Characteristics of Tau and Its Ligands in PET Imaging

A review of the flortaucipir literature for positron emission tomography imaging of tau neurofibrillary tangles

Alzheimer's disease is defined by the presence of β-amyloid plaques and neurofibrillary tau tangles potentially preceding clinical symptoms by many years. Previously only detectable post-mortem, these pathological hallmarks are now identifiable using biomarkers, permitting an in vivo definitive diagnosis of Alzheimer's disease. 18F-flortaucipir (previously known as 18F-T807; 18F-AV-1451) was the first tau positron emission tomography tracer to be introduced and is the only Food and Drug Administration-approved tau positron emission tomography tracer (Tauvid™). It has been widely adopted and validated in a number of independent research and clinical settings. In this review, we present an overview of the published literature on flortaucipir for positron emission tomography imaging of neurofibrillary tau tangles. We considered all accessible peer-reviewed literature pertaining to flortaucipir through 30 April 2022. We found 474 relevant peer-reviewed publications, which were organized into the following categories based on their primary focus: typical Alzheimer's disease, mild cognitive impairment and pre-symptomatic populations; atypical Alzheimer's disease; non-Alzheimer's disease neurodegenerative conditions; head-to-head comparisons with other Tau positron emission tomography tracers; and technical considerations. The available flortaucipir literature provides substantial evidence for the use of this positron emission tomography tracer in assessing neurofibrillary tau tangles in Alzheimer's disease and limited support for its use in other neurodegenerative disorders. Visual interpretation and quantitation approaches, although heterogeneous, mostly converge and demonstrate the high diagnostic and prognostic value of flortaucipir in Alzheimer's disease.

A Review of the Most Recent Clinical and Neuropathological Criteria for Chronic Traumatic Encephalopathy

(1) Background: Chronic traumatic encephalopathy (CTE) is a complex pathological condition characterized by neurodegeneration, as a result of repeated head traumas. Currently, the diagnosis of CTE can only be assumed postmortem. Thus, the clinical manifestations associated with CTE are referred to as traumatic encephalopathy syndrome (TES), for which diagnostic multiple sets of criteria can be used. (2) Objectives: In this study, we aimed to present and discuss the limitations of the clinical and neuropathological diagnostic criteria for TES/CTE and to suggest a diagnostic algorithm enabling a more accurate diagnostic procedure. (3) Results: The most common diagnostic criteria for TES/CTE discriminate between possible, probable, and improbable. However, several key variations between the available diagnostic criteria suggest that the diagnosis of CTE can still only be given with postmortem neurophysiological examination. Thus, a TES/CTE diagnosis during life imposes a different level of certainty. Here, we are proposing a comprehensive algorithm of diagnosis criteria for TES/CTE based on the similarities and differences between the previous criteria. (4) Conclusions: The diagnosis of TES/CTE requires a multidisciplinary approach; thorough investigation for other neurodegenerative disorders, systemic illnesses, and/or psychiatric conditions that can account for the symptoms; and also complex investigations of patient history, psychiatric assessment, and blood and cerebrospinal fluid biomarker evaluation.

Thiophene-Based Ligands: Design, Synthesis and Their Utilization for Optical Assignment of Polymorphic-Disease-Associated Protein Aggregates

The development of ligands for detecting protein aggregates is of great interest, as these aggregated proteinaceous species are the pathological hallmarks of several devastating diseases, including Alzheimer's disease. In this regard, thiophene-based ligands have emerged as powerful tools for fluorescent assessment of these pathological entities. The intrinsic conformationally sensitive photophysical properties of poly- and oligothiophenes have allowed optical assignment of disease-associated protein aggregates in tissue sections, as well as real-time in vivo imaging of protein deposits. Herein, we recount the chemical evolution of different generations of thiophene-based ligands, and exemplify their use for the optical distinction of polymorphic protein aggregates. Furthermore, the chemical determinants for achieving a superior fluorescent thiophene-based ligand, as well as the next generation of thiophene-based ligands targeting distinct aggregated species are described. Finally, the directions for future research into the chemical design of thiophene-based ligands that can aid in resolving the scientific challenges around protein aggregation diseases are discussed.

Neuroimaging in Alzheimer's Disease for Early Diagnosis: A Comprehensive Review

Alzheimer's disease (AD) is the most common cause of dementia in the elderly, affecting roughly half of those over the age of 85. We briefly discussed the risk factors, epidemiology, and treatment options for AD. The development of therapeutic therapies operating very early in the disease cascade has been spurred by the realization that the disease process begins at least a decade or more before the manifestation of symptoms. Thus, the clinical significance of early diagnosis was emphasized. Using various keywords, a literature search was carried out using PubMed and other databases. For inclusion, pertinent articles were chosen and reviewed. This article has reviewed different neuroimaging techniques that are considered advanced tools to aid in establishing a diagnosis and highlighted the advantages as well as disadvantages of those techniques. Besides, the prevalence of several in vivo biomarkers aided in discriminating affected individuals from healthy controls in the early stages of the disease. Each imaging method has its advantages and disadvantages, hence no single imaging approach can be the optimum modality for diagnosis. This article also commented on a better approach to using these techniques to increase the likelihood of an early diagnosis.

Discovery and evaluation of aza-fused tricyclic derivatives for detection of Tau pathology in Alzheimer's disease

For various neurodegenerative diseases, including Alzheimer's disease (AD), the abnormal aggregation of Tau is not only the predominant contributing factor but also a major biomarker for disease diagnosis. In this study, a series of aza-fused tricyclic derivatives were designed and synthesized. By changing the position and number of nitrogen atoms on the fused tricyclic core, the imidazonaphthyridine scaffold was screened and reported for the first time which could potentially detect Tau aggregates. Through a series of in vitro and in vivo biological evaluations, probe [125I]5 possessed exceptional binding affinity (IC50 = 1.63 nM) to neurofibrillary tangles in the AD brain, high selectivity over Aβ plaques (23.4-fold), clean off-target profile to monoamine oxidase A/B (MAO-A/B), and suitable pharmacokinetics (initial brain uptake = 3.22% ID/g).

PET molecular imaging for pathophysiological visualization in Alzheimer's disease

Alzheimer's disease (AD) is the most common dementia worldwide. The exact etiology of AD is unclear as yet, and no effective treatments are currently available, making AD a tremendous burden posed on the whole society. As AD is a multifaceted and heterogeneous disease, and most biomarkers are dynamic in the course of AD, a range of biomarkers should be established to evaluate the severity and prognosis. Positron emission tomography (PET) offers a great opportunity to visualize AD from diverse perspectives by using radiolabeled agents involved in various pathophysiological processes; PET imaging technique helps to explore the pathomechanisms of AD comprehensively and find out the most appropriate biomarker in each AD phase, leading to a better evaluation of the disease. In this review, we discuss the application of PET in the course of AD and summarized radiolabeled compounds with favorable imaging characteristics.

Amylin and Secretases in the Pathology and Treatment of Alzheimer's Disease

Alzheimer’s disease remains a prevailing neurodegenerative condition which has an array physical, emotional, and financial consequences to patients and society. In the past decade, there has been a greater degree of investigation on therapeutic small peptides. This group of biomolecules have a profile of fundamentally sound characteristics which make them an intriguing area for drug development. Among these biomolecules, there are four modulatory mechanisms of interest in this review: alpha-, beta-, gamma-secretases, and amylin. These protease-based biomolecules all have a contributory role in the amyloid cascade hypothesis. Moreover, the involvement of various biochemical pathways intertwines these peptides to have shared regulators (i.e., retinoids). Further clinical and translational investigation must occur to gain a greater understanding of its potential application in patient care. The aim of this narrative review is to evaluate the contemporary literature on these protease biomolecule modulators and determine its utility in the treatment of Alzheimer’s disease.

Monoamine oxidase binding not expected to significantly affect [18F]flortaucipir PET interpretation

Purpose

[¹⁸F]-labeled positron emission tomography (PET) radioligands permit in vivo assessment of Alzheimer’s disease biomarkers, including aggregated neurofibrillary tau (NFT) with [¹⁸F]flortaucipir. Due to structural similarities of flortaucipir with some monoamine oxidase A (MAO-A) inhibitors, this study aimed to evaluate flortaucipir binding to MAO-A and MAO-B and any potential impact on PET interpretation.

