI| = 2.9%), the parietal cortex (mean |AI| = 2.9%), and the thalamus (mean |AI| = 5.5%), correlating well with AI of early-phase Aβ-PET (mean R = 0.87, range 0.62–0.98). Visual assessments of hypoperfusion and hypometabolism were highly congruent.Conclusion: Early-phase Aβ-PET facilitates assessment of neuronal injury in CBS for cortical and subcortical areas. Known asymmetries in CBS are captured by this method, enabling assessment of Aβ-status and neuronal injury with a single radiation exposure at a single visit.

Neurodegenerative parkinsonian syndromes comprise a number of disorders that are characterized by similar clinical features but are separated on the basis of different pathologies, i.e., aggregates of α-synuclein or tau protein. Due to the overlap of signs and symptoms a precise differentiation is often difficult, especially early in the disease course. Enormous efforts have been taken to develop tau-selective PET imaging agents, but strong off-target binding to monoamine oxidase B (MAO-B) has been observed across first generation ligands. Nonetheless, astrogliosis-related MAO-B elevation is a common histopathological known feature of all parkinsonian syndromes and might be itself an interesting imaging target. Therefore, this study aimed to investigate the performance of [18F]-THK5351, a combined MAO-B and tau tracer for differential diagnosis of parkinsonian syndromes. [18F]-THK5351 PET was performed in 34 patients: six with Parkinson’s disease (PD), nine with multiple system atrophy with predominant parkinsonism (MSA-P), six with MSA with predominant cerebellar ataxia (MSA-C), and 13 with progressive supranuclear palsy (PSP) Richardson’s syndrome. Volume-of-interest-based quantification of standardized-uptake-values was conducted in different parkinsonian syndrome-related target regions. PET results were subjected to multinomial logistic regression to create a prediction model discriminating among groups. Furthermore, we correlated tracer uptake with clinical findings. Elevated [18F]-THK5351 uptake in midbrain and diencephalon differentiated PSP patients from PD and MSA-C. MSA-C patients were distinguishable by high tracer uptake in the pons and cerebellar deep white matter when compared to PSP and PD patients, whereas MSA-P patients tended to show higher tracer uptake in the lentiform nucleus. A multinomial logistic regression classified 33/34 patients into the correct clinical diagnosis group. Tracer uptake in the pons, cerebellar deep white matter, and striatum was closely associated with the presence of cerebellar and parkinsonian symptoms of MSA patients. The current study demonstrates that combined MAO-B and tau binding of THK5351 facilitates differential diagnosis of parkinsonian syndromes. Furthermore, our data indicate a correlation of MSA phenotype with [18F]-THK5351 uptake in certain brain regions, illustrating their relevance for the emergence of clinical symptoms and underlining the potential of THK5351 PET as a biomarker that correlates with pathological changes as well as with disease stage.

Objective: The aim of this cross−sectional single center study was to investigate 18kDa translocator protein (TSPO)−PET as a biomarker for microglial activation in the 4−repeat tauopathies corticobasal degeneration and progressive supranuclear palsy (PSP). Methods: Specific binding of the TSPO tracer 18F−GE−180 was determined by serial PET during pharmacological depletion of microglia in a 4−repeat tau mouse model. TSPO−PET was performed in 30 patients with corticobasal syndrome (CBS, 68±9 years, 16 female) and 14 patients with PSP (69±9 years, 8 female), and 13 control subjects (70±7 years, 7 female). Group comparisons and associations with parameters of disease progression and sTREM2 were assessed by region-based and voxel−wise analyses. Results: Tracer binding was significantly reduced after pharmacological depletion of microglia in 4−repeat tau mice. Elevated TSPO labeling (standardized−uptake−value−ratios) was observed in subcortical brain areas of CBS and PSP patients when compared to controls, most pronounced in the globus pallidus internus (CBS: 1.039 [95%CI 1.000−1.078, p<0.001], PSP: 1.046 [95%CI: 0.990−1.101, p<0.001], controls: 0.861 [95%CI 0.802−0.921]), whereas only CBS patients showed additionally elevated tracer binding in motor and supplemental motor areas. TSPO labeling was only correlated weakly with parameters of disease progression in CBS and PSP but allowed sensitive detection of 4−repeat tauopathy patients. sTREM2 did not differ between patients with CBS and controls. Interpretation: Our data indicate a potential of 18F−GE−180 PET to detect microglial activation in the brain of 4−repeat tauopathy patients, fitting to predilection sites of the phenotype. TSPO-PET may serve as a sensitive early disease stage biomarker in 4−repeat tauopathies.