18 F-flortaucipir tau positron emission tomography distinguishes established progressive supranuclear palsy from controls and Parkinson disease: A multicenter study

Accumulation of Tau Protein, Metabolism and Perfusion-Application and Efficacy of Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) Imaging in the Examination of Progressive Supranuclear Palsy (PSP) and Corticobasal Syndrome (CBS)

Neuroimaging in the context of examining atypical parkinsonian tauopathies is an evolving matter. Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) bring tools, which may be reasonable in supplementary examination, however cannot be interpreted as a gold standard for correct diagnosis. The review presents advantages and limitations of tau radiotracers in PET, metabolic PET and perfusion SPECT. The aim of this paper is to highlight the possibilities and boundaries in the supplementary examination of tauopathic parkinsonian syndromes.

Brain Iron Accumulation in Atypical Parkinsonian Syndromes: in vivo MRI Evidences for Distinctive Patterns

Recent data suggest mechanistic links among perturbed iron homeostasis, oxidative stress, and misfolded protein aggregation in neurodegenerative diseases. Iron overload and toxicity toward dopaminergic neurons have been established as playing a role in the pathogenesis of Parkinson's disease (PD). Brain iron accumulation has also been documented in atypical parkinsonian syndromes (APS), mainly comprising multiple system atrophy (MSA), and progressive supranuclear palsy (PSP). Iron-sensitive magnetic resonance imaging (MRI) has been applied to identify iron-related signal changes for the diagnosis and differentiation of these disorders. Topographic patterns of widespread iron deposition in deep brain nuclei have been described as differing between patients with MSA and PSP and those with PD. A disease-specific increase of iron occurs in the brain regions mainly affected by underlying disease pathologies. However, whether iron changes are a primary pathogenic factor or an epiphenomenon of neuronal degeneration has not been fully elucidated. Moreover, the clinical implications of iron-related pathology in APS remain unclear. In this review study, we collected data from qualitative and quantitative MRI studies on brain iron accumulation in APS to identify disease-related patterns and the potential role of iron-sensitive MRI.

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.

Recognizing and treating atypical Parkinson disorders

Though less common than Parkinson's disease (PD), the atypical Parkinson disorders such as such as dementia with Lewy bodies, multiple system atrophy, progressive supranuclear palsy, and corticobasal degeneration are increasingly recognized and important to distinguish from PD. Atypical or "Parkinson-plus" disorders are multisystem disorders and generally progress more rapidly and respond poorly to current therapies compared to PD. Recent advances in our understanding of the pathophysiology of these disorders, however, have generated new interest in the development of novel diagnostics and disease-modifying therapeutics aimed at identifying and treating these disorders. In this review we discuss the clinical approach to the atypical Parkinson disorders and the recent developments in diagnostic and research criteria that take into account the phenotypic heterogeneity and advances in our understanding of the pathophysiology of these disorders.

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.

MRI Outperforms [18F]AV-1451 PET as a Longitudinal Biomarker in Progressive Supranuclear Palsy

BACKGROUND

Elevated uptake of the [¹⁸ F]AV-1451 tau-PET ligand has been observed cross-sectionally in subjects with progressive supranuclear palsy (PSP). However, it is unknown how the ligand performs longitudinally in PSP. We aimed to determine how regional measures of change on [¹⁸ F]AV-1451 PET perform as longitudinal biomarkers of PSP compared with the more established biomarker of rate of midbrain atrophy.

METHODS

Sixteen subjects with PSP underwent 2 serial [¹⁸ F]AV-1451 tau-PET scans and 3-Tesla MRI over 12 months and were age- and sex-matched to 39 healthy controls with longitudinal [¹⁸ F]AV-1451 PET. Median [¹⁸ F]AV-1451 uptake was calculated for each scan for regions of interest across the brain and divided by uptake in cerebellar crus to create standard uptake value ratios. Midbrain volume on MRI was also calculated for each scan. Sample sizes required to power placebo-controlled treatment trials were calculated.

RESULTS

Rate of midbrain atrophy was significantly increased in PSP compared with controls. [¹⁸ F]AV-1451 regional change measures were significantly increased in PSP compared with controls in the pallidum, precentral cortex, dentate nucleus of the cerebellum, and midbrain. Change over time in the PSP Rating Scale correlated with change in midbrain volume but did not correlate with change in the [¹⁸ F]AV-1451 measures. Smallest sample-size estimates were obtained with rate of midbrain atrophy, followed by the PSP Rating Scale, with both outperforming [¹⁸ F]AV-1451 measures.

CONCLUSIONS

[¹⁸ F]AV-1451 tau-PET measures increase over time in subjects with PSP, but longitudinal [¹⁸ F]AV-1451 measures may not perform as well as rate of midbrain atrophy as biomarkers for PSP clinical trials. © 2018 International Parkinson and Movement Disorder Society.

