Tau Ligands Research Articles

Association between Tau and Cognition as a Function of Age

AbstractBackgroundThe relationship between tau pathology and cognition as a function of age and whether these relationships are consistent across samples is unknown. A growing number of studies now perform positron emission tomography with tau ligands (tau‐PET), making it possible for an improved understanding of the dynamics of tau and cognition in relation to other biomarkers.MethodWe used data from the Alzheimer’s Disease Neuroimaging Initiative (ADNI) cohort (ages 55‐90) and The 90+ Study (Table 1) to cover a broad age range. We examined the association between region‐specific tau uptake by composite Braak stage and cognitive outcomes at the visit at which tau‐PET was performed (ADNI) or the closest visit (90+). We examined two cognitive outcomes: the mini mental status exam (MMSE) and clinical dementia rating sum of box scores. Spearman‐rank correlation coefficients between tau‐PET standardized uptake value ratios (SUVRs) with cerebellar cortex normalization and cognitive outcome are calculated for four distinct age ranges in ADNI: <60, 60‐70, 70‐80, and 80+ and among the 90+ study participants. Associations adjusted for age and age‐squared, race/ethnicity, number of APOE‐ε4 alleles, educational attainment, and gender are determined using bootstrapped linear regression.ResultThe association between region‐specific tau uptake by composite Braak stage and cognition appears consistent for ages 60‐80 (Figures 1 and 2), with worse cognition with higher tau. Over age 80, tau does not appear to be associated with cognition. Based on point estimates, some associations appear to be reversed over age 80 in ADNI, which is not the case in the 90+ study, although these differences between cohorts are not statistically significant.ConclusionTau‐PET association varies with age, becoming less reliably associated with cognition over age 80. This is likely due to increased incidence of co‐occurring pathologies and selective survival. Associations may further depend on features of the source population.

Late Life Depression is Not Associated With Alzheimer-Type Tau: Preliminary Evidence From a Next-Generation Tau Ligand PET-MR Study

To investigate whether tau accumulation is higher in late life depression (LLD) compared to non-depressed cognitively unimpaired (CU) older adults. To situate these findings in the neurodegeneration model of LLD by assessing group differences in tau and grey matter volume (GMV) between LLD, non-depressed CU and mild cognitive impairment due to Alzheimer's Disease (MCI). Monocentric, cross-sectional study. University Psychiatric hospital, memory clinic and outpatient neurology practice. A total of 102 adults over age 60, of whom 19 currently depressed participants with LLD, 19 with MCI and 36 non-depressed CU participants completed neuropsychological testing and tau PET-MR imaging. PET-MRI: 18F-MK-6240 tracer SUVR for tau assessment; 3D T1-weighted structural MRI derived GMV in seven brain regions (temporal, cingulate, prefrontal and parietal regions); amyloid PET to assess amyloid positivity; Neuropsychological test scores: MMSE, RAVLT, GDS, MADRS. ANCOVA and Spearman's rank correlations to investigate group differences in tau and GMV, and correlations with neuropsychological test scores respectively. Compared to non-depressed CU participants, LLD patients showed lower GMV in temporal and anterior cingulate regions but similar tau accumulation and amyloid positivity rate. In contrast, MCI patients had significantly higher tau accumulation in all regions. Tau did not correlate with any neuropsychological test scores in LLD. Our findings suggest AD-type tau is not higher in LLD compared to non-depressed, cognitively unimpaired older adults and appears unlikely to contribute to lower gray matter volume in LLD, further underscoring the need to distinguish major depressive disorder from depressive symptoms occurring in early AD.

