Biomarkers for tau pathology

Assessment of Demographic, Genetic, and Imaging Variables Associated With Brain Resilience and Cognitive Resilience to Pathological Tau in Patients With Alzheimer Disease

Question

Which demographic, genetic, and neuroimaging factors are associated with cognitive and brain resilience to pathological tau in patients with Alzheimer disease?

Findings

In this multicenter, cross-sectional, longitudinal study of 260 cognitively impaired amyloid-β–positive participants, young age and female sex were associated with greater brain resilience, whereas higher educational level and cortical thickness were associated with greater cognitive resilience.

Meaning

Cognitive and brain resilience may be associated with differential mechanisms, which may help explain interindividual differences in how well patients tolerate pathological tau.

Importance

Better understanding is needed of the degree to which individuals tolerate Alzheimer disease (AD)–like pathological tau with respect to brain structure (brain resilience) and cognition (cognitive resilience).

Objective

To examine the demographic (age, sex, and educational level), genetic (APOE-ε4 status), and neuroimaging (white matter hyperintensities and cortical thickness) factors associated with interindividual differences in brain and cognitive resilience to tau positron emission tomography (PET) load and to changes in global cognition over time.

Design, Setting, an Participants

In this cross-sectional, longitudinal study, tau PET was performed from June 1, 2014, to November 30, 2017, and global cognition monitored for a mean [SD] interval of 2.0 [1.8] years at 3 dementia centers in South Korea, Sweden, and the United States. The study included amyloid-β–positive participants with mild cognitive impairment or AD dementia. Data analysis was performed from October 26, 2018, to December 11, 2019.

Exposures

Standard dementia screening, cognitive testing, brain magnetic resonance imaging, amyloid-β PET and cerebrospinal fluid analysis, and flortaucipir (tau) labeled with fluor-18 (¹⁸F) PET.

Main Outcomes and Measures

Separate linear regression models were performed between whole cortex [¹⁸F]flortaucipir uptake and cortical thickness, and standardized residuals were used to obtain a measure of brain resilience. The same procedure was performed for whole cortex [¹⁸F]flortaucipir uptake vs Mini-Mental State Examination (MMSE) as a measure of cognitive resilience. Bivariate and multivariable linear regression models were conducted with age, sex, educational level, APOE-ε4 status, white matter hyperintensity volumes, and cortical thickness as independent variables and brain and cognitive resilience measures as dependent variables. Linear mixed models were performed to examine whether changes in MMSE scores over time differed as a function of a combined brain and cognitive resilience variable.

Results

A total of 260 participants (145 [55.8%] female; mean [SD] age, 69.2 [9.5] years; mean [SD] MMSE score, 21.9 [5.5]) were included in the study. In multivariable models, women (standardized β = −0.15, P = .02) and young patients (standardized β = −0.20, P = .006) had greater brain resilience to pathological tau. Higher educational level (standardized β = 0.23, P < .001) and global cortical thickness (standardized β = 0.23, P < .001) were associated with greater cognitive resilience to pathological tau. Linear mixed models indicated a significant interaction of brain resilience × cognitive resilience × time on MMSE (β [SE] = −0.235 [0.111], P = .03), with steepest slopes for individuals with both low brain and cognitive resilience.

Conclusions and Relevance

Results of this study suggest that women and young patients with AD have relative preservation of brain structure when exposed to neocortical pathological tau. Interindividual differences in resilience to pathological tau may be important to disease progression because participants with both low brain and cognitive resilience had the most rapid cognitive decline over time.

Frequency of Biologically Defined Alzheimer Disease in Relation to Age, Sex, APOE ε4, and Cognitive Impairment

OBJECTIVE

To assess the frequency of biologically defined Alzheimer disease (AD) in relation to age, sex, APOE ε4, and clinical diagnosis in a prospective cohort study evaluated with amyloid-PET and tau-PET.

METHODS

We assessed cognitively unimpaired (CU) elderly (n = 166), patients with amnestic mild cognitive impairment (n = 77), and patients with probable AD dementia (n = 62) who underwent evaluation by dementia specialists and neuropsychologists in addition to amyloid-PET with [18F]AZD4694 and tau-PET with [18F]MK6240. Individuals were grouped according to their AD biomarker profile. Positive predictive value for biologically defined AD was assessed in relation to clinical diagnosis. Frequency of AD biomarker profiles was assessed using logistic regressions with odds ratios (ORs) and 95% confidence intervals (CIs).

RESULTS

The clinical diagnosis of probable AD dementia demonstrated good agreement with biologically defined AD (positive predictive value 85.2%). A total of 7.88% of CU were positive for both amyloid-PET and tau-PET. Frequency of biologically defined AD increased with age (OR 1.14; p < 0.0001) and frequency of APOE ε4 allele carriers (single ε4: OR 3.82; p < 0.0001; double ε4: OR 17.55, p < 0.0001).

CONCLUSION

Whereas we observed strong, but not complete, agreement between clinically defined probable AD dementia and biomarker positivity for both β-amyloid and tau, we also observed that biologically defined AD was not rare in CU elderly. Abnormal tau-PET was almost exclusively observed in individuals with abnormal amyloid-PET. Our results highlight that even in tertiary care memory clinics, detailed evaluation by dementia specialists systematically underestimates the frequency of biologically defined AD and related entities.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that biologically defined AD (abnormal amyloid PET and tau PET) was observed in 85.2% of people with clinically defined AD and 7.88% of CU elderly.

Diagnostic performance and prediction of clinical progression of plasma phospho-tau181 in the Alzheimer's Disease Neuroimaging Initiative

Whilst cerebrospinal fluid (CSF) and positron emission tomography (PET) biomarkers for amyloid-β (Aβ) and tau pathologies are accurate for the diagnosis of Alzheimer's disease (AD), their broad implementation in clinical and trial settings are restricted by high cost and limited accessibility. Plasma phosphorylated-tau181 (p-tau181) is a promising blood-based biomarker that is specific for AD, correlates with cerebral Aβ and tau pathology, and predicts future cognitive decline. In this study, we report the performance of p-tau181 in >1000 individuals from the Alzheimer's Disease Neuroimaging Initiative (ADNI), including cognitively unimpaired (CU), mild cognitive impairment (MCI) and AD dementia patients characterized by Aβ PET. We confirmed that plasma p-tau181 is increased at the preclinical stage of Alzheimer and further increases in MCI and AD dementia. Individuals clinically classified as AD dementia but having negative Aβ PET scans show little increase but plasma p-tau181 is increased if CSF Aβ has already changed prior to Aβ PET changes. Despite being a multicenter study, plasma p-tau181 demonstrated high diagnostic accuracy to identify AD dementia (AUC = 85.3%; 95% CI, 81.4-89.2%), as well as to distinguish between Aβ- and Aβ+ individuals along the Alzheimer's continuum (AUC = 76.9%; 95% CI, 74.0-79.8%). Higher baseline concentrations of plasma p-tau181 accurately predicted future dementia and performed comparably to the baseline prediction of CSF p-tau181. Longitudinal measurements of plasma p-tau181 revealed low intra-individual variability, which could be of potential benefit in disease-modifying trials seeking a measurable response to a therapeutic target. This study adds significant weight to the growing body of evidence in the use of plasma p-tau181 as a non-invasive diagnostic and prognostic tool for AD, regardless of clinical stage, which would be of great benefit in clinical practice and a large cost-saving in clinical trial recruitment.

Distinct Neurotoxic Effects of Extracellular Tau Species in Primary Neuronal-Glial Cultures

Recent data from various experimental models support the link between extracellular tau and neurodegeneration; however, the exact mechanisms by which extracellular tau or its modified forms or aggregates cause neuronal death remain unclear. We have previously shown that exogenously applied monomers and oligomers of the longest tau isoform (2N4R) at micromolar concentrations induced microglial phagocytosis of stressed-but-viable neurons in vitro. In this study, we investigated whether extracellular phosphorylated tau2N4R (p-tau2N4R), isoform 1N4R (tau1N4R) and K18 peptide can induce neuronal death or loss in primary neuronal-glial cell cultures. We found that p-tau2N4R at 30 nM concentration induced loss of viable neurons; however, 700 nM p-tau2N4R caused necrosis of both neurons and microglia, and this neuronal death was partially glial cell-dependent. We also found that extracellular tau1N4R oligomers, but not monomers, at 3 μM concentration caused neuronal death in mixed cell cultures: self-assembly tau1N4R dimers-tetramers induced neuronal necrosis and apoptosis, whereas Aβ-promoted tau1N4R oligomers caused glial cell-dependent loss of neurons without signs of increased cell death. Monomeric and pre-aggregated tau peptide containing 4R repeats (K18) had no effect in mixed cultures, suggesting that tau neurotoxicity might be dependent on N-terminal part of the protein. Taken together, our results show that extracellular p-tau2N4R is the most toxic form among investigated tau species inducing loss of neurons at low nanomolar concentrations and that neurotoxicity of tau1N4R is dependent on its aggregation state.