Methods

[¹⁸F]Flortaucipir autoradiography was performed on frozen human brain tissue slices, and PET imaging was conducted in rats. Dissociation constants were determined by saturation binding, association and dissociation rates were measured by kinetic binding experiments, and IC₅₀ values were determined by competition binding.

Results

Under stringent wash conditions, specific [¹⁸F]flortaucipir binding was observed on tau NFT-rich Alzheimer’s disease tissue and not control tissue. In vivo PET experiments in rats revealed no evidence of [¹⁸F]flortaucipir binding to MAO-A; pre-treatment with MAO inhibitor pargyline did not impact uptake or wash-out of [¹⁸F]flortaucipir. [¹⁸F]Flortaucipir bound with low nanomolar affinity to human MAO-A in a microsomal preparation in vitro but with a fast dissociation rate relative to MAO-A ligand fluoroethyl-harmol, consistent with no observed in vivo binding in rats of [¹⁸F]flortaucipir to MAO-A. Direct binding of flortaucipir to human MAO-B was not detected in a microsomal preparation. A high concentration of flortaucipir (IC₅₀ of 1.3 μM) was found to block binding of the MAO-B ligand safinamide to MAO-B on microsomes suggesting that, at micromolar concentrations, flortaucipir weakly binds to MAO-B in vitro.

Conclusion

These data suggest neither MAO-A nor MAO-B binding will contribute significantly to the PET signal in cortical target areas relevant to the interpretation of [¹⁸F]flortaucipir.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-022-05822-9.

Post-Concussion Syndrome and Chronic Traumatic Encephalopathy: Narrative Review on the Neuropathology, Neuroimaging and Fluid Biomarkers

Traumatic brain injury is a significant public health issue and represents the main contributor to death and disability globally among all trauma-related injuries. Martial arts practitioners, military veterans, athletes, victims of physical abuse, and epileptic patients could be affected by the consequences of repetitive mild head injuries (RMHI) that do not resume only to short-termed traumatic brain injuries (TBI) effects but also to more complex and time-extended outcomes, such as post-concussive syndrome (PCS) and chronic traumatic encephalopathy (CTE). These effects in later life are not yet well understood; however, recent studies suggested that even mild head injuries can lead to an elevated risk of later-life cognitive impairment and neurodegenerative disease. While most of the PCS hallmarks consist in immediate consequences and only in some conditions in long-termed processes undergoing neurodegeneration and impaired brain functions, the neuropathological hallmark of CTE is the deposition of p-tau immunoreactive pre-tangles and thread-like neurites at the depths of cerebral sulci and neurofibrillary tangles in the superficial layers I and II which are also one of the main hallmarks of neurodegeneration. Despite different CTE diagnostic criteria in clinical and research approaches, their specificity and sensitivity remain unclear and CTE could only be diagnosed post-mortem. In CTE, case risk factors include RMHI exposure due to profession (athletes, military personnel), history of trauma (abuse), or pathologies (epilepsy). Numerous studies aimed to identify imaging and fluid biomarkers that could assist diagnosis and probably lead to early intervention, despite their heterogeneous outcomes. Still, the true challenge remains the prediction of neurodegeneration risk following TBI, thus in PCS and CTE. Further studies in high-risk populations are required to establish specific, preferably non-invasive diagnostic biomarkers for CTE, considering the aim of preventive medicine.

Prospects for the use of graphene-based biological sensors in the early diagnosis of Alzheimer's disease (review of literature)

Among the most significant challenges presented to modern medicine is the problem of cognitive disorders. The relevance of her research is determined by the wide spread of disorders of the higher cortical functions, their significant negative impact on the quality of life of patients, as well as high economic costs on the part of the state and the patient's relatives aimed at organizing medical, diagnostic and rehabilitation processes. The main cause of cognitive impairment in the elderly is Alzheimer's disease. Currently, the criteria for the diagnosis of this nosological form have been developed and are widely used in practice. However, it should be noted that their use is most effective if the patient has a detailed clinical picture, at the stage of dementia. In addition, they provide for the study of biomarkers in a number of cases in the cerebrospinal fluid or using positron emission tomography, which presents certain technical difficulties. Especially significant problems arise in the pre-dement stages. This situation dictates the need to search for new promising diagnostic methods that will have high sensitivity and specificity, as well as the possibility of application in the early stages of Alzheimer's disease, including in outpatient settings. The article provides information about modern methods of computer neuroimaging, discusses the research directions of individual biomarkers, and also shows the prospects for using diagnostic test panels developed on the basis of graphene biosensors, taking into account the latest achievements of nanotechnology and their integration into medical science.

Tracers progress for positron emission tomography imaging of glial-related disease

Glial cells play an essential part in the neuron system. They can not only serve as structural blocks in the human brain but also participate in many biological processes. Extensive studies have shown that astrocytes and microglia play an important role in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, as well as glioma, epilepsy, ischemic stroke, and infections. Positron emission tomography is a functional imaging technique providing molecular-level information before anatomic changes are visible and has been widely used in many above-mentioned diseases. In this review, we focus on the positron emission tomography tracers used in pathologies related to glial cells, such as glioma, Alzheimer's disease, and neuroinflammation.

Tau Biomarkers in Dementia: Positron Emission Tomography Radiopharmaceuticals in Tauopathy Assessment and Future Perspective

Abnormal accumulation of Tau protein is closely associated with neurodegeneration and cognitive impairment and it is a biomarker of neurodegeneration in the dementia field, especially in Alzheimer’s disease (AD); therefore, it is crucial to be able to assess the Tau deposits in vivo. Beyond the fluid biomarkers of tauopathy described in this review in relationship with the brain glucose metabolic patterns, this review aims to focus on tauopathy assessment by using Tau PET imaging. In recent years, several first-generation Tau PET tracers have been developed and applied in the dementia field. Common limitations of first-generation tracers include off-target binding and subcortical white-matter uptake; therefore, several institutions are working on developing second-generation Tau tracers. The increasing knowledge about the distribution of first- and second-generation Tau PET tracers in the brain may support physicians with Tau PET data interpretation, both in the research and in the clinical field, but an updated description of differences in distribution patterns among different Tau tracers, and in different clinical conditions, has not been reported yet. We provide an overview of first- and second-generation tracers used in ongoing clinical trials, also describing the differences and the properties of novel tracers, with a special focus on the distribution patterns of different Tau tracers. We also describe the distribution patterns of Tau tracers in AD, in atypical AD, and further neurodegenerative diseases in the dementia field.

Advancements in Positron Emission Tomography/Magnetic Resonance Imaging and Applications to Diagnostic Challenges in Neuroradiology

Since the clinical adoption of magnetic resonance (MR) in medical imaging, MR has proven to be a workhorse in diagnostic neuroradiology, with the ability to provide superb anatomic detail as well as additional functional and physiologic data, depending on the techniques utilized. Positron emission tomography/computed tomography has also shown irreplaceable diagnostic value in certain disease processes of the central nervous system by providing molecular and metabolic information through the development of numerous disease-specific PET tracers, many of which can be utilized as a diagnostic technique in and of themselves or can provide a valuable adjunct to information derived from MR. Despite these advances, many challenges still remain in neuroradiology, particularly in malignancy, neurodegenerative disease, epilepsy, and cerebrovascular disease. Through improvements in attenuation correction, motion correction, and PET detectors, combining the 2 modalities of PET and MR through simultaneous imaging has proven feasible and allows for improved spatial and temporal resolution without compromising either of the 2 individual modalities. The complementary information offered by both technologies has provided increased diagnostic accuracy in both research and many clinical applications in neuroradiology.

The Evaluation of Tau Deposition with [18F]PI-2620 by Using a Semiquantitative Method in Cognitively Normal Subjects and Patients with Mild Cognitive Impairment and Alzheimer's Disease

Background

Some studies have reported the effectiveness of [¹⁸F]PI-2620 as an effective tau-binding radiotracer; however, few reports have applied semiquantitative analysis to the tracer. Therefore, this study's aim was to perform a semiquantitative analysis of [¹⁸F]PI-2620 in individuals with normal cognition and patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD).

