[18F]AV-1451 PET in behavioral variant frontotemporal dementia due to MAPT mutation

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.

Molecular imaging biomarkers in familial frontotemporal lobar degeneration: Progress and prospects

Familial frontotemporal lobar degeneration (FTLD) is a pathologically heterogeneous group of neurodegenerative diseases with diverse genotypes and clinical phenotypes. Three major mutations were reported in patients with familial FTLD, namely, progranulin (GRN), microtubule-associated protein tau (MAPT), and the chromosome 9 open reading frame 72 (C9orf72) repeat expansion, which could cause neurodegenerative pathological changes years before symptom onset. Noninvasive quantitative molecular imaging with PET or single-photon emission CT (SPECT) allows for selective visualization of the molecular targets in vivo to investigate brain metabolism, perfusion, neuroinflammation, and pathophysiological changes. There was increasing evidence that several molecular imaging biomarkers tend to serve as biomarkers to reveal the early brain abnormalities in familial FTLD. Tau-PET with ¹⁸F-flortaucipir and ¹¹C-PBB3 demonstrated the elevated tau position in patients with FTLD and also showed the ability to differentiate patterns among the different subtypes of the mutations in familial FTLD. Furthermore, dopamine transporter imaging with the ¹¹C-DOPA and ¹¹C-CFT in PET and the ¹²³I-FP-CIT in SPECT revealed the loss of dopaminergic neurons in the asymptomatic and symptomatic patients of familial FTLD. In addition, PET imaging with the ¹¹C-MP4A has demonstrated reduced acetylcholinesterase (AChE) activity in patients with FTLD, while PET with the ¹¹C-DAA1106 and ¹¹C-PK11195 revealed an increased level of microglial activation associated with neuroinflammation even before the onset of symptoms in familial FTLD. ¹⁸F-fluorodeoxyglucose (FDG)-PET indicated hypometabolism in FTLD with different mutations preceded the atrophy on MRI. Identifying molecular imaging biomarkers for familial FTLD is important for the in-vivo assessment of underlying pathophysiological changes with disease progression and future disease-modifying therapy. We review the recent progress of molecular imaging in familial FTLD with focused on the possible implication of these techniques and their prospects in specific mutation types.

The Role of Tau beyond Alzheimer’s Disease: A Narrative Review

Nowadays, there is a need for reliable fluid biomarkers to improve differential diagnosis, prognosis, and the prediction of treatment response, particularly in the management of neurogenerative diseases that display an extreme variability in clinical phenotypes. In recent years, Tau protein has been progressively recognized as a valuable neuronal biomarker in several neurological conditions, not only Alzheimer’s disease (AD). Cerebrospinal fluid and serum Tau have been extensively investigated in several neurodegenerative disorders, from classically defined proteinopathy, e.g., amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Parkinson’s disease (PD), but also in inflammatory conditions such as multiple sclerosis (MS), as a marker of axonal damage. In MS, total Tau (t-Tau) may represent, along with other proteins, a marker with diagnostic and prognostic value. In ALS, t-Tau and, mainly, the phosphorylated-Tau/t-Tau ratio alone or integrated with transactive DNA binding protein of ~43 kDa (TDP-43), may represent a tool for both diagnosis and differential diagnosis of other motoneuron diseases or tauopathies. Evidence indicated the crucial role of the Tau protein in the pathogenesis of PD and other parkinsonian disorders. This narrative review summarizes current knowledge regarding non-AD neurodegenerative diseases and the Tau protein.

Applied Bayesian Approaches for Research in Motor Neuron Disease

Statistical evaluation of empirical data is the basis of the modern scientific method. Available tools include various hypothesis tests for specific data structures, as well as methods that are used to quantify the uncertainty of an obtained result. Statistics are pivotal, but many misconceptions arise due to their complexity and difficult-to-acquire mathematical background. Even though most studies rely on a frequentist interpretation of statistical readouts, the application of Bayesian statistics has increased due to the availability of easy-to-use software suites and an increased outreach favouring this topic in the scientific community. Bayesian statistics take our prior knowledge together with the obtained data to express a degree of belief how likely a certain event is. Bayes factor hypothesis testing (BFHT) provides a straightforward method to evaluate multiple hypotheses at the same time and provides evidence that favors the null hypothesis or alternative hypothesis. In the present perspective, we show the merits of BFHT for three different use cases, including a clinical trial, basic research as well as a single case study. Here we show that Bayesian statistics is a viable addition of a scientist's statistical toolset, which can help to interpret data.

In Vivo 18 F-APN-1607 Tau Positron Emission Tomography Imaging in MAPT Mutations: Cross-Sectional and Longitudinal Findings

BACKGROUND

Frontotemporal lobar degeneration with tauopathy caused by MAPT (microtubule-associated protein tau) mutations is a highly heterogenous disorder. The ability to visualize and longitudinally monitor tau deposits may be beneficial to understand disease pathophysiology and predict clinical trajectories.

OBJECTIVE

The aim of this study was to investigate the cross-sectional and longitudinal ¹⁸ F-APN-1607 positron emission tomography/computed tomography (PET/CT) imaging findings in MAPT mutation carriers.

METHODS

Seven carriers of MAPT mutations (six within exon 10 and one outside of exon 10) and 15 healthy control subjects were included. All participants underwent ¹⁸ F-APN-1607 PET/CT at baseline. Three carriers of exon 10 mutations received follow-up ¹⁸ F-APN-1607 PET/CT scans. Standardized uptake value ratio (SUVR) maps were obtained using the cerebellar gray matter as the reference region. SUVR values observed in MAPT mutation carriers were normalized to data from healthy control subjects. A regional SUVR z score ≥ 2 was used as the criterion to define positive ¹⁸ F-APN-1607 PET/CT findings.

RESULTS

Although the seven study patients had heterogenous clinical phenotypes, all showed a significant ¹⁸ F-APN-1607 uptake characterized by high-contrast signals. However, the anatomical localization of tau deposits differed in patients with distinct clinical symptoms. Follow-up imaging data, which were available for three patients, demonstrated worsening trends in patterns of tau accumulation over time, which were paralleled by a significant clinical deterioration.