Milligram-scale assembly and NMR fingerprint of tau fibrils adopting the Alzheimer’s disease fold

In the Alzheimer’s disease (AD) brain, the microtubule-associated protein tau aggregates into paired helical filaments (PHFs) in which each protofilament has a C-shaped conformation. In vitro assembly of tau fibrils adopting this fold is highly valuable for both fundamental and applied studies of AD without requiring patient-brain extracted fibrils. To date, reported methods for forming AD-fold tau fibrils have been irreproducible and sensitive to subtle variations in fibrillization conditions. Here we describe a route to reproducibly assemble tau fibrils adopting the AD fold on the multi-milligram scale. We investigated the fibrilization conditions of two constructs, and found that a tau (297-407) construct that contains four AD phospho-mimetic glutamate mutations robustly formed the C-shaped conformation. Two- and three-dimensional correlation solid-state NMR spectra show a single predominant set of chemical shifts, indicating a single molecular conformation. Negative-stain electron microscopy and cryoelectron microscopy data confirm that the protofilament formed by 4E-tau (297-407) adopts the C-shaped conformation, which associates into paired, triple and quadruple helical filaments. In comparison, NMR spectra indicate that a previously reported construct, tau (297-391), forms a mixture of a four-layered dimer structure and the C-shaped structure, whose populations are highly sensitive to the environmental conditions. The determination of the NMR chemical shifts of the AD-fold tau opens the possibility for future studies of tau fibril conformations and ligand binding by NMR. The quantitative assembly of tau fibrils adopting the AD fold should facilitate the development of diagnostic and therapeutic compounds that target AD tau.

18F]PM-PBB3-PET Reveals Clinical and [18F]FDG-PET Mimics of 4-Repeat Tauopathy Caused by Creutzfeld-Jakob Disease.

This image depicts [18F] FDG and [18F]PM-PBB3 uptake surface projections of two male patients with complex movement disturbances. They were referred for PET imaging with the suspected diagnosis of a 4-repeat (4R)-tauopathy Figure 1. Patient #1 (79 years) presented with an aggressive, non-levodopa responsive hypokinetic-rigid syndrome with early falls and a vertical supranuclear gaze palsy. Patient #2 (68 years) presented with unusually rapidly progressive dystonia, right-sided rigor, bradykinesia, alien limb phenomenon and action myoclonus of the right arm, likewise without response to levodopa. Disease duration was 11 and 5 months, respectively. CSF levels of tau and amyloid in either patient were not indicative for Alzheimer's disease. In both, [18F] FDG-PET revealed a corticobasal degeneration-like bilateral reduction of cerebral glucose metabolism in frontoparietal cortical areas (arrowheads) and in the thalamus.1 A clear asymmetry to the detriment of the right hemisphere was observed in #1, whereas hypometabolism of the left hemisphere was only slightly pronounced in #2. Subsequent imaging with tau ligand [18F]PM-PBB32 showed elevated binding (arrowheads, asterisks indicate unspecific uptake of the choroid plexus) suggestive for pathological tau aggregates in frontal and parietal areas of #1, supporting the diagnosis of a 4R-tauopathy.2 In contrast, the lack of specific [18F]PM-PBB3 binding in patient #2 questioned the diagnosis of a 4R-tau related corticobasal syndrome (CBS; eg, progressive supranuclear palsy—CBS). Diagnosis of Creutzfeld-Jakob disease in this patient was made via 14–3-3 and PrPSc aggregation assays. In conclusion, [18F]PM-PBB3-PET reveals clinical and [18F] FDG-PET mimics of 4R-tau CBS caused by Creutzfeld-Jakob disease. (1) Research project: A. Conception, B. Organization, C. Execution; (2) Statistical analysis: A. Design, B. Execution, C. Review and critique; (3) Manuscript: A. Writing of the first draft, B. Review and critique. N.S.: 1A, B, 1C, 2C, 3A. G.B.: 1C, 2B, 2C, 3B. P.T.M.: 1B, 2A, 2C, 3B. M.R.: 1B, 2C, 3B. J.B.: 1C, 2A, 2C, 3A. Materials required for the synthesis of [18F] APN-1607 were generously provided by Aprinoia Therapeutics Limited. Open Access funding enabled and organized by Projekt DEAL. Ethical Compliance Statement: The authors confirm that the approval of an institutional review board was not required for this work. Both patients gave written informed consent for the scientific use of imaging and clinical data. Funding Sources and Conflicts of Interest: Nils Schröter, Berta-Ottenstein-Programme for Clinician Scientists, Faculty of Medicine, University of Freiburg. The authors have no potential conflict of interest to report. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. Financial Disclosures for the Previous 12 Months: The authors have nothing to disclose.