Methods

Twenty-six cognitively normal (CN) subjects, 7 patients with AD, and 36 patients with MCI were enrolled. A dynamic positron emission tomography (PET) scan was performed 30–75 min postinjection. PET and T1-weighted magnetic resonance imaging scans were coregistered. The standardized uptake value ratio (SUVr) was used for semiquantitative analysis. The P-Mod software was applied to create volumes of interest. The ANOVA and post hoc Tukey HSD were used for statistical analysis.

Results

In the AD group, the occipital lobe had a significantly higher mean SUVr (1.46 ± 0.57) than in the CN and MCI groups. Compared with the CN group, the AD group showed significantly higher mean SUVr in the fusiform gyrus (1.06 ± 0.09 vs. 1.49 ± 0.86), inferior temporal (1.07 ± 0.07 vs. 1.46 ± 0.08), parietal lobe, lingual gyrus, and precuneus regions. Similarly, the AD group demonstrated a higher mean SUVr than the MCI group in the precuneus, lingual, inferior temporal, fusiform, supramarginal, orbitofrontal, and superior temporal regions. The remaining observed regions, including the striatum, basal ganglia, thalamus, and white matter, showed a low SUVr across all groups with no statistically significant differences.

Conclusion

A significantly higher mean SUVr of [¹⁸F]PI-2620 was observed in the AD group; a significant area of the brain in the AD group demonstrated tau protein deposit in concordance with Braak Stages III–V, providing useful information to differentiate AD from CN and MCI. Moreover, the low SUVr in the deep striatum and thalamus could be useful for excluding primary tauopathies.

(S)-[18F]THK5117 brain uptake is associated with Aβ plaques and MAO-B enzyme in a mouse model of Alzheimer's disease

The mouse model of beta-amyloid (Aβ) deposition, APP/PS1-21, exhibits high brain uptake of the tau-tracer (S)-[¹⁸F]THK5117, although no neurofibrillary tangles are present in this mouse model. For this reason we investigated (S)-[¹⁸F]THK5117 off-target binding to Aβ plaques and MAO-B enzyme in APP/PS1-21 transgenic (TG) mouse model of Aβ deposition. APP/PS1-21 TG and wild-type (WT) control mice in four different age groups (2-26 months) were imaged antemortem by positron emission tomography with (S)-[¹⁸F]THK5117, and then brain autoradiography. Additional animals were used for immunohistochemical staining and MAO-B enzyme blocking study with deprenyl pre-treatment. Regional standardized uptake value ratios for the cerebellum revealed a significant temporal increase in (S)-[¹⁸F]THK5117 uptake in aged TG, but not WT, brain. Immunohistochemical staining revealed a similar increase in Aβ plaques but not endogenous hyper-phosphorylated tau or MAO-B enzyme, and ex vivo autography showed that uptake of (S)-[¹⁸F]THK5117 co-localized with the amyloid pathology. Deprenyl hydrochloride pre-treatment reduced the binding of (S)-[¹⁸F]THK5117 in the neocortex, hippocampus, and thalamus. This study's findings suggest that increased (S)-[¹⁸F]THK5117 binding in aging APP/PS1-21 TG mice is mainly due to increasing Aβ deposition, and to a lesser extent binding to MAO-B enzyme, but not hyper-phosphorylated tau.

Identifying degenerative effects of repetitive head trauma with neuroimaging: a clinically-oriented review

BACKGROUND AND SCOPE OF REVIEW

Varying severities and frequencies of head trauma may result in dynamic acute and chronic pathophysiologic responses in the brain. Heightened attention to long-term effects of head trauma, particularly repetitive head trauma, has sparked recent efforts to identify neuroimaging biomarkers of underlying disease processes. Imaging modalities like structural magnetic resonance imaging (MRI) and positron emission tomography (PET) are the most clinically applicable given their use in neurodegenerative disease diagnosis and differentiation. In recent years, researchers have targeted repetitive head trauma cohorts in hopes of identifying in vivo biomarkers for underlying biologic changes that might ultimately improve diagnosis of chronic traumatic encephalopathy (CTE) in living persons. These populations most often include collision sport athletes (e.g., American football, boxing) and military veterans with repetitive low-level blast exposure. We provide a clinically-oriented review of neuroimaging data from repetitive head trauma cohorts based on structural MRI, FDG-PET, Aβ-PET, and tau-PET. We supplement the review with two patient reports of neuropathology-confirmed, clinically impaired adults with prior repetitive head trauma who underwent structural MRI, FDG-PET, Aβ-PET, and tau-PET in addition to comprehensive clinical examinations before death.

REVIEW CONCLUSIONS

Group-level comparisons to controls without known head trauma have revealed inconsistent regional volume differences, with possible propensity for medial temporal, limbic, and subcortical (thalamus, corpus callosum) structures. Greater frequency and severity (i.e., length) of cavum septum pellucidum (CSP) is observed in repetitive head trauma cohorts compared to unexposed controls. It remains unclear whether CSP predicts a particular neurodegenerative process, but CSP presence should increase suspicion that clinical impairment is at least partly attributable to the individual's head trauma exposure (regardless of underlying disease). PET imaging similarly has not revealed a prototypical metabolic or molecular pattern associated with repetitive head trauma or predictive of CTE based on the most widely studied radiotracers. Given the range of clinical syndromes and neurodegenerative pathologies observed in a subset of adults with prior repetitive head trauma, structural MRI and PET imaging may still be useful for differential diagnosis (e.g., assessing suspected Alzheimer's disease).

An Update on the State of Tau Radiotracer Development: a Brief Review

Evolving scientific evidence has begun to point towards hyperphosphorylated tau as a major neurotoxic component in the pathophysiological development of many major neurodegenerative conditions. In response to a need for accurate and reliable diagnosis and disease monitoring in clinical and trial settings, there has been great effort put into the development of tau radiotracers. While first-generation and second-generation radiotracers have provided a basis for assessing tau, concerns of inadequate specificity and selectivity have continued to motivate further study of these radiotracers and the development of novel radiopharmaceuticals. Given the prospective scientific and clinical value of a valid tau radiotracer, the molecular neuroimaging community must be aware of the most recent developments in the realm of tau radiotracer development. This brief review article will critically overview the most established tau radiotracers and, most importantly, concentrate on the progress of more recently developed tau radiotracers.

Imaging Techniques in Alzheimer's Disease: A Review of Applications in Early Diagnosis and Longitudinal Monitoring

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder affecting many individuals worldwide with no effective treatment to date. AD is characterized by the formation of senile plaques and neurofibrillary tangles, followed by neurodegeneration, which leads to cognitive decline and eventually death.

INTRODUCTION

In AD, pathological changes occur many years before disease onset. Since disease-modifying therapies may be the most beneficial in the early stages of AD, biomarkers for the early diagnosis and longitudinal monitoring of disease progression are essential. Multiple imaging techniques with associated biomarkers are used to identify and monitor AD.

AIM

In this review, we discuss the contemporary early diagnosis and longitudinal monitoring of AD with imaging techniques regarding their diagnostic utility, benefits and limitations. Additionally, novel techniques, applications and biomarkers for AD research are assessed.

FINDINGS

Reduced hippocampal volume is a biomarker for neurodegeneration, but atrophy is not an AD-specific measure. Hypometabolism in temporoparietal regions is seen as a biomarker for AD. However, glucose uptake reflects astrocyte function rather than neuronal function. Amyloid-β (Aβ) is the earliest hallmark of AD and can be measured with positron emission tomography (PET), but Aβ accumulation stagnates as disease progresses. Therefore, Aβ may not be a suitable biomarker for monitoring disease progression. The measurement of tau accumulation with PET radiotracers exhibited promising results in both early diagnosis and longitudinal monitoring, but large-scale validation of these radiotracers is required. The implementation of new processing techniques, applications of other imaging techniques and novel biomarkers can contribute to understanding AD and finding a cure.