CONCLUSIONS

Our data represent a promising step in understanding the usefulness of ¹⁸ F-APN-1607 PET/CT imaging for detecting tau accumulation in MAPT mutation carriers. Our preliminary follow-up data also suggest the potential value of ¹⁸ F-APN-1607 PET/CT for monitoring the longitudinal trajectories of frontotemporal lobar degeneration caused by MAPT mutations. © 2021 International Parkinson and Movement Disorder Society.

[18F]Flortaucipir PET Across Various MAPT Mutations in Presymptomatic and Symptomatic Carriers

OBJECTIVE

To assess the [18F]flortaucipir binding distribution across MAPT mutations in presymptomatic and symptomatic carriers.

METHODS

We compared regional [18F]flortaucipir binding potential (BPND) derived from a 130-minute dynamic [18F]flortaucipir PET scan in 9 (pre)symptomatic MAPT mutation carriers (4 with P301L [1 symptomatic], 2 with R406W [1 symptomatic], 1 presymptomatic L315R, 1 presymptomatic S320F, and 1 symptomatic G272V carrier) with 30 cognitively normal controls and 52 patients with Alzheimer disease.

RESULTS

[18F]Flortaucipir BPND images showed overall highest binding in the symptomatic carriers. This was most pronounced in the symptomatic R406W carrier in whom tau binding exceeded the normal control range in the anterior cingulate cortex, insula, amygdala, temporal, parietal, and frontal lobe. Elevated medial temporal lobe BPND was observed in a presymptomatic R406W carrier. The single symptomatic carrier and 1 of the 3 presymptomatic P301L carriers showed elevated [18F]flortaucipir BPND in the insula, parietal, and frontal lobe compared to controls. The symptomatic G272V carrier exhibited a widespread elevated cortical BPND, with at neuropathologic examination a combination of 3R pathology and encephalitis. The L315R presymptomatic mutation carrier showed higher frontal BPND compared to controls. The BPND values of the S320F presymptomatic mutation carrier fell within the range of controls.

CONCLUSION

Presymptomatic MAPT mutation carriers already showed subtle elevated tau binding, whereas symptomatic MAPT mutation carriers showed a more marked increase in [18F]flortaucipir BPND. Tau deposition was most pronounced in R406W MAPT (pre)symptomatic mutation carriers, which is associated with both 3R and 4R tau accumulation. Thus, [18F]flortaucipir may serve as an early biomarker for MAPT mutation carriers in mutations that cause 3R/4R tauopathies.

Crossed cerebellar diaschisis on 18F-FDG PET: Frequency across neurodegenerative syndromes and association with 11C-PIB and 18F-Flortaucipir

We used 18F-FDG-PET to investigate the frequency of crossed cerebellar diaschisis (CCD) in 197 patients with various syndromes associated with neurodegenerative diseases. In a subset of 117 patients, we studied relationships between CCD and cortical asymmetry of Alzheimer's pathology (β-amyloid (11C-PIB) and tau (18F-Flortaucipir)). PET images were processed using MRIs to derive parametric SUVR images and define regions of interest. Indices of asymmetry were calculated in the cerebral cortex, basal ganglia and cerebellar cortex. Across all patients, cerebellar 18F-FDG asymmetry was associated with reverse asymmetry of 18F-FDG in the cerebral cortex (especially frontal and parietal areas) and basal ganglia. Based on our operational definition (cerebellar asymmetry >3% with contralateral supratentorial hypometabolism), significant CCD was present in 47/197 (24%) patients and was most frequent in corticobasal syndrome and semantic and logopenic variants of primary progressive aphasia. In β-amyloid-positive patients, mediation analyses showed that 18F-Flortaucipir cortical asymmetry was associated with cerebellar 18F-FDG asymmetry, but that cortical 18F-FDG asymmetry mediated this relationship. Analysis of 18F-FDG-SUVR values suggested that CCD might also occur in the absence of frank cerebellar 18F-FDG asymmetry due to symmetrical supratentorial degeneration resulting in a bilateral diaschisis process.

The Role of 18F-Flortaucipir (AV-1451) in the Diagnosis of Neurodegenerative Disorders

Tau protein plays a vital role in maintaining the structural and functional integrity of the nervous system; however, hyperphosphorylation or abnormal phosphorylation of tau protein plays an essential role in the pathogenesis of several neurodegenerative disorders. The development of radioligand such as the 18F-flortaucipir (AV-1451) has provided us with the opportunity to assess the underlying tau pathology in various etiologies of dementia. For the purpose of this article, we aimed to evaluate the utility of 18F-AV-1451 in the differential diagnosis of various neurodegenerative disorders. We used PubMed to look for the latest, peer-reviewed, and informative articles. The scope of discussion included the role of 18F-AV-1451 positron emission tomography (PET) to aid in the diagnosis of Alzheimer’s disease (AD), frontotemporal dementia (FTD), dementia with Lewy bodies (DLB), and Parkinson’s disease with dementia (PDD). We also discussed if the tau burden identified by neuroimaging correlated well with the clinical severity and identified the various challenges of 18F-AV-1451 PET. We concluded that although the role of 18F-AV-1451 seems promising in the neuroimaging of AD, the benefit appears uncertain when it comes to the non-Alzheimer’s tauopathies. More research is required to identify the off-target binding sites of 18F-AV-1451 to determine its clinical utility in the future.

Tau PET Imaging with [18F]PM-PBB3 in Frontotemporal Dementia with MAPT Mutation

BACKGROUND

Flortaucipir (AV-1451) and pyridinyl-butadienyl-benzothiazole 3 (PBB3) are newly developed and commonly used positron emission tomography (PET) tracers to detect tau deposition in tauopathies, including frontotemporal dementia (FTD). [18F]PM-PBB3, as a second-generation compound, has not been described in FTD so far.

OBJECTIVE

We aim to explore the in vivo performance of [18F]PM-PBB3 tau PET in an FTD case caused by microtubule-associated protein tau (MAPT) mutation and compare the binding to different tau strains between AV-1451 and PBB3.

METHODS

We reported the clinical and FDG, [18F]AV45 amyloid and [18F]PM-PBB3 tau PET findings in a patient with FTD of P301L MAPT mutation. Based on our results and published data, we summarized and compared the different utilities of tau PET tracers of AV-1451 and PBB3 in FTD with MAPT mutation.

