Tau Positron Emission Tomography (PET) Imaging: Past, Present, and

Subscriber access provided by NEW YORK UNIV

Perspective

Tau PET Imaging: Past, Present and Future Manuela Ariza, Hartmuth C. Kolb, Dieder Moechars, Frederik J.R. Rombouts, and José-Ignacio Andrés J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/jm5017544 • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 15, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Journal of Medicinal Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of Medicinal Chemistry

Tau PET Imaging: Past, Present and Future Manuela Ariza,a Hartmuth C. Kolb,b Dieder Moechars,c Frederik Rombouts,a and José Ignacio Andrésd,* a

Janssen Research and Development, Neuroscience Medicinal Chemistry, a division of Janssen

Pharmaceutica NV, Turnhoutseweg 30, B-2340 Beerse, Belgium. b

Janssen Research and Development, Neuroscience Biomarkers, 3210 Merryfield Row, San

Diego, CA 92121. c

Janssen Research and Development, Neuroscience Discovery Biology, a division of Janssen

Pharmaceutica NV, Turnhoutseweg 30, B-2340 Beerse, Belgium. d

Janssen Research and Development, Discovery Sciences, a division of Janssen-Cilag, Jarama

75, 45007 Toledo, Spain.

KEYWORDS. PET Imaging, PET Ligands, Tau protein, Neurofibrillary tangles, Alzheimer’s Disease.

ABSTRACT. Alzheimer’s Disease (AD) is a chronic neurodegenerative disorder and the most common cause of dementia among the elderly population. The good correlation of the density and neocortical spread of Neurofibrillary tangles (NFTs) with clinical AD disease progression offers an opportunity for the early diagnosis and staging using a non-invasive imaging technique such as Positron Emission Tomography (PET). Thus, PET imaging of NFTs holds promise not just as a diagnostic tool, but it may also enable the development of disease modifying

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 73

therapeutics for AD. In this Perspective, we focus on the structural diversity of tau PET tracers, the challenges related to the identification of high affinity and highly selective NFT ligands, as well as recent progress in the clinical development of tau PET radioligands.

1. Introduction Alzheimer’s Disease (AD) is a chronic neurodegenerative disorder and the most common cause of dementia among the elderly population, which is characterized by memory loss, spatial disorientation and cognitive impairment.1,2 In a constantly growing aging population, AD has become a major public health burden.3 Thus far only symptomatic treatments have been developed, which are able to slow the progression of cognitive decline only temporarily. Considerable progress has been made by numerous groups and organizations to understand the pathogenic mechanisms underlying AD, resulting in a number of promising disease modifying therapeutic approaches that are currently under development and clinical evaluation.4 Multiple factors are considered to be involved in the pathogenic mechanism leading to AD, such as age, genetic make-up, environmental factors, head trauma, depression, diabetes mellitus, hyperlipidemia and vascular factors.5 Apart from the intrinsic complexity of AD, the development of a cure for AD has been hampered by the lack of reliable tools for early diagnosis, staging and accurately monitoring disease progression. According to the new reviewed criteria and guidelines for diagnosing AD proposed by the National Institute on Aging (NIA) and the Alzheimer’s Association in 2011, three different stages have been accepted:6-9 1) Preclinical Alzheimer’s Disease, characterized by early brain changes without noticeable physical or clinical symptoms; 2) Mild Cognitive Impairment (MCI) due to Alzheimer’s Disease, outlined by very mild symptoms without affecting activities of daily living; and 3) Dementia due


2

Page 3 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to Alzheimer’s Disease, described by important memory and behavioral symptoms that hamper regular life. The initial neuropathological changes in the brain are thought to begin 20 years or more before symptoms appear in AD, which is in accordance to Braak and Braak stages,10,11 and during this period, individuals are able to function normally. The long duration of this pre-symptomatic phase offers an opportunity to elucidate the link between the pathophysiological process of AD and the emergence of the clinical disorder, which could allow both the early diagnosis and early intervention with a disease-modifying therapy. To date, the diagnosis of AD can only be confirmed by postmortem histological analysis of human brain samples.12 Postmortem studies of AD brains reveal two pathological hallmarks of AD: 1) senile plaques (SPs), composed by amyloid (Aβ) peptides, and 2) neurofibrillary tangles (NFT) that are composed of paired helical filaments (PHF) of hyperphosphorylated aggregated tau protein.13 Aβ peptide deposits are commonly found outside neurons in the brain while the tau protein tangles are formed inside the neurons, but eventually end up in the intercellular space after neuronal death.14 Currently, the clinical diagnosis of AD is based largely on history and statistical memory testing such as DSM-IV-TR15 (the Diagnostic and Statistical Manual of Mental Disorders) and NINCDS-ADRDA15 (the National Institute of Neurological Disorders and Stroke-Alzheimer Disease and Related Disorders). However, these examinations require clear evidence of impairment or dementia and are often inaccurate or insensitive due to the difficulty to distinguish AD from age-related cognitive decline.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 73

Measurement of Aβ peptides and total tau protein levels in cerebrospinal fluid (CSF) has been presented as a complementary useful tool in the diagnosis and monitoring of AD by the European Medicines Agency (EMA).16,17 However, CSF sample collection by lumbar puncture is invasive and may lead to adverse events.18 Modern imaging techniques are non-invasive and may be used to identify patients at risk of developing AD and/or monitor disease progression.19-21

Specifically, Positron Emission

Tomography (PET) is a sensitive molecular imaging technique that enables in vivo and in situ non-invasive visualization, characterization and quantification of physiological processes at the cellular or molecular levels.22 As such, PET has become an extremely powerful diagnostic tool, that is increasingly used in the drug discovery & development process to demonstrate target engagement and monitor disease progression.23,24 Since the deposition of neuritic plaques is one of the pathological hallmarks of AD, a significant effort has been devoted to developing PET imaging probes that visualize Aβ-plaques in vivo in human

brain

(Figure

1).25-29

Pittsburgh

compound

B,

2-[4-

([11C]methylamino)phenyl]benzothiazol-6-ol (1, [11C]-PiB), a thioflavin-T (ThT) derivative, was the first successful Aβ-selective PET radioligand30 and is currently among the most widely used agents for the delineation of Aβ-plaques in the brain.31 However, the short half-life of carbon-11 (11C, t1/2 = 20.3 min) limits the use of 1 to centers that have a cyclotron on-site. For a wider use, tracers with a longer half-life, i.e. fluorine-18 (18F, t1/2 = 109.8 min), have been developed. Significant

examples

include

[18F]fluoroethoxy)ethoxy]ethoxy]phenyl]vinyl]phenyl]-N-methylamine

N-[4-[2(E)-[4-[2-[2-(2(2,

[18F]-Florbetaben,

[18F]-AV-1, Neuroceq®),32 N-[4-[2(E)-[6-[2-[2-(2-[18F]fluoroethoxy)ethoxy]ethoxy]pyridin-3-


4

Page 5 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


yl]vinyl]pheny]-N-methylamine

(3,

[18F]-Florbetapir,

[18F]-AV-45,

Amyvid®),33

2-[3-

[18F]fluoro-4-(methylamino)phenyl]benzothiazol-6-ol (4, [18F]-Flutemetamol, Vizamyl®),34 and 2-[2-[18F]fluoro-6-(methylamino)pyridin-3-yl]-1-benzofuran-5-ol

(5,

[18F]-AZD4694,

NAV4694).35 Several articles and reviews have been published in recent years discussing the status and advances of this class of PET imaging agents.36-39

Figure 1. Structures of Aβ PET probes for AD. Generally, the density of amyloid plaques in brain tissue does not correlate well with neurodegeneration and cognitive impairment in AD.40 However, amyloid deposits have been shown to accumulate in the brain of individuals at risk of developing AD.41 As a result, amyloid imaging has not been approved for the diagnosis of AD or the measurement of the extent of cognitive impairment, but rather as a diagnostic method for the exclusion of AD in subjects that are cognitively impaired and amyloid PET-negative.42 Currently, it is being evaluated as a diagnostic tool for defining the preclinical stages of AD.41


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 73

In contrast to Aβ-plaques, the density and neocortical spread of NFTs correlate with progressive neuronal degeneration and cognitive impairment,40,43,44 thus making PET imaging of NFTs a desirable biomarker for AD. Recent findings provide additional support for the central role of tau in the pathogenesis of AD,43,45 supporting this protein both as a diagnostic and therapeutic target.46 For instance, imaging NFTs may allow the early detection of disease, possibly even before the onset of cognitive symptoms, as well as pre-symptomatic staging. PET imaging of NFTs, especially when done in conjunction with amyloid diagnosis, may also provide a means for distinguishing between AD and non-AD dementias: the absence of NFTs and amyloid would provide support for the latter diagnosis, while the presence of NFTs and amyloid would support for the former. For the research audience, NFT imaging provides insights into the relationships between tau aggregate accumulation and spread as a function of time, cognition and brain structure, across the continuum from normal aging to AD, and to investigate the effect of disease-modifying therapies on tau aggregation. Consequently, the development of radiotracers for tau aggregates has been pursued by numerous academic and industrial groups.47-52 The aim of this article is to chronologically review the progress that has been made in the field of tau aggregate imaging, from a Medicinal Chemistry perspective, as well as the recent evolution in the development of selective tau PET radioligands currently undergoing clinical evaluation, starting with an overview of the structural characteristics of tau aggregates and requirements that a suitable tau PET ligand must fulfill. 2. The Structure of Aggregated Tau The protein tau belongs to a large family of microtubule-associated proteins (MAPs) and is mainly localized in the neuronal axons. The best described biological function of tau protein is


6

Page 7 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the binding to tubulin to stabilize and control the polymerization of microtubules, which is critical to maintain cell shape, axonal transport and the neurotransmission machinery.53 Knowledge of the molecular structure and composition of tau protein is essential for the understanding of the protein self-assembly leading to the pathological tau aggregates and could allow for the rational design of NFT radiotracers. In human brain, six tau isoforms have been identified that arise from alternative mRNA splicing of a single gene composed of 16 exons located on the chromosome 17q21.31.54,55 The presence or absence of exon 2, exon 3 and exon 10 manifested in the protein constitute the main differences in these six isoforms. Exons 2 and 3 encode the insertion of 29-aminoacids in the Nterminus (called N), thus tau isoforms may have 2N (both exons inserted), 1N (only exon 2) or 0N (neither). Likewise, exon 10 encodes a 31-aminoacid stretch in the C-terminus, which is commonly known as the microtubule-binding repeat/domain (R). Hence, tau isoforms may be comprised of four repeated microtubule-binding domains called four repeat tau (4R, exon 10 included) or three repeated microtubule-binding domains called three repeat tau (3R, exon 10 excluded). Accordingly, the six tau isoforms are [4R 2N], [4R 1N], [4R 0N], [3R 2N], [3R 1N] and [3R 0N], of which the isoform [3R 0N] is expressed in fetal brains (Figure 2).56 Adult human 4R isoforms have been shown to be better at promoting microtubule assembly than the fetal 3R isoforms. In healthy human brains, there is an equal 4R/3R ratio and the alteration of this equilibrium is the underlying pathogenic mechanism of a certain clinical mutation in tau associated with FTLD-17.57 It is important to note that tau is a natively unfolded protein lacking a well-defined 3D structure. However, X-ray, Fourier transform infrared (FTIR) and circular dichroism (CD) studies suggest that the N- and C-terminal domains are folded to form a ‘paper clip’ conformation (Figure 2).58


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 73

Additionally, nuclear magnetic resonance (NMR) measurements revealed regions with transient secondary structure (α-helix, β-strand and polyproline II).59 Further studies have demonstrated that tau protein contains isolated short peptide motifs localized at the beginning of repeat domains R2-R4. Indeed, two of such motifs, called PHF6* (VQIINK) and PHF6 (VQIVYK), have a high tendency for β-structure formation and they play a crucial role in microtubule interactions but also in the tau aggregation process,60,61 which clearly illustrates the close relationship between physiological and pathological functions of tau. A protective effect against tau aggregation of the paper clip conformation has been postulated by shielding the hexapeptide motifs.58


8

Page 9 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Schematic representation of the human tau protein. A) Six human tau isoforms (reproduced with permission from [62]). B) Paper clip conformation (illustration based on [58]).