CONCLUSIONS

Several biomarkers are proposed for the early diagnosis and longitudinal monitoring of AD with imaging techniques, but all these biomarkers have their limitations regarding specificity, reliability and sensitivity. Future perspectives. Future research should focus on expanding the employment of imaging techniques and identifying novel biomarkers that reflect AD pathology in the earliest stages.

PET Agents in Dementia: An Overview

This article presents an overview of imaging agents for PET that have been applied for research and diagnostic purposes in patients affected by dementia. Classified by the target which the agents visualize, seven groups of tracers can be distinguished, namely radiopharmaceuticals for: (1) Misfolded proteins (ß-amyloid, tau, α-synuclein), (2) Neuroinflammation (overexpression of translocator protein), (3) Elements of the cholinergic system, (4) Elements of monoamine neurotransmitter systems, (5) Synaptic density, (6) Cerebral energy metabolism (glucose transport/ hexokinase), and (7) Various other proteins. This last category contains proteins involved in mechanisms underlying neuroinflammation or cognitive impairment, which may also be potential therapeutic targets. Many receptors belong to this category: AMPA, cannabinoid, colony stimulating factor 1, metabotropic glutamate receptor 1 and 5 (mGluR1, mGluR5), opioid (kappa, mu), purinergic (P2X7, P2Y12), sigma-1, sigma-2, receptor for advanced glycation endproducts, and triggering receptor expressed on myeloid cells-1, besides several enzymes: cyclooxygenase-1 and 2 (COX-1, COX-2), phosphodiesterase-5 and 10 (PDE5, PDE10), and tropomyosin receptor kinase. Significant advances in neuroimaging have been made in the last 15 years. The use of 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (FDG) for quantification of regional cerebral glucose metabolism is well-established. Three tracers for ß-amyloid plaques have been approved by the Food and Drug Administration and European Medicines Agency. Several tracers for tau neurofibrillary tangles are already applied in clinical research. Since many novel agents are in the preclinical or experimental stage of development, further advances in nuclear medicine imaging can be expected in the near future. PET studies with established tracers and tracers for novel targets may result in early diagnosis and better classification of neurodegenerative disorders and in accurate monitoring of therapy trials which involve these targets. PET data have prognostic value and may be used to assess the response of the human brain to interventions, or to select the appropriate treatment strategy for an individual patient.

Comparison of MRI-based and PET-based image pre-processing for quantification of 11C-PBB3 uptake in human brain

PURPOSE

Quantification of tau load using ¹¹C-PBB3-PET has the potential to improve diagnosis of neurodegenerative diseases. Although MRI-based pre-processing is used as a reference method, not all patients have MRI. The feasibility of a PET-based pre-processing for the quantification of ¹¹C-PBB3 tracer was evaluated and compared with the MRI-based method.

MATERIALS AND METHODS

Fourteen patients with decreased recent memory were examined with ¹¹C-PBB3-PET and MRI. The PET scans were visually assessed and rated as either PBB3(+) or PBB3(-). The image processing based on the PET-based method was validated against the MRI-based approach. The regional uptakes were quantified using the Mesial-temporal/Temporoparietal/Rest of neocortex (MeTeR) regions. SUVR values were calculated by normalizing to the cerebellar reference region to compare both methods within the patient groups.

RESULTS

Significant correlations were observed between the SUVRs of the MRI-based and the PET-based methods in the MeTeR regions (r_Me=0.91; r_Te=0.98; r_R=0.96; p<0.0001). However, the Bland-Altman plot showed a significant bias between both methods in the subcortical Me region (bias: -0.041; 95% CI: -0.061 to -0.024; p=0.003). As in the MRI-based method, the ¹¹C-PBB3 uptake obtained with the PET-based method was higher for the PBB3(+) group in each of the cortical regions and for the whole brain than for the PBB3(-) group (PET-based_Global: 1.11 vs. 0.96; Cliff's Delta (d)=0.68; p=0.04; MRI-based_Global: 1.11 vs. 0.97; d=0.70; p=0.03). To differentiate between positive and negative scans, Youden's index estimated the best cut-off of 0.99 from the ROC curve with good accuracy (AUC: 0.88±0.10; 95% CI: 0.67-1.00) and the same sensitivity (83%) and specificity (88%) for both methods.

CONCLUSION

The PET-based pre-processing method developed to quantify the tau burden with ¹¹C-PBB3 provided comparable SUVR values and effect sizes as the MRI-based reference method. Furthermore, both methods have a comparable discrimination accuracy between PBB3(+) and PBB3(-) groups as assessed by visual rating. Therefore, the presented PET-based method can be used for clinical diagnosis if no MRI image is available.

The Significance of Tau Aggregates in the Human Brain

Neurofibrillary degeneration has attracted the attention of neuroscientists as both a hallmark of the disease and a subject for experimentation for more than a century. Recent studies implicate phosphorylated tau (p-tau) directly in neurodegenerative disease pathogenesis, although the human data continue to raise questions. P-tau accumulates with age in a roughly hierarchical manner, but avoids abundance in the neocortex unless co-occurring with amyloid-β. Neurodegenerative tauopathies tend to have p-tau morphologies that differ from aging and Alzheimer’s disease. Tau isoforms (3R vs. 4R) have a tendency to vary with tauopathy phenotype for unknown reasons. Selective vulnerability to p-tau and spatial-temporal disconnect from amyloid-β are evident in aging. P-tau assessment at autopsy involves tissue decomposition, which may skew microanatomical observations toward limited biological meaning. Two major consensus guidelines for interpreting p-tau at autopsy emphasize the challenges of clinicopathologic correlation, and reinforce the observation that regional neurodegeneration is a better correlate of clinical signs than is proteinopathy. Despite the proliferation of interesting and novel theories related to tau-mediated pathogenesis, the weight of the human observations suggests that neurofibrillary degeneration is an epiphenomenal hallmark of aging and disease rather than an epicenter of neurotoxicity. This is consistent with numerous tau-targeted therapeutic strategies that have been unsuccessful to date.

Neuroimaging in the Diagnosis of Chronic Traumatic Encephalopathy: A Systematic Review

OBJECTIVE

Chronic traumatic encephalopathy (CTE) is a neurodegenerative tauopathy associated with repeated subconcussive and concussive head injury. Clinical features include cognitive, behavioral, mood, and motor impairments. Definitive diagnosis is only possible at postmortem. Here, the utility of neuroimaging in the diagnosis of CTE is evaluated by systematically reviewing recent evidence for changes in neuroimaging biomarkers in suspected cases of CTE compared with controls.

DATA SOURCES

Providing an update on a previous systematic review of articles published until December 2014, we searched for articles published between December 2014 and July 2016. We searched PubMed for studies assessing neuroimaging changes in symptomatic suspected cases of CTE with a history of repeated subconcussive or concussive head injury or participation in contact sports involving direct impact to the head. Exclusion criteria were case studies, review articles, and articles focusing on repetitive head trauma from military service, head banging, epilepsy, physical abuse, or animal models.

MAIN RESULTS

Seven articles met the review criteria, almost all of which studied professional athletes. The range of modalities were categorized into structural magnetic resonance imaging (MRI), diffusion MRI, and radionuclide studies. Biomarkers which differed significantly between suspected CTE and controls were Evans index (P = 0.05), cavum septum pellucidum (CSP) rate (P < 0.0006), length (P < 0.03) and ratio of CSP length to septum length (P < 0.03), regional differences in axial diffusivity (P < 0.05) and free/intracellular water fractions (P < 0.005), single-photon emission computed tomography perfusion abnormalities (P < 0.01), positron emission tomography (PET) signals from tau-binding, glucose-binding, and GABA receptor-binding radionuclides (P < 0.0001, P < 0.005, and P < 0.005, respectively). Important limitations include low specificity in identification of suspected cases of CTE across studies, the need for postmortem validation, and a lack of generalizability to nonprofessional athletes.

CONCLUSIONS

The most promising biomarker is tau-binding radionuclide PET signal because it is most specific to the underlying neuropathology and differentiated CTE from both controls and patients with Alzheimer disease (P < 0.0001). Multimodal imaging will improve specificity further. Future research should minimize variability in identification of suspected cases of CTE using published clinical criteria.