The Role of Tau Imaging in Parkinsonian Disorders

PURPOSE OF REVIEW

Differential diagnosis of atypical Parkinson syndromes (APS) is difficult as clinical presentations may vary and as there is a strong overlap between disease entities. Aggregations of misfolded and hyperphosphorylated tau proteins are the common denominator of many of these diseases.

RECENT FINDINGS

Several tau targeting positron emission tomography (PET) tracers have been evaluated as possible biomarkers in APS in the recent years. For Parkinson's disease, dementia with Lewy bodies, progressive supranuclear palsy, and corticobasal degeneration, promising results have been reported with regard to the ability to detect the presence of disease and to discriminate patients from controls. However, the discussion about the specificity of the first-generation radiotracers and their value in the clinical context is ongoing. A combined interpretation of signal strength and distribution pattern in PET scans with first- and second-generation tracers may be helpful in clinical diagnosis and follow-up of patients with APS.

Discriminative Accuracy of [18F]flortaucipir Positron Emission Tomography for Alzheimer Disease vs Other Neurodegenerative Disorders

IMPORTANCE

The positron emission tomography (PET) tracer [18F]flortaucipir allows in vivo quantification of paired helical filament tau, a core neuropathological feature of Alzheimer disease (AD), but its diagnostic utility is unclear.

OBJECTIVE

To examine the discriminative accuracy of [18F]flortaucipir for AD vs non-AD neurodegenerative disorders.

DESIGN, SETTING, AND PARTICIPANTS

In this cross-sectional study, 719 participants were recruited from 3 dementia centers in South Korea, Sweden, and the United States between June 2014 and November 2017 (160 cognitively normal controls, 126 patients with mild cognitive impairment [MCI], of whom 65.9% were amyloid-β [Aβ] positive [ie, MCI due to AD], 179 patients with AD dementia, and 254 patients with various non-AD neurodegenerative disorders).

EXPOSURES

The index test was the [18F]flortaucipir PET standardized uptake value ratio (SUVR) in 5 predefined regions of interest (ROIs). Cut points for tau positivity were determined using the mean +2 SDs observed in controls and Youden Index for the contrast AD dementia vs controls.

MAIN OUTCOMES AND MEASURES

The reference standard was the clinical diagnosis determined at the specialized memory centers. In the primary analysis, the discriminative accuracy (ie, sensitivity and specificity) of [18F]flortaucipir was examined for AD dementia vs all non-AD neurodegenerative disorders. In secondary analyses, the area under the curve (AUC) of [18F]flortaucipir SUVR was compared with 3 established magnetic resonance imaging measures (hippocampal volumes and AD signature and whole-brain cortical thickness), and sensitivity and specificity of [18F]flortaucipir in MCI due to AD vs non-AD neurodegenerative disorders were determined.

RESULTS

Among 719 participants, the overall mean (SD) age was 68.8 (9.2) years and 48.4% were male. The proportions of patients who were amyloid-β positive were 26.3%, 65.9%, 100%, and 23.8% among cognitively normal controls, patients with MCI, patients with AD dementia, and patients with non-AD neurodegenerative disorders, respectively. [18F]flortaucipir uptake in the medial-basal and lateral temporal cortex showed 89.9% (95% CI, 84.6%-93.9%) sensitivity and 90.6% (95% CI, 86.3%-93.9%) specificity using the threshold based on controls (SUVR, 1.34), and 96.8% (95% CI, 92.0%-99.1%) sensitivity and 87.9% (95% CI, 81.9%-92.4%) specificity using the Youden Index-derived cutoff (SUVR, 1.27) for distinguishing AD dementia from all non-AD neurodegenerative disorders. The AUCs for all 5 [18F]flortaucipir ROIs were higher (AUC range, 0.92-0.95) compared with the 3 volumetric MRI measures (AUC range, 0.63-0.75; all ROIs P < .001). Diagnostic performance of the 5 [18F]flortaucipir ROIs were lower in MCI due to AD (AUC range, 0.75-0.84).

CONCLUSIONS AND RELEVANCE

Among patients with established diagnoses at a memory disorder clinic, [18F]flortaucipir PET was able to discriminate AD from other neurodegenerative diseases. The accuracy and potential utility of this test in patient care require further research in clinically more representative populations.

