Candidate PET Radioligand Development for Neurofibrillary Tangles: Two Distinct Radioligand Binding Sites Identified in Postmortem Alzheimer's Disease Brain

Ligand-Based Virtual Screening as a Path to New Chemotypes for Candidate PET Radioligands for Imaging Tauopathies

Ligand-based virtual screening (LBVS) has rarely been tested as a method for discovering new structural scaffolds for PET radioligand development. This study used LBVS to discover potential chemotype leads for developing radioligands for PET imaging of tauopathies. ZINC12, a free database of over 12 million commercially available compounds, was searched to discover novel scaffolds based on similarities to four query compounds. Thirteen high-ranking hits were purchased and assayed for their ability to compete against three tritiated radioligands at their distinct binding sites in Alzheimer's disease brain tissue. Three hits were 2-substituted 6-methoxy naphthalenes. Synthetic elaboration of this new chemotype yielded three new ligands (25, 26, and 28) with high affinity for the [3H]6 (flortaucipur) neurofibrillary tangle binding site. Compound 28 showed remarkably high affinity (Ki, 7 nM) and other desirable properties for a candidate PET radioligand, including low topological polar surface area, moderate computed log D, and amenability for labeling with carbon-11. LBVS appears to be uniquely valuable for discovering new chemotypes for candidate PET radioligands.

Ligand Profiling as a Diagnostic Tool to Differentiate Patient-Derived α-Synuclein Polymorphs

Amyloid fibrils are characteristic of many neurodegenerative diseases, including Alzheimer's and Parkinson's diseases. While different diseases may have fibrils formed of the same protein, the supramolecular morphology of these fibrils is disease-specific. Here, a method is reported to distinguish eight morphologically distinct amyloid fibrils based on differences in ligand binding properties. Eight fibrillar polymorphs of α-synuclein (αSyn) were investigated: five generated de novo using recombinant αSyn and three generated using protein misfolding cyclic amplification (PMCA) of recombinant αSyn seeded with brain homogenates from deceased patients diagnosed with Parkinson's disease (PD), multiple system atrophy (MSA), and dementia with Lewy bodies (DLB). Fluorescence binding assays were carried out for each fibril using a toolkit of six different ligands. The fibril samples were separated into five categories based on a binary classification of whether they bound specific ligands or not. Quantitative binding measurements then allowed every fibrillar polymorph to be uniquely identified, and the PMCA fibrils derived from PD, MSA, and DLB patients could be unambiguously distinguished. This approach constitutes a novel and operationally simple method to differentiate amyloid fibril morphologies and to identify disease states using PMCA fibrils obtained by seeding with patient samples.

A closer look at amyloid ligands, and what they tell us about protein aggregates

The accumulation of amyloid fibrils is characteristic of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease. Detecting these fibrils with fluorescent or radiolabelled ligands is one strategy for diagnosing and better understanding these diseases. A vast number of amyloid-binding ligands have been reported in the literature as a result. To obtain a better understanding of how amyloid ligands bind, we have compiled a database of 3457 experimental dissociation constants for 2076 unique amyloid-binding ligands. These ligands target Aβ, tau, or αSyn fibrils, as well as relevant biological samples including AD brain homogenates. From this database significant variation in the reported dissociation constants of ligands was found, possibly due to differences in the morphology of the fibrils being studied. Ligands were also found to bind to Aβ(1-40) and Aβ(1-42) fibrils with similar affinities, whereas a greater difference was found for binding to Aβ and tau or αSyn fibrils. Next, the binding of ligands to fibrils was shown to be largely limited by the hydrophobic effect. Some Aβ ligands do not fit into this hydrophobicity-limited model, suggesting that polar interactions can play an important role when binding to this target. Finally several binding site models were outlined for amyloid fibrils that describe what ligands target what binding sites. These models provide a foundation for interpreting and designing site-specific binding assays.

Ligand Profiling to Characterize Different Polymorphic Forms of α-Synuclein Aggregates

The presence of amyloid fibrils is a characteristic feature of many diseases, most famously neurodegenerative disease. The supramolecular structure of these fibrils appears to be disease-specific. Identifying the unique morphologies of amyloid fibrils could, therefore, form the basis of a diagnostic tool. Here we report a method to characterize the morphology of α-synuclein (αSyn) fibrils based on profiling multiple different ligand binding sites that are present on the surfaces of fibrils. By employing various competition binding assays, seven different types of binding sites were identified on four different morphologies of αSyn fibrils. Similar binding sites on different fibrils were shown to bind ligands with significantly different affinities. We combined this information to construct individual profiles for different αSyn fibrils based on the distribution of binding sites and ligand interactions. These results demonstrate that ligand-based profiling can be used as an analytical method to characterize fibril morphologies with operationally simple fluorescence binding assays.