PET Radiopharmaceuticals for Alzheimer's Disease and Parkinson's Disease Diagnosis, the Current and Future Landscape

Ironically, population aging which is considered a public health success has been accompanied by a myriad of new health challenges, which include neurodegenerative disorders (NDDs), the incidence of which increases proportionally to age. Among them, Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common, with the misfolding and the aggregation of proteins being common and causal in the pathogenesis of both diseases. AD is characterized by the presence of hyperphosphorylated τ protein (tau), which is the main component of neurofibrillary tangles (NFTs), and senile plaques the main component of which is β-amyloid peptide aggregates (Aβ). The neuropathological hallmark of PD is α-synuclein aggregates (α-syn), which are present as insoluble fibrils, the primary structural component of Lewy body (LB) and neurites (LN). An increasing number of non-invasive PET examinations have been used for AD, to monitor the pathological progress (hallmarks) of disease. Notwithstanding, still the need for the development of novel detection tools for other proteinopathies still remains. This review, although not exhaustively, looks at the timeline of the development of existing tracers used in the imaging of Aβ and important moments that led to the development of these tracers.

Structure-Activity Relationships of Radioiodinated 6,5,6-Tricyclic Compounds for the Development of Tau Imaging Probes

Tau aggregate, which is the main component of the neurofibrillary tangle, is an attractive imaging target for diagnosing and monitoring the progression of Alzheimer's disease (AD). In this study, we designed and synthesized six radioiodinated 6,5,6-tricyclic compounds to explore novel scaffolds for tau imaging probes. On in vitro autoradiography of AD brain sections, pyridoimidazopyridine and benzimidazopyrimidine derivatives ([¹²⁵I]21 and [¹²⁵I]22, respectively) showed selective binding affinity for tau aggregates, whereas carbazole, pyrrolodipyridine, and pyridoimidazopyrimidine derivatives ([¹²⁵I]10, [¹²⁵I]12, and [¹²⁵I]23, respectively) bound β-amyloid aggregates. In a biodistribution study using normal mice, [¹²⁵I]21 and [¹²⁵I]22 showed high initial uptakes (5.73 and 5.66% ID/g, respectively, at 2 min postinjection) into and rapid washout (0.14 and 0.10% ID/g, respectively, at 60 min postinjection) from the brain. Taken together, two novel scaffolds including pyridoimidazopyridine and benzimidazopyrimidine may be applied to develop useful tau imaging probes.

Chronic Traumatic Encephalopathy

PURPOSE OF REVIEW

This article provides a discussion on the current state of knowledge of chronic traumatic encephalopathy (CTE), with an emphasis on clinical features and emerging biomarkers of the condition.

RECENT FINDINGS

The results of several large brain bank case series among subjects with a history of contact sports or repetitive head trauma have indicated that a high frequency of CTE may exist in this population. However, the true prevalence of CTE among individuals with a history of head trauma remains unknown, given that individuals who experienced cognitive, behavioral, and mood symptoms during life are more likely to have their brains donated for autopsy at death and epidemiologic studies of the condition are lacking. Neuropathologic consensus criteria have been published. Research-based clinical criteria have been proposed and are beginning to be applied, but the definitive diagnosis of CTE in a living patient remains impossible without effective biomarkers for the condition, which is an active area of study.

SUMMARY

The field of CTE research is rapidly growing and parallels many of the advances seen for other neurodegenerative conditions, such as Alzheimer disease decades ago.

In vivo detection of cerebral tau pathology in long-term survivors of traumatic brain injury

Traumatic brain injury (TBI) can trigger progressive neurodegeneration, with tau pathology seen years after a single moderate-severe TBI. Identifying this type of posttraumatic pathology in vivo might help to understand the role of tau pathology in TBI pathophysiology. We used flortaucipir positron emission tomography (PET) to investigate whether tau pathology is present many years after a single TBI in humans. We examined PET data in relation to markers of neurodegeneration in the cerebrospinal fluid (CSF), structural magnetic resonance imaging measures, and cognitive performance. Cerebral flortaucipir binding was variable, with many participants with TBI showing increases in cortical and white matter regions. At the group level, flortaucipir binding was increased in the right occipital cortex in TBI when compared to healthy controls. Flortaucipir binding was associated with increased total tau, phosphorylated tau, and ubiquitin carboxyl-terminal hydrolase L1 CSF concentrations, as well as with reduced fractional anisotropy and white matter tissue density in TBI. Apolipoprotein E (APOE) ε4 genotype affected the relationship between flortaucipir binding and time since injury, CSF β amyloid 1–42 (Aβ42) concentration, white matter tissue density, and longitudinal Mini-Mental State Examination scores in TBI. The results demonstrate that tau PET is a promising approach to investigating progressive neurodegeneration associated with tauopathy after TBI.

Emerging advances of in vivo detection of chronic traumatic encephalopathy and traumatic brain injury

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disorder that is of epidemic proportions in contact sports athletes and is linked to subconcussive and concussive repetitive head impacts (RHI). Although postmortem analysis is currently the only confirmatory method to diagnose CTE, there has been progress in early detection techniques of fluid biomarkers as well as in advanced neuroimaging techniques. Specifically, promising new methods of diffusion MRI and radionucleotide PET scans could aid in the early detection of CTE.The authors examine early detection methods focusing on various neuroimaging techniques. Advances in structural and diffusion MRI have demonstrated the ability to measure volumetric and white matter abnormalities associated with CTE. Recent studies using radionucleotides such as flortaucipir and ¹⁸F-FDDNP have shown binding patterns that are consistent with the four stages of neurofibrillary tangle (NFT) distribution postmortem. Additional research undertakings focusing on fMRI, MR spectroscopy, susceptibility-weighted imaging, and singlephoton emission CT are also discussed as are advanced MRI methods such as diffusiontensor imaging and arterial spin labeled. Neuroimaging is fast becoming a key instrument in early detection and could prove essential for CTE quantification. This review explores a global approach to in vivo early detection.Limited data of in vivo CTE biomarkers with postmortem confirmation are available. While some data exist, they are limited by selection bias. It is unlikely that a single test will be sufficient to properly diagnosis and distinguish CTE from other neurodegenerative diseases such as Alzheimer disease or Frontotemporal Dementia. However, with a combination of fluid biomarkers, neuroimaging, and genetic testing, early detection may become possible.

PET imaging of tau protein targets: a methodology perspective

The two neuropathological hallmarks of Alzheimer's disease (AD) are amyloid-[Formula: see text] plaques and neurofibrillary tangles of tau protein. Fifteen years ago, Positron Emission Tomography (PET) with Pittsburgh Compound B (11C-PiB) enabled selective in-vivo visualization of amyloid-[Formula: see text] plaque deposits and has since provided valuable information about the role of amyloid-[Formula: see text] deposition in AD. The progression of tau deposition has been shown to be highly associated with neuronal loss, neurodegeneration, and cognitive decline. Until recently it was not possible to visualize tau deposition in-vivo, but several tau PET tracers are now available in different stages of clinical development. To date, no tau tracer has been approved by the Food and Drug Administration for use in the evaluation of AD or other tauopathies, despite very active research efforts. In this paper we review the recent developments in tau PET imaging with a focus on in-vivo findings in AD and discuss the challenges associated with tau tracer development, the status of development and validation of different tau tracers, and the clinical information these provide.