What a neurologist should know about PET and SPECT functional imaging for parkinsonism: A practical perspective

The diagnosis of a parkinsonian syndrome based on clinical criteria remains sometimes difficult, especially at disease onset. Brain or heart molecular imaging techniques (SPECT or PET) can provide a major help to improve and speed up diagnosis, influencing treatment strategies. Presynaptic dopaminergic imaging using either [¹⁸F]-Dopa PET or ¹²³I -2β-Carbomethoxy-3β-(4-Iodophenyl)- N-(3-Fluoropropyl) Nortropane ([¹²³I]-Ioflupane)SPECT demonstrates or rules out the presence of a dopaminergic degenerative process. This allows to distinguish Parkinson's disease, Parkinson "plus" syndromes and dementia with Lewy bodies (reduced radiotracers binding) from essential tremor, psychogenic, post-neuroleptic or vascular parkinsonisms, dopa-responsive dystonia and Alzheimer's disease (normal radiotracers binding). For differential diagnosis between Parkinson's disease and Parkinson "plus" syndromes, brain molecular imaging with [¹⁸F]-Fluorodeoxyglucose ([¹⁸F]-FDG) PET or ^99mTc-HMPAO SPECT can provide useful information, whereas [¹⁸F]-Dopa PET or [¹²³I]-Ioflupane does not separate these entities. Finally, sympathetic cardiac [¹²³I]-Metaiodobenzylguanidine ([¹²³I]-MIBG) scintigraphy or SPECT can help distinguishing Parkinson's disease and dementia with Lew bodies (decreased binding) from multiple system atrophy and progressive supranuclear palsy (normal binding). New radiotracers notably those targeting the pathological process itself such as Tau aggregates are under development and may provide interesting informations to delineate the different Parkinson "plus" syndromes.

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.

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.

Lateralisation in Parkinson disease

Asymmetry of dopaminergic neurodegeneration and subsequent lateralisation of motor symptoms are distinctive features of Parkinson's disease compared to other forms of neurodegenerative or symptomatic parkinsonism. Even 200 years after the first description of the disease, the underlying causes for this striking clinicopathological feature are not yet fully understood. There is increasing evidence that lateralisation of disease is due to a complex interplay of hereditary and environmental factors that are reflected not only in the concept of dominant hemispheres and handedness but also in specific susceptibilities of neuronal subpopulations within the substantia nigra. As a consequence, not only the obvious lateralisation of motor symptoms occurs but also patterns of associated non-motor signs are defined, which include cognitive functions, sleep behaviour or olfaction. Better understanding of the mechanisms contributing to lateralisation of neurodegeneration and the resulting patterns of clinical phenotypes based on bilateral post-mortem brain analyses and clinical studies focusing on right/left hemispheric symptom origin will help to develop more targeted therapeutic approaches, taking into account subtypes of PD as a heterogeneous disorder.

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.

Progressive Supranuclear Palsy: an Update

PURPOSE OF REVIEW

Progressive supranuclear palsy (PSP) is a 4R tau neuropathologic entity. While historically defined by the presence of a vertical supranuclear gaze palsy and falls in the first symptomatic year, clinicopathologic studies identify alternate presenting phenotypes. This article reviews the new PSP diagnostic criteria, diagnostic approaches, and treatment strategies.

RECENT FINDINGS

The 2017 International Parkinson and Movement Disorder Society PSP criteria outline 14 core clinical features and 4 clinical clues that combine to diagnose one of eight PSP phenotypes with probable, possible, or suggestive certainty. Evidence supports the use of select imaging approaches in the classic PSP-Richardson syndrome phenotype. Recent trials of putative disease-modifying agents showed no benefit. The new PSP diagnostic criteria incorporating the range of presenting phenotypes have important implications for diagnosis and research. More work is needed to understand how diagnostic evaluations inform phenotype assessment and identify expected progression. Current treatment is symptomatic, but tau-based therapeutics are in active clinical trials.

Molecular imaging: What is right and what is an illusion?

Over the past 40 years, brain molecular imaging has evolved from measuring cerebral metabolism with fluorodeoxyglucose, to neuroreceptor imaging, to imaging pathological protein deposits. In the early going, the characteristics of successful molecular imaging radiotracers were defined, and a detailed “Process” was developed for the collection of basic pharmacodynamic and pharmacokinetic data. These data are essential for the interpretation of in vivo imaging data and for defining the strengths, weaknesses, and limitations of new tracers. This perspective discusses the use of this “Process” in the development of the amyloid β positron emission tomography radiotracer, Pittsburgh Compound-B, and discusses some of the current controversies and difficulties in the field of tau positron emission tomography in the context of human data that preceded completion of this radiotracer characterization process—which still remains to be completed. As a field, we must decide which data are valid and which are artifacts and determine that when the artifacts are so overwhelming, the data are merely an illusion.