Synthesis and structure-activity relationship (SAR) studies of 1,2,3-triazole, amide, and ester-based benzothiazole derivatives as potential molecular probes for tau protein.

The pyridinyl-butadienyl-benzothiazole (PBB3 15) scaffold was used to develop tau ligands with improved in vitro and in vivo properties for imaging applications to provide insights into the etiology and characteristics of Alzheimer's disease. The photoisomerisable trans-butadiene bridge of PBB3 was replaced with 1,2,3-triazole, amide, and ester moieties and in vitro fluorescence staining studies revealed that triazole derivatives showed good visualisation of Aβ plaques, but failed to detect the neurofibrillary tangles (NFTs) in human brain sections. However, NFTs could be observed using the amide 110 and ester 129. Furthermore, the ligands showed low to high affinities (Ki = >1.5 mM-0.46 nM) at the shared binding site(s) with PBB3.

Structure-Activity Relationships of Novel Tau Ligands: Passive Fibril Binders and Active Aggregation Inhibitors.

Intrinsically disordered proteins (IDPs) are core components of many biological processes and are central players in several pathologies. Despite being important drug targets, attempts to design small-molecule ligands that would help understand and attenuate their behavior are frustrated by the structural diversity exhibited by these flexible proteins. To accommodate the dynamic nature of IDPs, we developed a procedure that efficiently identifies active small-molecule ligands for disordered proteins. By exploring the chemical space around these ligands, we refined their effect on aggregation and identified molecular features critical for activity and affinity. Notably, the discovery of this new family of disordered protein ligands was achieved more quickly and with less expense than conventional high-throughput screening (HTS) or docking alone would have allowed. The resulting ligands include tau aggregation inhibitors as well as at least one compound that binds fibrils potently but does not appear to perturb the extent of kinetics of aggregation.

Clinical evaluation of SPARE‐TauAD, an AV‐1451‐derived tau index

AbstractBackgroundThe deposition of abnormal proteins defines neurodegenerative diseases. Among these proteins, tau deposits define frontotemporal lobar degeneration (FTLD) tau, and together with Aβ, it defines the more prevalent Alzheimer’s disease (AD). PET scans with ligands binding AD tau neurofibrillary tangles (NFT) topographically quantify the amount of pathology. However, for clinical purposes, this information needs to be summarized into easily applicable and meaningful indices. Here we develop a machine learning‐derived tau PET index, the Spatial Pattern of Abnormality for Recognition of Tau in AD (SPARE‐TauAD).Method603 subjects were included in the study. Their clinical diagnoses were: 366 cognitively normal (CN), 177 mild cognitive impairment (MCI), and 60 dementia subjects. These subjects were classified as Aβ positive and negative based on CSF Aβ1‐42 levels or florbetapir PET summary composite. SPARE‐TauAD was derived using a support vector machine classifier comparing AV‐1451 PET scans in 141 Aβ negative CN subjects against 108 Aβ positive MCI and dementia subjects. We then evaluated its association with CSF t‐tau, global cognition (measured using ADAS‐Cog 13), clinical diagnosis, and clinical progression.ResultSPARE‐TauAD and CSF t‐tau showed a moderate correlation (rho=0.51, p<0.0001). SPARE‐TauAD explained a larger variance of ADAS‐Cog13 scores (R2=0.60) compared to CSF t‐tau (R2=0.21) and the florbetapir PET summary composite (R2=0.23). Using 5‐fold cross‐validation in an independent sample of Aβ negative CN subjects (n=77) against Aβ positive MCI and dementia subjects (n=35 for CSF), SPARE‐TauAD showed higher balanced accuracy (79.6%) compared to CSF t‐tau (61.6%). SPARE‐TauAD was also associated with faster progression from CN to MCI/dementia (H.R.=6.83, p‐value<0.0001) and MCI to dementia (H.R.=3.43, p‐value<0.0001). Whereas CSF t‐tau was no associated with clinical progression.ConclusionSPARE‐TauAD showed, as expected, a moderate association with CSF t‐tau. However, it offered better cross‐sectional classification accuracy and correlation with cognitive scores. It also outperformed CSF t‐tau to predict longitudinal progression in this small with limited longitudinal follow‐up. One limitation is that the SPARE‐TauAD index has been developed in an AD MCI and dementia cohort. The used tau ligand, AV‐1451, has a greater affinity for NFT present in AD than tau pathology present in FTLD‐tau.

