Regional cerebral THK5351 accumulations correlate with neuropsychological test scores in Alzheimer continuum

Objectives

We evaluated the relationship between regional accumulations of the tau positron emission tomography (PET) tracer THK5351 and cognitive dysfunction in the Alzheimer's disease (AD) continuum.

Methods

The cases of 18 patients with AD or mild cognitive impairment (MCI) due to AD who underwent three-dimensional MRI, fluoro-2-deoxyglucose (FDG)-(PET), Pittsburgh compound B (PiB)-amyloid PET, and THK5351-tau PET were analyzed. Their mean age was 70.6±11.3, their mean Mini-Mental State Examination (MMSE) score was 22.3±6.8, and their mean Alzheimer Disease Assessment Scale-Cognitive Subtest (ADAS) score was 12.5±7.3. To determine the correlation between each patient's four imaging results and their MMSE and ADAS scores, we performed a voxel-wise statistical analysis with statistical parametric mapping (SPM).

Results

The SPM analysis showed that the bilateral parietotemporal FDG accumulations and MMSE scores were positively correlated, and the bilateral parietotemporal FDG accumulations were negatively correlated with ADAS scores. There were significant correlations between bilateral parietotemporal and left posterior cingulate/precuneus THK5351 accumulations and MMSE/ADAS scores.

Conclusion

In the AD brain, THK5351 correlates with neuropsychological test scores as well as or more additional than FDG due to its affinity for both tau and monoamine oxidase-B (MAO-B), and measurements of THK5351 may thus be useful in estimating the progression of AD.

Longitudinal Assessment of Tau-Associated Pathology by 18F-THK5351 PET Imaging: A Histological, Biochemical, and Behavioral Study

Several common and debilitating neurodegenerative disorders are characterized by the intracellular accumulation of neurofibrillary tangles (NFTs), which are composed of hyperphosphorylated tau protein. In Alzheimer's disease (AD), NFTs are accompanied by extracellular amyloid-beta (Aβ), but primary tauopathy disorders are marked by the accumulation of tau protein alone, including forms of frontotemporal dementia (FTD), corticobasal degeneration (CBD), and progressive supranuclear palsy (PSP), among others. ¹⁸F-THK5351 has been reported to bind pathological tau as well as associated reactive astrogliosis. The goal of this study was to validate the ability of the PET tracer ¹⁸F-THK5351 to detect early changes in tau-related pathology and its relation to other pathological hallmarks. We demonstrated elevated in vivo ¹⁸F-THK5351 PET signaling over time in transgenic P301S tau mice from 8 months that had a positive correlation with histological and biochemical tau changes, as well as motor, memory, and learning impairment. This study indicates that ¹⁸F-THK5351 may help fill a critical need to develop PET imaging tracers that detect aberrant tau aggregation and related neuropathology in order to diagnose the onset of tauopathies, gain insights into their underlying pathophysiologies, and to have a reliable biomarker to follow during treatment trials.

Binding of [18F]AV1451 in post mortem brain slices of semantic variant primary progressive aphasia patients

Purpose

In vivo tau-PET tracer retention in the anterior temporal lobe of patients with semantic variant primary progressive aphasia (SV PPA) has consistently been reported. This is unexpected as the majority of these patients have frontotemporal lobar degeneration TDP (FTLD-TDP).

Methods

We conducted an in vitro [¹⁸F]AV1451 autoradiography binding study in five cases with a clinical diagnosis of SV PPA constituting the range of pathologies (i.e., three FTLD-TDP, one Alzheimer’s disease (AD), and one Pick’s disease (PiD)). Binding was compared with two controls without neurodegeneration, two typical AD, one corticobasal syndrome with underlying AD, and one frontotemporal dementia behavioral variant with FTLD-TDP. The effect of blocking with the authentic reference material and with the MAO-B inhibitor deprenyl was assessed. Immunohistochemistry was performed on adjacent cryosections.

Results

Absence of specific [¹⁸F]AV1451 binding was observed for all three SV PPA FTLD-TDP cases. The absence of binding in controls as well as the successful blocking with authentic AV1451 in cases with tauopathy demonstrated specificity of the [¹⁸F]AV1451 signal for tau. The specific [¹⁸F]AV1451 binding was highest in AD, followed by PiD. This binding colocalized with the respective tau lesions and could not be blocked by deprenyl. Similar pilot findings were obtained with [¹⁸F]THK5351.

Conclusion

In vitro autoradiography showed no [¹⁸F]AV1451 binding in SV PPA due to FTLD-TDP, while specific binding was present in SV PPA due to AD and PiD. The discrepancy between in vitro and in vivo findings remains to be explained. The discordance is not related to [¹⁸F]AV1451 idiosyncrasies as [¹⁸F]THK5351 findings were similar.

Electronic supplementary material

The online version of this article (10.1007/s00259-019-04631-x) contains supplementary material, which is available to authorized users.

Tau positron emission tomography imaging in tauopathies: The added hurdle of off-target binding

Ligands targeting tau for use with positron emission tomography have rapidly been developed during the past several years, enabling the in vivo study of tau pathology in patients with Alzheimer's disease and related non-Alzheimer's disease tauopathies. Several candidate compounds have been developed, showing good in vitro characteristics with respect to their ability to bind tau deposits; off-target binding, however, has also been observed. In this short commentary, we briefly summarize the available in vivo and in vitro evidence pertaining to their off-target binding and discuss the different approaches that are needed for the future development of tau positron emission tomography tracers.

Head to head comparison of [18F] AV-1451 and [18F] THK5351 for tau imaging in Alzheimer's disease and frontotemporal dementia

PURPOSE

Tau accumulation is a core pathologic change in various neurodegenerative diseases including Alzheimer's disease and frontotemporal lobar degeneration-tau. Recently, tau positron emission tomography tracers such as [¹⁸F] AV-1451 and [¹⁸F] THK5351 have been developed to detect tau deposition in vivo. In the present study, we performed a head to head comparison of these two tracers in Alzheimer's disease and frontotemporal dementia cases and aimed to investigate which tracers are better suited to image tau in these disorders.

METHODS

A cross-sectional study was conducted using a hospital-based sample at a tertiary referral center. We recruited eight participants (two Alzheimer's disease, four frontotemporal dementia and two normal controls) who underwent magnetic resonance image, amyloid positron emission tomography with [¹⁸F]-Florbetaben and tau positron emission tomography with both THK5351 and AV-1451. To measure regional AV1451 and THK5351 uptakes, we used the standardized uptake value ratios by dividing mean activity in target volume of interest by mean activity in the cerebellar hemispheric gray matter.

RESULTS

Although THK5351 and AV-1451 uptakes were highly correlated, cortical uptake of AV-1451 was more striking in Alzheimer's disease, while cortical uptake of THK5351 was more prominent in frontotemporal dementia. THK5351 showed higher off-target binding than AV-1451 in the white matter, midbrain, thalamus, and basal ganglia.

CONCLUSIONS

AV-1451 is more sensitive and specific to Alzheimer's disease type tau and shows lower off-target binding, while THK5351 may mirror non-specific neurodegeneration.