Tau Positron Emission Tomography Imaging in Degenerative Parkinsonisms

In recent years, several radiotracers that selectively bind to pathological tau proteins have been developed. Evidence is emerging that binding patterns of in vivo tau positron emission tomography (PET) studies in Alzheimer’s disease (AD) patients closely resemble the distribution patterns of known neurofibrillary tangle pathology, with the extent of tracer binding reflecting the clinical and pathological progression of AD. In Lewy body diseases (LBD), tau PET imaging has clearly revealed cortical tau burden with a distribution pattern distinct from AD and increased cortical binding within the LBD spectrum. In progressive supranuclear palsy, the globus pallidus and midbrain have shown increased binding most prominently. Tau PET patterns in patients with corticobasal syndrome are characterized by asymmetrical uptake in the motor cortex and underlying white matter, as well as in the basal ganglia. Even in the patients with multiple system atrophy, which is basically a synucleinopathy, ¹⁸F-flortaucipir, a widely used tau PET tracer, also binds to the atrophic posterior putamen, possibly due to off-target binding. These distinct patterns of tau-selective radiotracer binding in the various degenerative parkinsonisms suggest its utility as a potential imaging biomarker for the differential diagnosis of parkinsonisms.

Tau-PET imaging with [18F]AV-1451 in primary progressive apraxia of speech

Apraxia of speech is a motor speech disorder characterized by combinations of slow speaking rate, abnormal prosody, distorted sound substitutions, and trial-and-error articulatory movements. Apraxia of speech is due to abnormal planning and/or programming of speech production. It is referred to as primary progressive apraxia of speech (PPAOS) when it is the only symptom of a neurodegenerative condition. Past reports suggest an association of PPAOS with primary 4-repeat (4R) tau (e.g., progressive supranuclear palsy, corticobasal degeneration), rather than amyloid, pathology. The goal of the current study was to investigate the distribution of tau tracer uptake using [18F]AV-1451 positron emission tomography (PET) imaging in patients with PPAOS. Fourteen PPAOS patients underwent [18F]AV-1451 PET (tau-PET) imaging, [C11] Pittsburgh Compound B (PiB) PET and structural MRI and were matched 3:1 by age and sex to 42 cognitively normal controls. Tau-PET uptake was assessed at the region-of-interest (ROI) level and at the voxel-level. The PPAOS group (n = 14) showed increased tau-PET uptake in the precentral gyrus, supplementary motor area and Broca's area compared to controls. To examine whether tau deposition in Broca's area was related to the presence of aphasia, we examined a subgroup of the PPAOS patients who had predominant apraxia of speech, with concomitant aphasia (PPAOSa; n = 7). The PPAOSa patients showed tau-PET uptake in the same regions as the whole group. However, the remaining seven patients who did not have aphasia showed uptake only in superior premotor and precentral cortices, with no uptake observed in Broca's area. This cross-sectional study demonstrates that elevated tau tracer uptake is observed using [18F]AV-1451 in PPAOS. Further, it appears that [18F]AV-1451 is sensitive to the regional distribution of tau deposition in different stages of PPAOS, given the relationship between tau signal in Broca's area and the presence of aphasia.

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.

Pittsburgh Compound B and AV-1451 positron emission tomography assessment of molecular pathologies of Alzheimer's disease in progressive supranuclear palsy

INTRODUCTION

Little is known about Alzheimer's disease molecular proteins, beta-amyloid and paired helical filament (PHF) tau, in progressive supranuclear palsy (PSP). Recent techniques have been developed to allow for investigations of these proteins in PSP. We determined the frequency of beta-amyloid deposition in PSP, and whether beta-amyloid deposition in PSP is associated with PHF-tau deposition pattern, or clinical features.

METHODS

Thirty probable PSP participants underwent MRI, [¹⁸F]AV-1451 PET and Pittsburgh compound B (PiB) PET. Apolipoprotein (APOE) genotyping was also performed. A global PiB standard-uptake value ratio (SUVR) was calculated. AV-1451 SUVRs were calculated for a set of Alzheimer's disease (AD)-related regions and a set of PSP-related regions. Voxel-level analyses were conducted to assess for differences in AV-1451 uptake patterns and MRI atrophy between PiB(+) and PiB(-) cases compared to 60 normal PiB(-) controls. Statistical testing for correlations and associations between variables of interest were also performed.

RESULTS

Twelve subjects (40%) showed beta-amyloid deposition. Higher PiB SUVR correlated with older age but not with AV-1451 SUVR in the AD- or PSP-related regions. Higher AV-1451 SUVR in AD-related regions was associated with higher AV-1451 SUVR in PSP-related regions. We found little evidence for beta-amyloid related differences in clinical metrics, proportion of APOE e4 carriers, pattern of AV-1451 uptake, or pattern of atrophy.

CONCLUSION

Beta-amyloid deposition occurs in a relatively high proportion of PSP subjects. Unlike in Alzheimer's disease, however, there is little evidence that beta-amyloid, and PHF-tau, play a significant role in neurodegeneration in PSP.