RESULTS

The patient demonstrated slightly diffuse [18F]PM-PBB3 tau deposition in cerebral lobes especially in the left frontal lobe overlapping with the hypometabolic region detected by FDG PET. From our analysis of 35 FTD patients with MAPT mutation who underwent tau PET, AV-1451 was positive in all (n = 11) patients with mutations known to cause three and four repeat (3R/4R) tau deposition and in 14.3% (n = 2/14) of 4R tauopathies, while positive PBB3 retention was found in all patients with both 3R/4R (n = 2) and 4R (n = 8) tau.

CONCLUSIONS

[18F]PM-PBB3 tau PET assisted the diagnosis of FTD with P301L MAPT mutation, and might be useful in the in vivo detection of both 3R/4R and 4R tau domains in the brain of FTD with MAPT mutation.

Progressive Supranuclear Palsy and Corticobasal Degeneration

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are neurodegenerative tauopathies with neuronal and glial lesions composed of tau that is composed predominantly of isomers with four repeats in the microtubule-binding domain (4R tau). The brain regions vulnerable to pathology in PSP and CBD overlap, but there are differences, particularly with respect to distribution of neuronal loss, the relative abundance of neuronal and glial lesions, the morphologic features of glial lesions, and the frequency of comorbid pathology. Both PSP and CBD have a wide spectrum of clinical manifestations, including disorders of movement and cognition. Recognition of phenotypic diversity in PSP and CBD may improve antemortem diagnostic accuracy, which tends to be very good for the most common presentation of PSP (Richardson syndrome), but poor for the most characteristic presentation of CBD (corticobasal syndrome: CBS). Development of molecular and imaging biomarkers may improve antemortem diagnostic accuracy. Currently, multidisciplinary symptomatic and supportive treatment with pharmacological and non-pharmacological strategies remains the standard of care. In the future, experimental therapeutic trials will be important to slow disease progression.

One-Stop Shop: 18F-Flortaucipir PET Differentiates Amyloid-Positive and -Negative Forms of Neurodegenerative Diseases

Tau protein aggregations are a hallmark of amyloid-associated Alzheimer disease and some forms of non-amyloid-associated frontotemporal lobar degeneration. In recent years, several tracers for in vivo tau imaging have been under evaluation. This study investigated the ability of ¹⁸F-flortaucipir PET not only to assess tau positivity but also to differentiate between amyloid-positive and -negative forms of neurodegeneration on the basis of different ¹⁸F-flortaucipir PET signatures. Methods: The ¹⁸F-flortaucipir PET data of 35 patients with amyloid-positive neurodegeneration, 19 patients with amyloid-negative neurodegeneration, and 17 healthy controls were included in a data-driven scaled subprofile model (SSM)/principal-component analysis (PCA) identifying spatial covariance patterns. SSM/PCA pattern expression strengths were tested for their ability to predict amyloid status in a receiver-operating-characteristic analysis and validated with a leave-one-out approach. Results: Pattern expression strengths predicted amyloid status with a sensitivity of 0.94 and a specificity of 0.83. A support vector machine classification based on pattern expression strengths in 2 different SSM/PCA components yielded a prediction accuracy of 98%. Anatomically, prediction performance was driven by parietooccipital gray matter in amyloid-positive patients versus predominant white matter binding in amyloid-negative patients. Conclusion: SSM/PCA-derived binding patterns of ¹⁸F-flortaucipir differentiate between amyloid-positive and -negative neurodegenerative diseases with high accuracy. ¹⁸F-flortaucipir PET alone may convey additional information equivalent to that from amyloid PET. Together with a perfusion-weighted early-phase acquisition (¹⁸F-FDG PET-equivalent), a single scan potentially contains comprehensive information on amyloid (A), tau (T), and neurodegeneration (N) status as required by recent biomarker classification algorithms (A/T/N).

Comparison of regional flortaucipir PET with quantitative tau immunohistochemistry in three subjects with Alzheimer's disease pathology: a clinicopathological study

Background

The objective of this study was to make a quantitative comparison of flortaucipir PET retention with pathological tau and β-amyloid across a range of brain regions at autopsy.

Methods

Patients with dementia (two with clinical diagnosis of AD, one undetermined), nearing the end of life, underwent 20-min PET, beginning 80 min after an injection of ~370 mBq flortaucipir [¹⁸F]. Neocortical, basal ganglia, and limbic tissue samples were obtained bilaterally from 19 regions at autopsy and subject-specific PET regions of interest corresponding to the 19 sampled target tissue regions in each hemisphere were hand drawn on the PET images. SUVr values were calculated for each region using a cerebellar reference region. Abnormally phosphorylated tau (Ptau) and amyloid-β (Aβ) tissue concentrations were measured for each tissue region with an antibody capture assay (Histelide) using AT8 and H31L21 antibodies respectively.

Results

The imaging-to-autopsy interval ranged from 4–29 days. All three subjects had intermediate to high levels of AD neuropathologic change at autopsy. Mean cortical SUVr averaged across all three subjects correlated significantly with the Ptau immunoassay (Pearson r = 0.81; p < 0.0001). When Ptau and Aβ_1-42 were both included in the model, the Ptau correlation with flortaucipir SUVr was preserved but there was no correlation of Aβ_1-42 with flortaucipir. There was also a modest correlation between limbic (hippocampal/entorhinal and amygdala) flortaucipir SUVr and Ptau (Pearson r = 0.52; p < 0.080). There was no significant correlation between SUVr and Ptau in basal ganglia.

Conclusions

The results of this pilot study support a quantitative relationship between cortical flortaucipir SUVr values and quantitative measures of Ptau at autopsy. Additional research including more cases is needed to confirm the generalizability of these results.