High Selectivity and Sensitivity of Oligomeric p-Phenylene Ethynylenes for Detecting Fibrillar and Prefibrillar Amyloid Protein Aggregates

Misfolding and aggregation of amyloid proteins into fibrillar aggregates is a central pathogenic event in neurodegenerative disorders such as Alzheimer's (AD) and Parkinson's diseases (PD). Currently, there is a lack of reliable sensors for detecting the range of protein aggregates involved in disease etiology, particularly the prefibrillar aggregate conformations that are more neurotoxic. In this study, the fluorescent sensing of two novel oligomeric p-phenylene ethynylenes (OPEs), anionic OPE1- and cationic OPE2+, for detecting prefibrillar and fibrillar aggregates of AD-associated amyloid-β (Aβ40 and Aβ42) and PD-associated α-synuclein proteins (wildtype, and single mutants A30P, E35K, and A53T) over their monomeric counterparts, were tested. Furthermore, the performance of OPEs was evaluated and compared to thioflavin T (ThT), the most widely used fibril dye. Our results show that OPE1- and OPE2+ exhibited aggregate-specific binding inducing large fluorescence turn-on and spectral shifts based on a combination of backbone planarization, hydrophobic unquenching, and superluminescent OPE complex formation sensing modes. OPEs exhibited higher selectivity, higher binding affinity, and comparable limits of detection for Aβ40 fibrils compared to ThT. OPE2+ exhibited the largest fluorescence turn-on and highest sensitivity. Significantly, OPEs detected prefibrillar aggregates of Aβ42 and α-synuclein that ThT failed to detect. The superior sensing performance, the nonprotein specific detection, and the ability to selectively detect fibrillar and prefibrillar amyloid protein aggregates point to the potential of OPEs to overcome the limitations of existing probes and promise significant advancement in the detection of the myriad of protein aggregates involved in the early stages of AD and PD.

Characterization of the binding of tau imaging ligands to melanin-containing cells: putative off-target-binding site

OBJECTIVE

Amyloid-β plaques and neurofibrillary tangles composed of tau protein are the neuropathological hallmarks of Alzheimer's disease. In recent years, marked progress has been made in Alzheimer's disease research using tau ligands for positron emission tomography (PET). However, the issue of off-target binding, that is, the binding of ligands to regions without tau pathology, remains unresolved. Tissues with melanin-containing cells (MCCs) have been suggested as binding targets for tau ligands. In the present study, we characterized the MCC-binding properties of representative tau PET ligands.

METHODS

Autoradiographic studies of [¹⁸F]AV-1451 and [¹⁸F]THK5351 were conducted using postmortem human midbrain sections. Saturation-binding assays of [¹⁸F]AV-1451 and [¹⁸F]THK5351 were performed with B16F10 melanoma cells. The blocking effects of 25 compounds against [¹⁸F]THK5351 binding to B16F10 cells were used to investigate the relationship between chemical structure and MCC binding.

RESULTS

Autoradiography demonstrated specific binding of the radioligands in the substantia nigra. [¹⁸F]AV-1451 and [¹⁸F]THK5351 exhibited saturable binding to melanoma cells ([¹⁸F]AV-1451: K_d = 669 ± 196 nM, B_max = 622 ± 269 pmol/mg protein; [¹⁸F]THK5351: K_d = 441 ± 126 nM, B_max = 559 ± 75.5 pmol/mg protein). In blocking studies with melanoma cells, compounds bearing multiple aromatic rings and an aminopyridine group, including tau ligands such as AV-1451, PBB3, and a lead compound of MK-6240, exhibited the inhibition of [¹⁸F]THK5351 binding comparable to self-blocking by THK5351 (> 70% at 10 µM).

CONCLUSIONS

These studies suggest that the binding properties of [¹⁸F]AV-1451 and [¹⁸F]THK5351 are sufficient to expect highlighting of tissues with a high density of MCCs. The findings of the present study should aid the development of neuroimaging ligands that do not bind to MCC.

Hybrid PET-MRI in Alzheimer's Disease Research

Multiple factors, namely amyloid, tau, inflammation, metabolic, and perfusion changes, contribute to the cascade of neurodegeneration and functional decline occurring in Alzheimer's disease (AD). These molecular and cellular processes and related functional and morphological changes can be visualized in vivo by two imaging modalities, namely positron emission tomography (PET) and magnetic resonance imaging (MRI). These imaging biomarkers are now part of the diagnostic algorithm and of particular interest for patient stratification and targeted drug development.In this field the availability of hybrid PET/MR systems not only offers a comprehensive evaluation in a single imaging session, but also opens new possibilities for the integration of the two imaging information. Here, we cover the clinical protocols and practical details of FDG, amyloid, and tau PET/MR imaging as applied in our institutions.

Tau PET imaging in neurodegenerative tauopathies-still a challenge

The accumulation of pathological misfolded tau is a feature common to a collective of neurodegenerative disorders known as tauopathies, of which Alzheimer's disease (AD) is the most common. Related tauopathies include progressive supranuclear palsy (PSP), corticobasal syndrome (CBS), Down's syndrome (DS), Parkinson's disease (PD), and dementia with Lewy bodies (DLB). Investigation of the role of tau pathology in the onset and progression of these disorders is now possible due the recent advent of tau-specific ligands for use with positron emission tomography (PET), including first- (e.g., [¹⁸F]THK5317, [¹⁸F]THK5351, [¹⁸F]AV1451, and [¹¹C]PBB3) and second-generation compounds [namely [¹⁸F]MK-6240, [¹⁸F]RO-948 (previously referred to as [¹⁸F]RO69558948), [¹⁸F]PI-2620, [¹⁸F]GTP1, [¹⁸F]PM-PBB3, and [¹⁸F]JNJ64349311 ([¹⁸F]JNJ311) and its derivative [¹⁸F]JNJ-067)]. In this review we describe and discuss findings from in vitro and in vivo studies using both initial and new tau ligands, including their relation to biomarkers for amyloid-β and neurodegeneration, and cognitive findings. Lastly, methodological considerations for the quantification of in vivo ligand binding are addressed, along with potential future applications of tau PET, including therapeutic trials.

The long-term consequences of repetitive head impacts: Chronic traumatic encephalopathy

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease associated with exposure to repetitive head impacts (RHI). Although described in boxers for almost a century, scientific and public interest in CTE grew tremendously following a report of postmortem evidence of CTE in the first former professional American football player in 2005. Neuropathologic diagnostic criteria for CTE have been defined, with abnormal perivascular deposition of hyperphosphorylated tau at the sulcal depths as the pathognomonic feature. CTE can currently only be diagnosed postmortem, but clinical research criteria for the in vivo diagnosis of CTE have been proposed. The clinical phenotype of CTE is still ill-defined and there are currently no validated biomarkers to support an in-life diagnosis of "Probable CTE." Many knowledge gaps remain regarding the neuropathologic and clinical make-up of CTE. An increased understanding of CTE is critical given the millions that could potentially be impacted by this disease. This chapter describes the state of the literature on CTE. The historical origins of CTE are first presented, followed by a comprehensive description of the neuropathologic and clinical features. The chapter concludes with discussion on future research directions, emphasizing the importance of diagnosing CTE during life to facilitate development of preventative and intervention strategies.

A Systematic Review of Positron Emission Tomography of Tau, Amyloid Beta, and Neuroinflammation in Chronic Traumatic Encephalopathy: The Evidence To Date

Chronic traumatic encephalopathy (CTE) is associated with pathological changes, yet detecting these changes during life has proven elusive. Positron emission tomography (PET) offers the potential for identifying such pathology. Few studies have been completed to date and their approaches and results have been diverse. It was the objective of this review to systematically examine relevant research using ligands for PET that bind to identified pathology in CTE. We focused on identification of patterns of binding and addressing gaps in knowledge of PET imaging for CTE. A comprehensive literature search was conducted. Data used were published on or before May 22, 2017. As the extant literature is limited, any peer-reviewed article assessing military, contact sports athletes, or professional fighters was considered for inclusion. The main outcomes were regional binding to brain regions identified through control comparisons or through clinical metrics (e.g., standardized uptake volume ratios). A total of 1207 papers were identified for review, of which six met inclusion criteria. Meta-analyses were planned but were deemed inappropriate given the small number of studies identified. Methodological concerns in these initial papers included small sample sizes, lack of a control comparison, use of nonstandard statistical procedures to quantify data, and interpretation of potentially off-target binding areas. Across studies, the hippocampi, amygdalae, and midbrain had reasonably consistent increased uptake. Evidence for increased uptake in cortical regions was less consistent. The evidence suggests that the field of PET imaging in those at risk for CTE remains nascent. As the field evolves to include more stringent studies, ligands for PET may prove an important tool in identifying CTE in vivo.