Towards a universal cortical tau sampling mask

AbstractBackgroundThe introduction of the AT(N) framework raised several issues in regards to the definition of T+. What brain regions should be sampled? Based on one or on multiple tracers? In this work, we developed a “universal” cortical tau mask for the AD continuum derived from all the major tau ligands. This “universal” cortical mask will serve as the common tau area for all tracers over which several different regional sampling VOI or composites can be then applied. Guaranteeing sampling of the same common regions is the first step to develop a common scale for all tau tracers: the CenTauR.Method464 participants underwent tau scans with either 18F‐AV1451 (CN=54/AD=24), 18F‐MK6240 (CN=157/AD=22), 18F‐PI2620 (CN=10/AD=21), 18F‐PM‐PBB3 (CN=30/AD=28), 18F‐GTP1 (CN=15/AD=38) or 18F‐RO948 (CN=35/AD=30). All CN were Aß‐ and all AD were Aß+. The tau scans were spatially normalized using CapAIBL and the cerebellar cortex was used as reference region. For each tracer, a difference image between the means of the Aß‐ CN and Aß+ AD patients was generated. Difference images were subsequently thresholded at 1/3 of the difference between Aß‐ CN and Aß+ AD in the inferior temporal lobe. A single tau specific mask was then constructed from the intersection of all the specific tau tracer masks.A MRI‐derived grey matter mask at PET resolution was applied to the composite mask only sampling grey matter regions. Finally, the mask was mirrored and fused to remove the hemispherical asymmetry of tau pathology. Agreement between masks was assessed by dice‐scores.ResultVisually, all the tracer‐specific masks appeared very similar. None of the known off‐target binding regions were discernible in the resulting masks (Figure 1). There was good agreement between all masks, with dice‐scores of 0.60 and 0.66 for cortical regions.ConclusionWe constructed an “universal” tau mask for the AD continuum based on all the commonly used tau tracers aiming at standardizing tau sampling and quantification across tracers and across centres. The “universal” tau mask demarcates a tau specific space that can then be sub‐segmented into smaller regions to focus on specific areas or composite regions that might better capture early tau deposition and spreading.

Chemoinformatics Analyses of Tau Ligands Reveal Key Molecular Requirements for the Identification of Potential Drug Candidates against Tauopathies.

Tau is a highly soluble protein mainly localized at a cytoplasmic level in the neuronal cells, which plays a crucial role in the regulation of microtubule dynamic stability. Recent studies have demonstrated that several factors, such as hyperphosphorylation or alterations of Tau metabolism, may contribute to the pathological accumulation of protein aggregates, which can result in neuronal death and the onset of a number of neurological disorders called Tauopathies. At present, there are no available therapeutic remedies able to reduce Tau aggregation, nor are there any structural clues or guidelines for the rational identification of compounds preventing the accumulation of protein aggregates. To help identify the structural properties required for anti-Tau aggregation activity, we performed extensive chemoinformatics analyses on a dataset of Tau ligands reported in ChEMBL. The performed analyses allowed us to identify a set of molecular properties that are in common between known active ligands. Moreover, extensive analyses of the fragment composition of reported ligands led to the identification of chemical moieties and fragment combinations prevalent in the more active compounds. Interestingly, many of these fragments were arranged in recurring frameworks, some of which were clearly present in compounds currently under clinical investigation. This work represents the first in-depth chemoinformatics study of the molecular properties, constituting fragments and similarity profiles, of known Tau aggregation inhibitors. The datasets of compounds employed for the analyses, the identified molecular fragments and their combinations are made publicly available as supplementary material.