Trial registration, NIH Clinicaltrials.gov NCT # 02516046. Registered August 27, 2015. https://clinicaltrials.gov/ct2/show/NCT02516046?term=02516046&draw=2&rank=1

Progressive supranuclear palsy: Advances in diagnosis and management

Progressive supranuclear palsy (PSP) is a complex clinicopathologic disease with no current cure or disease modulating therapies that can only be definitively confirmed at autopsy. Growing understanding of the phenotypic diversity of PSP has led to expanded clinical criteria and new insights into etiopathogenesis that coupled with improved in vivo biomarkers makes increased access to current clinical trials possible. Current standard-of-care treatment of PSP is multidisciplinary, supportive and symptomatic, and several trials of potentially disease modulating agents have already been completed with disappointing results. Current ongoing clinical trials target the abnormal aggregation of tau through a variety of mechanisms including immunotherapy and gene therapy offer a more direct method of treatment. Here we review PSP clinicopathologic correlations, in vivo biomarkers including MRI, PET, and CSF biomarkers. We additionally review current pharmacologic and non-pharmacologic methods of treatment, prior and ongoing clinical trials in PSP. Newly expanded clinical criteria and improved specific biomarkers will aid in identifying patients with PSP earlier and more accurately and expand access to these potentially beneficial clinical trials.

Neuroinflammation and protein aggregation co-localize across the frontotemporal dementia spectrum

The clinical syndromes of frontotemporal dementia are clinically and neuropathologically heterogeneous, but processes such as neuroinflammation may be common across the disease spectrum. We investigated how neuroinflammation relates to the localization of tau and TDP-43 pathology, and to the heterogeneity of clinical disease. We used PET in vivo with (i) 11C-PK-11195, a marker of activated microglia and a proxy index of neuroinflammation; and (ii) 18F-AV-1451, a radioligand with increased binding to pathologically affected regions in tauopathies and TDP-43-related disease, and which is used as a surrogate marker of non-amyloid-β protein aggregation. We assessed 31 patients with frontotemporal dementia (10 with behavioural variant, 11 with the semantic variant and 10 with the non-fluent variant), 28 of whom underwent both 18F-AV-1451 and 11C-PK-11195 PET, and matched control subjects (14 for 18F-AV-1451 and 15 for 11C-PK-11195). We used a univariate region of interest analysis, a paired correlation analysis of the regional relationship between binding distributions of the two ligands, a principal component analysis of the spatial distributions of binding, and a multivariate analysis of the distribution of binding that explicitly controls for individual differences in ligand affinity for TDP-43 and different tau isoforms. We found significant group-wise differences in 11C-PK-11195 binding between each patient group and controls in frontotemporal regions, in both a regions-of-interest analysis and in the comparison of principal spatial components of binding. 18F-AV-1451 binding was increased in semantic variant primary progressive aphasia compared to controls in the temporal regions, and both semantic variant primary progressive aphasia and behavioural variant frontotemporal dementia differed from controls in the expression of principal spatial components of binding, across temporal and frontotemporal cortex, respectively. There was a strong positive correlation between 11C-PK-11195 and 18F-AV-1451 uptake in all disease groups, across widespread cortical regions. We confirmed this association with post-mortem quantification in 12 brains, demonstrating strong associations between the regional densities of microglia and neuropathology in FTLD-TDP (A), FTLD-TDP (C), and FTLD-Pick's. This was driven by amoeboid (activated) microglia, with no change in the density of ramified (sessile) microglia. The multivariate distribution of 11C-PK-11195 binding related better to clinical heterogeneity than did 18F-AV-1451: distinct spatial modes of neuroinflammation were associated with different frontotemporal dementia syndromes and supported accurate classification of participants. These in vivo findings indicate a close association between neuroinflammation and protein aggregation in frontotemporal dementia. The inflammatory component may be important in shaping the clinical and neuropathological patterns of the diverse clinical syndromes of frontotemporal dementia.

A new perspective for advanced positron emission tomography-based molecular imaging in neurodegenerative proteinopathies

Recent studies in neurodegenerative conditions have increasingly highlighted that the same neuropathology can trigger different clinical phenotypes or, vice-versa, that similar phenotypes can be triggered by different neuropathologies. This evidence has called for the adoption of a pathology spectrum-based approach to study neurodegenerative proteinopathies. These conditions share brain deposition of abnormal protein aggregates, leading to aberrant biochemical, metabolic, functional, and structural changes. Positron emission tomography (PET) is a well-recognized and unique tool for the in vivo assessment of brain neuropathology, and novel PET techniques are emerging for the study of specific protein species. Today, key applications of PET range from early research and clinical diagnostic tools to their use in clinical trials for both participants screening and outcome evaluation. This position article critically reviews the role of distinct PET molecular tracers for different neurodegenerative proteinopathies, highlighting their strengths, weaknesses, and opportunities, with special emphasis on methodological challenges and future applications.

FTD spectrum: Neuroimaging across the FTD spectrum

Frontotemporal dementia is a complex and heterogeneous neurodegenerative disease that encompasses many clinical syndromes, pathological diseases, and genetic mutations. Neuroimaging has played a critical role in our understanding of the underlying pathophysiology of frontotemporal dementia and provided biomarkers to aid diagnosis. Early studies defined patterns of neurodegeneration and hypometabolism associated with the clinical, pathological and genetic aspects of frontotemporal dementia, with more recent studies highlighting how the breakdown of structural and functional brain networks define frontotemporal dementia. Molecular positron emission tomography ligands allowing the in vivo imaging of tau proteins have also provided important insights, although more work is needed to understand the biology of the currently available ligands.

Dementia spectrum disorders: lessons learnt from decades with PET research

The dementia spectrum encompasses a range of disorders with complex diagnosis, pathophysiology and limited treatment options. Positron emission tomography (PET) imaging provides insights into specific neurodegenerative processes underlying dementia disorders in vivo. Here we focus on some of the most common dementias: Alzheimer's disease, Parkinsonism dementias including Parkinson's disease with dementia, dementia with Lewy bodies, progressive supranuclear palsy and corticobasal syndrome, and frontotemporal lobe degeneration. PET tracers have been developed to target specific proteinopathies (amyloid, tau and α-synuclein), glucose metabolism, cholinergic system and neuroinflammation. Studies have shown distinct imaging abnormalities can be detected early, in some cases prior to symptom onset, allowing disease progression to be monitored and providing the potential to predict symptom onset. Furthermore, advances in PET imaging have identified potential therapeutic targets and novel methods to accurately discriminate between different types of dementias in vivo. There are promising imaging markers with a clinical application on the horizon, however, further studies are required before they can be implantation into clinical practice.

