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

European intersocietal recommendations for the biomarker-based diagnosis of neurocognitive disorders

The recent commercialisation of the first disease-modifying drugs for Alzheimer's disease emphasises the need for consensus recommendations on the rational use of biomarkers to diagnose people with suspected neurocognitive disorders in memory clinics. Most available recommendations and guidelines are either disease-centred or biomarker-centred. A European multidisciplinary taskforce consisting of 22 experts from 11 European scientific societies set out to define the first patient-centred diagnostic workflow that aims to prioritise testing for available biomarkers in individuals attending memory clinics. After an extensive literature review, we used a Delphi consensus procedure to identify 11 clinical syndromes, based on clinical history and examination, neuropsychology, blood tests, structural imaging, and, in some cases, EEG. We recommend first-line and, if needed, second-line testing for biomarkers according to the patient's clinical profile and the results of previous biomarker findings. This diagnostic workflow will promote consistency in the diagnosis of neurocognitive disorders across European countries.

Novel avenues of tau research

INTRODUCTION

The pace of innovation has accelerated in virtually every area of tau research in just the past few years.

METHODS

In February 2022, leading international tau experts convened to share selected highlights of this work during Tau 2022, the second international tau conference co-organized and co-sponsored by the Alzheimer's Association, CurePSP, and the Rainwater Charitable Foundation.

RESULTS

Representing academia, industry, and the philanthropic sector, presenters joined more than 1700 registered attendees from 59 countries, spanning six continents, to share recent advances and exciting new directions in tau research.

DISCUSSION

The virtual meeting provided an opportunity to foster cross-sector collaboration and partnerships as well as a forum for updating colleagues on research-advancing tools and programs that are steadily moving the field forward.

The Role of Dual-Phase 18 F-FP-CIT PET to Early Diagnosis of Corticobasal Syndrome

BACKGROUND

Corticobasal syndrome (CBS) is a neurodegeneration characterized by asymmetric parkinsonism, dystonia, myoclonus, and apraxia. In the early stage, CBS presents with asymmetric parkinsonism and cortical symptoms (apraxia and alien hand), and neuroimaging finding is often vague, making early clinical differentiation from idiopathic Parkinson disease (IPD) challenging. This study was performed to delineate the specific patterns of cortical hypoperfusion, dopamine transporter (DAT) uptake using dual-phase FP-CIT PET in discriminating between CBS and IPD at early stage.

PATIENTS AND METHODS

The study enrolled clinically diagnosed CBS (n = 11) and IPD (n = 22) patients (age and sex matched). All participants underwent dual-phase 18 F-FP-CIT PET, and regional SUV ratio (SUVR) was obtained by semiquantitative analysis. The early perfusion imaging and DAT imaging were compared between groups.

RESULTS

The regional SUVRs (early phase) of the frontal lobe, thalamus, cingulate, and caudate were significantly lower in patients with CBS, whereas the SUVR of occipital lobe was lower in the IPD group. The CBS group exhibited more prominent asymmetry than the IPD group, particularly in the perirolandic area, superior frontal gyrus, and anterior parietal lobe in early phase PET. Striatal DAT uptake (delayed phase) revealed that the caudate showed lower SUVR and prominent asymmetry in the CBS group, and the caudate-to-putamen ratio (CP ratio) was significantly lower in CBS patients ( P < 0.001). Among the parameters (early and delayed), the CP ratio in DAT exhibited the most powerful discriminative power from receiver operating characteristic curve comparison (area under curve = 0.983).

CONCLUSIONS

This study demonstrated that the dual-phase FP-CIT PET is useful in differentiating CBS and IPD in the early stage of the disease, and a lower CP ratio of DAT imaging is highly informative for distinguishing between corticobasal degeneration and IPD.

Individual regional associations between Aβ-, tau- and neurodegeneration (ATN) with microglial activation in patients with primary and secondary tauopathies