Early brainstem [18F]THK5351 uptake is linked to cortical hyperexcitability in healthy aging

BACKGROUND

Neuronal hyperexcitability characterizes the early stages of Alzheimer’s disease (AD). In animals, early misfolded tau and amyloid-β (Aβ) protein accumulation — both central to AD neuropathology — promote cortical excitability and neuronal network dysfunction. In healthy humans, misfolded tau and Aβ aggregates are first detected, respectively, in the brainstem and frontomedial and temporobasal cortices, decades prior to the onset of AD cognitive symptoms. Whether cortical excitability is related to early brainstem tau — and its associated neuroinflammation — and cortical Aβ aggregations remains unknown.

METHODS

We probed frontal cortex excitability, using transcranial magnetic stimulation combined with electroencephalography, in a sample of 64 healthy late-middle–aged individuals (50–69 years; 45 women and 19 men). We assessed whole-brain ^[18F]THK5351 PET uptake as a proxy measure of tau/neuroinflammation, and we assessed whole-brain Aβ burden with ^[18F]Flutemetamol or ^[18F]Florbetapir radiotracers.

RESULTS

We found that higher ^[18F]THK5351 uptake in a brainstem monoaminergic compartment was associated with increased cortical excitability (r = 0.29, P = 0.02). By contrast, ^[18F]THK5351 PET signal in the hippocampal formation, although strongly correlated with brainstem signal in whole-brain voxel-based quantification analyses (P value corrected for family-wise error [P_{FWE-corrected}] < 0.001), was not significantly associated with cortical excitability (r = 0.14, P = 0.25). Importantly, no significant association was found between early Aβ cortical deposits and cortical excitability (r = –0.20, P = 0.11).

CONCLUSION

These findings reveal potential brain substrates for increased cortical excitability in preclinical AD and may constitute functional in vivo correlates of early brainstem tau accumulation and neuroinflammation in humans.

TRIAL REGISTRATION

EudraCT 2016-001436-35.

FUNDING

F.R.S.-FNRS Belgium, Wallonie-Bruxelles International, ULiège, Fondation Simone et Pierre Clerdent, European Regional Development Fund.

18F-THK 5351 and 11C-PiB PET of the Thai normal brain template

Objectives

The aim of the study was to create a local normal database brain template of Thai individuals for ¹¹C-Pittsburgh compound B (¹¹C-PiB) and ¹⁸F-THK 5351 depositions using statistical parametric mapping (SPM) software, and to validate and optimize the established specific brain template for use in clinical practice with a highly reliability and reproducibility.

Methods

This prospective study was conducted in 24 healthy right-handed volunteers (13 men, 11 women; aged: 42-79 years) who underwent ¹⁸F-THK 5351 and ¹¹C-PiB PET/CT scans. SPM was used for the ¹⁸F-THK 5351 and ¹¹C-PiB PET/CT image analysis. All PET images were processed individually using Diffusion Tensor Image -Magnetic Resonance Imaging-weighted images (DTI-MRI images), which involved: (1) conversion of Digital Imaging and Communications in Medicine (DICOM) files into an analyzable file extension (.NIFTI) for statistical parametric mapping, (2) setting of the origin (the anterior commissure was used as the anatomical landmark), (3) re-alignment, (4) co-registration of PET with B0 (T1W) and DTI-MRI images, (5) normalization, and (6) normal verification using the Thai MRI standard. We then compared the normal PET template with the abnormal deposition area of different dementia syndromes, including Alzheimer's disease and progressive supranuclear palsy.

Results

This method was able to differentiate cognitively normal from Alzheimer's disease and progressive supranuclear palsy subjects.

Conclusions

This normal brain template was able to be integrated into clinical practice and research using PET analyses at our center.

Comparing ATN-T designation by tau PET visual reads, tau PET quantification, and CSF PTau181 across three cohorts

PURPOSE

To compare rates of tau biomarker positivity (T-status) per the 2018 Alzheimer's Disease (AD) Research Framework derived from [¹⁸F]flortaucipir (FTP) PET visual assessment, FTP quantification, and cerebrospinal fluid (CSF) phosphorylated Tau-181 (PTau181).

METHODS

We included 351 subjects with varying clinical diagnoses from three cohorts with available FTP PET and CSF PTau181 within 18 months. T-status was derived from (1) FTP visual assessment by two blinded raters; (2) FTP standardized uptake value ratio (SUVR) quantification from a temporal meta-ROI (threshold: SUVR ≥1.27); and (3) Elecsys® Phospho-Tau (181P) CSF (Roche Diagnostics) concentrations (threshold: PTau181 ≥ 24.5 pg/mL).

RESULTS

FTP visual reads yielded the highest rates of T+, while T+ by SUVR increased progressively from cognitively normal (CN) through mild cognitive impairment (MCI) and AD dementia. T+ designation by CSF PTau181 was intermediate between FTP visual reads and SUVR values in CN, similar to SUVR in MCI, and lower in AD dementia. Concordance in T-status between modality pairs ranged from 68 to 76% and varied by clinical diagnosis, being highest in patients with AD dementia. In discriminating Aβ + MCI and AD subjects from healthy controls and non-AD participants, FTP visual assessment was most sensitive (0.96) but least specific (0.60). Specificity was highest with FTP SUVR (0.91) with sensitivity of 0.89. Sensitivity (0.73) and specificity (0.72) were balanced for PTau181.

CONCLUSION

The choice of tau biomarker may differ by disease stage and research goals that seek to maximize sensitivity or specificity. Visual interpretations of tau PET enhance sensitivity compared to quantification alone, particularly in early disease stages.

Biomarkers for Alzheimer's disease-preparing for a new era of disease-modifying therapies

Clinical trial results presented in 2019 suggest that antibody-based removal of cerebral amyloid β (Aβ) plaques may possibly clear tau tangles and modestly slow cognitive decline in symptomatic Alzheimer's disease (AD). Although regulatory approval of this approach is still pending, preparing the healthcare system for the advent of disease-modifying therapies against AD is imperative. In particular, it will be necessary to identify the most suitable biomarkers to facilitate appropriate treatment of AD. Here, we give an update on recent developments in fluid and imaging biomarkers for AD-related pathologies and discuss potential approaches that could be adopted to screen for and clarify the underlying pathology in people seeking medical advice because of cognitive symptoms. We succinctly review recent data regarding biomarkers for Aβ and tau pathology, neurodegeneration, synaptic dysfunction, and inflammation, highlight the need for further research into common copathologies, and suggest how different biomarkers could be used (most likely in combination) to facilitate the development and clinical implementation of novel drug candidates against AD.

Increasing the reproducibility of fluid biomarker studies in neurodegenerative studies

Biomarkers have revolutionized scientific research on neurodegenerative diseases, in particular Alzheimer's disease, transformed drug trial design, and are also increasingly improving patient management in clinical practice. A few key cerebrospinal fluid biomarkers have been robustly associated with neurodegenerative diseases. Several novel biomarkers are very promising, especially blood-based markers. However, many biomarker findings have had low reproducibility despite initial promising results. In this perspective, we identify possible sources for low reproducibility of studies on fluid biomarkers for neurodegenerative diseases, with a focus on Alzheimer's disease. We suggest guidelines for researchers and journal editors, with the aim to improve reproducibility of findings.