18F-flortaucipir (AV-1451) tau PET in frontotemporal dementia syndromes

BACKGROUND

The tau positron emission tomography (PET) ligand 18F-flortaucipir binds to paired helical filaments of tau in aging and Alzheimer's disease (AD), but its utility in detecting tau aggregates in frontotemporal dementia (FTD) is uncertain.

METHODS

We performed 18F-flortaucipir imaging in patients with the FTD syndromes (n = 45): nonfluent variant primary progressive aphasia (nfvPPA) (n = 11), corticobasal syndrome (CBS) (n = 10), behavioral variant frontotemporal dementia (bvFTD) (n = 10), semantic variant primary progressive aphasia (svPPA) (n = 2) and FTD associated pathogenic genetic mutations microtubule-associated protein tau (MAPT) (n = 6), chromosome 9 open reading frame 72 (C9ORF72) (n = 5), and progranulin (GRN) (n = 1). All patients underwent MRI and β-amyloid biomarker testing via 11C-PiB or cerebrospinal fluid. 18F-flortaucipir uptake in patients was compared to 53 β-amyloid negative normal controls using voxelwise and pre-specified region of interest approaches.

RESULTS

On qualitative assessment, patients with nfvPPA showed elevated 18F-flortacupir binding in the left greater than right inferior frontal gyrus. Patients with CBS showed elevated binding in frontal white matter, with higher cortical gray matter uptake in a subset of β-amyloid-positive patients. Five of ten patients with sporadic bvFTD demonstrated increased frontotemporal binding. MAPT mutation carriers had elevated 18F-flortaucipir retention primarily, but not exclusively, in mutations with Alzheimer's-like neurofibrillary tangles. However, tracer retention was also seen in patients with svPPA, and the mutations C9ORF72, GRN predicted to have TDP-43 pathology. Quantitative region-of-interest differences between patients and controls were seen only in inferior frontal gyrus in nfvPPA and left insula and bilateral temporal poles in MAPT carriers. No significant regional differences were found in CBS or sporadic bvFTD. Two patients underwent postmortem neuropathological examination. A patient with C9ORF72, TDP-43-type B pathology, and incidental co-pathology of scattered neurofibrillary tangles in the middle frontal, inferior temporal gyrus showed corresponding mild 18F-flortaucipir retention without additional uptake matching the widespread TDP-43 type B pathology. A patient with sporadic bvFTD demonstrated punctate inferior temporal and hippocampus tracer retention, corresponding to the area of severe argyrophilic grain disease pathology.

CONCLUSIONS

18F-flortaucipir in patients with FTD and predicted tauopathy or TDP-43 pathology demonstrated limited sensitivity and specificity. Further postmortem pathological confirmation and development of FTD tau-specific ligands are needed.

In vivo evidence for pre-symptomatic neuroinflammation in a MAPT mutation carrier

Neuroinflammation occurs in frontotemporal dementia, however its timing relative to protein aggregation and neuronal loss is unknown. Using positron emission tomography and magnetic resonance imaging to quantify these processes in a pre‐symptomatic carrier of the 10 + 16 MAPT mutation, we show microglial activation in frontotemporal regions, despite a lack of protein aggregation or atrophy in these areas. The distribution of microglial activation better discriminated the carrier from controls than did protein aggregation at this pre‐symptomatic disease stage. Our findings suggest an early role for microglial activation in frontotemporal dementia. Longitudinal studies are needed to explore the causality of this pathophysiological association.

[18F]AV-1451 binding in vivo mirrors the expected distribution of TDP-43 pathology in the semantic variant of primary progressive aphasia

INTRODUCTION

Semantic dementia, including the semantic variant of primary progressive aphasia (svPPA), is strongly associated with TAR-DNA binding protein 43 (TDP-43) type C pathology. It provides a useful model in which to test the specificity of in vivo binding of the putative tau ligand [¹⁸F]AV-1451, which is elevated in frontotemporal lobar degeneration tauopathies.

METHODS AND RESULTS

Seven patients (five with svPPA and two with 'right' semantic dementia) and 12 healthy controls underwent positron emission tomography brain imaging with [¹⁸F]AV-1451. Two independent preprocessing methods were used. For both methods, all patients had clearly elevated binding potential (BP_ND (non-displaceable binding potential)) in temporal lobes, lateralising according to their clinical syndrome and evident in raw images. Region of interest analyses confirmed that BP_ND was significantly increased in temporal regions, insula and fusiform gyrus, consistent with those areas known to be most affected in semantic dementia. Hierarchical cluster analysis, based on the distribution of [¹⁸F]AV-1451 binding potential, separated semantic dementia from controls with 86% sensitivity and 100% specificity.

CONCLUSIONS

[¹⁸F]AV-1451 binds in vivo regions that are likely to contain TDP-43 and not significant tau pathology. While this suggests a non-tau target for [¹⁸F]AV-1451, the pathological regions in semantic dementia do not normally contain significant levels of recently proposed 'off target' binding sites for [¹⁸F]AV-1451, such as neuronal monoamine oxidase or neuromelanin. Postmortem and longitudinal data will be useful to assess the utility of [¹⁸F]AV-1451 to differentiate and track different types of frontotemporal lobar degeneration.

[18F]AV-1451 binding is increased in frontotemporal dementia due to C9orf72 expansion

The PET ligand [¹⁸F]AV‐1451 was developed to bind tau pathology in Alzheimer's disease, but increased binding has been shown in both genetic tauopathies and in semantic dementia, a disease strongly associated with TDP‐43 pathology. Here we assessed [¹⁸F]AV‐1451 binding in behavioral variant frontotemporal dementia due to a hexanucleotide repeat expansion in C9orf72, characterized by TDP‐43 pathology. We show that the C9orf72 mutation increases binding in frontotemporal cortex, with a distinctive distribution of binding compared with healthy controls.

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.