Imaging Protein Misfolding in the Brain Using β-Sheet Ligands

Neurodegenerative diseases characterized by pathological protein accumulation in cells are termed “proteinopathies.” Although various protein aggregates share cross-β-sheet structures, actual conformations vary among each type of protein deposit. Recent progress in the development of radiotracers for positron emission tomography (PET) has enabled the visualization of protein aggregates in living brains. Amyloid PET tracers have been developed, and are widely used for the diagnosis of Alzheimer’s disease and non-invasive assessment of amyloid burden in clinical trials of anti-dementia drugs. Furthermore, several tau PET tracers have been successfully developed and used in the clinical studies. However, recent studies have identified the presence of off-target binding of radiotracers in areas of tau deposition, suggesting that concomitant neuroinflammatory changes might affect tracer binding. In contrast to amyloid and tau PET, there are no established tracers for imaging Lewy bodies in the human brain. In this review, we describe lessons learned from the development of PET tracers and discuss the future direction of tracer development for protein misfolding diseases.

4',6-Diamidino-2-Phenylindole Distinctly Labels Tau Deposits

Tau deposits have distinct biochemical characteristics and vary morphologically based on identification with tau antibodies and several chemical dyes. Here, we report 4',6-diamidino-2-phenylindole (DAPI)-positivity of tau deposits. Furthermore, we investigated the cause for this positivity. DAPI was positive in 3R/4R (3-repeat/4-repeat) tau deposits in Alzheimer's disease, myotonic dystrophy, and neurodegeneration with brain iron accumulation, and in 4R tau deposits in corticobasal degeneration, but negative in 4R tau deposits in frontotemporal dementia with parkinsonism-17 and progressive supranuclear palsy. The peak emission wavelength of DAPI after binding to a tau deposit was similar to that after binding to a nucleus. This DAPI-positivity was conspicuous at the optimum concentration of 2 μg/ml. DAPI-positivity was diminished after formic acid treatment, but preserved after nucleic acid elimination and phosphate moiety blocking. Our results suggest that staining with 2 μg/ml DAPI is a common but useful tool to differentially detect tau deposits in various tauopathies.

Evaluation of 18F-RO-948 PET for Quantitative Assessment of Tau Accumulation in the Human Brain

The availability of tau PET radioligands enables quantitative assessment of tau density and distribution in the human brain. We evaluated the kinetics of a novel radioligand, ¹⁸F-RO-948 (previously referred to as ¹⁸F-RO6958948), and its ability to identify tau positivity in individual patients with mild Alzheimer disease (AD). Methods: Eleven subjects with amyloid-positive mild AD, 5 amyloid-negative older control subjects (OC), and 5 younger control subjects (YC) completed 1 or 2 (4 AD and 5 OC) PET scans with ¹⁸F-RO-948 for 90, 120, or 200 min. The kinetics of the radioligand was evaluated with standard compartmental and noncompartmental models (with plasma data in 70% of cases), tissue-reference methods, and SUV ratio. These approaches were applied to assess the ability of ¹⁸F-RO-948 to discriminate AD subjects from OC subjects. Results: The plasma reference graphical analysis appeared to be the optimal method of quantification for ¹⁸F-RO-948, yielding strictly time-consistent values of distribution volume and distribution volume ratio at 90 min against the analyses at 120 and 200 min. The reference tissue graphical analysis and SUV ratio were cross-validated against plasma reference graphical analysis. Test-retest evaluation showed excellent reproducibility. A proposed novel index of tau load, the regional tau-positive fraction, showed high values in the medial and lateral temporal and parietal regions in AD and successfully separated AD subjects from OC and YC subjects with a significant margin. Conclusion: ¹⁸F-RO-948 appears to be a promising radioligand for quantitative imaging of tau in the brain of AD patients.

The development and validation of tau PET tracers: current status and future directions

Purpose

To provide an overview on positron emission tomography (PET) imaging of tau pathology in Alzheimer’s disease (AD) and other neurodegenerative disorders.

Results

Different classes of tau tracers such as flortaucipir, THK5317, and PBB3 have been developed and utilized in previous clinical studies. In AD, the topographical distribution of tracer binding follows the known distribution of neurofibrillary tangles and is closely associated with neurodegeneration as well as the clinical phenotype of dementia. Significant retention of tracers has also been observed in the frequent site of the 4-repeat (4R) tau isoform deposits in non-AD tauopathies, such as in progressive supranuclear palsy. However, in vitro binding studies indicate that most tau tracers are less sensitive to straight tau filaments, in contrast to their high binding affinity to paired helical filaments of tau (PHF-tau). The first-generation of tau tracers shows off-target binding in the basal ganglia, midbrain, thalamus, choroid plexus, and venous sinus. Off-target binding of THK5351 to monoamine oxidase B (MAO-B) has been observed in disease-associated brain regions linked to neurodegeneration and is associated with astrogliosis in areas of misfolded protein accumulation. The second generation of tau tracers, such as [¹⁸F]MK-6240, is highly selective to PHF-tau with little off-target binding and have enabled the reliable assessment of PHF-tau burden in aging and AD.

Conclusions

Tau PET tracers have enabled in vivo quantification of PHF-tau burden in human brains. Tau PET can help in understanding the underlying cause of dementia symptoms, and in patient selection for clinical trials of anti-dementia therapies.

Dissociation of Tau Deposits and Brain Atrophy in Early Alzheimer's Disease: A Combined Positron Emission Tomography/Magnetic Resonance Imaging Study

The recent advent of tau-specific positron emission tomography (PET) has enabled in vivo assessment of tau pathology in Alzheimer’s disease (AD). However, because PET scanners have limited spatial resolution, the measured signals of small brain structures or atrophied areas are underestimated by partial volume effects (PVEs). The aim of this study was to determine whether partial volume correction (PVC) improves the precision of measures of tau deposits in early AD. We investigated tau deposits in 18 patients with amyloid-positive early AD and in 36 amyloid-negative healthy controls using ¹⁸F-THK5351 PET. For PVC, we applied the SPM toolbox PETPVE12. The PET images were then spatially normalized and subjected to voxel-based group analysis using SPM12 for comparison between the early AD patients and healthy controls. We also compared these two groups in terms of brain atrophy using voxel-based morphometry of MRI. We found widespread neocortical tracer retention predominantly in the posterior cingulate and precuneus areas, but also in the inferior temporal lobes, inferior parietal lobes, frontal lobes, and occipital lobes in the AD patients compared with the controls. The pattern of tracer retention was similar between before and after PVC, suggesting that PVC had little effect on the precision of tau load measures. Gray matter atrophy was detected in the medial/lateral temporal lobes and basal frontal lobes in the AD patients. Interestingly, only a few associations were found between atrophy and tau deposits, even after PVC. In conclusion, PVC did not significantly affect ¹⁸F-THK5351 PET measures of tau deposits. This discrepancy between tau deposits and atrophy suggests that tau load precedes atrophy.

Untangling the Tauopathy for Alzheimer's disease and parkinsonism

Tau is a microtubule-associated protein that mainly localizes to the axon to stabilize axonal microtubule structure and neuronal connectivity. Tau pathology is one of the most common proteinopathies that associates with age-dependent neurodegenerative diseases including Alzheimer's disease (AD), and various Parkinsonism. Tau protein undergoes a plethora of intra-molecular modifications and some altered forms promote the production of toxic oligomeric tau and paired helical filaments, and through which further assemble into neurofibrillary tangles, also known as tauopathy. In this review, we will discuss the recent advances of the tauopathy research, primarily focusing on its association with the early axonal manifestation of axonal transport defect, axonal mitochondrial stress, autophagic vesicle accumulation and the proceeding of axon destruction, and the pathogenic Tau spreading across the synapse. Two alternative strategies either by targeting tau protein itself or by improving the age-related physiological decline are currently racing to find the hopeful treatment for tauopathy. Undoubtedly, more studies are needed to combat this devastating condition that has already affected millions of people in our aging population.