Regional Tau Effects on Prospective Cognitive Change in Cognitively Normal Older Adults

Studies suggest that tau deposition starts in the anterolateral entorhinal cortex (EC) with normal aging, and that the presence of β-amyloid (Aβ) facilitates its spread to neocortex, which may reflect the beginning of Alzheimer's disease (AD). Functional connectivity between the anterolateral EC and the anterior-temporal (AT) memory network appears to drive higher tau deposition in AT than in the posterior-medial (PM) memory network. Here, we investigated whether this differential vulnerability to tau deposition may predict different cognitive consequences of EC, AT, and PM tau. Using ¹⁸F-flortaucipir (FTP) and ¹¹C-Pittsburgh compound-B (PiB) positron emission tomography (PET) imaging, we measured tau and Aβ in 124 cognitively normal human older adults (74 females, 50 males) followed for an average of 2.8 years for prospective cognition. We found that higher FTP in all three regions was individually related to faster memory decline, and that the effects of AT and PM FTP, but not EC, were driven by Aβ+ individuals. Moreover, when we included all three FTP measures competitively in the same model, only AT FTP significantly predicted memory decline. Our data support a model whereby tau, facilitated by Aβ, transits from EC to cortical regions that are most closely associated with the anterolateral EC, which specifically affects memory in the initial stage of AD. Memory also appears to be affected by EC tau in the absence of Aβ, which may be less clinically consequential. These findings may provide clarification of differences between normal aging and AD, and elucidate the transition between the two stages.SIGNIFICANCE STATEMENT Tau and β-amyloid (Aβ) are hallmarks of Alzheimer's disease (AD) but are also found in cognitively normal people. It is unclear whether, and how, this early deposition of tau and Aβ may affect cognition in normal aging and the asymptomatic stage of AD. We show that tau deposition in the entorhinal cortex (EC), which is common in advanced age, predicts memory decline in older adults independent of Aβ, likely reflecting normal, age-related memory loss. In contrast, tau in anterior-temporal (AT) regions is most predictive of memory decline in Aβ+ individuals. These data support the idea that tau preferentially spreads to specific cortical regions, likely through functional connections, which plays a primary role in memory decline in the early stage of AD.

Novel tau biomarkers phosphorylated at T181, T217 or T231 rise in the initial stages of the preclinical Alzheimer's continuum when only subtle changes in Aβ pathology are detected

In Alzheimer's disease (AD), tau phosphorylation in the brain and its subsequent release into cerebrospinal fluid (CSF) and blood is a dynamic process that changes during disease evolution. The main aim of our study was to characterize the pattern of changes in phosphorylated tau (p-tau) in the preclinical stage of the Alzheimer's continuum. We measured three novel CSF p-tau biomarkers, phosphorylated at threonine-181 and threonine-217 with an N-terminal partner antibody and at threonine-231 with a mid-region partner antibody. These were compared with an automated mid-region p-tau181 assay (Elecsys) as the gold standard p-tau measure. We demonstrate that these novel p-tau biomarkers increase more prominently in preclinical Alzheimer, when only subtle changes of amyloid-β (Aβ) pathology are detected, and can accurately differentiate Aβ-positive from Aβ-negative cognitively unimpaired individuals. Moreover, we show that the novel plasma N-terminal p-tau181 biomarker is mildly but significantly increased in the preclinical stage. Our results support the idea that early changes in neuronal tau metabolism in preclinical Alzheimer, likely in response to Aβ exposure, can be detected with these novel p-tau assays.

Plasma tau predicts cerebral vulnerability in aging

Identifying cerebral vulnerability in late life may help prevent or slow the progression of aging-related chronic diseases. However, non-invasive biomarkers aimed at detecting subclinical cerebral changes in the elderly are lacking. Here, we have examined the potential of plasma total tau (t-tau) for identifying cerebral and cognitive deficits in normal elderly subjects. Patterns of cortical thickness and cortical glucose metabolism were used as outcomes of cerebral vulnerability. We found that increased plasma t-tau levels were associated with widespread reductions of cortical glucose uptake, thinning of the temporal lobe, and memory deficits. Importantly, tau-related reductions of glucose consumption in the orbitofrontal cortex emerged as a determining factor of the relationship between cortical thinning and memory loss. Together, these results support the view that plasma t-tau may serve to identify subclinical cerebral and cognitive deficits in normal aging, allowing detection of individuals at risk for developing aging-related neurodegenerative conditions.

Amyloid-β PET and CSF in an autopsy-confirmed cohort

OBJECTIVE

Accumulation of amyloid-β is among the earliest changes in Alzheimer's disease (AD). Amyloid-β positron emission tomography (PET) and Aβ42 in cerebrospinal fluid (CSF) both assess amyloid-β pathology in-vivo, but 10-20% of cases show discordant (CSF+/PET- or CSF-/PET+) results. The neuropathological correspondence with amyloid-β CSF/PET discordance is unknown.

METHODS

We included 21 patients from our tertiary memory clinic who had undergone both CSF Aβ42 analysis and amyloid-β PET, and had neuropathological data available. Amyloid-β PET and CSF results were compared with neuropathological ABC scores (comprising of Thal (A), Braak (B), and CERAD (C) stage, all ranging from 0 [low] to 3 [high]) and neuropathological diagnosis.

RESULTS

Neuropathological diagnosis was AD in 11 (52%) patients. Amyloid-β PET was positive in all A3, C2, and C3 cases and in one of the two A2 cases. CSF Aβ42 was positive in 92% of ≥A2 and 90% of ≥C2 cases. PET and CSF were discordant in three of 21 (14%) cases: CSF+/PET- in a patient with granulomatosis with polyangiitis (A0B0C0), CSF+/PET- in a patient with FTLD-TDP type B (A2B1C1), and CSF-/PET+ in a patient with AD (A3B3C3). Two CSF+/PET+ cases had a non-AD neuropathological diagnosis, that is FTLD-TDP type E (A3B1C1) and adult-onset leukoencephalopathy with axonal spheroids (A1B1C0).

INTERPRETATION

Our study demonstrates neuropathological underpinnings of amyloid-β CSF/PET discordance. Furthermore, amyloid-β biomarker positivity on both PET and CSF did not invariably result in an AD diagnosis at autopsy, illustrating the importance of considering relevant comorbidities when evaluating amyloid-β biomarker results.

High-yield synthesis of a tau PET radioligand and its nonradioactive ligand using an alternative protection and deprotection strategy

Recently developed tau imaging radiopharmaceuticals show specific uptake in tau protein-rich regions in human brains without off-target binding. These radiopharmaceuticals and their nonradioactive reference ligands are generally obtained in low (radio)chemical yields. In the present study, we investigated high-yield synthesis of ¹⁸ F-RO948 ([¹⁸ F]1) and its nonradioactive ligand (1). The ligand 1 was synthesized by a Suzuki-Miyaura coupling reaction between 9-(4-methoxybenzyl)-9H-pyrrolo[2,3-b:4,5-c']dipyridin-2-yl trifluoromethanesulfonate (3) and 2-fluoro-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (4), followed by oxidative removal of the para-methoxybenzyl (PMB) group with ceric ammonium nitrate (CAN). This two-step reaction gave 1 in 55.8% yield. The precursor for [¹⁸ F]1 was synthesized from 3 and 2-nitro-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (6). The resulting PMB-protected precursor 8 was obtained in 74.5% yield. [¹⁸ F]1 was synthesized by radiofluorination of 8 (radiochemical conversion (RCC): 95.7 ± 1.7%), followed by deprotection of the PMB group with CAN. This one-pot, two-step radiochemical synthesis followed by HPLC purification gave [¹⁸ F]1 in high decay-corrected radiochemical yield (54-60%). The RCC of [¹⁸ F]fluoride to [¹⁸ F]1 in our two-step synthesis method was similar to that in a one-step radiofluorination reaction of a tert-butoxycarbonyl (BOC)-protected precursor 10 that proceeds with concomitant thermal deprotection of the BOC group. Taken together, the results of this study suggest that this high-yield synthesis method is useful for the synthesis of ¹⁸ F-labeled (NH)heteroarene compounds.