β-amyloid (Aβ) and tau aggregation as well as neuronal injury and atrophy (ATN) are the major hallmarks of Alzheimer's disease (AD), and biomarkers for these hallmarks have been linked to neuroinflammation. However, the detailed regional associations of these biomarkers with microglial activation in individual patients remain to be elucidated. We investigated a cohort of 55 patients with AD and primary tauopathies and 10 healthy controls that underwent TSPO-, Aβ-, tau-, and perfusion-surrogate-PET, as well as structural MRI. Z-score deviations for 246 brain regions were calculated and biomarker contributions of Aβ (A), tau (T), perfusion (N1), and gray matter atrophy (N2) to microglial activation (TSPO, I) were calculated for each individual subject. Individual ATN-related microglial activation was correlated with clinical performance and CSF soluble TREM2 (sTREM2) concentrations. In typical and atypical AD, regional tau was stronger and more frequently associated with microglial activation when compared to regional Aβ (AD: βT = 0.412 ± 0.196 vs. βA = 0.142 ± 0.123, p < 0.001; AD-CBS: βT = 0.385 ± 0.176 vs. βA = 0.131 ± 0.186, p = 0.031). The strong association between regional tau and microglia reproduced well in primary tauopathies (βT = 0.418 ± 0.154). Stronger individual associations between tau and microglial activation were associated with poorer clinical performance. In patients with 4RT, sTREM2 levels showed a positive association with tau-related microglial activation. Tau pathology has strong regional associations with microglial activation in primary and secondary tauopathies. Tau and Aβ related microglial response indices may serve as a two-dimensional in vivo assessment of neuroinflammation in neurodegenerative diseases.

Interplay of tau and functional network connectivity in progressive supranuclear palsy: a [18F]PI-2620 PET/MRI study

PURPOSE

Progressive supranuclear palsy (PSP) is primary 4-repeat tauopathy. Evidence spanning from imaging studies indicate aberrant connectivity in PSPs. Our goal was to assess functional connectivity network alterations in PSP patients and the potential link between regional tau-burden and network-level functional connectivity using the next-generation tau PET tracer [¹⁸F]PI-2620 and resting-state functional MRI (fMRI).

MATERIAL AND METHODS

Twenty-four probable PSP patients (70.9 ± 6.9 years, 13 female), including 14 Richardson syndrome (RS) and 10 non-RS phenotypes, underwent [¹⁸F]PI-2620 PET/MRI imaging. Distribution volume ratios (DVRs) were estimated using non-invasive pharmacokinetic modeling. Resting-state fMRI was also acquired in these patients as well as in thirteen older non-AD MCI reference group (64 ± 9 years, 4 female). The functional network was constructed using 141 by 141 region-to-region functional connectivity metrics (RRC) and network-based statistic was carried out (connection threshold p < 0.001, cluster threshold pFDR < 0.05).

RESULTS

In total, 9870 functional connections were analyzed. PSPs compared to aged non-AD MCI reference group expressed aberrant connectivity evidenced by the significant NBS network consisting of 89 ROIs and 118 connections among them (NBS mass 4226, pFDR < 0.05). Tau load in the right globus pallidus externus (GPe) and left dentate nucleus (DN) showed significant effects on functional network connectivity. The network linked with increased tau load in the right GPe was associated with hyperconnectivity of low-range intra-opercular connections (NBS mass 356, pFDR < 0.05), while the network linked with increased tau load in the left cerebellar DN was associated with cerebellar hyperconnectivity and cortico-cerebellar hypoconnectivity (NBS mass 517, pFDR < 0.05).

CONCLUSIONS

PSP patients show altered functional connectivity. Network incorporating deep gray matter structures demonstrate hypoconnectivity, cerebellum hyperconnectivity, while cortico-cortical connections show variable changes. Tau load in the right GPe and left DN is associated with functional networks which strengthen low-scale intra-opercular and intra-cerebellar connections and weaken opercular-cerebellar connections. These findings support the concept of tau load-dependent functional network changes in PSP, by that providing evidence for downstream effects of neuropathology on brain functionality in this primary tauopathy.

Evolving concepts in progressive supranuclear palsy and other 4-repeat tauopathies

Tauopathies are classified according to whether tau deposits predominantly contain tau isoforms with three or four repeats of the microtubule-binding domain. Those in which four-repeat (4R) tau predominates are known as 4R-tauopathies, and include progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, globular glial tauopathies and conditions associated with specific MAPT mutations. In these diseases, 4R-tau deposits are found in various cell types and anatomical regions of the brain and the conditions share pathological, pathophysiological and clinical characteristics. Despite being considered 'prototype' tauopathies and, therefore, ideal for studying neuroprotective agents, 4R-tauopathies are still severe and untreatable diseases for which no validated biomarkers exist. However, advances in research have addressed the issues of phenotypic overlap, early clinical diagnosis, pathophysiology and identification of biomarkers, setting a road map towards development of treatments. New clinical criteria have been developed and large cohorts with early disease are being followed up in prospective studies. New clinical trial readouts are emerging and biomarker research is focused on molecular pathways that have been identified. Lessons learned from failed trials of neuroprotective drugs are being used to design new trials. In this Review, we present an overview of the latest research in 4R-tauopathies, with a focus on progressive supranuclear palsy, and discuss how current evidence dictates ongoing and future research goals.