Associations of quantitative susceptibility mapping with Alzheimer's disease clinical and imaging markers

Altered iron metabolism has been hypothesized to be associated with Alzheimer's disease pathology, and prior work has shown associations between iron load and beta amyloid plaques. Quantitative susceptibility mapping (QSM) is a recently popularized MR technique to infer local tissue susceptibility secondary to the presence of iron as well as other minerals. Greater QSM values imply greater iron concentration in tissue. QSM has been used to study relationships between cerebral iron load and established markers of Alzheimer's disease, however relationships remain unclear. In this work we study QSM signal characteristics and associations between susceptibility measured on QSM and established clinical and imaging markers of Alzheimer's disease. The study included 421 participants (234 male, median age 70 years, range 34–97 years) from the Mayo Clinic Study of Aging and Alzheimer's Disease Research Center; 296 (70%) had a diagnosis of cognitively unimpaired, 69 (16%) mild cognitive impairment, and 56 (13%) amnestic dementia. All participants had multi-echo gradient recalled echo imaging, PiB amyloid PET, and Tauvid tau PET. Variance components analysis showed that variation in cortical susceptibility across participants was low. Linear regression models were fit to assess associations with regional susceptibility. Expected increases in susceptibility were found with older age and cognitive impairment in the deep and inferior gray nuclei (pallidum, putamen, substantia nigra, subthalamic nucleus) (betas: 0.0017 to 0.0053 ppm for a 10 year increase in age, p = 0.03 to <0.001; betas: 0.0021 to 0.0058 ppm for a 5 point decrease in Short Test of Mental Status, p = 0.003 to p<0.001). Effect sizes in cortical regions were smaller, and the age associations were generally negative. Higher susceptibility was significantly associated with higher amyloid PET SUVR in the pallidum and putamen (betas: 0.0029 and 0.0012 ppm for a 20% increase in amyloid PET, p = 0.05 and 0.02, respectively), higher tau PET in the basal ganglia with the largest effect size in the pallidum (0.0082 ppm for a 20% increase in tau PET, p<0.001), and with lower cortical gray matter volume in the medial temporal lobe (0.0006 ppm for a 20% decrease in volume, p = 0.03). Overall, these findings suggest that susceptibility in the deep and inferior gray nuclei, particularly the pallidum and putamen, may be a marker of cognitive decline, amyloid deposition, and off-target binding of the tau ligand. Although iron has been demonstrated in amyloid plaques and in association with neurodegeneration, it is of insufficient quantity to be reliably detected in the cortex using this implementation of QSM.

PET Tau Imaging and Motor Impairments Differ Between Corticobasal Syndrome and Progressive Supranuclear Palsy With and Without Alzheimer's Disease Biomarkers.

Introduction: Frontotemporal lobar degeneration (FTLD)-related syndrome includes progressive supranuclear palsy (PSP) and corticobasal syndrome (CBS). PSP is usually caused by a tauopathy but can have associated Alzheimer's disease (AD) while CBS can be caused by tauopathy, transactive response DNA binding protein 43 kDa, or AD pathology. Our aim was to compare the parkinsonian syndromes presenting without AD biomarkers (CBS/PSP-non-AD) to parkinsonian syndromes with AD biomarkers (CBS/PSP-AD).Materials and Methods: Twenty-four patients [11 males, 13 females; age (68.46 ± 7.23)] were recruited for this study. The whole cohort was divided into parkinsonian syndromes without AD biomarkers [N = 17; diagnoses (6 CBS, 11 PSP)] and parkinsonian syndromes with AD biomarkers [N = 7; diagnoses (6 CBS-AD, 1 PSP-AD)]. Anatomical MRI and PET imaging with tau ligand [18F]-AV1451 tracer was completed. Cerebrospinal fluid analysis or [18F]-AV1451 PET imaging was used to assess for the presence of AD biomarkers. Progressive supranuclear palsy rating scale (PSPRS) and unified Parkinson's disease rating scale (UPDRS) motor exam were implemented to assess for motor disturbances. Language and cognitive testing were completed.Results: The CBS/PSP-non-AD group [age (70.18 ± 6.65)] was significantly older (p = 0.028) than the CBS/PSP-AD group [age (64.29 ± 7.32)]. There were no differences between the groups in terms of gender, education, years of disease duration, and disease severity as measured with the Clinical Dementia Rating scale. The CBS/PSP-non-AD group had significantly lower PET Tau Standard Volume Uptake Ratio (SUVR) values compared to the CBS/PSP-AD group in multiple frontal and temporal areas, and inferior parietal (all p < 0.03). The CBS/PSP-non-AD group had significantly higher scores compared to the CBS/PSP-AD group on PSPRS (p = 0.004) and UPDRS motor exam (p = 0.045). The CBS/PSP-non-AD group had higher volumes of inferior parietal, precuneus, and hippocampus (all p < 0.02), but lower volume of midbrain (p = 0.02), compared to the CBS/PSP-AD group.Discussion: The CBS/PSP-non-AD group had higher motor disturbances compared to the CBS/PSP-AD group; however, both groups performed similarly on neuropsychological measures. The AD biomarker group had increased global uptake of PET Tau SUVR and lower volumes in AD-specific areas. These results show that the presenting phenotype of CBS and PSP syndromes and the distribution of injury are strongly affected by the presence of AD biomarkers.