Discovery of 2-(4-(2-fluoroethoxy)piperidin-1-yl)-9-methyl-9H-pyrrolo[2,3-b:4,5-c']dipyridine ([18F]PI-2014) as PET tracer for the detection of pathological aggregated tau in Alzheimer's disease and other tauopathies

The compound screening was initiated with a direct staining assay to identify compounds binding to Tau aggregates and not Abeta plaques using human brain sections derived from late stage Alzheimer's disease donors. The binding of Tau aggregate selective compounds was then quantitatively assessed with human brain derived paired helical filaments utilizing the label-free Back Scattering Interferometry assay. In vivo biodistribution experiments of selected fluorine-18 labeled compounds were performed in mice to assess brain uptake, brain washout, and defluorination. Compound 11 emerged as the most promising candidate, displaying high in vitro binding affinity and selectivity to neurofibrillary tangles. Fluorine-18 labeled compound 11 showed high brain uptake and rapid washout from the mouse brain with no observed bone uptake. Furthermore, compound 11 was able to detect Tau aggregates in tauopathy brain sections from corticobasal degeneration, progressive supranuclear palsy, and Pick's disease donors. Thus, 2-(4-(2-fluoroethoxy)piperidin-1-yl)-9-methyl-9H-pyrrolo[2,3-b:4,5-c']dipyridine (PI-2014, compound 11) was selected for characterization in a first-in-human study.

Analysis of amyloid and tau deposition in Alzheimer's disease using 11C-Pittsburgh compound B and 18F-THK 5351 positron emission tomography imaging

This study aims to determine the deposition of ¹¹C-Pittsburgh compound B (¹¹C-PiB) and ¹⁸F-THK 5351 using a normal database of the optimal cut-off-points for standardized uptake value ratios (SUVRs) in Alzheimer's disease (AD) patients. Sixteen AD patients and 24 cognitively normal individuals were enrolled in this study. The optimal cutoff points for the SUVR from the normal database were used for quantitative analysis. P-mod software with the Automated Anatomical Labeling merged atlas was employed to generate automatic volumes of interest to identify different brain regions, and the SUVRs of AD patients were compared with those of the age-matched normal controls. The correlation between PiB and THK5351 deposition at matching brain regions was identified. The mean regional ¹¹C-PiB SUVRs of the AD patients were significantly higher than the healthy controls (P < 0.05). The ¹¹C-PiB SUVR cut-offs were 1.46–1.81, with sensitivity ranging from 81.25% to 93.75% and specificity of 100%. The mean SUVRs of ¹⁸F-THK 5351 in various regions were also significantly higher in the AD patients than in the healthy controls (P < 0.05). The inferior temporal gyrus yielded an optimum SUVR cut-off-points of 1.5 with 80% sensitivity and 83.33% specificity. The correlation of PiB and THK5351 SUVR was reported at precuneus, parietal, and occipital brain areas, with spearman's rho of 0.67, 0.66, and 0.72, respectively. Our findings allow determination of the SUVRs of ¹¹C-PiB and ¹⁸F-THK-5351 amyloid and tau positron emission tomography tracers for clinical use, according to the normal database of the optimal cut-off-points for SUVRs in AD patients.

Discordant Alzheimer's neurodegenerative biomarkers and their clinical outcomes

OBJECTIVE

In the 2018 ATN framework, Alzheimer's neurodegenerative biomarkers comprised cerebrospinal fluid (CSF) total tau, 18 F-fluorodeoxyglucose-positron emission tomography, and brain atrophy. We aimed to assess the clinical outcomes of having discordant Alzheimer's neurodegenerative biomarkers.

METHODS

A total of 721 non-demented individuals from the Alzheimer's Disease Neuroimaging Initiative database were included and then further categorized into concordant-negative, discordant, and concordant-positive groups. Demographic distributions of the groups were compared. Longitudinal changes in clinical outcomes and risk of conversion were assessed using linear mixed-effects models and multivariate Cox proportional hazard models, respectively.

RESULTS

Discordant group was intermediate to concordant-negative and concordant-positive groups in terms of APOE ε4 positivity, CSF amyloid-beta, and phosphorylated tau. Compared with concordant-negative group, discordant group deteriorated faster in cognitive scores (Mini-Mental State Examination, the Clinical Dementia Rating Scale-Sum of Boxes, and the Functional Activities Questionnaire) and demonstrated greater rates of atrophy in brain structures (hippocampus, entorhinal cortex, and whole brain), and concordant-positive group performed worse over time than discordant group. Moreover, the risk of cognitive decline increased from concordant-negative to discordant to concordant-positive. The results from longitudinal analyses were validated in A+T+, cognitively normal, and mild cognitive impairment individuals, and were also validated by applying different cutoffs and neurodegenerative biomarkers.

INTERPRETATION

Discordant neurodegenerative status denotes a stage of cognitive function which is intermediate between concordant-negative and concordant-positive. Identification of discordant cases would provide insights into intervention and new therapy approaches, particularly in A+T+ individuals. Moreover, this work may be a complement to the ATN scheme.

Bridging the Gap Between Fluid Biomarkers for Alzheimer's Disease, Model Systems, and Patients

Alzheimer’s disease (AD) is a debilitating neurodegenerative disease characterized by the accumulation of two proteins in fibrillar form: amyloid-β (Aβ) and tau. Despite decades of intensive research, we cannot yet pinpoint the exact cause of the disease or unequivocally determine the exact mechanism(s) underlying its progression. This confounds early diagnosis and treatment of the disease. Cerebrospinal fluid (CSF) biomarkers, which can reveal ongoing biochemical changes in the brain, can help monitor developing AD pathology prior to clinical diagnosis. Here we review preclinical and clinical investigations of commonly used biomarkers in animals and patients with AD, which can bridge translation from model systems into the clinic. The core AD biomarkers have been found to translate well across species, whereas biomarkers of neuroinflammation translate to a lesser extent. Nevertheless, there is no absolute equivalence between biomarkers in human AD patients and those examined in preclinical models in terms of revealing key pathological hallmarks of the disease. In this review, we provide an overview of current but also novel AD biomarkers and how they relate to key constituents of the pathological cascade, highlighting confounding factors and pitfalls in interpretation, and also provide recommendations for standardized procedures during sample collection to enhance the translational validity of preclinical AD models.

Plasma p-tau181 accurately predicts Alzheimer's disease pathology at least 8 years prior to post-mortem and improves the clinical characterisation of cognitive decline

The neuropathological confirmation of amyloid-β (Aβ) plaques and tau neurofibrillary tangles (NFT) remains the gold standard for a definitive diagnosis of Alzheimer’s disease (AD). Nowadays, the in vivo diagnosis of AD is greatly aided by both cerebrospinal fluid (CSF) and positron emission tomography (PET) biomarkers. Although highly accurate, their broad implementation is restricted by high cost, limited accessibility and invasiveness. We recently developed a high-performance, ultrasensitive immunoassay for the quantification of tau phosphorylated at threonine-181 (p-tau181) in plasma, which identifies AD pathophysiology with high accuracy. However, it remains unclear whether plasma p-tau181, measured years before the death, can predict the eventual neuropathological confirmation of AD, and successfully discriminates AD from non-AD dementia pathologies. We studied a unique cohort of 115 individuals with longitudinal blood collections with clinical evaluation at 8, 4 and 2 years prior to neuropathological assessment at death. The results demonstrate that plasma p-tau181 associates better with AD neuropathology and Braak staging than a clinical diagnosis 8 years before post-mortem. Moreover, while all patients had a diagnosis of AD dementia during life, plasma p-tau181 proved to discriminate AD from non-AD pathologies with high accuracy (AUC = 97.4%, 95% CI = 94.1–100%) even 8 years before death. Additionally, the longitudinal trajectory of plasma p-tau181 was assessed in all patients. We found that the main increases in plasma p-tau181 occurred between 8 and 4 years prior to death in patients with AD neuropathology and later plateauing. In contrast, non-AD pathologies and controls exhibited minor, albeit significant, increases in p-tau181 up until death. Overall, our study demonstrates that plasma p-tau181 is highly predictive and specific of AD neuropathology years before post-mortem examination. These data add further support for the use of plasma p-tau181 to aid clinical management in primary care and recruitment for clinical trials.

The pathway to secondary prevention of Alzheimer's disease

Alzheimer's disease (AD) is a continuum consisting of a preclinical stage that occurs decades before symptoms appear. As researchers make advances in investigating the continuum, the importance of developing drugs for secondary prevention is garnering increased discussion. For efficacious drug development for secondary prevention it is important to define what are the earliest biological stages of AD. The Alzheimer's Association Research Roundtable convened November 27 to 28, 2018 to focus on pre-clinical AD. This review will address the biological approach to defining pre-clinical AD, detection, identification of at-risk individuals, and lessons learned from trials such as A4 and TOMMORROW.