Apart from the primary structure diversity, tau protein is subject to several post-translational modifications such as glycosylation, phosphorylation, nitration, acetylation or ubiquitination, of which phosphorylation is considered the most common and important modification. Whereas phosphorylation is essential for the normal protein function under physiological conditions, abnormalities in tau phosphorylation have been proposed to play a crucial role in tau aggregation.62 In fact, under pathological conditions in AD, higher concentrations of tau protein appear dissociated from microtubules, becoming more prone to hyperphosphorylation. This abnormally hyperphosphorylated tau is able to bind to normal tau protein disrupting the usual microtubule network and promoting self-assembly into filaments and NFTs.63 Aggregation of tau is characteristic of different neurodegenerative diseases known as ‘tauopathies’.64 Although the most prevalent is AD, they additionally include tangle-only dementia (TD), argyrophilic grain disease (AGD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Pick disease (PiD) and frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). The heterogeneity of these disorders is closely related to the wide-range of human tau isoforms and post-translational modifications. Indeed, tau aggregates may appear ultrastructurally as paired helical filaments (PHF), straight filaments (SF), randomly coiled filaments (RCF) or twisted filaments (TF). This variability translates into polymorphism, whereby either the same tau isoform adopts different conformations and tau aggregate ultrastructure, or different isoforms appear with the same


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 73

ultrastructure. For example, in AD and TD tau inclusions appear as both 4R and 3R isoforms in the shape of NFTs, whereas 3R isoforms predominate in PiD as Pick’s bodies in the shape of RCFs and SFs. In CBD, tau deposits are found as astrocytic plaques composed of 4R tau isoform in the form of SFs and TFs.65 Recent studies of the ultrastructure of the tau fibrils in AD brains showed that the core of NFTs is composed of hyperphosphorylated tau filaments in the form of PHF which were found to be a twisted double-helical fibril with a crossover repeat of ~80 nm and a width varying between 8 and 20 nm.66 The region around the hexapeptide motifs exhibits three major β-strands in a crossβ conformation, which constitutes the rigid core of the fibril. In addition, the tau fibril core is stabilized by the formation of distinct salt bridges as well as by hydrophobic and π-stacking interactions.67 Even though structure determination of PHF cannot be achieved with present techniques, the structure of the amino-acid 386 to 391 core of the C-terminal tail has been elucidated by using conformation-dependent monoclonal antibodies.68 This

386

TDHGAE391

fragment was isolated and crystallized from PHFs in a complex with the antibody MN423 (Figure 3).69


10

Page 11 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. 3D structure of the 386TDHGAE391 core of the PHF C-terminus.69

It is also noteworthy that the occurrence of each of the six tau isoforms is highly variable along different species. For instance, both rats and mice are characterized by the complete absence of tau isoforms with three tubulin binding regions, while both 4R and 3R tau isoforms are expressed in pigs, although only four out of the six possible isoforms are found.70 Tau research benefits from the availability of pre-clinical in vivo models, such as transgenic tauopathy mouse models,71,72 most of which are based on the overexpression of the known MAPT mutations discovered in FTDP-17 patients or expression of the 6 isoforms of wild type tau.73 More recently, tauopathies have been induced by injection of tau aggregates into the brains of experimental animals, providing support for a prion-like seeding mechanism.74


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 73

3. Properties of a Tau PET Ligand The ideal PET radioligand for brain imaging is almost exclusively a small molecule (MW < 500), which must fulfill a wide range of requirements that have been reviewed on multiple occasions.24,75-78 First of all, a suitable CNS PET ligand must show high affinity and selectivity towards its biological target, and this binding needs to be reversible to ensure that equilibrium is reached quickly enough to image the subject. The density of the target in the brain region of interest is an important factor for consideration, and in this respect the maximum concentration of target binding sites (Bmax) is key, as it has a direct impact on the level of affinity (Kd) required for a successful radiotracer, most usually in the low nM range. It is generally accepted that the ratio Bmax/Kd should be ≥ 10.79 Importantly, a CNS PET radiotracer should be able to cross the blood brain barrier (BBB), and hence should not be a substrate for efflux transporters. At the same time it should exhibit low non-specific binding, in order to avoid attenuation of the signalto-noise ratio due to undesired binding to brain tissue. An adequate balance between lipophilicity and fraction of radiotracer that is freely available in plasma is required, and in general a free fraction of the tracer in plasma of > 5% and log D values ranging from 2 to 3.5 are considered optimal.75 However, there may exist exceptions to these two general rules, provided that the dissociation from plasma proteins occurs fast enough to ensure that sufficient concentration of radiotracer is available for an adequate brain uptake.80,81 Ideally, a CNS PET ligand does not form BBB permeable radioactive metabolites, which would complicate the efficient quantification of the PET imaging data due to signals stemming from the parent radiotracer and the radiometabolite(s). In the case of

18

F radiolabeled PET ligands,

extensive defluorination should be prevented as it would lead to high signals from bones and skull, which would thwart accurate measurement of cortical uptake. A suitable brain


12

Page 13 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pharmacokinetic profile, i.e. observable brain uptake and washout kinetics, is also a must for a successful CNS PET tracer.75 Finally, PET ligand candidates must be synthetically accessible using quick (within 3 half-lives) and efficient (enough radiochemical yield and specific activity) processes, due to the short half-lives of the two most widely used positron emission radionuclides, 11C (half-life = 20.4 min) and 18F (half-life = 109.7 min). PET imaging of NFTs in human brain presents a number of specific challenges.40,51 First of all, the tracer needs to discriminate PHF-Tau from the other β-sheet-structured aggregates present in Alzheimer’s brain, i.e. amyloid plaques. Structural similarity aside, amyloid aggregates different from tau aggregates are also in the order of 5-20 fold more abundant in brain than tau aggregates,40 and are co-localized in certain brain regions. Furthermore, recently it has been reported that α-synuclein-containing aggregates are abundant in AD, hence tau PET tracers should be selective over these protein aggregates as well.82,83 In addition, given the intracellular localization of tau, the radiotracer must be able to cross not just the BBB, but also the cell membrane. Furthermore, since tau aggregates are present also in white matter, a tau PET ligand should be devoid of non-specific binding in this brain area. Finally, although the structural variability displayed by tau protein might be an advantage for the differential diagnosis of tauopathies, selective discrimination among different forms of aggregated tau offers an additional challenge for a PET ligand due to similarities in isoform composition, posttranslational modifications and β-sheet structure. 4. Tau PET Ligands Non-selective β-sheet binders, such as thioflavin T (ThT) have been instrumental to the discovery of specific tau aggregate binders. Since Aβ-plaques and other amyloid aggregates and


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 73

NFTs share a similar β-sheet structure, it is not surprising that structural similarities exist between imaging agents for both species. The structural diversity of tau protein and its aggregates as well as their post-translational modification has thwarted the systematic evaluation of potential binding sites for a PET ligand using crystal structure or bioinformatical methods. For this reason, compound screening approaches were utilized for ligand discovery, guided by the hypothesis that fused aromatic systems are more likely to interact with the β-sheet fibrillar aggregates present in PHF-tau. These efforts led to the discovery of leads based on benzothiazoles, benzimidazoles, benzoxazoles, naphthalenes and quinolines. Compound screening is generally based on competitive in vitro assays using either synthetic heparin-induced tau polymers (HITP), or post-mortem AD human brain homogenates (ADPHFs) or sections. HITP are composed only of 3R and/or 4R tau, and are unlikely to have similar hyperphosphorylation or glycated or ubiquitinated residues as the native AD-PHFs. Furthermore, HITPs and AD-PHFs differ with respect to the structure and variability in twist in the observed Tau filaments.84 Such differences could be the reason why screening campaigns based on synthetic HITP have not been very successful for identifying ligands for native AD-PHF tau.67,70 Assays based on native protein, as found in brain homogenates or brain slices from AD patients, have yielded the most promising tau ligands.47-51 Up to our knowledge there are not any reports providing a systematic SAR analysis on the features that are required to achieve tau selectivity over amyloid aggregates. Only a few of the papers describing tau ligands discuss briefly the SAR around the respective chemical series,47 but do not provide any robust conclusions.


14

Page 15 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.1. Lead Compounds One of the earliest ligand screening campaigns targeting tau aggregates for PET tracer discovery was performed by Okamura et al. in 2005.85 In this study, more than 2,000 small molecules were tested, which resulted in the discovery of quinolines and benzimidazoles leads, such as 6 (BF126), 7 (BF-158) and 8 (BF-170) (Figure 4).85 Structurally, compound 6 resembles 1, with the benzothiazole core being replaced by benzimidazole and a carbon-carbon double bond spacer being inserted between the planar aromatic ring and the aniline-like substituent at position 2. Compounds 7 and 8 also carry an aniline moiety, which is directly attached to a quinoline ring at the 2-position. In vitro fluorescence binding affinity assay data as well as neuropathological staining results suggested quinolines 7 and 8 to be better tau ligands than the benzimidazole 6, yet all compounds showed a relatively low selectivity over amyloid plaques (Table 1). Additionally, compounds 6, 7 and 8 were unable to bind to tau present in non-AD tauopathies such as PiD and PSP. Compound 7 was radiolabeled with

11

C for in vitro autoradiographic

studies, which confirmed the accumulation of this tracer in tangle-rich brain regions. Biodistribution studies of 11C-7 in mice showed good brain uptake (11.3 % ID/g at 2 min postinjection) but rather slow clearance from the brain (3.1 % ID/g at 30 min post-injection). Despite their low affinity (EC50 > 200 nM) and high non-specific binding, these compounds first highlighted the potential utility of quinolines and benzimidazoles as tau PET radioligands in AD.85 A subsequent screening campaign of 72,455 compounds with the aim of distinguishing tau aggregates from Aβ plaques was performed by Kuret et al. in 2007.86 This effort identified phenyldiazenyl benzothiazole (PDB) derivative 9 with only two-fold selectivity for tau tangles over amyloid. Compound 9 is structurally very similar to 6, and a benzimidazole replaces the


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 73

benzothiazole and a diazo group the carbon-carbon double bond. Based on these structures, Matsumura et al. developed two novel PDB agents by substituting the methoxy group with iodine-125 (10,

125

I-PDB-3)87 and by replacing it with a fluoro-polyethylene glycol (FPEG)

chain, respectively (11, 18F-FPPDB)88 (Figure 4). Compound 10 showed a clear improvement in binding affinity to synthetic tau aggregates (Ki = 0.48 nM) and selectivity over amyloid plaques. However, unfavorable pharmacokinetics made this compound unsuitable for imaging NFTs in vivo probably due to the high lipophilicity (log P = 3.84, Table 1). The fluoro-PDB analogue 11 with a lower log P of 2.05 demonstrated improved pharmacokinetics, increased initial brain uptake and accelerated clearance versus 10, however affinity and selectivity were compromised (Table 1).


16

Page 17 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

2

11C-BF-158

(7): R1 = H, R2 = 11CH3 BF-170 (8): R1 = R2 = H

BF-126 (6)

9: R = OMe 125 I-PDB-3 (10): R = 125I 18F-FPPDB (11): R = (OCH CH ) 18 2 2 3 F

BF-188 (13)

125I-ISBIM-3

(12)

Lansoprazole (15): R = H Methyl-Lansoprazole (17): R1= 11CH3, R2 = F 18 F-Methyl-Lansoprazole (18): R1 = CH3, R2 = 18F

Astemizole (14): R = 11CH3 16: R = (CH2)218F Figure 4. Structures of benzimidazoles 6, 12-18; quinolines 7-8; benzothiazoles 9-11.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 73

The substitution of the benzothiazole scaffold by a benzimidazole ring proved an effective strategy to reduce lipophilicity, as expected from the lower calculated logP for an unsubstituted benzimidazole ring than for an unsubstituted benzothiazole ring (clogP = 1.57 versus 2.08, using Daylight software and Biobyte). Thus, radioiodinated styrylbenzimidazole derivatives similar to compound 10 were synthesized with the goal of reducing the non-specific binding.89 Although clearance was improved (0.33% ID/g at 60 min post-injection), the best candidate, 12 (125IISBIM-3)89 (Figure 4), did not display sufficient potency nor Aβ/Tau selectivity (2.73 fold), which may not be surprising, given the close structural similarity with the previously reported compound 6. More recently, the butadienyl benzimidazole derivative 1390 was identified as a modestly tau selective multispectral fluorescent imaging (MSFI) probe, which gives different fluorescent spectra when bound to tau or Aβ fibrils (Figure 4, Table 1). Although 13 was specifically developed for fluorescence-based imaging, it is an equally promising lead for the development of new tau PET probes. Besides the planar aromatic bicycle, compounds 6-13 are characterized by the presence of an aniline terminus either directly linked to the aromatic ring or via a sp2 hybridized 2 or 4 atom spacer. Interestingly, the aniline moiety is also present in previously reported Aβ ligands, which may explain the low selectivity achieved with those compounds and illustrates the challenge in differentiating between both fibrillar proteins. The binding characteristics of the benzimidazole drugs Astemizole (14) and Lansoprazole (15) to aggregated tau were recently reported by Rojo et. al (Figure 4, Table 1).91 Compound 14 appears to have high affinity for both HITP and AD-PHF tau, although no selectivity over Aβ plaques


18

Page 19 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and only minor brain uptake and clearance were observed. The latter is likely due to the fact that 14 is a substrate of the ATP-dependent efflux transporter p-glycoprotein (P-gP) in rats, which may limit its utility as a human PET tracer as well. Table 1. LogP, affinity for tau, selectivity Aβ/Tau and brain uptake for compounds 6-15, and 1720.