Synthesis and Bioevaluation of 2-Styrylquinoxaline Derivatives as Tau-PET Tracers

This study focused on designing and evaluating Tau-PET tracers for noninvasive positron emission computed tomography (PET) imaging of neurofibrillary tangles (NFTs), a hallmark pathology of Alzheimer's disease (AD). The tracers were synthesized with a 2-styrylquinoxaline scaffold and varying lengths of FPEG chains. The compound [18F]15, which had two ethoxy units, showed high affinity for recombinant K18-Tau aggregates (Ki = 41.48 nM) and the highest selectivity versus Aβ1-42 aggregates (8.83-fold). In vitro autoradiography and fluorescent staining profiles further validated the binding of [18F]15 or 15 toward NFTs in brain sections from AD patients and Tau-transgenic mice. In normal ICR mice, [18F]15 exhibited an ideal initial brain uptake (11.21% ID/g at 2 min) and moderate washout ratio (2.29), and micro-PET studies in rats confirmed its ability to penetrate the blood-brain barrier with the peak SUV value of 1.94 in the cortex. These results suggest that [18F]15 has the potential to be developed into a useful Tau-PET tracer for early AD diagnosis and evaluation of anti-Tau therapeutics.

The Sensitivity of Tau Tracers for the Discrimination of Alzheimer's Disease Patients and Healthy Controls by PET

Hyperphosphorylated tau aggregates, also known as neurofibrillary tangles, are a hallmark neuropathological feature of Alzheimer's disease (AD). Molecular imaging of tau by positron emission tomography (PET) began with the development of [18F]FDDNP, an amyloid β tracer with off-target binding to tau, which obtained regional specificity through the differing distributions of amyloid β and tau in AD brains. A concerted search for more selective and affine tau PET tracers yielded compounds belonging to at least eight structural categories; 18F-flortaucipir, known variously as [18F]-T807, AV-1451, and Tauvid®, emerged as the first tau tracer approved by the American Food and Drug Administration. The various tau tracers differ concerning their selectivity over amyloid β, off-target binding at sites such as monoamine oxidase and neuromelanin, and degree of uptake in white matter. While there have been many reviews of molecular imaging of tau in AD and other conditions, there has been no systematic comparison of the fitness of the various tracers for discriminating between AD patient and healthy control (HC) groups. In this narrative review, we endeavored to compare the binding properties of the various tau tracers in vitro and the effect size (Cohen's d) for the contrast by PET between AD patients and age-matched HC groups. The available tracers all gave good discrimination, with Cohen's d generally in the range of two-three in culprit brain regions. Overall, Cohen's d was higher for AD patient groups with more severe illness. Second-generation tracers, while superior concerning off-target binding, do not have conspicuously higher sensitivity for the discrimination of AD and HC groups. We suppose that available pharmacophores may have converged on a maximal affinity for tau fibrils, which may limit the specific signal imparted in PET studies.

Selective In Vitro and Ex Vivo Staining of Brain Neurofibrillary Tangles and Amyloid Plaques by Novel Ethylene Ethynylene-Based Optical Sensors

The identification of protein aggregates as biomarkers for neurodegeneration is an area of interest for disease diagnosis and treatment development. In this work, we present novel super luminescent conjugated polyelectrolyte molecules as ex vivo sensors for tau-paired helical filaments (PHFs) and amyloid-β (Aβ) plaques. We evaluated the use of two oligo-p-phenylene ethynylenes (OPEs), anionic OPE12- and cationic OPE24+, as stains for fibrillar protein pathology in brain sections of transgenic mouse (rTg4510) and rat (TgF344-AD) models of Alzheimer's disease (AD) tauopathy, and post-mortem brain sections from human frontotemporal dementia (FTD). OPE12- displayed selectivity for PHFs in fluorimetry assays and strong staining of neurofibrillary tangles (NFTs) in mouse and human brain tissue sections, while OPE24+ stained both NFTs and Aβ plaques. Both OPEs stained the brain sections with limited background or non-specific staining. This novel family of sensors outperformed the gold-standard dye Thioflavin T in sensing capacities and co-stained with conventional phosphorylated tau (AT180) and Aβ (4G8) antibodies. As the OPEs readily bind protein amyloids in vitro and ex vivo, they are selective and rapid tools for identifying proteopathic inclusions relevant to AD. Such OPEs can be useful in understanding pathogenesis and in creating in vivo diagnostically relevant detection tools for neurodegenerative diseases.