Neuroimmune Tau Mechanisms: Their Role in the Progression of Neuronal Degeneration

Progressive neurodegenerative pathologies in aged populations are an issue of major concern worldwide. The microtubule-associated protein tau is able to self-aggregate to form abnormal supramolecular structures that include small oligomers up to complex polymers. Tauopathies correspond to a group of diseases that share tau pathology as a common etiological agent. Since microglial cells play a preponderant role in innate immunity and are the main source of proinflammatory factors in the central nervous system (CNS), the alterations in the cross-talks between microglia and neuronal cells are the main focus of studies concerning the origins of tauopathies. According to evidence from a series of studies, these changes generate a feedback mechanism reactivating microglia and provoking constant cellular damage. Thus, the previously summarized mechanisms could explain the onset and progression of different tauopathies and their functional/behavioral effects, opening the window towards an understanding of the molecular basis of anomalous tau interactions. Despite clinical and pathological differences, increasing experimental evidence indicates an overlap between tauopathies and synucleinopathies, considering that neuroinflammatory events are involved and the existence of protein misfolding. Neurofibrillary tangles of pathological tau (NFT) and Lewy bodies appear to coexist in certain brain areas. Thus, the co-occurrence of synucleinopathies with tauopathies is evidenced by several investigations, in which NFT were found in the substantia nigra of patients with Parkinson’s disease, suggesting that the pathologies share some common features at the level of neuroinflammatory events.

In vivo 18F-AV-1451 tau PET signal in MAPT mutation carriers varies by expected tau isoforms

OBJECTIVE

To evaluate 18F-AV-1451 tau PET binding among microtubule-associated protein tau (MAPT) mutation carriers.

METHODS

Using a case-control study, we quantitatively and qualitatively compared tau PET scans in 10 symptomatic and 3 asymptomatic MAPT mutation carriers (n = 13, age range 42-67 years) with clinically normal (CN) participants (n = 241, age range 42-67 years) and an Alzheimer disease (AD) dementia cohort (n = 30, age range 52-67 years). Eight participants had MAPT mutations that involved exon 10 (N279K n = 5, S305N n = 2, P301L n = 1) and tend to form 4R tau pathology, and 5 had mutations outside exon 10 (V337M n = 2, R406W n = 3) and tend to form mixed 3R/4R tau pathology.

RESULTS

Tau PET signal was qualitatively and quantitatively different between participants with AD, CN participants, and MAPT mutation carriers, with the greatest signal intensity in those with AD and minimal regional signal in MAPT mutation carries with mutations in exon 10. However, MAPT mutation carriers with mutations outside exon 10 had uptake levels within the AD range, which was significantly higher than both MAPT mutation carriers with mutations in exon 10 and controls.

CONCLUSIONS

Tau PET shows higher magnitude of binding in MAPT mutation carriers who harbor mutations that are more likely to produce AD-like tau pathology (e.g., in our series, the non-exon 10 families tend to accumulate mixed 3R/4R aggregates). Exon 10 splicing determines the balance of 3R and 4R tau isoforms, with some mutations involving exon 10 predisposing to a greater proportion of 4R aggregates and consequently a lower level of AV-1451 binding, as seen in this case series, thus supporting the notion that this tau PET ligand has specific binding properties for AD-like tau pathology.

Predominant subcortical accumulation of 18F-flortaucipir binding in behavioral variant frontotemporal dementia

Behavioral variant frontotemporal dementia (bvFTD) is the most common form of frontotemporal dementia, and tau pathology can be found in 40%-50% of bvFTD patients. In this study, we sought to investigate ¹⁸F-flortaucipir-binding patterns and their correlates in clinically diagnosed bvFTD patients by comparing with results for Alzheimer's disease (AD) patients. We enrolled 20 bvFTD, 20 AD, and 20 age-matched healthy subjects who underwent neuropsychological tests, magnetic resonance imaging, and tau positron emission tomography scans with ¹⁸F-flortaucipir. Regional standardized uptake value ratios for the cerebral cortex and underlying white matter were compared between the 2 groups. The bvFTD patients showed increased ¹⁸F-flortaucipir binding in the putamen and globus pallidus when compared to the healthy controls. In addition, bvFTD was associated with increased binding in the white matter regions underlying the frontal, anterior cingulate, and insula cortices. The bvFTD patients may exhibit predominantly subcortical ¹⁸F-flortaucipir-binding pattern that is distinct from the patterns seen in AD patients. We hypothesize that the clinical characteristics of bvFTD patients may be attributable to the dysfunctional frontal-subcortical networks. However, concerns remain regarding unknown "off-target" binding in the white matter and the basal ganglia.

Tau Imaging in Parkinsonism: What Have We Learned So Far?

Background

Positron emission tomography ligands are now available that bind to tau proteins in the brain, providing the exciting opportunity to assess the presence and distribution of tau in vivo in living patients.

Methods

This manuscript performed a systematic review of studies that have performed tau PET imaging in patients with parkinsonian disorders. Pubmed was searched up to November 2017, and the review included case reports and patient-control studies.

Results

Most tau-PET studies have utilized the [¹⁸F]AV-1451 ligand, with a few using the [¹¹C]PBB3 and [¹⁸F]THK-5351 ligands. Elevated cortical tau-PET uptake has been observed in Parkinson's disease dementia and dementia with Lewy bodies, presumed to be related to Alzheimer's disease-related pathology. Mild patterns of tau-PET uptake have been observed in subcortical structures in progressive supranuclear palsy and subcortical structures and motor cortex in corticobasal syndrome, although discrepancy with autoradiographic studies that show lack of binding to 4-repeat tau and "off-target" binding observed in subcortical structures limits interpretation of these findings. Findings in frontotemporal dementia with tau mutations are variable, but elevated signal is most pronounced in mutations with deposition of both 3 and 4-repeat tau. Elevated tau-PET uptake has also been observed in multiple system atrophy, a synucleinopathy.

Conclusion

The value of the current generation of tau-PET ligands varies across Parkinsonian syndromes depending upon underlying variability in tau pathology and "off-target" binding. More work is needed to understand the biological basis of binding and more specific tau PET ligands are needed to study parkinsonian disorders.