Tau Affinity (nM)

Brain Uptake (% ID/g) Selectivity

Compound

LogP

HITP

AD-PHF

Tau/Aβ β

2 min

30 min

60 min

6

1.09

EC50 = 583

N/A

2.20a

7.2

0.16

N/A

C-7

1.67

EC50 = 399

N/A

1.60a

11.3

3.1

N/A

8

1.85

EC50 = 221

N/A

3.50a

9.1

0.25

N/A

9

N/A

EC50 = 83

N/A

1.70a

N/A

N/A

N/A

I-10

3.84

Ki = 0.48

N/A

17.20b

0.94

N/A

2.89

F-11

2.05

Ki = 13.0

N/A

1.54b

4.28

N/A

2.53

I-12

N/A

Kd = 300

N/A

2.73c

3.28

N/A

0.33

13

N/A

Ki = 3.9

N/A

1.60b

4.8

0.12

N/A

C-14

5.57

Kd = 1.86

Kd = 13.4

1.13c

N/A

~6 x 10-3

~8 x 10-3

15

1.47

Ki = 2.5

Ki = 833.3

N/Ab

N/A

~3 x 10-3

~2.7 x 10-3

C-17

2.18

Kd = 0.7

N/A

11.7c

N/A

~0.1

~0.05

11

125

18

125

11

11


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

125

18

Page 20 of 73

I-19

N/A

Ki = 64

N/A

7.33b

1.54

0.70

0.25

F-20

N/A

N/A

N/A

N/A

6.50

2.83

2.35

Selectivity Tau vs Aβ. LogP values are the partition coefficient (octanol/water) measured for each compound as reported in the respective references: a EC50 (Aβ)/EC50 (Tau); b Ki (Aβ)/Ki (Tau); c Kd (Aβ)/Kd (Tau).

Nevertheless, several radioactive derivatives were synthesized by

11

C-methylation or

18

F-

alkylation of the p-hydroxy phenyl position (16), but it proved impossible to overcome the pharmacokinetics and permeability issues.92 Unlike 14, compound 15 displayed higher affinity for HITP than for AD-PHF tau protein, probably due to differences in the structure of induced tau fibrils and human isolated tau aggregates. Despite the low AD-PHF tau affinity, the low permeability and lack of selectivity, several radioactive derivatives of 15 were prepared, e.g. 11C1793 and

18

F-18.94 Both tracers display excellent affinity to HITP, and preliminary studies with

transgenic mice expressing human AD and PSP tau revealed their promise as tau PET probes.93,94 Additionally, autoradiography staining with human AD brain sections co-localized with tau immunostaining, but not with amyloid plaques, yet white-matter non-specific binding was observed as well. In vivo metabolic studies of

11

C-17 in rodents and non-human primates

showed the presence of a single polar radioactive metabolite that is likely the hydroxyl or sulfone derivative, in accordance with the known metabolism of lansoprazole. It is important to note that both radiotracers are substrates for the rodent P-gP transporter but not for the corresponding primate P-gP transporter. This suggests a potential utility of these compounds in humans, although additional studies are clearly needed.


20

Page 21 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The growing interest for imaging tau aggregates in human brain and the need to find a more potent and selective ligand, has spurred a quest for structurally different compounds from the ones originally reported. For example, compounds 1995,96 and 2097,98 (Figure 5) were considered as tau PET tracers when studying the effect and properties of similar compounds as potential tau aggregation inhibitors.99-101 Compound 19 is a thiohydantoin derivative linked via rigid Zmethylidene to a 5-phenylfuryl group at the 5-position, and a flexible 2-(1H-imidazol-4-yl)ethyl chain at the 1-position. Even though the affinity and selectivity of compound 19 were moderate (Table 1), autoradiography and immunohistochemical studies in sections of human AD brain show that this tracer binds to tangle-rich brain regions.95 Brain section staining experiments suggest that 7-aminothieno[3,4-d]pyridazin-1(2H)-one (20) is able to discriminate NFTs from Aβ plaques. A biodistribution evaluation of 20 in normal mice revealed relatively rapid brain uptake and clearance (Table 1). However, additional studies are needed to test the utility of 20 as a tau PET ligand, such as the determination of the binding affinity for NFTs and Tau/Aß selectivity. A recent Hoffman-La Roche patent application claims imidazo[2,1]thiazol-3-one derivatives102 (21, Figure 5) as potent (Ki (HITP tau) ~ 0.010 nM) and exceptionally selective tau aggregate binders (Tau/Aβ ~ 100-1000 fold), suggesting that they have potential as tau PET tracers. Structurally, these derivatives are similar to the thiohydantoin compound 19, and it is interesting to note the highly polar phenolic terminus on one side, and a lipophilic aromatic terminus on the opposite side. Previous studies with amphiphilic molecules had suggested that it may be possible to increase potency and selectivity by incorporating lipophilic and hydrophilic features in the same compound.103 However, the true potential of this chemical class has yet to be shown, since the affinity and selectivity in human AD brain has not yet been disclosed. These compounds


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 73

might be the subject of a recently registered clinical Phase 1 study on the “Evaluation of [11C]RO6924963, [11C]RO6931643, and [18F]RO6958948 as Tracers for Positron Emission Tomography (PET) Imaging of tau in Healthy and Alzheimer's Disease Participants”,104 which does, however, not disclose the exact chemical nature of the test compounds.


22

Page 23 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

125I-TH2

18F-ATPZ-38

(19)

Imidazo[2,1]thiazol-3-ones (21)

18F-SKT04-137

(20)

Bis(aryl)urea azaindole (22)

(23)

Bis(arylvinyl)pyrimidines (24)

18F-FDDNP

(25)

Figure 5. Diverse tau PET ligands: structures of compounds 19-25.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 73

Several additional scaffolds have been proposed in recent years as potential tau imaging agents, such as bis(aryl)urea azaindole 22, developed by Merck105 or SKT04-137 (23) by Wista Laboratories LTD,106 bearing an urea or an amide moiety, respectively, as linkers between two aromatic partners. Bis(arylvinyl)pyrimidines such as 24107 are characterized by a linear πconjugated symmetric structure having two styryl moieties bound to a pyrimidine central core. This compound displayed good affinity (IC50 (HITP tau) = 2 nM) and selectivity (26 fold), although no pharmacokinetic studies assessing brain penetration have been reported. Finally, the naphthalene derivative 2-[1-[6-[N-(2[18F]-fluoroethyl)-N-methylamino]naphthalen-2yl]ethylidene]malononitrile (25,

18

F-FDDNP) is an older radioligand, first described as a

potential PET tracer for Aß and NFTs by Barrio et. al. in 1999.108 Although the authors have touted the potential efficacy of 25,109-112 the apparently low affinity for amyloid structures113 and the low Aß/Tau selectivity of 25 is likely to limit its utility as a PET tracer. Different studies reported with this radioligand have been covered in several recent review articles.40,47,50,51 4.2. Selective Tau Binders Evaluated in Human Subjects Recently, considerable progress in the development of high-affinity, tau-selective ligands has been made independently by three groups: the Tohoku University/Austin Hospital group, led by N. Okamura, has described a series of 18F-labeled THK compounds; the Chiba group, led by M. Maruyama and H. Shimada, has described a new series of 11C-labeled PBB compounds; and the Siemens group, led by H. Kolb, has reported two

18

F-labeled compounds, which are now being

developed by Avid/Lilly. 4.2.1. THK compounds


24

Page 25 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The THK tau imaging probes comprise a family of arylquinolines first reported by Okamura et al.

after

structural

optimization

of

lead

compounds

7

and

8.85

4-[6-(2-

[18F]Fluoroethoxy)quinolin-2-yl]phenylamine (26, THK-523)114 was the first candidate described with good affinity for synthetic tau fibrils and reasonable selectivity over amyloid plaques (Table 2). The introduction of an alkyl ether at the 6-position of the arylquinoline not only allowed introduction of

18

F, but was also accompanied by an improvement in affinity and selectivity

compared to the initial hits 7 and 8. Table 2. Binding affinities and pharmacokinetics of compounds 26-28. Brain uptake Tau affinity (nM) (% ID/g) Selectivity Compound

LogP

HITP

AD-PHF

2 min

30 min

Tau/Aβ β Kd1 = 1.67

6

2.40 18

F-THK-523 (26)

Kd2 = 21.74

Kd = 86.50

10a

2.72

1.47

Kd = 2.63

25a

9.20

3.61

Kd = 5.19

30b,*

6.06

0.59

Ki = 59.30 Kd1 = 1.45 3.03

18

Ki = 7.80

F-THK-5105 (27)

2.32 18

Kd2 = 7.40

Ki = 10.50

F-THK-5117 (28) Selectivity Tau vs Aβ. LogP values are the partition coefficient (octanol/water) measured for each compound as reported in the respective references: a Kd (Aβ)/Kd (Tau); b Ki (Aβ)/Ki (Tau); *Unpublished data, AAIC meeting, Copenhagen. 2014.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 73

Initial in vitro saturation binding studies with 26 suggested two different binding sites,114 and further competition studies revealed the affinity for recombinant tau fibrils to be relatively low (Ki = 59.30 nM, Table 3).115 Unfortunately, the binding affinity for AD-PHF, present in AD brain homogenates, was even lower (Kd = 86.50 nM) than for synthetic HITP, which again demonstrated the shortcomings of synthetic tau preparations. Even though the Tau/Aβ selectivity was initially reported to be 10 fold, based on the synthetic tau affinity data (Kd1 = 1.67 nM), 26 is actually non-selective, based on the other tau binding data.115 Interestingly, in vivo retention of 26 in tau transgenic mice (rTg4510) was significantly higher compared to the analogue model for Aβ plaques (APP/PS1 mice),114 suggesting that 26 may be selective for tau pathology in vivo, with a favorable brain uptake and washout. In vivo PET imaging with 26 showed higher cortical retention in AD patients compared to healthy controls (HC), and the tracer distribution was in agreement with the reported histopathological brain distribution of PHF in AD.116 However, the high white matter retention of 26 hampers a clear visualization of PET scans and consequently precluded its further development. The optimization of the alkyl chain at the quinoline 6-position provided two novel arylquinolines, 1-([2-[4-(dimethylamino)phenyl]quinolin-6-yl]oxy)-3-[18F]fluoropropan-2-ol (27, THK-5105) and 1-([2-[4-(methylamino)phenyl]quinolin-6-yl]oxy)-3-[18F]fluoropropan-2-ol (28, THK-5117).115 Both compounds showed higher in vitro tau affinities than 26 for synthetic HITP tau fibrils as well as for human AD-PHF tau aggregates (Table 2) with Kd values of 2.6 nM (27) and 5.2 nM (28) for the latter. Additionally, 27 and 28 show higher in vitro Aβ/Tau selectivity.