Impairments in Brain Bioenergetics in Aging and Tau Pathology: A Chicken and Egg Situation?

The brain is the most energy-consuming organ of the body and impairments in brain energy metabolism will affect neuronal functionality and viability. Brain aging is marked by defects in energetic metabolism. Abnormal tau protein is a hallmark of tauopathies, including Alzheimer's disease (AD). Pathological tau was shown to induce bioenergetic impairments by affecting mitochondrial function. Although it is now clear that mutations in the tau-coding gene lead to tau pathology, the causes of abnormal tau phosphorylation and aggregation in non-familial tauopathies, such as sporadic AD, remain elusive. Strikingly, both tau pathology and brain hypometabolism correlate with cognitive impairments in AD. The aim of this review is to discuss the link between age-related decrease in brain metabolism and tau pathology. In particular, the following points will be discussed: (i) the common bioenergetic features observed during brain aging and tauopathies; (ii) how age-related bioenergetic defects affect tau pathology; (iii) the influence of lifestyle factors known to modulate brain bioenergetics on tau pathology. The findings compiled here suggest that age-related bioenergetic defects may trigger abnormal tau phosphorylation/aggregation and cognitive impairments after passing a pathological threshold. Understanding the effects of aging on brain metabolism may therefore help to identify disease-modifying strategies against tau-induced neurodegeneration.

Dual-Phase β-Amyloid PET Captures Neuronal Injury and Amyloidosis in Corticobasal Syndrome

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

Cortical [18 F]PI-2620 Binding Differentiates Corticobasal Syndrome Subtypes

BACKGROUND

Corticobasal syndrome is associated with cerebral protein aggregates composed of 4-repeat (~50% of cases) or mixed 3-repeat/4-repeat tau isoforms (~25% of cases) or nontauopathies (~25% of cases).

OBJECTIVES

The aim of this single-center study was to investigate the diagnostic value of the tau PET-ligand [¹⁸ F]PI-2620 in patients with corticobasal syndrome.

METHODS

Forty-five patients (71.5 ± 7.6 years) with corticobasal syndrome and 14 age-matched healthy controls underwent [¹⁸ F]PI-2620-PET. Beta-amyloid status was determined by cerebral β-amyloid PET and/or CSF analysis. Subcortical and cortical [¹⁸ F]PI-2620 binding was quantitatively and visually compared between β-amyloid-positive and -negative patients and controls. Regional [¹⁸ F]PI-2620 binding was correlated with clinical and demographic data.

RESULTS

Twenty-four percent (11 of 45) were β-amyloid-positive. Significantly elevated [¹⁸ F]PI-2620 distribution volume ratios were observed in both β-amyloid-positive and β-amyloid-negative patients versus controls in the dorsolateral prefrontal cortex and basal ganglia. Cortical [¹⁸ F]PI-2620 PET positivity was distinctly higher in β-amyloid-positive compared with β-amyloid-negative patients with pronounced involvement of the dorsolateral prefrontal cortex. Semiquantitative analysis of [¹⁸ F]PI-2620 PET revealed a sensitivity of 91% for β-amyloid-positive and of 65% for β-amyloid-negative cases, which is in excellent agreement with prior clinicopathological data. Regardless of β-amyloid status, hemispheric lateralization of [¹⁸ F]PI-2620 signal reflected contralateral predominance of clinical disease severity.

CONCLUSIONS

Our data indicate a value of [¹⁸ F]PI-2620 for evaluating corticobasal syndrome, providing quantitatively and regionally distinct signals in β-amyloid-positive as well as β-amyloid-negative corticobasal syndrome. In corticobasal syndrome, [¹⁸ F]PI-2620 may potentially serve for a differential diagnosis and for monitoring disease progression. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Molecular Imaging of Extrapyramidal Movement Disorders With Dementia: The 4R Tauopathies

Two pathologically distinct neurodegenerative conditions, progressive supranuclear palsy and corticobasal degeneration, share in common deposits of tau proteins that differ both molecularly and ultrastructurally from the common tau deposits diagnostic of Alzheimer disease. The proteinopathy in these disorders is characterized by fibrillary aggregates of 4R tau proteins. The clinical presentations of progressive supranuclear palsy and of corticobasal degeneration are often confused with more common disorders such as Parkinson disease or subtypes of frontotemporal lobar degeneration. Neither of these 4R tau disorders has effective therapy, and while there are emerging molecular imaging approaches to identify patients earlier in the course of disease, there are as yet no reliably sensitive and specific approaches to diagnoses in life. In this review, aspects of the clinical syndromes, neuropathology, and molecular biomarker imaging studies applicable to progressive supranuclear palsy and to corticobasal degeneration will be presented. Future development of more accurate molecular imaging approaches is proposed.