Neuroimaging in Dementia

Although the diagnosis of dementia still is primarily based on clinical criteria, neuroimaging is playing an increasingly important role. This is in large part due to advances in techniques that can assist with discriminating between different syndromes. Magnetic resonance imaging remains at the core of differential diagnosis, with specific patterns of cortical and subcortical changes having diagnostic significance. Recent developments in molecular PET imaging techniques have opened the door for not only antemortem but early, even preclinical, diagnosis of underlying pathology. This is vital, as treatment trials are underway for pharmacological agents with specific molecular targets, and numerous failed trials suggest that earlier treatment is needed. This article provides an overview of classic neuroimaging findings as well as new and cutting-edge research techniques that assist with clinical diagnosis of a range of dementia syndromes, with an emphasis on studies using pathologically proven cases.

Tau-imaging in neurodegeneration

Pathological cerebral aggregations of proteins are suggested to play a crucial role in the development of neurodegenerative disorders. For example, aggregation of the protein ß-amyloid in form of extracellular amyloid-plaques as well as intraneuronal depositions of the protein tau in form of neurofibrillary tangles represent hallmarks of Alzheimer's disease (AD). Recently, novel tracers for in vivo molecular imaging of tau-aggregates in the brain have been introduced, complementing existing tracers for imaging amyloid-plaques. Available data on these novel tracers indicate that the subject of Tau-PET may be of considerable complexity. On the one hand this refers to the various forms of appearance of tau-pathology in different types of neurodegenerative disorders. On the other hand, a number of hurdles regarding validation of these tracers still need to be overcome with regard to comparability and standardization of the different tracers, observed off-target/non-specific binding and quantitative interpretation of the signal. These issues will have to be clarified before systematic clinical application of this exciting new methodological approach may become possible. Potential applications refer to early detection of neurodegeneration, differential diagnosis between tauopathies and non-tauopathies and specific patient selection and follow-up in therapy trials.

Clinical Applications of Small-molecule PET Radiotracers: Current Progress and Future Outlook

Radiotracers, or radiopharmaceuticals, are bioactive molecules tagged with a radionuclide used for diagnostic imaging or radiotherapy and, when a positron-emitting radionuclide is chosen, the radiotracers are used for PET imaging. The development of novel PET radiotracers in many ways parallels the development of new pharmaceuticals, and small molecules dominate research and development pipelines in both disciplines. The 4 decades since the introduction of [¹⁸F]FDG have seen the development of many small molecule PET radiotracers. Ten have been approved by the US Food and Drug Administration as of 2016, whereas hundreds more are being evaluated clinically. These radiotracers are being used in personalized medicine and to support drug discovery programs where they are greatly improving our understanding of and ability to treat diseases across many areas of medicine including neuroscience, cardiovascular medicine, and oncology.

In vivo tau PET imaging in dementia: Pathophysiology, radiotracer quantification, and a systematic review of clinical findings

In addition to the deposition of β-amyloid plaques, neurofibrillary tangles composed of aggregated hyperphosphorylated tau are one of the pathological hallmarks of Alzheimer's disease and other neurodegenerative disorders. Until now, our understanding about the natural history and topography of tau deposition has only been based on post-mortem and cerebrospinal fluid studies, and evidence continues to implicate tau as a central driver of downstream neurodegenerative processes and cognitive decline. Recently, it has become possible to assess the regional distribution and severity of tau burden in vivo with the development of novel radiotracers for positron emission tomography (PET) imaging. In this article, we provide a comprehensive discussion of tau pathophysiology, its quantification with novel PET radiotracers, as well as a systematic review of tau PET imaging in normal aging and various dementia conditions: mild cognitive impairment, Alzheimer's disease, frontotemporal dementia, progressive supranuclear palsy, and Lewy body dementia. We discuss the main findings in relation to group differences, clinical-cognitive correlations of tau PET, and multi-modal relationships among tau PET and other pathological markers. Collectively, the small but growing literature of tau PET has yielded consistent anatomical patterns of tau accumulation that recapitulate post-mortem distribution of neurofibrillary tangles which correlate with cognitive functions and other markers of pathology. In general, AD is characterised by increased tracer retention in the inferior temporal lobe, extending into the frontal and parietal regions in more severe cases. It is also noted that the spatial topography of tau accumulation is markedly distinct to that of amyloid burden in aging and AD. Tau PET imaging has also revealed characteristic spatial patterns among various non-AD tauopathies, supporting its potential role for differential diagnosis. Finally, we propose novel directions for future tau research, including (a) longitudinal imaging in preclinical dementia, (b) multi-modal mapping of tau pathology onto other pathological processes such as neuroinflammation, and (c) the need for more validation studies against post-mortem samples of the same subjects.

18F-AV-1451 positron emission tomography in Alzheimer's disease and progressive supranuclear palsy

The ability to assess the distribution and extent of tau pathology in Alzheimer's disease and progressive supranuclear palsy in vivo would help to develop biomarkers for these tauopathies and clinical trials of disease-modifying therapies. New radioligands for positron emission tomography have generated considerable interest, and controversy, in their potential as tau biomarkers. We assessed the radiotracer 18F-AV-1451 with positron emission tomography imaging to compare the distribution and intensity of tau pathology in 15 patients with Alzheimer's pathology (including amyloid-positive mild cognitive impairment), 19 patients with progressive supranuclear palsy, and 13 age- and sex-matched controls. Regional analysis of variance and a support vector machine were used to compare and discriminate the clinical groups, respectively. We also examined the 18F-AV-1451 autoradiographic binding in post-mortem tissue from patients with Alzheimer's disease, progressive supranuclear palsy, and a control case to assess the 18F-AV-1451 binding specificity to Alzheimer's and non-Alzheimer's tau pathology. There was increased 18F-AV-1451 binding in multiple regions in living patients with Alzheimer's disease and progressive supranuclear palsy relative to controls [main effect of group, F(2,41) = 17.5, P < 0.0001; region of interest × group interaction, F(2,68) = 7.5, P < 0.00001]. More specifically, 18F-AV-1451 binding was significantly increased in patients with Alzheimer's disease, relative to patients with progressive supranuclear palsy and with control subjects, in the hippocampus and in occipital, parietal, temporal, and frontal cortices (t's > 2.2, P's < 0.04). Conversely, in patients with progressive supranuclear palsy, relative to patients with Alzheimer's disease, 18F-AV-1451 binding was elevated in the midbrain (t = 2.1, P < 0.04); while patients with progressive supranuclear palsy showed, relative to controls, increased 18F-AV-1451 uptake in the putamen, pallidum, thalamus, midbrain, and in the dentate nucleus of the cerebellum (t's > 2.7, P's < 0.02). The support vector machine assigned patients' diagnoses with 94% accuracy. The post-mortem autoradiographic data showed that 18F-AV-1451 strongly bound to Alzheimer-related tau pathology, but less specifically in progressive supranuclear palsy. 18F-AV-1451 binding to the basal ganglia was strong in all groups in vivo. Postmortem histochemical staining showed absence of neuromelanin-containing cells in the basal ganglia, indicating that off-target binding to neuromelanin is an insufficient explanation of 18F-AV-1451 positron emission tomography data in vivo, at least in the basal ganglia. Overall, we confirm the potential of 18F-AV-1451 as a heuristic biomarker, but caution is indicated in the neuropathological interpretation of its binding. Off-target binding may contribute to disease profiles of 18F-AV-1451 positron emission tomography, especially in primary tauopathies such as progressive supranuclear palsy. We suggest that 18F-AV-1451 positron emission tomography is a useful biomarker to assess tau pathology in Alzheimer's disease and to distinguish it from other tauopathies with distinct clinical and pathological characteristics such as progressive supranuclear palsy.