26

Page 27 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In vitro autoradiography staining of human AD brain sections showed that the distribution of 27 and 28 in the mesial temporal sections coincided with Gallyas-Braak staining and tau immunostaining but not with the distribution of 1. Compared to 26, both compounds 27 and 28 showed higher contrast of tau pathology, mostly due to their higher affinity and selectivity. The presence of the secondary alcohol again introduces a polar terminus which might explain the higher selectivity observed for 27 and 28 compared to 26. Additionally, in view of the structural similarities of these compounds, mono and di-methylation of the aniline moiety appears to be the reason for the enhanced tau affinity and improved pharmacokinetic profile, with 27 and 28 showing a better brain uptake and washout than 26. First-in-human PET studies of 27 revealed this radiotracer’s ability to differentiate between AD patients and healthy controls.117 Furthermore, the pattern distribution in mesial and lateral lobes of AD patients agrees with the reported NFT distribution in AD brain10 and did not correlate with 11

C-1 retention. These results suggest that 27 is selective for NFT over Aβ plaques, although

non-specific binding to brainstem, thalamus and subcortical white matter hamper data interpretation.118 The relatively slow kinetics and clearance displayed by this ligand compared to other known PET ligands has limited its further use. First-in-human PET studies with 28 are currently underway (Figure 6)52 and preliminary results show faster kinetics and better signal-to-noise ratio than observed for 27, as expected from its higher hydrophilicity and the pre-clinical pharmacokinetic studies. The S-enantiomer has been selected as the clinical candidate based on the faster kinetics and better signal-to-noise ratio than the R-enantiomer (Villemagne et al., unpublished data, AAIC meeting, Copenhagen, July 2014).


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 73

Autoradiography studies with the tritiated ligand 3H-28 in human brain slices exhibited a good correlation with AT8 immunostaining and the Bmax was calculated to be 704 fmol/mg (Lemoine et al., unpublished data, AAIC meeting, Copenhagen, July 2014). Additionally, tracer retentions were associated with severity of dementia and human brain atrophy (Figure 6). In order to further improve the pharmacokinetic properties of arylquinolines, further structural modifications are currently underway.119 Very recently another analogue was reported: 18F-THK5351, which is the 4-methylaminopyridyl- analogue of 28. The authors claim that this new tau PET tracer shows faster kinetics, lower white matter retention and higher signal-to-noise ratio than 27 and 28.51,120

Figure 6. [18F]-28 PET images in mild, moderate and severe AD patients. In the mild AD case, specific 28 binding is confined to the medial, anterior and inferior temporal cortex. The moderate AD case shows additional 28 retention in association areas. The severe AD case shows more


28

Page 29 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


extensive and higher 28 retention in the neocortex.52 (Reproduced with copyright permission from [52]).

4.2.2. Phenyl-butadienyl-benzothiazole (PBB) compounds A class of phenyl- and pyridyl-butadienyl-benzothiazoles (PBBs) has been identified as potential tau imaging probes by the National Institute of Radiological Sciences (NIRS) in Chiba, Japan (Figure 7).121 These scaffolds are characterized by a π-electron conjugated backbone with a specific extent ranging from 13 to 19 Å, which the authors deem essential for getting selectivity for AD and non-AD tau aggregates. Structurally, the PBB series can be seen as analogues of fluorescent amyloid dye (ThT) in which the two aromatic moieties are spaced with an all-trans butadiene bridge. Taking advantage of the fluorescent properties of PBBs, NFT-like pathology in the brain stem and spinal cord of transgenic mouse models could be visualized ex vivo in both PS19 and rTg4510

strains.

Among

five

different

PBB

compounds,

2-[(1E,3E)-4-[6-(11C-

methylamino)pyridin-3-yl]buta-1,3-dien-1-yl]-1,3-benzothiazol-6-ol (29, PBB3)121 was found to be the best candidate with a good tau affinity in the nanomolar range (Kd (HITP) = 2.55 nM) and selectivity for tau over Aβ (50 fold). This compound exhibited reasonable biostability (unmetabolized compound ~ 20% at 3 min post-injection) and sufficient uptake and clearance from mouse brain.121 Radiolabeling of 29 with

11

C afforded the PET tracer

11

C-29 that yielded superior signals with

less non-specific binding in the brain stem of PS19 mice compared to other tracers. However,


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 73

micro-PET studies seemed to show that Tg4510 mice are better for visualizing tau aggregates with 11C-29 (Higuchi et al., unpublished data, AAIC meeting, Copenhagen, July 2014). Based on these pre-clinical findings in mice, compound

11

C-29 was selected as a candidate for

further evaluation in humans. Despite its relatively fast metabolism in humans, accumulation of 11

C-29 in the medial temporal region of AD patients as compared to controls was demonstrated.

Furthermore, these preliminary results suggested the possibility of capturing the temporospatial spreading of neurofibrillary tau pathologies from the limbic system (Braak stage III/IV) to neocortical areas (Braak stage V/VI). Distribution of from that of amyloid tracer although

11

11

11

C-29 in AD human brains was different

C-1 showing minimal non-specific binding to white matter,

C-29 is accumulated in dural venous sinuses both in control and AD brains.

Additional PET scans using

11

C-29 with a CBD patient exhibited tracer retention in the basal

ganglia, suggesting it could be useful for imaging non-AD tauopathies as well.121 Accordingly, compound 29 appeared to be a promising candidate for in vivo imaging of tau pathology. However, this compound exhibits stability issues, largely due to the rapid interconversion of E/Z isomers in the presence of light (photoisomerization),122 which is a wellknown phenomenon in stilbenes.123,124 This photoisomerization can be largely suppressed by shielding the compound from light during chemical synthesis, radiosynthesis and purification. Metabolite analysis in mouse and human plasma of

11

C-29 revealed a polar radioactive

metabolite.122 The percentage of unchanged 29 was less than 2 % as early as 1 min after injection in mice, although relatively slower in human (29 < 8% at 3 min post-injection). This may explain the lower brain uptake of 29 (1.92 % ID/g at 1 min post-injection) when compared to other radiotracers. One of the key disadvantages of 29 stems from the presence of this undesirable


30

Page 31 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


radioactive metabolite in mouse which confounds image analysis. Consequently, determination of the metabolic pathway for 29 as well as identification of the metabolite chemical structure is required in order to improve biostability. These additional studies would support the consistency of properties and data previously displayed by 29. As 18F is a more preferred radionuclide than 11C, new fluorinated PBB compounds are currently under investigation, such as 30, which apparently exhibited promising preliminary results. (Higuchi et al., unpublished data, AAIC meeting, Copenhagen, July 2014).

11

Example of fluorinated 18F-PBB3 (30)

C-PBB3 (29)

Figure 7. Structure of 11C-PBB3 (29) and a novel fluorinated 18F-PBB3 derivative (30) 4.2.3. Siemens – Avid/Lilly compounds An in vitro autoradiography screening campaign of human AD brain sections of over 800 compounds at Siemens MI Biomarker Research revealed two lead series, carbazoles and benzimidazoles. Compound optimization eventually provided two novel promising tracers: 7-[6[18F]fluoropyridin-3-yl]-5H-pyrido[4,3-b]indole (31, AV-1451,

18

F-T807)125 and 2-[4-(2--

[18F]fluoroethyl)piperidin-1-yl]pyrimido[1,2-a]benzimidazole (32, AV-680,

18

F-T808).126 As

PET imaging probes, 31 and 32 satisfy all criteria for quantitative imaging of tau pathology, which include high affinity for Tau in the nanomolar range, more than 25-fold selectivity for


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 73

AD-PHF over Aβ, lack of white matter binding and a favorable pharmacokinetic profile (Table 3).127,128 Table 3. Binding affinity and pharmacokinetics of 31 and 32. Brain uptake Tau affinity* (nM) (%ID/g) Selectivity Compound

LogP

AD-PHF

2 min

20 min

Tau/Aβ β

18

1.67

Kd = 14.6

25a

4.16

1.1

N/A

Kd = 22

27a

6.7

2.3

F-T807

(AV-1451, 31)

18

F-T808

(AV-680, 32) * Based on Scatchard plot analysis of autoradiography staining of human AD brain slices, containing PHF-Tau. LogP values are the partition coefficient (octanol/water) measured for each compound as reported in the respective references: a Selectivity Tau vs Aβ: Kd (Aβ)/Kd (Tau).

31 brain section autoradiography comparison with immunostaining demonstrates good agreement with the Tau staining pattern, but not with the ß-amyloid pattern (Figure 8), suggesting that 31 binding co-localizes with Tau but not ß-amyloid. Additionally, a linear correlation was observed between immunostaining-based NFT loads of brain sections from 26


32

Page 33 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


human donors and the 31-based autoradiography staining intensity. Such a correlation did not exist between 31 and ß-amyloid immunostaining.128

Figure 8. [18F]31 autoradiography on brain sections from different groups and its comparison with PHF-tau and Aβ double IHC.128 [18F]31 co-localized with PHF-tau but not with Aβ plaques. Top: low magnification, bottom: high magnification from the framed areas. Images of PHF-tau (left) and Aβ (right) IHC double immunostaining and autoradiogram image (middle) from two adjacent sections (10 µm) from a PHF-tau rich AD brain (frontal lobe). Fluorescent and autoradiographic images were obtained using a Fuji Film FLA-7000 imaging instrument. Scale bars= 2 mm.

Interestingly, pre-clinical in vivo studies with 31 and 32 using β-amyloid plaque bearing APPSWE-PS19 transgenic mice129 showed no noticeable differences in retention between tau


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 73

transgenic mice and wild-type animals, suggesting that these compounds are specific binders for human aggregated tau, but not the aggregated β-amyloid PHF-tau present in these mouse models. The initial clinical PET scans of 31 with AD and MCI patients and healthy controls showed a radiotracer accumulation in line with the known distribution of PHF in brain according to Braak,130 where increasing neocortical spread was clearly associated with severity of dementia (Figure 9).131

Figure 9. [18F]31 PET images in a healthy subject (left), mild MCI, and mild and severe AD patients. SUVR means the target-to-cerebellum standardized uptake value. The PET data show increasing Tau spread with increasing disease severity. The sensory motor cortex and the


34

Page 35 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


primary visual cortex (not visible in this projection) are largely spared even in the severe case, in agreement with Braak staging.131,132

The radiotracer displays favorable kinetics with rapid brain uptake and clearance and minimal non-specific binding to white matter as well as cortical gray matter of healthy control subjects.

Figure 10. [18F]31 target-to-cerebellum SUV ratio. SUVR measured from 80 to 100 min post injection images showed the highest cortical tracer retention by the severe AD subject. The increasing neocortical spread detected in the PET scans with increasing dementia severity agrees with the pathology-derived Braak staging system.131


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 73

First-in-human PET studies of 32 showed faster kinetics than 31, matching earlier pre-clinical rodent PET studies. The cortical retention of 31131 (Figure 10) and 32133 were correlated with increasing disease severity. Additionally, post-mortem human brain analysis by fluorescence tau staining of one subject who died after undergoing a PET scan with 32 was consistent with the observed pattern of regional in vivo uptake.134 However, substantial accumulation of

18

F in the

skull, due to metabolic de-fluorination of 32, was observed in some cases, which may hamper PET images and the use of this radiotracer in vivo. For this reason, the more stable 31 was selected for further clinical development. Further in vivo studies of 31 in large groups of AD and non-AD tauopathy patients are currently under way. 5. Conclusions The development of tau PET radioligands complements earlier work on Aß imaging, since it is now possible to detect both pathological hallmarks of AD in living patients. Together, both types of tracer may enable earlier diagnosis of AD, possibly even before cognitive symptoms arise, as well as more accurate staging of this neurodegenerative disease, and differentiation from nonAD dementia. This may pave the way for early intervention with a disease-modifying therapy, which may have a better chance of halting the irreversible neurodegenerative processes. The well-established correlation of the density and neocortical spread of NFTs with memory decline in AD, may provide the basis for tau PET imaging as a means for monitoring disease progression or success of therapy. This may be applicable not only to therapies that target amyloid and concomitant tau pathology in AD, but, importantly, also tau-centric therapies.