Longitudinal changes of tau PET imaging in relation to hypometabolism in prodromal and Alzheimer's disease dementia

The development of tau-specific positron emission tomography (PET) tracers allows imaging in vivo the regional load of tau pathology in Alzheimer's disease (AD) and other tauopathies. Eighteen patients with baseline investigations enroled in a 17-month follow-up study, including 16 with AD (10 had mild cognitive impairment and a positive amyloid PET scan, that is, prodromal AD, and six had AD dementia) and two with corticobasal syndrome. All patients underwent PET scans with [¹⁸F]THK5317 (tau deposition) and [¹⁸F]FDG (glucose metabolism) at baseline and follow-up, neuropsychological assessment at baseline and follow-up and a scan with [¹¹C]PIB (amyloid-β deposition) at baseline only. At a group level, patients with AD (prodromal or dementia) showed unchanged [¹⁸F]THK5317 retention over time, in contrast to significant decreases in [¹⁸F]FDG uptake in temporoparietal areas. The pattern of changes in [¹⁸F]THK5317 retention was heterogeneous across all patients, with qualitative differences both between the two AD groups (prodromal and dementia) and among individual patients. High [¹⁸F]THK5317 retention was significantly associated over time with low episodic memory encoding scores, while low [¹⁸F]FDG uptake was significantly associated over time with both low global cognition and episodic memory encoding scores. Both patients with corticobasal syndrome had a negative [¹¹C]PIB scan, high [¹⁸F]THK5317 retention with a different regional distribution from that in AD, and a homogeneous pattern of increased [¹⁸F]THK5317 retention in the basal ganglia over time. These findings highlight the heterogeneous propagation of tau pathology among patients with symptomatic AD, in contrast to the homogeneous changes seen in glucose metabolism, which better tracked clinical progression.

Clinical Routine FDG-PET Imaging of Suspected Progressive Supranuclear Palsy and Corticobasal Degeneration: A Gatekeeper for Subsequent Tau-PET Imaging?

Background: F-18-fluordeoxyglucose positron emission tomography (FDG-PET) is widely used for discriminative diagnosis of tau-positive atypical parkinsonian syndromes (T+APS). This approach now stands to be augmented with more specific tau tracers. Therefore, we retrospectively analyzed a large clinical routine dataset of FDG-PET images for evaluation of the strengths and limitations of stand-alone FDG-PET.

Methods: A total of 117 patients (age 68.4 ± 11.1 y) underwent an FDG-PET exam. Patients were followed clinically for a minimum of one year and their final clinical diagnosis was recorded. FDG-PET was rated visually (positive/negative) and categorized as high, moderate or low likelihood of T+APS and other neurodegenerative disorders. We then calculated positive and negative predictive values (PPV/NPV) of FDG-PET readings for the different subgroups relative to their final clinical diagnosis.

Results: Suspected diagnoses were confirmed by clinical follow-up (≥1 y) for 62 out of 117 (53%) patients. PPV was excellent when FDG-PET indicated a high likelihood of T+APS in combination with low to moderate likelihood of another neurodegenerative disorder. PPV was distinctly lower when FDG-PET indicated only a moderate likelihood of T+APS or when there was deemed equal likelihood of other neurodegenerative disorder. NPV of FDG-PET with a low likelihood for T+APS was high.

Conclusions: FDG-PET has high value in clinical routine evaluation of suspected T+APS, gaining satisfactory differential diagnosis in two thirds of the patients. One third of patients would potentially profit from further evaluation by more specific radioligands, with FDG-PET serving gatekeeper function for the more expensive methods.

Dual tracer tau PET imaging reveals different molecular targets for 11C-THK5351 and 11C-PBB3 in the Alzheimer brain

Purpose

Several tau PET tracers have been developed, but it remains unclear whether they bind to the same molecular target on the heterogeneous tau pathology. In this study we evaluated the binding of two chemically different tau-specific PET tracers (¹¹C-THK5351 and ¹¹C-PBB3) in a head-to-head, in vivo, multimodal design.

Methods

Nine patients with a diagnosis of mild cognitive impairment or probable Alzheimer’s disease and cerebrospinal fluid biomarker evidence supportive of the presence of Alzheimer’s disease brain pathology were recruited after thorough clinical assessment. All patients underwent imaging with the tau-specific PET tracers ¹¹C-THK5351 and ¹¹C-PBB3 on the same day, as well as imaging with the amyloid-beta-specific tracer ¹¹C-AZD2184, a T1-MRI sequence, and neuropsychological assessment.

Results

The load and regional distribution of binding differed between ¹¹C-THK5351 and ¹¹C-PBB3 with no statistically significant regional correlations observed between the tracers. The binding pattern of ¹¹C-PBB3, but not that of ¹¹C-THK5351, in the temporal lobe resembled that of ¹¹C-AZD2184, with strong correlations detected between ¹¹C-PBB3 and ¹¹C-AZD2184 in the temporal and occipital lobes. Global cognition correlated more closely with ¹¹C-THK5351 than with ¹¹C-PBB3 binding. Similarly, cerebrospinal fluid tau measures and entorhinal cortex thickness were more closely correlated with ¹¹C-THK5351 than with ¹¹C-PBB3 binding.

Conclusion

This research suggests different molecular targets for these tracers; while ¹¹C-PBB3 appeared to preferentially bind to tau deposits with a close spatial relationship to amyloid-beta, the binding pattern of ¹¹C-THK5351 fitted the expected distribution of tau pathology in Alzheimer’s disease better and was more closely related to downstream disease markers.

Electronic supplementary material

The online version of this article (10.1007/s00259-018-4012-5) contains supplementary material, which is available to authorized users.

Large inter- and intra-case variability of first generation tau PET ligand binding in neurodegenerative dementias

Imaging of pathological tau with positron emission tomography (PET) has the potential to allow early diagnosis of the dementias and monitoring of disease progression, including assessment of therapeutic interventions, in vivo. The first generation of tau PET tracers, including the carbazole flortaucipir and the 2-arylquinolines of the THK series, are now used in clinical research; however, concerns have been raised about off-target binding and low sensitivity.

With the aim to determine the nature of tau pathology depicted by structurally distinct tau ligands we carried out a microscopic neuropathological evaluation in post-mortem human brain tissue of cases with primary and secondary tauopathies. Carbazole and 2-arylquinoline binding was only observed in cases with Alzheimer’s disease and one case with frontotemporal dementia and parkinsonism linked to chromosome 17 exhibiting a R406W MAPT mutation. In end stage Alzheimer’s disease cases, fluorescent imaging with the carbazole T726 and the 2-arylquinoline THK-5117 revealed high inter- and intra-case variability of tracer binding, and this was corroborated by quantitative phosphorimaging with the PET tracer [¹⁸F]THK-5117. Microscopic analysis of the pathological inclusions revealed that the fluorescent tracers preferentially bind to premature tau aggregates. Whilst T726 binding was limited to neuronal tau, THK-5117 additionally depicted neuritic tau. Neither tracer depicted tau in pre-symptomatic disease.

Our results highlight limitations of the first generation of tau PET tracers, in particular lack of correlation between pathological tau load and tracer binding, limited sensitivity to tau in early disease, and high variability in tracer binding between and within cases. Concerns remain that these limitations may also affect the next generation tracers as they target the same high affinity binding site. Therefore, it is crucial to assess inter- and intra-subject correlation of tracer binding with pathological tau load in post-mortem tissue studies, and to rigorously assess novel tau PET tracers before translation into clinical studies.

Tau positron emission tomography imaging in tauopathies: The added hurdle of off-target binding

Ligands targeting tau for use with positron emission tomography have rapidly been developed during the past several years, enabling the in vivo study of tau pathology in patients with Alzheimer's disease and related non-Alzheimer's disease tauopathies. Several candidate compounds have been developed, showing good in vitro characteristics with respect to their ability to bind tau deposits; off-target binding, however, has also been observed. In this short commentary, we briefly summarize the available in vivo and in vitro evidence pertaining to their off-target binding and discuss the different approaches that are needed for the future development of tau positron emission tomography tracers.

[18F]-THK5351 PET Correlates with Topology and Symptom Severity in Progressive Supranuclear Palsy

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.