Design and synthesis of novel diosgenin-triazole hybrids targeting inflammation as potential neuroprotective agents

Alzheimer's disease is a progressive neurodegenerative disease, and its incidence is expected to increase as the global population ages. Recent studies provide increasing evidence that inflammation plays a key role in the pathogenesis and progression of AD. Diosgenin, an active ingredient in Dioscorea nipponica Makino, is a promising bioactive lead compound in the treatment of Alzheimer's disease, which exhibited anti-inflammatory activity. To search for more efficient anti-Alzheimer agents, a series of novel diosgenin-triazolyl hybrids were designed, synthesized, and their neuroprotective effects against oxygen-glucose deprivation-induced neurotoxicity and LPS-induced NO production were evaluated. Most of these new hybrids displayed better activities than DIO. In particular, the promising compound L6 not only demonstrated an excellent neuroprotective effect but also showed the best anti-inflammatory activity. The structure-activity relationship study illustrated that the introduction of benzyl or phenyl triazole did improve the activity, and the introduction of benzyl triazole was better than that of phenyl triazole. The results we obtained showed that the diosgenin skeleton could be a promising structural template for the development of new anti-Alzheimer drug candidates, and compound L6 has the potential to be an important lead compound for further research.

Synthesis and Evaluation of Fluorine-18 Labeled 2-Phenylquinoxaline Derivatives as Potential Tau Imaging Agents

In this study, three pairs of optically pure ¹⁸F-labeled 2-phenylquinoxaline derivatives were evaluated as Tau imaging agents for the diagnosis of Alzheimer's disease (AD). The chiral 2-fluoromethyl-1,2-ethylenediol side chain was attached to the 2-phenylquinoxaline backbone to increase hydrophilicity, thereby improving the binding affinity of the probe to tangles and their selectivity toward Tau tangles over β-amyloid plaques (Aβ). These probes displayed excellent fluorescent properties and high selectivity for tangles on brain sections from transgenic mice (rTg4510) and AD patients. Quantitative binding assays with AD homogenates showed that the probes (R)-5 and (S)-16 have a high affinity (K_i = 4.1 and 10.3 nM, respectively) and high selectivity (30.5-fold and 34.6-fold, respectively) for tangles over Aβ. The high affinity and selectivity of (R)-[¹⁸F]5 and (S)-[¹⁸F]16 for tangles were further confirmed with autoradiography on AD brain tissue in vitro. In addition, they displayed sufficient blood-brain barrier penetration (7.06% and 10.95% ID/g, respectively) and suitable brain kinetics (brain_2 min/brain_60 min = 10.1, 6.5 respectively) in normal mice. Ex vivo metabolism studies and micro-positron emission computed tomography (PET) revealed high brain biostability, good brain kinetic properties, and low nonspecific binding for (S)-[¹⁸F]16. Together, these results demonstrate that (R)-[¹⁸F]5 and (S)-[¹⁸F]16 are promising PET probes for Tau tangles imaging.

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.

Crystal structure of 3-(2-methylbenzyl)thiazolidin-2-one, C11H13ONS

C11H13ONS, monoclinic, P21/c (no. 14), a = 8.0795(6) Å, b = 7.2294(5) Å, c = 17.5898(14) Å, β = 98.354(8)°, V = 1016.52(13) Å3, Z = 4, R gt(F) = 0.0332, wR ref(F 2) = 0.0825, T = 120.00(10) K.

KOt-Bu-promoted selective ring-opening N-alkylation of 2-oxazolines to access 2-aminoethyl acetates and N-substituted thiazolidinones

An efficient and simple KOt-Bu-promoted selective ring-opening N-alkylation of 2-methyl-2-oxazoline or 2-(methylthio)-4,5-dihydrothiazole with benzyl halides under basic conditions is described for the first time. The method provides a convenient and practical pathway for the synthesis of versatile 2-aminoethyl acetates and N-substituted thiazolidinones with good functional group tolerance and selectivity. KOt-Bu not only plays an important role to promote this ring-opening N-alkylation, but also acts as an oxygen donor.

Imaging Protein Misfolding in the Brain Using β-Sheet Ligands

Neurodegenerative diseases characterized by pathological protein accumulation in cells are termed “proteinopathies.” Although various protein aggregates share cross-β-sheet structures, actual conformations vary among each type of protein deposit. Recent progress in the development of radiotracers for positron emission tomography (PET) has enabled the visualization of protein aggregates in living brains. Amyloid PET tracers have been developed, and are widely used for the diagnosis of Alzheimer’s disease and non-invasive assessment of amyloid burden in clinical trials of anti-dementia drugs. Furthermore, several tau PET tracers have been successfully developed and used in the clinical studies. However, recent studies have identified the presence of off-target binding of radiotracers in areas of tau deposition, suggesting that concomitant neuroinflammatory changes might affect tracer binding. In contrast to amyloid and tau PET, there are no established tracers for imaging Lewy bodies in the human brain. In this review, we describe lessons learned from the development of PET tracers and discuss the future direction of tracer development for protein misfolding diseases.