Fluid Biomarkers for Synaptic Dysfunction and Loss

Synapses are the site for brain communication where information is transmitted between neurons and stored for memory formation. Synaptic degeneration is a global and early pathogenic event in neurodegenerative disorders with reduced levels of pre- and postsynaptic proteins being recognized as a core feature of Alzheimer's disease (AD) pathophysiology. Together with AD, other neurodegenerative and neurodevelopmental disorders show altered synaptic homeostasis as an important pathogenic event, and due to that, they are commonly referred to as synaptopathies. The exact mechanisms of synapse dysfunction in the different diseases are not well understood and their study would help understanding the pathogenic role of synaptic degeneration, as well as differences and commonalities among them and highlight candidate synaptic biomarkers for specific disorders. The assessment of synaptic proteins in cerebrospinal fluid (CSF), which can reflect synaptic dysfunction in patients with cognitive disorders, is a keen area of interest. Substantial research efforts are now directed toward the investigation of CSF synaptic pathology to improve the diagnosis of neurodegenerative disorders at an early stage as well as to monitor clinical progression. In this review, we will first summarize the pathological events that lead to synapse loss and then discuss the available data on established (eg, neurogranin, SNAP-25, synaptotagmin-1, GAP-43, and α-syn) and emerging (eg, synaptic vesicle glycoprotein 2A and neuronal pentraxins) CSF biomarkers for synapse dysfunction, while highlighting possible utilities, disease specificity, and technical challenges for their detection.

Tracking the potential involvement of metabolic disease in Alzheimer's disease-Biomarkers and beyond

There is a vast literature linking systemic metabolic conditions to dementia due to Alzheimer's disease (AD). Advances in in vivo measurements of AD neuropathology using brain imaging, cerebrospinal fluid (CSF), and/or blood biomarkers have led to research in AD that uses in vivo biomarkers as outcomes, focusing primarily on amyloid, tau, and neurodegeneration as constructs. Studies of Type 2 Diabetes Mellitus (T2DM) and AD biomarkers seem to show that T2DM is not related to amyloid deposition, but is related to neurodegeneration and tau deposition. There is a dearth of studies examining adiposity, insulin resistance, and metabolic syndrome in relation to AD biomarkers and the associations in these studies are inconsistent. Metabolomics studies have reported associations of unsaturated fatty acids with AD neuropathology at autopsy, and sphingolipids and glycerophospholipids in relation to neurodegeneration and amyloid and tau. There are other neurodegenerative diseases, such as Lewy body disease that may overlap with AD, and specific biomarkers for these pathologies are being developed and should be integrated into AD biomarker research. More longitudinal studies are needed with concurrent assessment of metabolic factors and AD biomarkers in order to improve the opportunity to assess causality. Ideally, AD biomarkers should be integrated into clinical trials of interventions that affect metabolic factors. Advances in blood-based AD biomarkers, which are less costly and invasive compared with CSF and brain imaging biomarkers, could facilitate widespread implementation of AD biomarkers in studies examining the metabolic contribution to AD.

Tau PET imaging with 18F-PI-2620 in aging and neurodegenerative diseases

PURPOSE

In vivo measurement of the spatial distribution of neurofibrillary tangle pathology is critical for early diagnosis and disease monitoring of Alzheimer's disease (AD).

METHODS

Forty-nine participants were scanned with ¹⁸F-PI-2620 PET to examine the distribution of this novel PET ligand throughout the course of AD: 36 older healthy controls (HC) (age range 61 to 86), 11 beta-amyloid+ (Aβ+) participants with cognitive impairment (CI; clinical diagnosis of either mild cognitive impairment or AD dementia, age range 57 to 86), and 2 participants with semantic variant primary progressive aphasia (svPPA, age 66 and 78). Group differences in brain regions relevant in AD (medial temporal lobe, posterior cingulate cortex, and lateral parietal cortex) were examined using standardized uptake value ratios (SUVRs) normalized to the inferior gray matter of the cerebellum.

RESULTS

SUVRs in target regions were relatively stable 60 to 90 min post-injection, with the exception of very high binders who continued to show increases over time. Robust elevations in ¹⁸F-PI-2620 were observed between HC and Aβ+ CI across all AD regions. Within the HC group, older age was associated with subtle elevations in target regions. Mildly elevated focal uptake was observed in the anterior temporal pole in one svPPA patient.

CONCLUSION

Preliminary results suggest strong differences in the medial temporal lobe and cortical regions known to be impacted in AD using ¹⁸F-PI-2620 in patients along the AD trajectory. This work confirms that ¹⁸F-PI-2620 holds promise as a tool to visualize tau aggregations in AD.

Tau pathology and relative cerebral blood flow are independently associated with cognition in Alzheimer's disease

Purpose

We aimed to investigate associations between tau pathology and relative cerebral blood flow (rCBF), and their relationship with cognition in Alzheimer’s disease (AD), by using a single dynamic [¹⁸F]flortaucipir positron emission tomography (PET) scan.

Methods

Seventy-one subjects with AD (66 ± 8 years, mini-mental state examination (MMSE) 23 ± 4) underwent a dynamic 130-min [¹⁸F]flortaucipir PET scan. Cognitive assessment consisted of composite scores of four cognitive domains. For tau pathology and rCBF, receptor parametric mapping (cerebellar gray matter reference region) was used to create uncorrected and partial volume-corrected parametric images of non-displaceable binding potential (BP_ND) and R₁, respectively. (Voxel-wise) linear regressions were used to investigate associations between BP_ND and/or R₁ and cognition_.

Results

Higher [¹⁸F]flortaucipir BP_ND was associated with lower R₁ in the lateral temporal, parietal and occipital regions. Higher medial temporal BP_ND was associated with worse memory, and higher lateral temporal BP_ND with worse executive functioning and language. Higher parietal BP_ND was associated with worse executive functioning, language and attention, and higher occipital BP_ND with lower cognitive scores across all domains. Higher frontal BP_ND was associated with worse executive function and attention. For [¹⁸F]flortaucipir R₁, lower values in the lateral temporal and parietal ROIs were associated with worse executive functioning, language and attention, and lower occipital R₁ with lower language and attention scores. When [¹⁸F]flortaucipir BP_ND and R₁ were modelled simultaneously, associations between lower R₁ in the lateral temporal ROI  and worse attention remained, as well as for lower parietal R₁ and worse executive functioning and attention.

Conclusion

Tau pathology was associated with locally reduced rCBF. Tau pathology and low rCBF were both independently associated with worse cognitive performance. For tau pathology, these associations spanned widespread neocortex, while for rCBF, independent associations were restricted to lateral temporal and parietal regions and the executive functioning and attention domains. These findings indicate that each biomarker may independently contribute to cognitive impairment in AD.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04831-w) contains supplementary material, which is available to authorized users.

Relevance of biomarkers across different neurodegenerative diseases

Background

The panel of fluid- and imaging-based biomarkers available for neurodegenerative disease research is growing and has the potential to close important gaps in research and the clinic. With this growth and increasing use, appropriate implementation and interpretation are paramount. Various biomarkers feature nuanced differences in strengths, limitations, and biases that must be considered when investigating disease etiology and clinical utility. For example, neuropathological investigations of Alzheimer’s disease pathogenesis can fall in disagreement with conclusions reached by biomarker-based investigations. Considering the varied strengths, limitations, and biases of different research methodologies and approaches may help harmonize disciplines within the neurodegenerative disease field.

Purpose of review

Along with separate review articles covering fluid and imaging biomarkers in this issue of Alzheimer’s Research and Therapy, we present the result of a discussion from the 2019 Biomarkers in Neurodegenerative Diseases course at the University College London. Here, we discuss themes of biomarker use in neurodegenerative disease research, commenting on appropriate use, interpretation, and considerations for implementation across different neurodegenerative diseases. We also draw attention to areas where biomarker use can be combined with other disciplines to understand issues of pathophysiology and etiology underlying dementia. Lastly, we highlight novel modalities that have been proposed in the landscape of neurodegenerative disease research and care.