Brain and blood biomarkers of tauopathy and neuronal injury in humans and rats with neurobehavioral syndromes following blast exposure

Traumatic brain injury (TBI) is a risk factor for the later development of neurodegenerative diseases that may have various underlying pathologies. Chronic traumatic encephalopathy (CTE) in particular is associated with repetitive mild TBI (mTBI) and is characterized pathologically by aggregation of hyperphosphorylated tau into neurofibrillary tangles (NFTs). CTE may be suspected when behavior, cognition, and/or memory deteriorate following repetitive mTBI. Exposure to blast overpressure from improvised explosive devices (IEDs) has been implicated as a potential antecedent for CTE amongst Iraq and Afghanistan Warfighters. In this study, we identified biomarker signatures in rats exposed to repetitive low-level blast that develop chronic anxiety-related traits and in human veterans exposed to IED blasts in theater with behavioral, cognitive, and/or memory complaints. Rats exposed to repetitive low-level blasts accumulated abnormal hyperphosphorylated tau in neuronal perikarya and perivascular astroglial processes. Using positron emission tomography (PET) and the [18F]AV1451 (flortaucipir) tau ligand, we found that five of 10 veterans exhibited excessive retention of [18F]AV1451 at the white/gray matter junction in frontal, parietal, and temporal brain regions, a typical localization of CTE tauopathy. We also observed elevated levels of neurofilament light (NfL) chain protein in the plasma of veterans displaying excess [18F]AV1451 retention. These findings suggest an association linking blast injury, tauopathy, and neuronal injury. Further study is required to determine whether clinical, neuroimaging, and/or fluid biomarker signatures can improve the diagnosis of long-term neuropsychiatric sequelae of mTBI.

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.MethodsWe 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.ResultsNeocortical 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.

THK5351 and flortaucipir PET with pathological correlation in a Creutzfeldt-Jakob disease patient: a case report

BackgroundTHK5351 and flortaucipir tau ligands have high affinity for paired helical filament tau, yet diverse off-target bindings have been reported. Recent data support the hypothesis that THK5351 binds to monoamine oxidase B (MAO-B) expressed from reactive astrocytes and that flortaucipir has an affinity toward MAO-A and B; however, pathological evidence is lacking. We performed a head-to-head comparison of the two tau ligands in a sporadic Creutzfeldt-Jakob disease (CJD) patient and performed an imaging-pathological correlation study.Case presentationA 67-year-old man visited our clinic a history of 6 months of rapidly progressive dementia, visual disturbance, and akinetic mutism. Diffusion-weighted imaging showed cortical diffusion restrictions in the left temporo-parieto-occipital regions. 18F-THK5351 PET, but not 18F-flortaucipir PET showed high uptake in the left temporo-parieto-occipital regions, largely overlapping with the diffusion restricted areas. Cerebrospinal fluid analysis was weakly positive for 14–3-3 protein and pathogenic prion protein was found. The patient showed rapid cognitive decline along with myoclonic seizures and died 13 months after his first visit. A post-mortem study revealed immunoreactivity for PrPsc, no evidence of neurofibrillary tangles, and abundant astrocytosis which was reactive for MAO-B antibody.ConclusionsOur findings add pathological evidence that increased THK5351 uptake in sporadic CJD patients might be caused by an off-target binding driven by its high affinity for MAO-B.