Different Positron Emission Tomography Tau Tracers Bind to Multiple Binding Sites on the Tau Fibril: Insight from Computational Modeling

Using the recently reported cryo-EM structure for the tau fibril [ Fitzpatrick et al. (2017) Nature 547, 185-190 ], which is a potential target concerning Alzheimer's disease, we present the first molecular modeling studies on its interaction with various positron emission tomography (PET) tracers. Experimentally, based on the binding assay studies, at least three different high-affinity binding sites have been reported for tracers in the tau fibril. Herein, through integrated modeling using molecular docking, molecular dynamics, and binding free energy calculations, we provide insight into the binding patterns of various tracers to the tau fibril. We suggest that there are four different high-affinity binding sites available for many of the studied tracers showing varying binding affinity to different binding sites. Thus, PBB3 binds most strongly to site 4, and interestingly, this site is not a preferable site for any other tracers. For THK5351, our data show that it strongly binds to sites 3 and 1, the former one being more preferable. We also find that MK6240 and T807 bind to site 1 specifically. The modeling data also give some insight into whether a tracer bound to a specific site can be replaced by others or not. For example, the displacement of T807 by PBB3 as reported experimentally can also be explained and attributed to the larger binding affinity of the latter compound in all binding sites. The binding free energy results explain very well the small binding affinity of THK523 compared to all the aryl quinoline moieties containing THK tracers. The ability of certain tau tracers, like FDDNP and THK523, to bind to amyloid fibrils has also been investigated. Furthermore, such off-target interaction of tau tracers with amyloid beta fibrils has been validated using a quantum mechanical fragmentation approach.

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.

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

Background

The 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.

Methods

Binding 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.

Results

Regional 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 (K_d1 = 5.6 nM, B_max = 76 pmol/g; K_d2 = 1 nM, B_max = 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 K_i = 0.1 pM; THK5117 K_i = 0.3 pM; T807 K_i = 0.2 pM) and an additional high-affinity site (THK5351 K_i = 16 nM; THK5117 K_i = 20 nM; T807 K_i = 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 K_i = 286 nM; T807 K_i = 227 nM) and the putamen (THK5117 K_i = 148 nM; T807 K_i = 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.

Conclusions

THK5351, 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.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-017-0325-z) contains supplementary material, which is available to authorized users.

Quinuclidine and DABCO Enhance the Radiofluorinations of 5-Substituted 2-Halopyridines

Positron emission tomography (PET) is an important molecular imaging technique for medical diagnosis, biomedical research and drug development. PET tracers for molecular imaging contain β+-emitting radionuclides, such as carbon-11 (t1/2 = 20.4 min) or fluorine-18 (t1/2 = 109.8 min). The [18F]2-fluoro-pyridyl moiety features in a few prominent PET radiotracers, not least because this moiety is usually resistant to unwanted radiodefluorination in vivo. Various methods have been developed for labeling these radiotracers from cyclotron-produced no-carrier-added [18F]fluoride ion, mainly based on substitution of a leaving group, such as halide (Cl or Br), or preferably a better leaving group, such as nitro or trimethylammonium. However, precursors with a good leaving group are sometimes more challenging or lengthy to prepare. Methods for enhancing the reactivity of more readily accessible 2-halopyridyl precursors are therefore desirable, especially for early radiotracer screening programs that may require the quick labeling of several homologous radiotracer candidates. In this work, we explored a wide range of additives for beneficial effect on nucleophilic substitution by [18F]fluoride ion in 5-subsituted 2-halopyridines (halo = Cl or Br). The nucleophilic cyclic tertiary amines, quinuclidine and DABCO, proved effective for increasing yields to practically useful levels (> 15%). Quinuclidine and DABCO likely promote radiofluorination through reversible formation of quaternary ammonium intermediates.

In Vivo Tau Imaging for a Diagnostic Platform of Tauopathy Using the rTg4510 Mouse Line

Association of tau deposition with neurodegeneration in Alzheimer's disease (AD) and related tau-positive neurological disorders collectively referred to as tauopathies indicates contribution of tau aggregates to neurotoxicity. The discovery of tau gene mutations in FTDP-17-tau kindreds has provided unequivocal evidence that tau abnormalities alone can induce neurodegenerative disorders. Therefore, visualization of tau accumulation would offer a reliable, objective index to aid in the diagnosis of tauopathy and to assess the disease progression. Positron emission tomography (PET) imaging of tau lesions is currently available using several tau PET ligands. Because most tau PET ligands have the property of an extrinsic fluorescent dye, these ligands are considered to be useful for both PET and fluorescence imaging. In addition, small-animal magnetic resonance imaging (MRI) is available for both structural and functional imaging. Using these advanced imaging techniques, in vivo studies on a mouse model of tauopathy will provide significant insight into the translational research of neurodegenerative diseases. In this review, we will discuss the utilities of PET, MRI, and fluorescence imaging for evaluating the disease progression of tauopathy.

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.