Laetitia Lemoine

Tau PET imaging: present and future directions.

Abnormal aggregation of tau in the brain is a major contributing factor in various neurodegenerative diseases. The role of tau phosphorylation in the pathophysiology of tauopathies remains unclear. Consequently, it is important to be able to accurately and specifically target tau deposits in vivo in the brains of patients. The advances of molecular imaging in the recent years have now led to the recent development of promising tau-specific tracers for positron emission tomography (PET), such as THK5317, THK5351, AV-1451, and PBB3. These tracers are now available for clinical assessment in patients with various tauopathies, including Alzheimer’s disease, as well as in healthy subjects. Exploring the patterns of tau deposition in vivo for different pathologies will allow discrimination between neurodegenerative diseases, including different tauopathies, and monitoring of disease progression. The variety and complexity of the different types of tau deposits in the different diseases, however, has resulted in quite a challenge for the development of tau PET tracers. Extensive work remains in order to fully characterize the binding properties of the tau PET tracers, and to assess their usefulness as an early biomarker of the underlying pathology. In this review, we summarize recent findings on the most promising tau PET tracers to date, discuss what has been learnt from these findings, and offer some suggestions for the next steps that need to be achieved in a near future.

Comparative binding properties of the tau PET tracers THK5117, THK5351, PBB3, and T807 in postmortem Alzheimer brains

BackgroundThe aim of this study was to compare the binding properties of several tau positron emission tomography tracers—THK5117, THK5351, T807 (also known as AV1451; flortaucipir), and PBB3—head to head in the same human brain tissue.MethodsBinding assays were performed to compare the regional distribution of 3H-THK5117 and 3H-THK5351 in postmortem tissue from three Alzheimer’s disease (AD) cases and three control subjects in frontal and temporal cortices as well as in the hippocampus. Competition binding assays between THK5351, THK5117, PBB3, and T807, as well as off-target binding of THK5117 and T807 toward monoamine oxidase B (MAO-B), were performed using binding assays in brain homogenates and autoradiography of three AD cases.ResultsRegional binding of 3H-THK5117 and 3H-THK5351 was similar, except in the temporal cortex, which showed higher 3H-THK5117 binding. Saturation studies demonstrated two binding sites for 3H-THK5351 (Kd1 = 5.6 nM, Bmax = 76 pmol/g; Kd2 = 1 nM, Bmax = 40 pmol/g). Competition studies in the hippocampus between 3H-THK5351 and unlabeled THK5351, THK5117, and T807 revealed super-high-affinity sites for all three tracers (THK5351 Ki = 0.1 pM; THK5117 Ki = 0.3 pM; T807 Ki = 0.2 pM) and an additional high-affinity site (THK5351 Ki = 16 nM; THK5117 Ki = 20 nM; T807 Ki = 78nM). 18F-T807, 11C-THK5351, and 11C-PBB3 autoradiography of large frozen sections from three AD brains showed similar regional binding for the three tracers, with lower binding intensity for 11C-PBB3. Unlabeled THK5351 and T807 displaced 11C-THK5351 to a similar extent and a lower extent, respectively, compared with 11C-PBB3. Competition with the MAO-B inhibitor 3H-l-deprenyl was observed for THK5117 and T807 in the hippocampus (THK5117 Ki = 286 nM; T807 Ki = 227 nM) and the putamen (THK5117 Ki = 148 nM; T807 Ki = 135 nM). 3H-THK5351 binding was displaced using autoradiography competition with unlabeled THK5351 and T807 in cortical areas by 70–80% and 60–77%, respectively, in the basal ganglia, whereas unlabeled deprenyl displaced 3H-THK5351 binding by 40% in the frontal cortex and 50% in the basal ganglia.ConclusionsTHK5351, THK5117, and T807 seem to target similar binding sites, but with different affinities, whereas PBB3 seems to target its own binding site. Both THK5117 and T807 demonstrated off-target binding in the hippocampus and putamen with a ten times lower binding affinity to the MAO-B inhibitor deprenyl compared with 3H-THK5351.