36

Page 37 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Considerable progress has been made in the development of high-affinity, tau-selective ligands, and three series of tracers, 28 and analogues, 29 and fluorinated derivatives, and 31 are currently undergoing clinical evaluation. The heterogeneity of tau protein aggregates and the fact that the binding site to these is probably ill-defined135 complicates the interpretation of binding data and classification of ligands. For instance, saturation binding studies with

18

F-26 suggested the

presence of two different binding sites.114 Competition binding studies performed at Janssen R&D also suggest that there is an overlap between 29 and 31 binding sites (unpublished results). The reason for this differential binding is still poorly understood. One possibility is that ligands show varying affinity for different forms of aggregated tau, due to differences in primary structure and/or post-translational modifications. Secondly, the lack of a well-defined binding site across the fiber axis opens the possibility that multiple binding modes exist to one particular form of aggregated tau.135 Studying further the structure and binding characteristics of tau aggregates isolated from preclinical models and human tauopathies at various disease stages could be instrumental to gain more understanding in the binding characteristics of tau ligands. It will be important to investigate longitudinal imaging in context of disease progression, and to compare the in vivo PET results with ex vivo pathology. Additionally, the dynamic range of the tracers needs to be evaluated, as well as their test/re-test reproducibility. These tracers are currently being studied across various tauopathies, and resulting insights will guide the development of disease modifying therapies, enable early intervention, and hopefully improve outcomes. Contributors All authors have contributed equally to the preparation of this Review and have read and accepted the final revised version prior to submission


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 73

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Phone: 0034 925 24 57 80 Biographies Dr. Manuela Ariza had her academic training at the University of Malaga, Spain, where she obtained her Ph.D. in Organic Chemistry in 2011 in the group of Prof. Dr. Rafael Suau. During this period, she spent three months as a visiting researcher at the École Supérieure de Physique et Chemie Industrielles (ESPCI ParisTech), Paris (France) under the supervision of Prof. Dr. Janine Cossy. In 2011 she obtained a one-year Research Internship from the Company-University Foundation of the Autonomous University of Madrid to collaborate with the Neuroscience team at Janssen Pharmaceutica in Toledo. In 2013 she became a Post-doctoral Scientist at the Neuroscience Department of Janssen Pharmaceutica in Belgium, where she has been focused on the design and synthesis of novel potential Tau PET Ligands. Dr. Hartmuth Kolb received his Ph.D. in Organic Chemistry in 1991 at Imperial College of Science, Technology and Medicine, London. Following postdoctoral work with Barry Sharpless, he joined Ciba-Geigy in 1993. In 1997, Dr. Kolb became the Head of Chemistry at Coelacanth Corporation. In this role, he and Dr. Sharpless developed the Click Chemistry approach to drug discovery. In 2002, he joined The Scripps Research Institute as an Associate Professor. From 2004 to 2013, he was the head of Siemens Biomarker Research, where his team developed novel oncology and neurodegenerative disease PET tracers, a key highlight being the PHF-Tau tracer, [18F]-T807. Dr. Kolb joined Johnson & Johnson in January 2014 as the Head of Neuroscience Biomarkers. He is an author on over 75 peer-reviewed publications.


38

Page 39 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dr. Dieder Moechars obtained his doctorate in Biomedical Sciences at the Katholieke Universiteit Leuven (Belgium) in 1996. Next he became a post-doctoral researcher at the Centre for Human and Experimental Genetics, K.U.Leuven under the supervision of Prof. F. Van Leuven. In 1999 he joined Janssen Pharmaceutica, Belgium, where he currently holds the position of Scientific Director. He is a Preclinical Biologist with over 20 years of experience in AD research, internationally recognized for work in basic neurobiology and pharmaceutical R&D with more than 70 peer reviewed articles and several patents. His research interests include a multitude of drug discovery projects from TI-TV to NME delivery and currently he is actively involved on the development of β-amyloid and tau based therapies for disease modification in AD. Dr. Frederik Rombouts obtained his doctorate in Organic Chemistry at the Katholieke Universiteit Leuven (Belgium) in 2002 in the group of Prof. Georges Hoornaert. Next he became a post-doctoral researcher at the Université de Montreal (Canada) under supervision of Prof. William D. Lubell. In 2003 he joined Janssen Pharmaceutica, Belgium, where he currently holds the position of Principal Scientist. In 2008, Dr. Rombouts joined the Neuroscience Medicinal Chemistry department, focused on developing therapies and imaging agents for Alzheimer’s Disease. Since then he has led the Hit Generation Team, which supports all pre-HTL, HTL and LO projects with novel chemical starting points. His research interests include especially the creative aspects of drug design and synthesis, structure- and fragment-based drug discovery, novel synthetic methodologies and target identification. Dr. José Ignacio Andrés obtained his doctorate in Organic Chemistry in 1985 at the Complutense University of Madrid, Spain. That year he joined Janssen Research & Development to start up the Medicinal Chemistry Department in Toledo (Spain), and he currently holds the


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 73

position of Scientific Director & Fellow. In 2001 he was appointed Medicinal Chemistry Associate for CNS - Psychiatry Discovery, and in 2007 he was appointed Head of the newly created PET Ligand Discovery Team in Toledo. During his scientific career he has been directly involved in a large number of discovery programs, several of which have successfully brought compounds into the clinic including two new PET radioligands. He is co-author of around 50 peer-reviewed publications and co-inventor in around 50 patent applications. ACKNOWLEDGMENT The authors thank Katleen Fierens, PhD, Discovery Sciences – Janssen R&D, for her key contribution to the progress of our Tau PET discovery program. This work was supported in part by the Institute for the Promotion of Innovation by Science and Technology (IWT120492) in Flanders. ABBREVIATIONS Aβ, amyloid; AD, Alzheimer’s Disease; AGD, argyrophilic grain disease; BBB, blood brain barrier; CBD, corticobasal degeneration; CD, circular dichroism; CSF, cerebrospinal fluid; DSM-IV-TR, Diagnostic and Statistical Manual of Mental Disorders; EMA, European Medicines Agency; FPEG, fluoro-polyethylenglycol; FTDP-17, frontotemporal dementia and parkinsonism linked to chromosome 17; FTIR, Fourier transform infrared; HC, Healthy Control; MAPs, microtubule-associated

proteins;

MCI,

Mild

Cognitive

Impairment;

MSFI,

selective

multispectral fluorescent imaging; NIA, National Institute on Aging; NFT, neurofibrillary tangles; NINCDS-ADRDA, National Institute of Neurological Disorders and Stroke-Alzheimer Disease and Related Disorders; NMR, nuclear magnetic resonance; PDB, phenyldiazenyl benzothiazole; PiD, Pick disease; PET, Positron Emission Tomography; PiB, Pittsburgh compound B; P-gP, p-glycoprotein; PHF, paired helical filaments; PSP, progressive supranuclear


40

Page 41 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


palsy; RCF, randomly coiled filaments; SF, straight filaments; SPs, senile plaques;

SUV,

standardized uptake values; SUVR, target-to-cerebellum standardized uptake value; TD, tangleonly dementia; TF, twisted filaments; ThT, thioflavin-T.

REFERENCES 1.

Khachaturian, Z. S. Diagnosis of Alzheimer's disease. Arch. Neurol. 1985, 42,

1097−1105. 2.

Citron, M. Alzheimer’s disease: strategies for disease modification. Nat. Rev. Drug Disc.

2010, 9, 387−398. 3.

www.alz.org

4.

Anand, R.; Dip Gill, K.; Ali Mahdi, A. Therapeutics of Alzheimer's disease: past, present

and future. Neuropharmacology 2014, 76, 27−50. 5.

Fargo, K.; Bleiler, L. Alzheimer's Association report. Alzheimers Dement. 2014, 10,

e47−92. 6.

Jack, C. R.; Albert, M. S.; Knopman, D. S.; McKhann, G. M.; Sperling, R. A.; Carrillo,

M. C.; Thies, B.; Phelps, C. H. Introduction to the recommendations from the National Institute on Aging-Alzheimer's Association workgroups on diagnostic guidelines for Alzheimer's disease. Alzheimers Dement. 2011, 7, 257–262. 7.

McKhann, G. M.; Knopman, D. S.; Chertkow, H.; Hyman, B. T.; Jack, C. R.; Kawash, C.

H.; Klunk, W. E.; Koroshetz, W. J.; Manly, J. J.; Mayeux, R.; Mohs, R. C.; Morris, J. C.; Rossor, M. N.; Scheltens, P.; Carrillo, M. C.; Thies, B.; Weintraub, S.; Phelps, C. H. The diagnosis of


41


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 73

dementia due to Alzheimer's disease: recommendations from the National Institute on AgingAlzheimer's Association workgroups on diagnostic guidelines for Alzheimer's disease. Alzheimers Dement. 2011, 7, 263–269. 8.

Albert, M. S.; DeKosky, S. T.; Dickson, D.; Dubois, B.; Feldman, H. H.; Fox, N. C.;

Gamst, A.; Holtzman, D. M.; Jagust, W. J.; Petersen, R. C.; Snyder, P. J.; Carrillo, M. C.; Thies, B.; Phelps, C. H. The diagnosis of mild cognitive impairment due to Alzheimer’s disease: recommendations from the National Institute on Aging-Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer’s disease. Alzheimers Dement. 2011, 7, 270–279. 9.

Sperling, R. A.; Aisen, P. S.; Beckett, L. A.; Bennett, D. A.; Craft, S.; Fagan, A. M.;

Iwatsubo, T.; Jack, C. R.; Kaye, J.; Montine, T. J.; Park, D. C.; Reiman, E. M.; Rowe, C. C.; Siemers, E.; Stern, Y.; Yaffe, K.; Carrillo, M. C.; Thies, B.; Morrison-Bogorad, M.; Wagster, M. V.; Phelps, C. H. Toward defining the preclinical stages of Alzheimer’s disease: recommendations from the National Institute on Aging-Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer’s disease. Alzheimers Dement. 2011, 7, 280–292. 10.

Braak, H.; Braak, E. Neuropathological stageing of Alzheimer-related changes. Acta

Neuropathol. 1991, 82, 239–259. 11.

Braak, H.; Braak, E. Frequency of stages of Alzheimer-related lesions in different age

categories. Neurobiol. Aging 1997, 18, 351–357. 12.

Delacourte, A. Diagnosis of Alzheimer's disease. Ann. Biol. Clin. 1998, 56, 133–142.


42

Page 43 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13.

Trojanowski, J. Q.; Clark, C. M.; Schmidt, M. L.; Arnold, S. E.; Lee, V. M. Strategies for

improving the postmortem neuropathological diagnosis of Alzheimer's disease. Neurobiol. Aging 1997, 18, S75–S79. 14.

Karran, E.; Mercken, M.; De Strooper, B., The amyloid cascade hypothesis for

Alzheimer's disease: an appraisal for the development of therapeutics. Nat. Rev. Drug Disc. 2011, 10 (9), 698–712. 15.

Dubois, B.; Feldman, H. H.; Jacova, C.; DeKosky, S. T.; Barberger-Gateau, P.;

Cummings, J.; Delacourte, A.; Galasko, D.; Gauthier, S.; Jicha, G.; Meguro, K.; O’Brien, J.; Pasquier, F.; Robert, P.; Rossor, M.; Salloway, S.; Stern, Y.; Visser, P. J.; Scheltens, P. Research criteria for the diagnosis of Alzheimer's disease: revising the NINCDS-ADRDA criteria. Lancet Neurol. 2007, 6, 734–746. 16.

Isaac, M.; Vamvakas, S.; Abadie, E.; Jonsson, B.; Gispen, C.; Pani, L. Qualification

opinion of novel methodologies in the predementia stage of Alzheimer's disease: cerebro-spinalfluid related biomarkers for drugs affecting amyloid burden - regulatory considerations by European Medicines Agency focusing in improving benefit/risk in regulatory trials. Eur. Neuropsychopharmacol. 2011, 21, 781–788. 17.

Blennow, K.; Hampel, H.; Zetterberg, H. Biomarkers in amyloid-β immunotherapy trials

in Alzheimer's disease. Neuropsychopharmacol. 2014, 39, 189–201. 18.

Zetterberg, H.; Tullhӧg, K.; Hansson, O.; Minthon, L.; Londos, E.; Blennow, K. Low

incidence of post-lumbar puncture headache in 1,089 consecutive memory clinic patients. Eur. Neurol. 2010, 63, 326–330.


43


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

19.

Page 44 of 73

DeKosky, S.T.; Marek, K. Looking backward to move forward: early detection of

neurodegenerative disorders. Science 2003, 302, 830–834. 20.

Reiman, E. M.; Jagust, W. J. Brain imaging in the study of Alzheimer's disease.

Neuroimage 2012, 61, 505–516. 21.

Zhou, M.; Wang, X.; Liu, Z.; Yu, L.; Hu, S.; Chen, L.; Zeng, W. Advances of molecular

imaging probes for the diagnosis of Alzheimer's disease. Curr. Alzheimer Res. 2014, 11, 221– 231. 22.

P. E. Valk, D. L. Bailey, D. W. Townsend, M. N. Maisey, Positron emission tomography:

basic science, Springer, New York, 2005. 23.

Matthews, P. M.; Rabiner, E. A.; Passchier, J.; Gunn, R. N. Positron emission

tomography molecular imaging for drug development. Br. J. Clin. Pharmacol. 2011, 73, 175186. 24.

Piel, M.; Vernaleken, I.; Rösch, F. Positron emission tomography in CNS drug discovery

and drug monitoring. J. Med. Chem. 2014, 57, 9232-9258. 25.