Tau PET imaging: present and future directions

Abnormal aggregation of tau in the brain is a major contributing factor in various neurodegenerative diseases. The role of tau phosphorylation in the pathophysiology of tauopathies remains unclear. Consequently, it is important to be able to accurately and specifically target tau deposits in vivo in the brains of patients. The advances of molecular imaging in the recent years have now led to the recent development of promising tau-specific tracers for positron emission tomography (PET), such as THK5317, THK5351, AV-1451, and PBB3. These tracers are now available for clinical assessment in patients with various tauopathies, including Alzheimer's disease, as well as in healthy subjects. Exploring the patterns of tau deposition in vivo for different pathologies will allow discrimination between neurodegenerative diseases, including different tauopathies, and monitoring of disease progression. The variety and complexity of the different types of tau deposits in the different diseases, however, has resulted in quite a challenge for the development of tau PET tracers. Extensive work remains in order to fully characterize the binding properties of the tau PET tracers, and to assess their usefulness as an early biomarker of the underlying pathology. In this review, we summarize recent findings on the most promising tau PET tracers to date, discuss what has been learnt from these findings, and offer some suggestions for the next steps that need to be achieved in a near future.

Neuroimaging of Inflammation in Memory and Related Other Disorders (NIMROD) study protocol: a deep phenotyping cohort study of the role of brain inflammation in dementia, depression and other neurological illnesses

INTRODUCTION

Inflammation of the central nervous system is increasingly regarded as having a role in cognitive disorders such as dementia and depression, but it is not clear how such inflammation relates to other aspects of neuropathology, structural and functional changes in the brain and symptoms (as assessed via clinical and neuropsychological assessment and MRI). This study will explore these pathophysiological mechanisms using positron emission tomography (PET) which allows in vivo imaging of inflammation, amyloid and τ deposition, together with neuropsychological profiling, MRI and peripheral biomarker analysis.

METHODS AND ANALYSIS

Using PET imaging of the ligand [¹¹C]PK11195, we will test for increased neuroinflammation in vivo in patients with Alzheimer's disease, Lewy body dementia, frontotemporal dementia, progressive supranuclear palsy, late-onset depression and mild cognitive impairment, when compared to healthy controls. We will assess whether areas of inflammatory change are associated with amyloid and τ deposition (assessed using ¹¹C-labelled Pittsburgh Compound B ([¹¹C]PiB) and ¹⁸F-labelled AV-1451, respectively), as well as structural and connectivity markers found on MRI. Inflammatory biomarker analysis and immune-phenotyping of peripheral blood monocytes will determine the correlation between central inflammation and peripheral inflammation. Finally, we will examine whether central inflammatory markers seen on PET imaging are associated with global and domain specific cognitive impairments or predict cognitive decline over 12 months.

ETHICS AND DISSEMINATION

The study protocol was approved by the local ethics committee, East of England-Cambridge Central Research Ethics Committee (reference: 13/EE/0104). The study is also Administration of Radioactive Substances Advisory Committee (ARSAC) approved as part of this process. Data will be disseminated by presentation at national and international conferences and by publication, predominantly in journals of clinical neuroscience, neurology and psychiatry.

PET Tau and Amyloid-β Burden in Mild Alzheimer's Disease: Divergent Relationship with Age, Cognition, and Cerebrospinal Fluid Biomarkers

BACKGROUND

Combining PET amyloid-β (Aβ) and tau imaging may be critical for tracking disease progression in Alzheimer's disease (AD).

OBJECTIVE

We sought to characterize the relationship between Aβ and tau ligands as well as with other measures of pathology.

METHODS

We conducted a multi-center observational study in early AD (MMSE >20) participants aged 50 to 85 y. The schedule included cognitive assessments (ADAS-Cog) and CSF measurement of Aβ and tau at baseline and 6 months; PET-CT imaging with Aβ ([18F]AV45) and tau ([18F]AV1451) ligands at baseline.

RESULTS

22 participants took part in the study with 20 completing its 6-month duration and 12 having both tau and amyloid PET. The PET biomarker analysis revealed a strong negative correlation between age and tau in multiple regions. Entorhinal cortex tau and age interacted significantly in terms of cognitive change over 6 months which may have been to older participants deteriorating faster despite lower levels of cortical tau. Cortical Aβ associated with entorhinal cortex tau while CSF tau/Aβ ratio correlated strongly with cortical tau but not Aβ.

CONCLUSION

The negative relationship between age and cortical tau whereby younger patients with mild AD had relatively greater tau burden is potentially important. It suggests that younger-age onset AD may be primarily driven by tau pathology while AD developing later may depend on a multitude of pathological mechanisms. These data also suggest that PET-tau performs better than PET-amyloid in predicting the best validated AD diagnostic marker- the CSF total tau/Aβ ratio.