HEAD-TO-HEAD IN VIVO COMPARISON OF TAU POSITRON EMISSION TOMOGRAPHY LIGANDS 18F-FLORTAUCIPIR AND 18F-RO948

18F-Flortaucipir (FTP) is the most widely used PET ligand for the in vivo assessment of tau pathology. It binds to paired-helical filament tau and has been shown to be useful in the diagnosis of AD. However, off-target binding e.g. in the choroid plexus and the basal ganglia hamper accurate quantification of FTP in these and adjacent regions. Recently, several other tau PET ligands have been developed. Here, we compare 18F-FTP with the novel tau ligand 18F-RO948 head-to-head in vivo. Within this study, 33 individuals have undergone 18F-FTP PET at 80–100 min and 18F-RO948 at 70- 90 min post ligand injection. Four subjects have undergone dynamic scanning 0–100 min. Subject diagnoses were: 18 AD (Aβpositive), three MCI (Aβpositive), four Progressive Supranuclear Palsy patients, three with semantic variant Primary Progressive Aphasia, one Dementia with Lewy Bodies and four neurologically healthy controls (mean age 71.6 ± 8.2 y). The average time between PET scans was 21 ± 41 days. Standardized uptake value ratios (SUVR) were created using an inferior cerebellar reference region. The neocortical retention of the two radioligands was highly similar when assessed using SUVR-values. A small, but significant, increase in retention was found in the entorhinal cortex (Braak stage I-II) using 18F-RO948 (Figure 1). On visual inspection, 18F-RO948 uptake was substantially lower in the basal ganglia, the thalamus, and the choroid plexus, but not in the substantia nigra compared to 18F-FTP uptake (Figure 2). Statistical comparison confirmed significantly lower 18F-RO948 uptake in the choroid plexus and the basal ganglia (Figure 3). However, some cases exhibited strong 18F-RO948 signal in the skull or the meninges, which was not observed for 18F-FTP, and at a group level the skull/meningeal binding was significantly higher in the 18F-RO948 scans.

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

Amyloid-β plaques and neurofibrillary tangles composed of tau protein are the neuropathological hallmarks of Alzheimer's disease. In recent years, marked progress has been made in Alzheimer's disease research using tau ligands for positron emission tomography (PET). However, the issue of off-target binding, that is, the binding of ligands to regions without tau pathology, remains unresolved. Tissues with melanin-containing cells (MCCs) have been suggested as binding targets for tau ligands. In the present study, we characterized the MCC-binding properties of representative tau PET ligands. Autoradiographic studies of [18F]AV-1451 and [18F]THK5351 were conducted using postmortem human midbrain sections. Saturation-binding assays of [18F]AV-1451 and [18F]THK5351 were performed with B16F10 melanoma cells. The blocking effects of 25 compounds against [18F]THK5351 binding to B16F10 cells were used to investigate the relationship between chemical structure and MCC binding. Autoradiography demonstrated specific binding of the radioligands in the substantia nigra. [18F]AV-1451 and [18F]THK5351 exhibited saturable binding to melanoma cells ([18F]AV-1451: Kd = 669 ± 196nM, Bmax = 622 ± 269pmol/mg protein; [18F]THK5351: Kd = 441 ± 126nM, Bmax = 559 ± 75.5pmol/mg protein). In blocking studies with melanoma cells, compounds bearing multiple aromatic rings and an aminopyridine group, including tau ligands such as AV-1451, PBB3, and a lead compound of MK-6240, exhibited the inhibition of [18F]THK5351 binding comparable to self-blocking by THK5351 (> 70% at 10µM). These studies suggest that the binding properties of [18F]AV-1451 and [18F]THK5351 are sufficient to expect highlighting of tissues with a high density of MCCs. The findings of the present study should aid the development of neuroimaging ligands that do not bind to MCC.