Kung, H. F. The β-amyloid hypothesis in Alzheimer's disease: seeing is believing ACS

Med. Chem. Lett. 2012, 3, 265–267. 26.

Mason, N. S.; Mathis, C. A.; Klunk, W. E. Positron emission tomography radioligands

for in vivo imaging of Aβ plaques J. Label. Compd. Radiopharm. 2013, 56, 89–95. 27.

Noёl, S.; Cadet, S.; Gras, E.; Hureau, C. The benzazole scaffold: a SWAT to combat

Alzheimer's disease. Chem. Soc. Rev. 2013, 42, 7747–7762.


44

Page 45 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28.

Rowe, C. C.; Villemagne, V. L. Amyloid imaging with PET in early Alzheimer disease

diagnosis. Med. Clin. North Am. 2013, 97, 377–398. 29.

Holland, J. P.; Liang, S. H.; Rotstein, B. H.; Collier, T. L.; Stephenson, N. A.; Greguric,

I.; Vasdev, N. Alternative approaches for PET radiotracer development in Alzheimers disease: imaging beyond plaque. J. Label. Compd. Radiopharm. 2014, 57, 323–331. 30.

Klunk, W. E.; Engler, H.; Nordberg, A.; Wang, Y.; Blomqvist, G.; Holt, D. P.;

Bergstrӧm, M.; Savitcheva, I.; Huang, G. F.; Estrada, S.; Ausén, B.; Debnath, M. L.; Barletta, J.; Price, J. C.; Sandell, J.; Lopresti, B. J.; Wall, A.; Koivisto, P.; Antoni, G.; Mathis, C. A.; Långström, B. Imaging brain amyloid in Alzheimer's disease with Pittsburgh compound-B Ann. Neurol. 2004, 55, 306–319. 31.

Cohen, A. D.; Rabinovici, G. D.; Mathis, C. A.; Jagust, W. J.; Klunk, W. E.; Ikonomovic,

M. D. Using Pittsburgh compound B for in vivo PET imaging of fibrillar amyloid-beta. Adv. Pharmacol. 2012, 64, 27–81. 32.

Zhang, W.; Oya, S.; Kung, M.-P.; Hou, C.; Maier, D. L.; Kung, H. F. F-18

Polyethyleneglycol stilbenes as PET imaging agents targeting Aβ aggregates in the brain. Nucl. Med. Biol. 2005, 32, 799–809. 33.

Dollé, F. Carbon-11 and fluorine-18 chemistry devoted to molecular probes for imaging

the brain with positron emission tomography J. Label. Compd. Radiopharm. 2013, 56, 65–67. 34.

Serdons, K.; Verduyckt, T.; Vanderghinste, D.; Cleynhens, J.; Borghgraef, P.;

Vermaelen, P.; Terwinghe, C.; Van Leuven, F.; Van Laere, K.; Kung, H.; Bormans, G.; Verbruggen, A. Synthesis of 18F-labeled 2-(4'-fluorophenyl)-1,3-benzothiazole and evaluation as


45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 73

amyloid imaging agent in comparison with [11C]-PIB. Bioorg. Med. Chem. Lett. 2009, 19, 602– 605. 35.

Jureus, A.; Swahn, B.-M.; Sandell, J.; Jeppsson, F.; Johnson, A. E.; Johnstroem, P.;

Neelissen, J. A. M.; Sunnemark, D.; Farde, L.; Svensson, S. P. S. Characterization of AZD4694, a novel fluorinated Aβ plaque neuroimaging PET radioligand. J. Neurochem. 2010, 114, 784– 794. 36.

Landau, S. M.; Thomas, B. A.; Thurfjell, L.; Schmidt, M.; Margolin, R.; Mintun, M.;

Pontecorvo, M.; Baker, S. L.; Jagust, W. J. Amyloid PET imaging in Alzheimer’s disease: a comparison of three radiotracers. Eur. J. Nucl. Med. Mol. Imaging 2014, 41, 1398-1407. 37.

Rowe, C. C.; Pejoska, S.; Mulligan, R. S.; Jones, G.; Chan, J. G.; Svensson, S.; Cselényi,

Z.; Masters, C. L.; Villemagne, V. L. Head-to-head comparison of

11

C-PiB and

18

F-AZD4694

(NAV4694) for β-amyloid imaging in aging and dementia. J. Nucl. Med. 2013, 54, 880–886. 38.

Kepe, V.; Moghbel, M. C.; Langström, B.; Zaidi, H.; Vinters, H. V.; Huang, S.-C.;

Satyamurthy, N.; Doudeet, D.; Mishani, E.; Cohen, R. M.; Hoilund-Carlsen, P. F.; Alavi, A.; Barrio, J. R. Amyloid-β positron emission tomography imaging probes: a critical review. J. Alzheimers Dis. 2013, 36, 613-631. 39.

Eckroat, T. J.; Mayhoub, A. S.; Garneau-Tsodikova, S. Amyloid-β probes: review of

structure-activity and brain-kinetics relationships. Beilstein J. Org. Chem. 2013, 9, 1012-1044. 40.

Villemagne, V. L.; Furumoto, S.; Fodero-Tavoletti, M.; Harada, R.; Mulligan, R. S.;

Kudo, Y.; Masters, C. L.; Yanai, K.; Rowe, C. C.; Okamura, N. The challenges of tau imaging. Future Neurol. 2012, 7, 409-421.


46

Page 47 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


41.

Sperling, R. A.; Aisen, P. S.; Beckett, L. A.; Bennett, D. A.; Craft, S.; Fagan, A. M.;

Iwatsubo, T.; Jack, C. R. Jr.; Kaye, J.; Montine, T. J.; Park, D. C.; Reiman, E. M.; Rowe, C. C.; Siemers, E.; Stern, Y.; Yaffe, K.; Carrillo, M. C.; Thies, B.; Morrison-Bogorad, M.; Wagster, M. V.; Phelps, C. H. Toward defining the preclinical stages of Alzheimer’s disease: recommendations from the National Institute on Aging and the Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer's disease. Alzheimers Dement. 2011, 7, 280– 292. 42.

www.fda.gov

43.

Maccioni, R. B.; Farías, G.; Morales, I.; Navarrete, L. The revitalized tau hypothesis on

Alzheimer's disease. Arch. Med. Res. 2010, 41, 226−231. 44.

Arriagada, P. V.; Growdon, J. H.; Hedley-Whyte, E. T.; Hyman, B. T. Neurofibrillary

tangles but not senile plaques parallel duration and severity of Alzheimer's disease. Neurology 1992, 42, 631−639. 45.

Ke, Y. D.; Suchowerska, A. K.; van der Hoven, J.; De Silva, D. M.; Wu, C. W.; van

Eersel, J.; Ittner, A.; Ittner, L. M. Lessons from tau-deficient mice. Int. J. Alzheimers Dis. 2012, 2012, Article ID 873270, 8 pages. 46.

Medeiros, R.; Baglietto-Vargas, D.; LaFerla, F. M. The role of tau in Alzheimer’s disease

and related disorders. CNS Neurosci. Ther. 2011, 17, 514−524. 47.

Villemagne, V. L.; Okamura, N. In vivo tau imaging: obstacles and progress. Alzheimers

Dement. 2014, 10, S254−S264.


47


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

48.

Page 48 of 73

Shah, M.; Catafau, A. M. Molecular imaging insights into neurodegeneration: focus on

tau PET radiotracers. J. Nucl. Med. 2014, 55, 871−874. 49.

Mach, R. H. New targets for the development of PET tracers for imaging

neurodegeneration in Alzheimer disease. J. Nucl. Med. 2014, 55, 1221-1224. 50.

Zimmer, E. R.; Leuzy, A.; Gauthier, S.; Rosa-Neto, P. Developments in tau PET

imaging. Can. J. Neurol. Sci. 2014, 41, 547-553. 51.

Villemagne, V. L.; Fodero-Tavoletti, M. T.; Masters, C. L.; Rowe, C. C. Tau imaging:

early progress and future directions. Lancet Neurol. 2015, 14, 114-124. 52.

Okamura, N.; Harada, R.; Furumoto, S.; Arai, H.; Yanai, K.; Kudo, Y. Tau PET imaging

in Alzheimer’s disease. Curr. Neurol. Neurosci. Rep. 2014, 14: 500. 53.

Weingarten, M. D.; Lockwood, A. H.; Hwo, S.-Y.; Kirschner, M. W. A protein factor

essential for microtubule assembly. Proc. Nat. Acad. Sci. USA 1975, 72, 1858−1862. 54.

Andreadis, A..; Broderick, J. A.; Kosik, K. S. Relative exon affinities and suboptimal

splice site signals lead to non-equivalence of two cassette exons. Nucleic Acids Res. 1995, 23, 3585−3593. 55.

Andreadis, A.; Brown, W. M.; Kosik, K. S. Structure and novel exons of the human tau

gene, Biochemistry 1992, 31, 10626−10633. 56.

Buée, L.; Bussière, T.; Buée-Scherrerb, V.; Delacourte, A.; Hof, P. R. Tau protein

isoforms, phosphorylation and role in neurodegenerative disorders. Brain Res. Rev. 2000, 33, 95–130.


48

Page 49 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


57.

Levy, S. F.; Leboeuf, A. C.; Massie, M. R.; Jordan, M. A.; Wilson, L.; Feinstein, S. C.

Three- and four-repeat tau regulate the dynamic instability of two distinct microtubule subpopulations in qualitatively different manners. Implications for neurodegeneration. J. Biol. Chem. 2005, 280, 13520−13528. 58.

Jeganathan, S.; Hascher, A.; Chinnathambi, S.; Biernat, J.; Mandelkow, E. M.;

Mandelkow, E. Proline-directed pseudophosphorylation at AT8 and PHF1 epitopes induces a compaction of the paperclip folding of tau and generates a pathological (MC-1) conformation. J. Biol. Chem. 2008, 283, 32066−32076. 59.

Mukrasch, M. D.; Bibow, S.; Korukottu, J.; Jeganathan, S.; Biernat, J.; Griesinger, C.;

Mandelkow, E.; Zweckstetter, M. Structural polymorphism of 441-residue tau at single residue resolution. PLoS Biol. 2009, 7, e34. 60.

von Bergen, M.; Friedhoff, P.; Biernat, J.; Heberle, J.; Mandelkow, E. M.; Mandelkow,

E. Assembly of tau protein into Alzheimer paired helical filaments depends on a local sequence motif ((306)VQIVYK(311)) forming β structure. Proc. Natl. Acad. Sci. USA 2000, 97, 5129−5134. 61.

Von Bergen, M.; Barghorn, S.; Li, L.; Marx, A.; Biernat, J.; Mandelkow, E. M.;

Mandelkow, E. Mutations of tau protein in frontotemporal dementia promote aggregation of paired helical filaments by enhancing local β-structure. J. Biol. Chem. 2001, 276, 48165−48174. 62.

Martin, L.; Latypova, X.; Terro, F. Post-translational modifications of tau protein:

implications for Alzheimer’s disease. Neurochem. Int. 2011, 58, 458−471.


49


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

63.

Page 50 of 73

Iqbal, K.; Gong, C.-X.; Liu, F. Microtubule-associated protein tau as a therapeutic target

in Alzheimer’s disease. Expert Opin. Ther. Targets 2014, 18, 307−318. 64.

Spillantini, M. G.; Goedert, M. Tau pathology and neurodegeneration. Lancet Neurol.

2013; 12: 609−622. 65.

Clavaguera, F.; Grueninger, F.; Tolnay, M. Intercellular transfer of tau aggregates and

spreading of tau pathology: implications for therapeutic strategies. Neuropharmacology 2014, 76, 9−15. 66.

Bulic, B.; Pickhardt, M.; Mandelkow, E. Progress and developments in tau aggregation

inhibitors for Alzheimer disease. J. Med. Chem. 2013, 56, 4135−4155. 67.

Ramachandran, G.; Udgaonkar, J. B. Mechanistic studies unravel the complexity inherent

in tau aggregation leading to Alzheimer’s disease and the tauopathies. Biochemistry 2013, 52, 4107−4126. 68.

Sevcik, J.; Skrabana, R.; Dvorsky, R.; Csokova, N.; Iqbal, K.; Novak, M. X-ray structure

of the PHF core C terminus: insight into the folding of the intrinsically disordered protein tau in Alzheimer’s disease. FEBS Lett. 2007, 581, 5872–5878. 69.