Microglial activation and tau burden predict cognitive decline in Alzheimer's disease

Tau pathology, neuroinflammation, and neurodegeneration are key aspects of Alzheimer's disease. Understanding whether these features predict cognitive decline, alone or in combination, is crucial to develop new prognostic measures and enhanced stratification for clinical trials. Here, we studied how baseline assessments of in vivo tau pathology (measured by 18F-AV-1451 PET), neuroinflammation (measured by 11C-PK11195 PET) and brain atrophy (derived from structural MRI) predicted longitudinal cognitive changes in patients with Alzheimer's disease pathology. Twenty-six patients (n = 12 with clinically probable Alzheimer's dementia and n = 14 with amyloid-positive mild cognitive impairment) and 29 healthy control subjects underwent baseline assessment with 18F-AV-1451 PET, 11C-PK11195 PET, and structural MRI. Cognition was examined annually over the subsequent 3 years using the revised Addenbrooke's Cognitive Examination. Regional grey matter volumes, and regional binding of 18F-AV-1451 and 11C-PK11195 were derived from 15 temporo-parietal regions characteristically affected by Alzheimer's disease pathology. A principal component analysis was used on each imaging modality separately, to identify the main spatial distributions of pathology. A latent growth curve model was applied across the whole sample on longitudinal cognitive scores to estimate the rate of annual decline in each participant. We regressed the individuals' estimated rate of cognitive decline on the neuroimaging components and examined univariable predictive models with single-modality predictors, and a multi-modality predictive model, to identify the independent and combined prognostic value of the different neuroimaging markers. Principal component analysis identified a single component for the grey matter atrophy, while two components were found for each PET ligand: one weighted to the anterior temporal lobe, and another weighted to posterior temporo-parietal regions. Across the whole-sample, the single-modality models indicated significant correlations between the rate of cognitive decline and the first component of each imaging modality. In patients, both stepwise backward elimination and Bayesian model selection revealed an optimal predictive model that included both components of 18F-AV-1451 and the first (i.e. anterior temporal) component for 11C-PK11195. However, the MRI-derived atrophy component and demographic variables were excluded from the optimal predictive model of cognitive decline. We conclude that temporo-parietal tau pathology and anterior temporal neuroinflammation predict cognitive decline in patients with symptomatic Alzheimer's disease pathology. This indicates the added value of PET biomarkers in predicting cognitive decline in Alzheimer's disease, over and above MRI measures of brain atrophy and demographic data. Our findings also support the strategy for targeting tau and neuroinflammation in disease-modifying therapy against Alzheimer's disease.

Propagation of Tau Pathology: Integrating Insights From Postmortem and In Vivo Studies

Cellular accumulation of aggregated forms of the protein tau is a defining feature of so-called tauopathies such as Alzheimer's disease, progressive supranuclear palsy, and chronic traumatic encephalopathy. A growing body of literature suggests that conformational characteristics of tau filaments, along with regional vulnerability to tau pathology, account for the distinct histopathological morphologies, biochemical composition, and affected cell types seen across these disorders. In this review, we describe and discuss recent evidence from human postmortem and clinical biomarker studies addressing the differential vulnerability of brain areas to tau pathology, its cell-to-cell transmission, and characteristics of the different strains that tau aggregates can adopt. Cellular biosensor assays are increasingly used in human tissue to detect the earliest forms of tau pathology, before overt histopathological lesions (i.e., neurofibrillary tangles) are apparent. Animal models with localized tau expression are used to uncover the mechanisms that influence spreading of tau aggregates. Further, studies of human postmortem-derived tau filaments from different tauopathies injected in rodents have led to striking findings that recapitulate neuropathology-based staging of tau. Furthermore, the recent advent of tau positron emission tomography and novel fluid-based biomarkers render it possible to study the temporal progression of tau pathology in vivo. Ultimately, evidence from these approaches must be integrated to better understand the onset and progression of tau pathology across tauopathies. This will lead to improved methods for the detection and monitoring of disease progression and, hopefully, to the development and refinement of tau-based therapeutics.

Imaging biomarkers in neurodegeneration: current and future practices

There is an increasing role for biological markers (biomarkers) in the understanding and diagnosis of neurodegenerative disorders. The application of imaging biomarkers specifically for the in vivo investigation of neurodegenerative disorders has increased substantially over the past decades and continues to provide further benefits both to the diagnosis and understanding of these diseases. This review forms part of a series of articles which stem from the University College London/University of Gothenburg course "Biomarkers in neurodegenerative diseases". In this review, we focus on neuroimaging, specifically positron emission tomography (PET) and magnetic resonance imaging (MRI), giving an overview of the current established practices clinically and in research as well as new techniques being developed. We will also discuss the use of machine learning (ML) techniques within these fields to provide additional insights to early diagnosis and multimodal analysis.

An update on blood-based biomarkers for non-Alzheimer neurodegenerative disorders

Cerebrospinal fluid analyses and neuroimaging can identify the underlying pathophysiology at the earliest stage of some neurodegenerative disorders, but do not have the scalability needed for population screening. Therefore, a blood-based marker for such pathophysiology would have greater utility in a primary care setting and in eligibility screening for clinical trials. Rapid advances in ultra-sensitive assays have enabled the levels of pathological proteins to be measured in blood samples, but research has been predominantly focused on Alzheimer disease (AD). Nonetheless, proteins that were identified as potential blood-based biomarkers for AD, for example, amyloid-β, tau, phosphorylated tau and neurofilament light chain, are likely to be relevant to other neurodegenerative disorders that involve similar pathological processes and could also be useful for the differential diagnosis of clinical symptoms. This Review outlines the neuropathological, clinical, molecular imaging and cerebrospinal fluid features of the most common neurodegenerative disorders outside the AD continuum and gives an overview of the current status of blood-based biomarkers for these disorders.

Aβ deposition is associated with increases in soluble and phosphorylated tau that precede a positive Tau PET in Alzheimer's disease

The links between β-amyloid (Aβ) and tau in Alzheimer's disease are unclear. Cognitively unimpaired persons with signs of Aβ pathology had increased cerebrospinal fluid (CSF) phosphorylated tau (P-tau181 and P-tau217) and total-tau (T-tau), which increased over time, despite no detection of insoluble tau aggregates [normal Tau positron emission tomography (PET)]. CSF P-tau and T-tau started to increase before the threshold for Amyloid PET positivity, while Tau PET started to increase after Amyloid PET positivity. Effects of Amyloid PET on Tau PET were mediated by CSF P-tau, and high CSF P-tau predicted increased Tau PET rates. Individuals with MAPT mutations and signs of tau deposition (but without Aβ pathology) had normal CSF P-tau levels. In 5xFAD mice, CSF tau increased when Aβ aggregation started. These results show that Aβ pathology may induce changes in soluble tau release and phosphorylation, which is followed by tau aggregation several years later in humans.

Plasma P-tau181 in Alzheimer's disease: relationship to other biomarkers, differential diagnosis, neuropathology and longitudinal progression to Alzheimer's dementia

Plasma phosphorylated tau181 (P-tau181) might be increased in Alzheimer's disease (AD), but its usefulness for differential diagnosis and prognosis is unclear. We studied plasma P-tau181 in three cohorts, with a total of 589 individuals, including cognitively unimpaired participants and patients with mild cognitive impairment (MCI), AD dementia and non-AD neurodegenerative diseases. Plasma P-tau181 was increased in preclinical AD and further increased at the MCI and dementia stages. It correlated with CSF P-tau181 and predicted positive Tau positron emission tomography (PET) scans (area under the curve (AUC) = 0.87-0.91 for different brain regions). Plasma P-tau181 differentiated AD dementia from non-AD neurodegenerative diseases with an accuracy similar to that of Tau PET and CSF P-tau181 (AUC = 0.94-0.98), and detected AD neuropathology in an autopsy-confirmed cohort. High plasma P-tau181 was associated with subsequent development of AD dementia in cognitively unimpaired and MCI subjects. In conclusion, plasma P-tau181 is a noninvasive diagnostic and prognostic biomarker of AD, which may be useful in clinical practice and trials.