Intensity normalization methods in brain FDG-PET quantification

BACKGROUND

The lack of standardization of intensity normalization methods and its unknown effect on the quantification output is recognized as a major drawback for the harmonization of brain FDG-PET quantification protocols. The aim of this work is the ground truth-based evaluation of different intensity normalization methods on brain FDG-PET quantification output.

METHODS

Realistic FDG-PET images were generated using Monte Carlo simulation from activity and attenuation maps directly derived from 25 healthy subjects (adding theoretical relative hypometabolisms on 6 regions of interest and for 5 hypometabolism levels). Single-subject statistical parametric mapping (SPM) was applied to compare each simulated FDG-PET image with a healthy database after intensity normalization based on reference regions methods such as the brain stem (RRBS), cerebellum (RRC) and the temporal lobe contralateral to the lesion (RRTL), and data-driven methods, such as proportional scaling (PS), histogram-based method (HN) and iterative versions of both methods (iPS and iHN). The performance of these methods was evaluated in terms of the recovery of the introduced theoretical hypometabolic pattern and the appearance of unspecific hypometabolic and hypermetabolic findings.

RESULTS

Detected hypometabolic patterns had significantly lower volumes than the introduced hypometabolisms for all intensity normalization methods particularly for slighter reductions in metabolism . Among the intensity normalization methods, RRC and HN provided the largest recovered hypometabolic volumes, while the RRBS showed the smallest recovery. In general, data-driven methods overcame reference regions and among them, the iterative methods overcame the non-iterative ones. Unspecific hypermetabolic volumes were similar for all methods, with the exception of PS, where it became a major limitation (up to 250 cm³) for extended and intense hypometabolism. On the other hand, unspecific hypometabolism was similar far all methods, and usually solved with appropriate clustering.

CONCLUSIONS

Our findings showed that the inappropriate use of intensity normalization methods can provide remarkable bias in the detected hypometabolism and it represents a serious concern in terms of false positives. Based on our findings, we recommend the use of histogram-based intensity normalization methods. Reference region methods performance was equivalent to data-driven methods only when the selected reference region is large and stable.

Clinical, ocular motor, and imaging profile of Niemann-Pick type C heterozygosity

Objective

To characterize subclinical abnormalities in asymptomatic heterozygote NPC1 mutation carriers as markers of neurodegeneration.

Methods

Motor function, cognition, mood, sleep, and smell function were assessed in 20 first-degree heterozygous relatives of patients with Niemann-Pick disease type C (NPC) (13 male, age 52.7 ± 9.9 years). Video-oculography and abdominal ultrasound with volumetry were performed to assess oculomotor function and size of liver and spleen. NPC biomarkers in blood were analyzed. 18F-fluorodesoxyglucose PET was performed (n = 16) to detect patterns of brain hypometabolism.

Results

NPC heterozygotes recapitulated characteristic features of symptomatic NPC disease and demonstrated the oculomotor abnormalities typical of NPC. Hepatosplenomegaly (71%) and increased cholestantriol (33%) and plasma chitotriosidase (17%) levels were present. The patients also showed signs seen in other neurodegenerative diseases, including hyposmia (20%) or pathologic screening for REM sleep behavior disorder (24%). Cognitive function was frequently impaired, especially affecting visuoconstructive function, verbal fluency, and executive function. PET imaging revealed significantly decreased glucose metabolic rates in 50% of participants, affecting cerebellar, anterior cingulate, parieto-occipital, and temporal regions, including 1 with bilateral abnormalities.

Conclusion

NPC heterozygosity, which has a carrier frequency of 1:200 in the general population, is associated with abnormal brain metabolism and functional consequences. Clinically silent heterozygous gene variations in NPC1 may be a risk factor for late-onset neurodegeneration, similar to the concept of heterozygous GBA mutations underlying Parkinson disease.

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.