Skrabana, R.; Dvorsky, R.; Sevcik, J.; Novak, M. Monoclonal antibody MN423 as a

stable mold facilitates structure determination of disordered tau protein. J. Struct. Biol. 2010, 171, 74–81.


50

Page 51 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70.

Janke, C.; Beck, M.; Stahl, T.; Holzer, M.; Brauer, K.; Bigl, V.; Arendt, T. Phylogenetic

diversity of the expression of the microtubule-associated protein tau: implications for neurodegenerative disorders. Mol. Brain Res. 1999, 68, 119–128. 71.

Noble, W.; Hanger, D.P.; Gallo, J.-M. Transgenic mouse models of tauopathy in drug

discovery. CNS Neurol. Disord. Drug Targets 2010, 9, 403–428. 72.

Denk, F.; Wade-Martins, R.; Knock-out and transgenic mouse models of tauopathies.

Neurobiol. Aging 2009, 30, 1–13. 73.

Roberson, E. D. Mouse models of frontotemporal dementia. Ann. Neurol. 2012, 72, 837–

849. 74.

Peeraer, E.; Bottelbergs, A.; Van Kolen, K.; Stancu, I. C.; Vasconcelos, B.; Mahieu, M.;

Duytschaever, H.; Ver Donck, L.; Torremans, A.; Sluydts, E.; Van Acker, N.; Kemp, J. A.; Mercken, M.; Brunden, K. R.; Trojanowski, J. Q.; Dewachter, I.; Lee, V. M.; Moechars, D. Intracerebral injection of preformed synthetic tau fibrils initiates widespread tauopathy and neuronal loss in the brains of tau transgenic mice. Neurobiol. Dis. 2015, 73, 83–95. 75.

Pike, V. W. PET radiotracers: crossing the blood-brain barrier and surviving metabolism.

Trends Pharmacol. Sci. 2009, 30, 431–440. 76.

Laruelle, M.; Slifstein, M.; Huang, Y. Relationships between radiotracer properties and

image quality in molecular imaging of the brain with positron emission tomography. Mol. Imaging Biol. 2003, 5, 363-375.


51


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

77.

Page 52 of 73

Zhang, L.; Villalobos, A.; Beck, E. M.; Bocan, T.; Chappie, T. A.; Chen, L.; Grimwood,

S.; Hech, S. D.; Helal, C. J.; Hou, X.; Humphrey, J. M.; Lu, J.; Skaddan, M. B.; McCarthy, T. J.; Verhoest, P. R.; Wager, T. T.; Zasadny, K. Design and selection parameters to accelerate the discovery of novel central nervous system positron emission tomography (PET) ligands and their application in the development of a novel phosphodiesterase 2A PET ligand. J. Med. Chem. 2013, 56, 4568-4579. 78.

Honer, M.; Gobbi, L.; Martarello, L.; Comley, R. A. Radioligand development for

molecular imaging of the central nervous system with positron emission tomography. Drug Discov. Today 2014, 19, 1936-1944. 79.

Patel, S.; Gibson, R. In vivo site-directed radiotracers: a minireview. Nucl. Med. Biol.

2008, 35, 805-815. 80.

Celen, S.; Koole, M.; De Angelis, M.; Sannen, I.; Chitneni, S. K.; Alcazar, J.;

Dedeurwaerdere, S.; Moechars, D.; Schmidt, M.; Verbruggen, A.; Langlois, X.; Van Laere, K.; Andrés, J. I.; Bormans, G. Preclinical evaluation of

18

F-JNJ41510417 as a radioligand for PET

imaging of phosphodiesterase-10A in the brain. J. Nucl. Med. 2010, 51, 1584-1591. 81.

Andrés, J. I.; Alcázar, J.; Cid, J. M.; De Angelis, M.; Iturrino, L.; Langlois, X.;

Lavreysen, H.; Trabanco, A. A.; Celen, S.; Bormans, G. Synthesis, evaluation and radiolabeling of new potent positive allosteric modulators of the metabotropic glutamate receptor 2 as potential tracers for positron emission tomography imaging. J. Med. Chem. 2012, 55, 8685-8699.


52

Page 53 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


82.

Clinton, L. K.; Blurton-Jones, M.; Myczek, K.; Trojanowski, J. Q.; La Ferla, F. M.

Synergistic interactions between Aβ, tau, and α-synuclein: acceleration of neuropathology and cognitive decline. J. Neurosci. 2010, 30, 7281-7289. 83.

Irwin, D. J.; Lee, V. M.-Y.; Trojanowski, J. Q. Parkinson's disease dementia:

convergence of α-synuclein, tau and amyloid-β pathologies. Nat. Rev. Neurosci. 2013, 14, 626636. 84.

Wegmann, S.; Jung, Y.-J.; Chinnathambi, S.; Mandelkow, E.-M.; Mandelkow, E.;

Muller, D. J. Human tau isoforms assemble into ribbon-like fibrils that display polymorphic structure and stability. J. Biol. Chem. 2010, 285, 27302-27313. 85.

Okamura, N.; Suemoto, T.; Furumoto, S.; Suzuki, M.; Shimadzu, H.; Akatsu, H.;

Yamamoto, T.; Fujiwara, H.; Nemoto, M.; Maruyama, M.; Arai, H.; Yanai, K.; Sawada, T.; Kudo, Y. Quinoline and benzimidazole derivatives: candidate probes for in vivo imaging of tau pathology in Alzheimer's disease. J. Neurosci. 2005, 25, 10857–10862. 86.

Honson, N. S.; Johnson, R. L.; Huang, W.; Inglese, J.; Austin, C. P.; Kuret, J.

Differentiating Alzheimer disease-associated aggregates with small molecules. Neurobiol. Dis. 2007, 28, 251–260. 87.

Matsumura, K.; Ono, M.; Hayashi, S.; Kimura, H.; Okamoto, Y.; Ihara, M.; Takahashi,

R.; Moric, H.; Saji, H. Phenyldiazenyl benzothiazole derivatives as probes for in vivo imaging of neurofibrillary tangles in Alzheimer’s disease brains. Med. Chem. Commun. 2011, 2, 596–600. 88.

Matsumura, K.; Ono, M.; Kimura, H.; Ueda, M.; Nakamoto, Y.; Togashi, K.; Okamoto,

Y.; Ihara, M.; Takahashi, R.; Saji, H.

18

F-Labeled phenyldiazenyl benzothiazole for in vivo


53


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 73

imaging of neurofibrillary tangles in Alzheimer's disease brains. ACS Med. Chem. Lett. 2012, 3, 58–62. 89.

Matsumura, K.; Ono, M.; Yoshimura, M.; Kimura, H.; Watanabe, H.; Okamoto, Y.;

Ihara, M.; Takahashi, R.; Saji, H. Synthesis and biological evaluation of novel styryl benzimidazole derivatives as probes for imaging of neurofibrillary tangles in Alzheimer’s disease. Bioorg. Med. Chem. 2013, 21, 3356–3362. 90.

Harada, R.; Okamura, N.; Furumoto, S.; Yoshikawa, T.; Arai, H.; Yanai, K.; Kudo, Y.

Use of a benzimidazole derivative BF-188 in fluorescence multispectral imaging for selective visualization of tau protein fibrils in the Alzheimer's disease brain. Mol. Imaging Biol. 2014, 16, 19–27. 91.

Rojo, L. E.; Alzate-Morales, J.; Saavedraa, I. N.; Davies, P.; Maccioni, R. B. Selective

interaction of lansoprazole and astemizole with tau polymers: potential new clinical use in diagnosis of Alzheimer’s disease. J. Alzheimers Dis. 2010, 19, 573–589. 92.

Riss, P. J.; Brichard, L.; Ferrari, V.; Williamson, D. J.; Fryer, T. D.; Hong, Y. T.; Baron,

J.-C.; Aigbirhio, F. I. Radiosynthesis and characterization of astemizole derivatives as lead compounds toward PET imaging of τ-pathology. Med. Chem. Commun. 2013, 4, 852–855. 93.

Shao, X.; Carpenter, G. M.; Desmond, T. J.; Sherman, P.; Quesada, C. A.; Fawaz, M.;

Brooks, A. F.; Kilbourn, M. R.; Albin, R. L.; Frey, K. A.; Scott, P. J. H. Evaluation of [11C]N‑ methyl lansoprazole as a radiopharmaceutical for PET imaging of tau neurofibrillary tangles. ACS Med. Chem. Lett. 2012, 3, 936–941.


54

Page 55 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


94.

Fawaz, M. V.; Brooks, A. F.; Rodnick, M. E.; Carpenter, G. M.; Shao, X.; Desmond, T.

J.; Sherman, P.; Quesada, C. A.; Hockley, B. G.; Kilbourn, M. R.; Albin, R. L.; Frey, K. A.; Scott, P. J. H. High affinity radiopharmaceuticals based upon lansoprazole for PET imaging of aggregated tau in Alzheimer’s disease and progressive supranuclear palsy: synthesis, pre-clinical evaluation and lead selection. ACS Chem. Neurosci. 2014, 5, 718-730. 95.

Ono, M.; Hayashi, S.; Matsumura, K.; Kimura, H.; Okamoto, Y.; Ihara, M.; Takahashi,

R.; Mori, H.; Saji, H. Rhodanine and thiohydantoin derivatives for detecting tau pathology in Alzheimer’s brains. ACS Chem. Neurosci. 2011, 2, 269–275. 96.

Saji, H.; Ono, M.; Seki, I. Radioactive iodine labeled organic compound or salt thereof.

International Patent 2011. WO2011/108236 A1. 97.

Ballatore, C.; Smith III, A. B.; Lee, V. M.-Y.; Trojanowski, J. Q.; Brunden, K. R.

Aminothienopyridazines as imaging probes of tau pathology: a patent evaluation of WO2013090497. Expert Opin. Ther. Patents 2014, 24, 355–360. 98.

Jones, C.; Glaser, M. E.; Wynn, D.; Nairne, J.; Mokkapati, U. P.; Newington, I. M.;

Rangaswamy, C.; Jose, J.; Johansson, S. Heterocyclic compounds as imaging probes of tau pathology. International Patent 2013. WO2013/090497. 99.

Bulic, B.; Pickhardt, M.; Khlistunova, I.; Biernat, J.; Mandelkow, E. M.; Mandelkow, E.;

Waldmann, H. Rhodanine-based tau aggregation inhibitors in cell models of tauopathy. Angew. Chem., Int. Ed. Engl. 2007, 46, 9215–9219. 100.

Ballatore, C.; Brunden, K. R.; Piscitelli, F.; James, M. J.; Crowe, A.; Yao, Y.; Hyde, E.;

Trojanowski, J. Q.; Lee, V. M.-Y.; Smith III, A. B. Discovery of brain-penetrant, orally


55


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 73

bioavailable aminothienopyridazine inhibitors of tau aggregation. J. Med. Chem. 2010, 53, 3739–3747. 101.

Crowe, A.; James, M. J.; Lee, V. M.-Y.; Smith III, A. B.; Trojanowski, J. Q.; Ballatore,

C.; Brunden, K. R. Aminothienopyridazines and methylene blue affect tau fibrillization via cysteine oxidation. J. Biol. Chem. 2013, 288, 11024–11037. 102.

Gobbi, L.; Knust, H.; Koblet, A. Imidazo[2,1]thiazol-3-one derivatives useful as

diagnostic agents for Alzheimer’s disease. International Patent 2014. WO2014/026881 A1. 103.

Taghavi, A.; Nasir, S.; Pickhardt, M.; Heyny-von Hauβen, R.; Mall, G.; Mandelkow, E.;

Mandelkow, E.-M.; Schmidt, B. N-Benzylidene-benzohydrazides as novel and selective tau-PHF ligands. J. Alzheimers Dis. 2011, 27, 835–843. 104.

http://clinicaltrials.gov/ct2/show/record/NCT02187627

105.

Altman, M. D.; Fischer, C.; Katz, J. D.; Williams, T. M.; Zhang, X.-F.; Zhou, H.

Isotopically labeled biaryl urea compounds useful as tau tracers. International Patent 2013. WO 2013/181075 A1. 106.

Kemp, S. J.; Storey, L. J.; Storey, J. M. D.; Rickard, J.; Harrington, C. R.; Wischik, C.

M.; Clunas, S.; Heinrich, T. K. Ligands for aggregates tau molecules. International Patent 2010. WO 2010/034982 A1. 107.