Diagnostic value of plasma phosphorylated tau181 in Alzheimer's disease and frontotemporal lobar degeneration

With the potential development of new disease-modifying Alzheimer's disease (AD) therapies, simple, widely available screening tests are needed to identify which individuals, who are experiencing symptoms of cognitive or behavioral decline, should be further evaluated for initiation of treatment. A blood-based test for AD would be a less invasive and less expensive screening tool than the currently approved cerebrospinal fluid or amyloid β positron emission tomography (PET) diagnostic tests. We examined whether plasma tau phosphorylated at residue 181 (pTau181) could differentiate between clinically diagnosed or autopsy-confirmed AD and frontotemporal lobar degeneration. Plasma pTau181 concentrations were increased by 3.5-fold in AD compared to controls and differentiated AD from both clinically diagnosed (receiver operating characteristic area under the curve of 0.894) and autopsy-confirmed frontotemporal lobar degeneration (area under the curve of 0.878). Plasma pTau181 identified individuals who were amyloid β-PET-positive regardless of clinical diagnosis and correlated with cortical tau protein deposition measured by 18F-flortaucipir PET. Plasma pTau181 may be useful to screen for tau pathology associated with AD.

Genetic Risk for Alzheimer's Disease Moderates the Association Between Medial Temporal Lobe Volume and Episodic Memory Performance Among Older Adults

BACKGROUND

A complex set of interactions between biological, genetic, and environmental factors likely underlies the development of Alzheimer's disease (AD). Identifying which of these factors is most associated with AD is important for early diagnosis and treatment.

OBJECTIVE

We sought to examine genetic risk and structural brain volume on episodic memory in a sample of older adults ranging from cognitively normal to those diagnosed with AD.

METHODS

686 adults (55-91 years old) completed a 3T MRI scan, baseline cognitive assessments, and biospecimen collection through the Alzheimer's Disease Neuroimaging Initiative. Hierarchical linear regression analyses examined main and interaction effects of medial temporal lobe (MTL) volume and polygenic hazard score (PHS), indicating genetic risk for AD, on a validated episodic memory composite score.

RESULTS

Genetic risk moderated the relationship between MTL volume and memory, such that individuals with high PHS and lower hippocampal and entorhinal volume had lower memory composite scores [ΔF (1,677) = 4.057, p = 0.044, ΔR2 = 0.002]. Further analyses showed this effect was driven by the left hippocampus [ΔF(1,677) = 5.256, p = 0.022, ΔR2 = 0.003] and right entorhinal cortex [ΔF (1,677) = 6.078, p = 0.014, ΔR2 = 0.003].

CONCLUSIONS

Among those with high genetic risk for AD, lower volume was associated with poorer memory. Results suggest that the interaction between AD genetic risk and MTL volume increases the likelihood for memory impairment among older adults. Results from this study suggest that genetic risk and brain volume should be considered key factors in tracking cognitive decline.

Does early cognitive decline require the presence of both tau and amyloid-β?

This scientific commentary refers to ‘Amyloid and tau imaging biomarkers explain cognitive decline from late middle-age’, by Betthauser et al. (doi: 10.1093/brain/awz378).

Latest advances in cerebrospinal fluid and blood biomarkers of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease and its diagnosis has classically been based on clinical symptoms. Recently, a biological rather than a syndromic definition of the disease has been proposed that is based on biomarkers that reflect neuropathological changes. In AD, there are two main biomarker categories, namely neuroimaging and fluid biomarkers [cerebrospinal fluid (CSF) and blood]. As a complex and multifactorial disease, AD biomarkers are important for an accurate diagnosis and to stage the disease, assess the prognosis, test target engagement, and measure the response to treatment. In addition, biomarkers provide us with information that, even if it does not have a current clinical use, helps us to understand the mechanisms of the disease. In addition to the pathological hallmarks of AD, which include amyloid-β and tau deposition, there are multiple concomitant pathological events that play a key role in the disease. These include, but are not limited to, neurodegeneration, inflammation, vascular dysregulation or synaptic dysfunction. In addition, AD patients often have an accumulation of other proteins including α-synuclein and TDP-43, which may have a pathogenic effect on AD. In combination, there is a need to have biomarkers that reflect different aspects of AD pathogenesis and this will be important in the future to establish what are the most suitable applications for each of these AD-related biomarkers. It is unclear whether sex, gender, or both have an effect on the causes of AD. There may be differences in fluid biomarkers due to sex but this issue has often been neglected and warrants further research. In this review, we summarize the current state of the principal AD fluid biomarkers and discuss the effect of sex on these biomarkers.

An update on fluid biomarkers for neurodegenerative diseases: recent success and challenges ahead

Over the last twenty years, the characterization of Alzheimer's disease (AD) patients has progressed from a description of clinical symptomatology followed by neuropathological findings at autopsy to in vivo pathophysiological signatures using cerebrospinal fluid (CSF) and positron emission tomography (PET). Additionally, CSF biomarkers now reflect synaptic pathology, axonal injury and neuroinflammation. Novel techniques are capable of measuring proteins of pathophysiological importance at femtomolar concentrations in blood (e.g. amyloid, tau species and neurofilaments), which enable screening of large populations in the near future. This will be essential for secondary prevention trials and clinical management. However, common diseases such as dementia with Lewy bodies, Parkinson's disease and frontotemporal dementias, are still without reliable diagnostic biomarkers, although emerging techniques show promising pilot results for some of these diseases. This is likely to change in the next few years, which will be crucial to stratify populations enrolling in clinical trials, since pathologies often coexist.

Advances and considerations in AD tau-targeted immunotherapy

The multifactorial and complex nature of Alzheimer’s disease (AD) has made it difficult to identify therapeutic targets that are causally involved in the disease process. However, accumulating evidence from experimental and clinical studies that investigate the early disease process point towards the required role of tau in AD etiology. Importantly, a large number of studies investigate and characterize the plethora of pathological forms of tau protein involved in disease onset and propagation. Immunotherapy is one of the most clinical approaches anticipated to make a difference in the field of AD therapeutics. Tau –targeted immunotherapy is the new direction after the failure of amyloid beta (Aß)-targeted immunotherapy and the growing number of studies that highlight the Aß-independent disease process. It is now well established that immunotherapy alone will most likely be insufficient as a monotherapy. Therefore, this review discusses updates on tau-targeted immunotherapy studies, AD-relevant tau species, updates on promising biomarkers and a prospect on combination therapies to surround the disease propagation in an efficient and timely manner.

Salivary Biomarkers for Alzheimer's Disease and Related Disorders

The search for accessible and cost-effective biomarkers to complement current cerebrospinal fluid (CSF) and imaging biomarkers in the accurate detection of Alzheimer disease (AD) and other common neurodegenerative disorders remains a challenging task. The advances in ultra-sensitive detection methods has highlighted blood biomarkers (e.g. amyloid-β and neurofilament light) as a valuable and realistic tool in a diagnostic or screening process. Saliva, however, is also a rich source of potential biomarkers for disease detection and offers several practical advantages over biofluids that are currently examined for neurodegenerative disorders. However, while this may be true for the general population, challenges in collecting saliva from an elderly population should be seriously considered. In this review, we begin by discussing how saliva is produced and how age-related conditions can modify saliva production and composition. We then focus on the data available which support the concept of salivary amyloid-β, tau species and novel biomarkers in detecting AD and alpha-synuclein (α-syn) in Parkinson's disease (PD).

Tau PET imaging with 18F-PI-2620 in Patients with Alzheimer Disease and Healthy Controls: A First-in-Humans Study

¹⁸F-PI-2620 is a PET tracer with high binding affinity for aggregated tau, a key pathologic feature of Alzheimer disease (AD) and other neurodegenerative disorders. Preclinically, ¹⁸F-PI-2620 binds to both 3-repeat and 4-repeat tau isoforms. The purpose of this first-in-humans study was to evaluate the ability of ¹⁸F-PI-2620 to detect tau pathology in AD patients using PET imaging, as well as to assess the safety and tolerability of this new tau PET tracer. Methods: Participants with a clinical diagnosis of probable AD and healthy controls (HCs) underwent dynamic ¹⁸F-PI-2620 PET imaging for 180 min. ¹⁸F-PI-2620 binding was assessed visually and quantitatively using distribution volume ratios (DVR) estimated from noninvasive tracer kinetics and SUV ratio (SUVR) measured at different time points after injection, with the cerebellar cortex as the reference region. Time-activity curves and SUVR were assessed in AD and HC subjects, as well as DVR and SUVR correlations and effect size (Cohen's d) over time. Results: ¹⁸F-PI-2620 showed peak brain uptake around 5 min after injection and fast washout from nontarget regions. In AD subjects, focal asymmetric uptake was evident in temporal and parietal lobes, precuneus, and posterior cingulate cortex. DVR and SUVR in these regions were significantly higher in AD subjects than in HCs. Very low background signal was observed in HCs. ¹⁸F-PI-2620 administration was safe and well tolerated. SUVR time-activity curves in most regions and subjects achieved a secular equilibrium after 40 min after injection. A strong correlation (R ² > 0.93) was found between noninvasive DVR and SUVR for all imaging windows starting at more than 30 min after injection. Similar effect sizes between AD and HC groups were obtained across the different imaging windows. ¹⁸F-PI-2620 uptake in neocortical regions significantly correlated with the degree of cognitive impairment. Conclusion: Initial clinical data obtained in AD and HC subjects demonstrated a high image quality and excellent signal-to-noise ratio of ¹⁸F-PI-2620 PET for imaging tau deposition in AD subjects. Noninvasive quantification using DVR and SUVR for 30-min imaging windows between 30 and 90 min after injection-for example, 45-75 min-provides robust and significant discrimination between AD and HC subjects. ¹⁸F-PI-2620 uptake in expected regions correlates strongly with neurocognitive performance.