The Rational Discovery of a Tau Aggregation Inhibitor.

Intrinsically disordered proteins play vital roles in biology, and their dysfunction contributes to many major disease states. These proteins remain challenging targets for rational ligand discovery or drug design because they are highly dynamic and fluctuate through a diverse set of conformations, frustrating structure-based approaches. To meet this challenge, we have developed protocols to efficiently identify active small molecule ligands of disordered proteins. Our approach utilizes enhanced sampling molecular dynamics and conformational analysis approaches optimized for disordered targets, coupled with computational docking and machine learning-based screens of compound libraries. By applying this protocol to an amyloid-forming segment of microtubule-associated protein tau, we successfully identified novel, chemically diverse tau ligands, including an inhibitor that delays the aggregation reaction in vitro without affecting the amount of aggregate formed at the steady state. Our results indicate that we have expanded the toolkit of protein aggregation inhibitors into new areas of chemical space and demonstrate the feasibility of our ligand discovery strategy.

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

Imaging of pathological tau with positron emission tomography (PET) has the potential to allow early diagnosis of the dementias and monitoring of disease progression, including assessment of therapeutic interventions, in vivo. The first generation of tau PET tracers, including the carbazole flortaucipir and the 2-arylquinolines of the THK series, are now used in clinical research; however, concerns have been raised about off-target binding and low sensitivity.With the aim to determine the nature of tau pathology depicted by structurally distinct tau ligands we carried out a microscopic neuropathological evaluation in post-mortem human brain tissue of cases with primary and secondary tauopathies. Carbazole and 2-arylquinoline binding was only observed in cases with Alzheimer’s disease and one case with frontotemporal dementia and parkinsonism linked to chromosome 17 exhibiting a R406W MAPT mutation. In end stage Alzheimer’s disease cases, fluorescent imaging with the carbazole T726 and the 2-arylquinoline THK-5117 revealed high inter- and intra-case variability of tracer binding, and this was corroborated by quantitative phosphorimaging with the PET tracer [18F]THK-5117. Microscopic analysis of the pathological inclusions revealed that the fluorescent tracers preferentially bind to premature tau aggregates. Whilst T726 binding was limited to neuronal tau, THK-5117 additionally depicted neuritic tau. Neither tracer depicted tau in pre-symptomatic disease.Our results highlight limitations of the first generation of tau PET tracers, in particular lack of correlation between pathological tau load and tracer binding, limited sensitivity to tau in early disease, and high variability in tracer binding between and within cases. Concerns remain that these limitations may also affect the next generation tracers as they target the same high affinity binding site. Therefore, it is crucial to assess inter- and intra-subject correlation of tracer binding with pathological tau load in post-mortem tissue studies, and to rigorously assess novel tau PET tracers before translation into clinical studies.

Corticobasal degeneration: key emerging issues.

Corticobasal degeneration (CBD) was first described by Rebeiz et al. in 1967, and was called corticodentatonigral degeneration with neuronal achromasia [1]. Since then, our knowledge of the clinical features and underlying tau pathology has grown tremendously. Clinical antemortem diagnosis of CBD pathology remains challenging and has led to the development of revised diagnostic criteria. As various clinical phenotypes may have CBD pathology, accurate prevalence studies are lacking. Recently, pooled prevalence of fronto-temporal lobar degeneration, PSP and CBS was reported as 10.6 per 100,000 [2]. Although rare, CBD is an important disease to understand because it provides a model of a specific proteinopathy (tauopathy) and, therefore, opportunity to study pathophysiology of tauopathies and efficacy of tau-directed therapies. In the past few years, identification of tau specific ligands has advanced neuroimaging of tauopathies such as CBD and progressive supranuclear palsy. However, clinical prediction of CBD pathology remains challenging and an active are of research. In this review, we highlight key emerging issues in CBD pathophysiology, genetics and novel neuroimaging techniques with tau ligands.

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

IntroductionSemantic dementia, including the semantic variant of primary progressive aphasia (svPPA), is strongly associated with TAR-DNA binding protein 43 (TDP-43) type C pathology. It provides a useful model in which...