Development of [11C]/[3H]THK-5351 – A potential novel carbon-11 tau imaging PET radioligand

INTRODUCTION Due to the rise in the number of patients with dementia the imperative for finding new diagnostic and treatment options becomes ever more pressing. While significant progress has been made in PET imaging of Aβ aggregates both in vitro and in vivo, options for imaging tau protein aggregates selectively are still limited. Based on the work previously published by researchers from the Tohoku University, Japan, that resulted in the development of [18F]THK-5351, we have undertaken an effort to develop a carbon-11 version of the identical structure - [11C]THK-5351. In parallel, THK-5351 was also labeled with tritium ([3H]THK-5351) for use in in vitro autoradiography (ARG). METHODS The carbon-11 labeling was performed starting with di-protected enantiomeric pure precursor - tert-butyl 5-(6-((2S)-3-fluoro-2-(tetrahydro-2H-pyran-2-yloxy)propoxy)quinolin-2-yl)pyridin-2-yl carbamate, which was reacted with [11C]MeI, using DMF as the solvent and NaH as base, followed by deprotection with trifluoroacetic acid/water mixture, resulting in enantiomerically pure carbon-11 radioligand, [11C]THK-5351 - (S)-1-fluoro-3-(2-(6-([11C]methylamino)pyridin-3-yl)quinolin-6-yloxy)propan-2-ol. Tritium labeling and purification of [3H]THK-5351 were undertaken using similar approach, resulting in [3H]THK-5351 with RCP >99.8% and specific radioactivity of 1.3GBq/μmol. RESULTS [11C]THK-5351 was produced in good yield (1900±355MBq), specific radioactivity (SRA) (361±119GBq/μmol at EOS+20min) and radiochemical purity (RCP) (>99.8%), with enantiomeric purity of 98.7%. [3H]THK-5351 was evaluated for ARG of tau binding in post-mortem human brain tissue using cortical sections from one AD patient and one control subject. [3H]THK-5351 binding density was higher in the AD patient compared to the control subject, the binding was displaced by unlabeled THK-5351 confirming specific [3H]THK-5351 binding.

Cortical laminar tau deposits and activated astrocytes in Alzheimer’s disease visualised by 3 H-THK5117 and 3 H-deprenyl autoradiography

Hyperphosphorylated tau protein deposits and, inflammatory processes are characteristic components of Alzheimer disease (AD) pathology. We here aimed to visualize in vitro the distribution of tau deposits and activated astrocytes across the cortical layers in autopsy AD brain tissue using the radiotracers 3H-THK5117 and 3H-deprenyl. 3H-THK5117 and 3H-deprenyl autoradiographies were carried out on frozen brain sections from three AD patients and one healthy control. 3H-THK5117 showed a distinct laminar cortical binding similar to 3H-deprenyl autoradiography, with an extensive binding in the superficial and deep layers of the temporal neocortices, whereas the middle frontal gyrus showed an even binding throughout the layers. Globally, eventhough some differences could be observed, AT8 (tau) and GFAP (astrocyte) immunostaining showed a laminar pattern comparable to their corresponding radiotracers within each AD case. Some variability was observed between the AD cases reflecting differences in disease phenotype. The similar laminar cortical brain distribution of tau deposits and activated astrocytes supports the hypothesis of a close pathological interconnection. The difference in regional binding patterns of 3H-THK5117 and AT8 antibody staining suggest additional tau binding sites detectable by 3H-THK5117.

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 Alzheimers disease and related non‐Alzheimers 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.

COMPARISON OF BINDING PROPERTIES OF THK5117, THK5351, PBB3 AND T807 IN AUTOPSIES OF ALZHEIMER DISEASE CASES

of interest between AD, CBD, and PSP (Table 2). However, pairwise comparisons using Tukey and Kramer test revealed a significant difference (p1⁄4 0.02) in the proportion of total neurons with ht-NCI between AD and CBD, with AD showing the lower proportion (Figure 1-2). We also detected a significant difference (p1⁄4 0.0023) in the proportion of orexin positive neurons with ht-NCI between AD and CBD, with AD showing the lower proportion (Figure 3-4). Conclusions:Our unbiased stereological investigation detected moderate to high accumulation of phospho-tau in PFN in all three tauopathies. Tau burden in orexinergic neurons show a clear distinction between AD and CBD. Quantitative characterization of tau pathology and neuronal loss in sleep-related nuclei may inform on neurobiological basis of sleep disturbances. Further studies should focus on the role of degeneration of orexinergic neurons in exacerbating sleep fragmentation in AD patients.