Evaluation of Dosimetry, Quantitative Methods, and Test-Retest Variability of 18F-PI-2620 PET for the Assessment of Tau Deposits in the Human Brain

¹⁸F-PI-2620 is a next-generation tau PET tracer that has demonstrated ability to image the spatial distribution of suspected tau pathology. The objective of this study was to assess the tracer biodistribution, dosimetry, and quantitative methods of ¹⁸F-PI-2620 in the human brain. Full kinetic modeling to quantify tau load was investigated. Noninvasive kinetic modeling and semiquantitative methods were evaluated against the full tracer kinetics. Finally, the reproducibility of PET measurements from test and retest scans was assessed. Methods: Three healthy controls (HCs) and 4 Alzheimer disease (AD) subjects underwent 2 dynamic PET scans, including arterial sampling. Distribution volume ratio (DVR) was estimated using full tracer kinetics (reversible 2-tissue-compartment [2TC] model and Logan graphical analysis [LGA]) and noninvasive kinetic models (noninvasive LGA [NI-LGA] and the multilinear reference tissue model [MRTM2]). SUV ratio (SUVR) was determined at different imaging windows after injection. The correlation between DVR and SUVR, effect size (Cohen's d), and test-retest variability (TRV) were evaluated. Additionally, 6 HCs received 1 tracer administration and underwent whole-body PET for dosimetry calculation. Organ doses and the whole-body effective dose were calculated using OLINDA 2.0. Results: A strong correlation was found across different kinetic models (R ² > 0.97) and between DVR(2TC) and SUVR between 30 and 90 min, with an R ² of more than 0.95. Secular equilibrium was reached at around 40 min after injection in most regions and subjects. TRV and effect size for SUVR across different regions were similar at 30-60 min (TRV, 3.8%; Cohen's d, 3.80), 45-75 min (TRV, 4.3%; Cohen's d, 3.77) and 60-90 min (TRV, 4.9%; Cohen's d, 3.73) and increased at later time points. Elimination was via the hepatobiliary and urinary systems. The whole-body effective dose was 33.3 ± 2.1 μSv/MBq for an adult female and 33.1 ± 1.4 μSv/MBq for an adult male, with a 1.5-h urinary bladder voiding interval. Conclusion: ¹⁸F-PI-2620 exhibits fast kinetics, suitable dosimetry, and low TRV. DVR measured using the 2TC model with arterial sampling correlated strongly with DVR measured by NI-LGA, MRTM2, and SUVR. SUVR can be used for ¹⁸F-PI-2620 PET quantification of tau deposits, avoiding arterial blood sampling. Static ¹⁸F-PI-2620 PET scans between 45 and 75 min after injection provide excellent quantification accuracy, a large effect size, and low TRV.

Head-to-head comparison of tau positron emission tomography tracers [18F]flortaucipir and [18F]RO948

PURPOSE

[¹⁸F]flortaucipir binds to paired helical filament tau and accurately identifies tau in Alzheimer's disease (AD). However, "off-target" binding interferes with the quantification of [¹⁸F]flortaucipir in several brain regions. Recently, other tau PET tracers have been developed. Here, we compare [¹⁸F]flortaucipir with the novel tau tracer [¹⁸F]RO948 head-to-head in vivo.

METHODS

We included 18 participants with AD, three with amyloid-β-positive amnestic mild cognitive impairment, and four healthy controls. All underwent [¹⁸F]flortaucipir (80-100 min) and [¹⁸F]RO948 (70-90) PET scans within approximately 1 month. Four study participants underwent 0-100-min dynamic scanning. Standardized uptake value ratios (SUVRs) were created using an inferior cerebellar reference region.

RESULTS

Neocortical tracer retention was highly comparable using both SUVR and distribution volume ratio-1 values obtained from dynamic scans. However, [¹⁸F]RO948 retention was significantly higher in the entorhinal cortex and lower in the basal ganglia, thalamus, and choroid plexus compared with [¹⁸F]flortaucipir. Increased off-target binding was observed with age for both tracers. Several cases exhibited strong [¹⁸F]RO948 retention in the skull/meninges. This extra-cerebral signal, however, did not affect diagnostic accuracy and remained relatively unchanged when re-examining a subsample after 1 year. Kinetic modeling showed an increase in [¹⁸F]flortaucipir SUVR over the scanning interval, compared with a plateau for [¹⁸F]RO948.

CONCLUSION

[¹⁸F]RO948 and [¹⁸F]flortaucipir bound comparably in neocortical regions, but [¹⁸F]RO948 showed higher retention in the medial temporal lobe and lower intracerebral "off-target" binding. Time-dependent bias of SUVR estimates may prove less of a factor with [¹⁸F]RO948, compared with previous tau ligands.

Cortical tau deposition follows patterns of entorhinal functional connectivity in aging

Tau pathology first appears in the transentorhinal and anterolateral entorhinal cortex (alEC) in the aging brain. The transition to Alzheimer's disease (AD) is hypothesized to involve amyloid-β (Aβ) facilitated tau spread through neural connections. We contrasted functional connectivity (FC) of alEC and posteromedial EC (pmEC), subregions of EC that differ in functional specialization and cortical connectivity, with the hypothesis that alEC-connected cortex would show greater tau deposition than pmEC-connected cortex. We used resting state fMRI to measure FC, and PET to measure tau and Aβ in cognitively normal older adults. Tau preferentially deposited in alEC-connected cortex compared to pmEC-connected or non-connected cortex, and stronger connectivity was associated with increased tau deposition. FC-tau relationships were present regardless of Aβ, although strengthened with Aβ. These results provide an explanation for the anatomic specificity of neocortical tau deposition in the aging brain and reveal relationships between normal aging and the evolution of AD.

Blood-based molecular biomarkers for Alzheimer's disease

A major barrier to the effective conduct of clinical trials of new drug candidates against Alzheimer’s disease (AD) and to identifying patients for receiving future disease-modifying treatments is the limited capacity of the current health system to find and diagnose patients with early AD pathology. This may be related in part to the limited capacity of the current health systems to select those people likely to have AD pathology in order to confirm the diagnosis with available cerebrospinal fluid and imaging biomarkers at memory clinics. In the current narrative review, we summarize the literature on candidate blood tests for AD that could be implemented in primary care settings and used for the effective identification of individuals at increased risk of AD pathology, who could be referred for potential inclusion in clinical trials or future approved treatments following additional testing. We give an updated account of blood-based candidate biomarkers and biomarker panels for AD-related brain changes. Our analysis centres on biomarker candidates that have been replicated in more than one study and discusses the need of further studies to achieve the goal of a primary care-based screening algorithm for AD.

The elusive tau molecular structures: can we translate the recent breakthroughs into new targets for intervention?

Insights into tau molecular structures have advanced significantly in recent years. This field has been the subject of recent breakthroughs, including the first cryo-electron microscopy structures of tau filaments from Alzheimer’s and Pick’s disease inclusions, as well as the structure of the repeat regions of tau bound to microtubules. Tau structure covers various species as the tau protein itself takes many forms. We will here address a range of studies that help to define the many facets of tau protein structures and how they translate into pathogenic forms. New results shed light on previous data that need now to be revisited in order to up-date our knowledge of tau molecular structure. Finally, we explore how these data can contribute the important medical aspects of this research - diagnosis and therapeutics.