Boländer, A.; Kieser, D.; Voss, C.; Bauer, S.; Schön, C.; Burgold, S.; Bittner, T.; Hölzer,

J.; Heyny-von Haußen, R.; Mall, G.; Goetschy, V.; Czech, C.; Knust, H.; Berger, R.; Herms, J.; Hilger, I.; Schmidt, B. Bis(arylvinyl)pyrazines, -pyrimidines, and -pyridazines as imaging agents


56

Page 57 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for tau fibrils and β-amyloid plaques in Alzheimer’s disease models. J. Med. Chem. 2012, 55, 9170−9180. 108.

Barrio, J. R.; Huang, S. C.; Cole, G.; Satyamurthy, N.; Petric, A.; Phelps, M. E.; Small,

G. PET imaging of tangles and plaques in Alzheimer disease with a highly hydrophobic probe. J. Labelled Compd. Radiopharm. 1999, 42, S194–195. 109.

Small, G. W.; Kepe, V.; Ercoli, L. M.; Siddarth, P.; Bookheimer, S. Y.; Miller, K. J.;

Lavretsky, H.; Burggren, A. C.; Cole, G. M.; Vinters, H. V.; Thompson, P. M.; Huang, S.-C.; Satyamurthy, N.; Phelps, M. E.; Barrio, J. R. PET of brain amyloid and tau in mild cognitive impairment. N. Engl. J. Med. 2006, 355, 2652–2663 110.

Shin, J.; Kepe, V.; Barrio, J. R. Small, G. W. The merits of FDDNP-PET imaging in

Alzheimer’s disease. J. Alzheimers Dis. 2011, 26, 135–145. 111.

Nelson, L. D.; Siddarth, P.; Kepe, V.; Scheibel, K. E.; Huang, S.-C.; Barrio, J. R.; Small,

G. W. Positron emission tomography of brain beta-amyloid and tau levels in adults with Down syndrome. Arch. Neurol. 2011, 68, 768–774. 112.

Small, G. W.; Kepe, V.; Siddarth, P.; Ercoli, L. M.; Merrill, D. A.; Donoghue, N.;

Bookheimer, S. Y.; Martinez, J.; Omalu, B.; Bailes, J.; Barrio, J. R. PET scanning of brain tau in retired National Football League players: preliminary findings. Am. J. Geriatr. Psychiatry 2013, 21, 138–44. 113.

Thompson, P. W.; Ye, L.; Morgenstern, J. L.; Sue, L.; Beach, T. G.; Judd, D. J.; Shipley,

N. J.; Libri, V.; Lockhart, A. Interaction of the amyloid imaging tracer FDDNP with hallmark Alzheimer's disease pathologies. J. Neurochem. 2009, 109, 623–630.


57


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

114.

Page 58 of 73

Fodero-Tavoletti, M. T.; Okamura, N.; Furumoto, S.; Mulligan, R. S.; Connor, A. R.;

McLean, C. A.; Cao, D.; Rigopoulos, A.; Cartwright, G. A.; O’Keefe, G.; Gong, S.; Adlard, P. A.; Barnham, K. J.; Rowe, C. C.; Masters, C. L.; Kudo, Y.; Cappai, R.; Yanai, K.; Villemagne, V. L.

18

F-THK-523: a novel in vivo tau imaging ligand for Alzheimer’s disease. Brain 2011,

134, 1089–1100. 115.

Okamura, N.; Furumuto, S.; Harada, R.; Tago, T.; Yoshikawa, T.; Fodero-Tavoletti, M.;

Mulligan, R. S.; Villemagne, V. L.; Akatsu, H.; Yamamoto, T.; Arai, H.; Iwata, R.; Yanai, K.; Kudo, Y. Novel 18F-labeled arylquinoline derivatives for noninvasive imaging of tau pathology in Alzheimer disease. J. Nucl. Med. 2013, 54, 1420–1427. 116.

Villemagne, V. L.; Furumoto, S.; Fodero-Tavoletti, M. T.; Mulligan, R. S.; Hodges, J.;

Harada, R.; Yates, P.; Piguet, O.; Pejoska, S.; Doré, V.; Yanai, K.; Masters, C. L.; Kudo, Y.; Rowe, C. C.; Okamura, N. In vivo evaluation of a novel tau imaging tracer for Alzheimer's disease. Eur. J. Nucl. Med. Mol. Imaging 2014, 41, 816–826. 117.

Okamura, N.; Furumoto, S.; Fodero-Tavoletti, M. T.; Mulligan, R. S.; Harada, R.; Yates,

P.; Pejoska, S.; Kudo, Y.; Masters, C. L.; Yanai, K.; Rowe, C. C; Villemagne, V. L. Noninvasive assessment of Alzheimer's disease neurofibrillary pathology using

18

F-THK5105 PET.

Brain 2014, 137, 1762–1771. 118.

Bakhti, M.; Aggarwal, S.; Simons, M. Myelin architecture: zippering membranes tightly

together. Cell. Mol. Life Sci. 2014, 71, 1265–1277.


58

Page 59 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


119.

Tago, T.; Furumoto, S.; Okamura, N.; Harada, R.; Ishikawa, Y.; Arai, H.; Yanai, K.;

Iwata, R.; Kudo, Y. Synthesis and preliminary evaluation of 2-arylhydroxyquinoline derivatives for tau imaging. J. Label Compd. Radiopharm. 2014, 57, 18–24. 120.

Okamura, N.; Furumoto, S.; Harada, R.; Tago, T.; Iwata, R.; Tashiro, M.; Furukawa. K.;

Arai, H.; Yanai, K.; Kudo,Y. Characterization of 18F-THK-5351, a novel PET tracer for imaging tau pathology in Alzheimer’s disease. Eur. J. Nucl. Med. Mol. Imaging 2014, 41, S260. 121.

Maruyama, M.; Shimada, H.; Suhara, T.; Shinotoh, H.; Ji, B.; Maeda, J.; Zhang, M.-R.;

Trojanowski, J. Q.; Lee, V. M.-Y.; Ono, M.; Masamoto, K.; Takano, H.; Sahara, N.; Iwata, N.; Okamura, N.; Furumoto, S.; Kudo, Y.; Chang, Q.; Saido, T. C.; Takashima, A.; Lewis, J.; Jang, M.-K.; Aoki, I.; Ito, H.; Higuchi, M. Imaging of tau pathology in mouse model and in Alzheimer patients compared to normal controls. Neuron 2013, 79, 1–15. 122.

Hashimoto, H.; Kawamura, K.; Igarashi, N.; Takei, M.; Fujishiro, T.; Aihara, Y.; Shiomi,

S.; Muto, M.; Ito, T.; Furutsuka, K.; Yamasaki, T.; Yui, J.; Xie, L.; Ono, M.; Hatori, A.; Nemoto, K.; Suhara, T.; Higuchi, M.; Zhang, M.-R. Radiosynthesis, photoisomerization, biodistribution, and metabolite analysis of

11

C-PBB3 as a clinically useful PET probe for imaging of tau

pathology. J. Nucl. Med 2014, 55, 1532–1538. 123.

Lei, Y.; Yu, L.; Zhou, B.; Zhu, C.; Wen, Z.; Lin, S. H. Landscapes of four-enantiomer

conical intersections for photoisomerization of stilbene: CASSCF calculation. J. Phys. Chem. A 2014, 118 , 9021–903. 124.

Waldeck, D. H. Photoisomerization dynamics of stilbene. Chem Rev. 1991, 91, 415–436.


59


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

125.

Page 60 of 73

Szardenings, A. K.; Kolb, H. C.; Walsh, J. C.; Chen, G.; Gangadharmath, U. B.; Kasi, D.;

Liu, C.; Sinha, A.; Wang, E.; Yu, C.; Zhang, W.; Chen, K.; Mocharla, V. P.; Scott, P. J. H. Imaging agents for detecting neurological dysfunction. International Patent 2012 US 2012/0302755 A1. 126.

Cashion, D. K.; Cheng, G.; Kasi, D.; Kolb, H. C. Imaging agents for detecting

neurological disorders. International Patent 2011 WO 2011/119565 A1. 127.

Zhang, W.; Arteaga, J.; Cashion, D. K.; Chen, G.; Gangadharmath, U.; Gomez, L. F.;

Kasi, D.; Lam, C.; Liang, Q.; Liu, C.; Mocharla, V. P.; Mu, F.; Sinha, A.; Szardenings, A. K.; Wang, E.; Walsh, J. C.; Xia, C.; Yu, C.; Zhao, T.; Kolb, H. C. A highly selective and specific PET tracer for imaging of tau pathologies J. Alzheimers Dis. 2012, 31, 601–612. 128.

Xia, C.-F.; Arteaga, J.; Chen, G.; Gangadharmath, U.; Gomez, L. F.; Kasi, D.; Lam, C.;

Liang, Q.; Liu, C.; Mocharla, V. P.; Mu, F.; Sinha, A.; Su, H.; Szardenings, A. K.; Walsh, J. C.; Wang, E.; Yu, C.; Zhang, W.; Zhao, T.; Kolb, H. C. [18F]T807, a novel tau positron emission tomography imaging agent for Alzheimer’s disease. Alzheimers Dement. 2013, 9, 666–676. 129.

Radde, R.; Bolmont, T.; Kaeser, S. A.; Coomaraswamy, J.; Lindau, D.; Stoltze, L.;

Calhoun, M. E.; Jaeggi, F.; Wolburg, H.; Gengler, S.; Haass, C.; Ghetti, B.; Czech, C.; Hoelscher, C.; Mathews, P. M.; Jucker, M. Aβ42-driven cerebral amyloidosis in transgenic mice reveals early and robust pathology. EMBO Rep. 2006, 7, 940-946. 130.

Chien, D. T.; Bahri, S.; Szardenings, A. K.; Walsh, J. C.; Mu, F.; Su, M.-Y.; Shankle, W.

R.; Elizarov, A.; Kolb, H. C. Early clinical PET imaging results with the novel PHF-tau radioligand [F-18]-T807. J. Alzheimers Dis. 2013, 34, 457–468.


60

Page 61 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


131.

Mintun, M.; Schwarz, A.; Joshi, A.; Shcherbinin, S.; Chien, D.; Elizarov, A.; Su, M.-Y.;

Shankle, W.; Pontecorvo, M.; Tauscher, J.; Skovronsky, D.; Kolb, H. C. Exploratory analyses of regional human brain distribution of the PET tau tracer F18-labeled T807 (AV-1541) in subjects with normal cognitive function or cognitive impairment thought to be due to AD. Alzheimers Dement. 2013, 9, P842. 132.

Kolb,

H.C.

Human

amyloid

imaging

conference,

2013,

www.alzforum.org/news/conference-coverage/hai-spotlight-tau-tracers-human-amyloidimaging-meeting 133.

Chien, D. T.; Katrin, A.; Bahri, S.; Walsh, J. C.; Mu, F.; Xia, C.; Shankle, W. R.; Lerner,

A. J.; Su, M.-Y.; Elizarov, A.; Kolb, H. C. Early clinical PET imaging results with the novel PHF-tau radioligand [F18]-T808. J. Alzheimers Dis. 2014, 38, 171–184. 134.

Kolb, H. C.; Attardo, G.; Mintun, M.; Chien, D.; Elizarov, A.; Conti, P.; Miller, C.; Joshi,

A.; Skovronsky, D. First case report: image to autopsy correlation for tau imaging with [18F]T808 (AV-680). Alzheimers Dement 2013, 9, P844–845. 135.

Landau, M.; Sawaya, M. R.; Faull, K. F.; Laganowsky, A.; Jiang, L.; Sievers, S. A.; Liu,

J.; Barrio, J. R.; Eisenberg, D. Towards a pharmacophore for amyloid. PLoS Biol. 2011, 9, e1001080.


61


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 62 of 73

Insert Table of Contents Graphic and Synopsis Here


62

Page 63 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


178x84mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

254x190mm (96 x 96 DPI)


Page 64 of 73

Page 65 of 73


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

156x192mm (96 x 96 DPI)


Page 66 of 73

Page 67 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


161x192mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

257x140mm (96 x 96 DPI)


Page 68 of 73

Page 69 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


167x45mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

257x128mm (96 x 96 DPI)


Page 70 of 73

Page 71 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


402x243mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

190x142mm (96 x 96 DPI)


Page 72 of 73

Page 73 of 73

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


338x190mm (96 x 96 DPI)


Tau Positron Emission Tomography (PET) Imaging: Past, Present, and

Recommend Documents