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Background: Alzheimer's disease (AD) is characterized by diffuse amyloid-β (Aβ) and phosphorylated Tau (p-Tau) aggregates as well as neuroinflammation. Exogenously-induced 40 Hz gamma oscillations have been showing to reduce Aβ and p-Tau deposition presumably via microglia activation in AD mouse models.Objective: We aimed to translate preclinical data on gamma-induction in AD patients by means of transcranial alternating current stimulation (tACS).Methods: Four participants with mild-to-moderate AD received 1 h of daily 40 Hz (gamma) tACS for 4 weeks (Monday to Friday) targeting the bitemporal lobes (20 h treatment duration). Participant underwent Aβ, p-Tau, and microglia PET imaging with [11C]-PiB, [18F]-FTP, and [11C]-PBR28 respectively, before and after the intervention along with electrophysiological assessment.Results: No adverse events were reported, and an increase in gamma spectral power on EEG was observed after the treatment. [18F]-FTP PET revealed a significant decrease over 2% of p-Tau burden in 3/4 patients following the tACS treatment, primarily involving the temporal lobe regions targeted by tACS and especially mesial regions (e.g., entorhinal cortex). The amount of intracerebral Aβ as measured by [11C]-PiB was not significantly influenced by tACS, whereas 1/4 reported a significant decrease of microglia activation as measured by [11C]-PBR28.Conclusion: tACS seems to represent a safe and feasible option for gamma induction in AD patients, with preliminary evidence of a possible effect on protein clearance partially mimicking what is observed in animal models. Longer interventions and placebo control conditions are needed to fully evaluate the potential for tACS to slow disease progression. [ABSTRACT FROM AUTHOR]

Purpose: [18F]MK6240 was developed for PET imaging of tau aggregates, which are implicated in Alzheimer's disease. The goal of this work was to evaluate the kinetics of [18F]MK6240 and to investigate different strategies for in-vivo quantification of tau aggregates in humans. Methods: Thirty-five subjects, consisting of 18 healthy controls (CTRL), 11 subjects with mild cognitive impairment (MCI) and six with Alzheimer's Disease (AD), underwent dynamic [18F]MK6240 PET scans. Arterial blood measurements were collected in 16 subjects (eight CTRLs, six MCIs and two AD) to measure whole blood and plasma concentration time courses. Radiometabolite analysis was performed on a subset of plasma samples. Various compartmental model configurations as well as the Logan and multilinear analysis (MA1) graphical methods with arterial plasma input function were tested. Simplified reference tissue methods were investigated, including Logan distribution volume ratio (DVR), multilinear reference tissue method (MRTM2), and static SUV ratio using the cerebellum as a reference region. Results: Whole blood:plasma ratio stabilized to 0.66 ± 0.01 after 15 min. Percent parent in plasma (%PP) followed a single exponential and ranged from 0 to 10% at 90 min. [18F]MK6240 in gray matter peaked quickly (SUV > 2 at ~3 min). The preferred compartmental model was a reversible two-tissue compartment model, with the blood contribution included as a model parameter (2T4k1v). Compartmental and graphical analysis methods with arterial input functions yielded concordant results, but rapid metabolism raised challenges for blood-based quantification. MCI and AD subjects demonstrated a broad range of VT as compared to CTRL subjects. DVR from MRTM2 and Logan reference tissue methods correlated with DVR calculated indirectly from compartmental modeling, but underestimation was observed in data sets with very high binding (DVR > 3). SUVR also underestimated indirect DVR from blood-based analyses in high binding regions, although a non-linear relationship was exhibited. Conclusions: [18F]MK6240 exhibited a wide dynamic range of uptake, with binding patterns in MCI/AD subjects consistent with neurofibrillary tau deposition patterns. Linearized reference tissue methods using an estimated average tissue-to-plasma efflux constant k ′ 2 ¯ and static SUVR agreed well with blood-based methods for most data sets; however, discrepancies were noted in the highest binding cases. Caution should therefore be exercised in application of simplified methods to such data sets, and in quantitative interpretation of corresponding outcomes. [ABSTRACT FROM AUTHOR]

Introduction: Chronic traumatic encephalopathy (CTE) is a tauopathy associated to repetitive head trauma. There are no validated in vivo biomarkers of CTE and a definite diagnosis can only be made at autopsy. Recent studies have shown that positron emission tomography (PET) tracer AV-1451 (Flortaucipir) exhibits high binding affinity for paired helical filament (PHF)-tau aggregates in Alzheimer (AD) brains but relatively low affinity for tau lesions in other tauopathies like temporal lobal degeneration (FTLD)-tau, progressive supranuclear palsy (PSP) or corticobasal degeneration (CBD). Little is known, however, about the binding profile of this ligand to the tau-containing lesions of CTE. Objective: To study the binding properties of [18F]-AV-1451 on pathologically confirmed CTE postmortem brain tissue samples. Methods: We performed [18F]-AV-1451 phosphor screen and high resolution autoradiography, quantitative tau measurements by immunohistochemistry and Western blot and tau seeding activity assays in brain blocks containing hippocampus, superior temporal cortex, superior frontal cortex, inferior parietal cortex and occipital cortex from 5 cases of CTE, across the stages of disease: stage II-III (n = 1), stage III (n = 3), and stage IV (n = 1). Importantly, low or no concomitant classic AD pathology was present in these brains. Results: Despite the presence of abundant tau aggregates in multiple regions in all CTE brains, only faint or no [18F]-AV-1451 binding signal could be detected by autoradiography. The only exception was the presence of a strong signal confined to the region of the choroid plexus and the meninges in two of the five cases. Tau immunostaining and Thioflavin-S staining ruled out the presence of tau aggregates in those regions. High resolution nuclear emulsion autoradiography revealed the presence of leptomeningeal melanocytes as the histologic source of this off-target binding. Levels of abnormally hyperphosphorylated tau species, as detected by Western Blotting, and tau seeding activity were both found to be lower in extracts from cases CTE when compared to AD. Conclusion: AV-1451 may have limited utility for in vivo selective and reliable detection of tau aggregates in CTE. The existence of disease-specific tau conformations may likely explain the differential binding affinity of this tracer for tau lesions in different tauopathies. [ABSTRACT FROM AUTHOR]