In vitro characterization of fibrillar amyloid, tau deposits, and activated astrocytes in Arctic APP and sporadic Alzheimer's disease brain using, 3H-PIB and 3H-THK5117 and 3H-Deprenyl in comparison to immunostaining

In Vitro Characterization Of Fibrillar Amyloid, Tau Deposits, And Activated Astrocytes In Arctic App And Sporadic Alzheimers Disease Brain Using, 3H-Pib And 3H-Thk5117 And 3H-Deprenyl In Comparison To Immunostaining

IN VITRO CHARACTERIZATION OF FIBRILLAR AMYLOID, TAU DEPOSITION, AND ACTIVATED ASTROCYTES IN ARCTIC AD BRAIN IN COMPARISON WITH SPORADIC AD BRAIN USING 3H-PIB, 3H-THK5117 AND 3H-DEPRENYL

(MCI) during the follow-up, we found close matches between PK11195 images and DBSI neuroinflammation images (Fig. 1 Left). More strikingly, DBSI neuroinflammation increased over the course of the follow-up visits (Fig.1 Right), consistent with the participant’s clinical outcome. For an amyloid-positive participant who remained CN, the distribution and severity of DBSI-detected inflammation did not change from 2008 to 2015, suggesting slow neuroinflammation progression.We also examined two amyloid-positive CN participants who later converted to very mild AD. Increased levels of DBSI neuroinflammation in the follow-up examination were found. Five grey and white matter ROIs in these four participants were selected to quantitatively assess the temporal evolution of neuroinflammation change detected by DBSI. Whereas the level of DBSI neuroinflammation remained stable in the participant who remained CN, DBSI neuroinflammation levels dramatically increased in participants who developed symptoms at follow-up visits (Fig. 2). Additionally, we found the longitudinal neuroinflammation rate of change, not the baseline DBSI neuroinflammation strength, related strongly to disease progression. Conclusions: DBSI provides a noninvasive, non-radiative means to quantify neuroinflammation and predict clinical AD progression. Additionally, we demonstrate that a 3T clinical MRl scanner running FDA-approved diffusion sequences could reliably generate DBSI neuroinflammation images.

CHARACTERIZATION OF THK5117 BINDING IN AD BRAIN TISSUE: IMPLICATION FOR DEVELOPMENT OF PET TAU IMAGING

Background: Averting and clearing the aberrant accumulation of tau proteins in neurofibrillary tangles is the current focus of therapeutic strategies in Alzheimer’s disease (AD).New approaches for direct targeting of tau demand safe, non-invasive methods to detect and quantify tau in vivo [1, 2]. To assess tau progression together with the induced brain pathology, we longitudinally applied MRI on a conditional bigenic mouse model of tauopathy, rTg4510, in which tau expression can be supressedwith doxycycline [3].We investigated four, clinically relevant, quantitative techniques: cerebral blood flow (CBF) using arterial spin labelling (ASL); aggregated protein detection using amide proton transfer (APT); microstructural diffusion using diffusion tensor imaging (DTI), which were compared with the established gold standard structural MRI. Methods: 19 rTg4510, and 8 wild-type(WT) littermatched mice were imaged at baseline (4.5 months) using a 9.4T scanner for ASL, DTI, APT and structural data using parameters previously described [4,5]. 10 rTg4510 were then treated orally with doxycycline hyclate (10mg/kg) and maintained on a doxycycline diet for the remainder of the study to supress tau expression. The doxycycline-treated rTg4510, untreated rTg4510 and WT groups were imaged again at 5.5 and 7.5 months for longitudinal ASL, DTI, APTand structural data. During imaging, anaesthesia was maintained using 1.5-2% isoflurane and 1L/min O 2. Results:At 4.5 months, prior to doxycycline treatment, we observed marked changes in CBF, ATP and diffusion in rTg4510s due to increased tau pathology in comparison toWT controls. This was most apparent in the CBF study. Following only one month of treatment to supress tau, (imaging at 5.5 months), CBF had returned to control levels. This pattern was also observed for APT and diffusion imaging and the trend continued through to 7.5 months. Conclusions: This study demonstrates the sensitivity of multi-parametric MRI to the regulation of tau-driven pathological processes and provides an opportunity to assess novel therapeutic strategies. These approaches could give insights into the early onset of tauopathies, at a time when non-invasive imaging biomarkers are urgently needed for understanding the progressive pathology. This is the first demonstration that suppression of tau expression can be observed with the clinically relevant MRI parameters CBF, ATP and diffusion.