Development of small molecule probes for the beta-amyloid protein of Alzheimer's disease

The detection methods currently available for protein aggregation in neurological diseases

Protein aggregation is a pathological feature in various neurodegenerative diseases and is thought to play a crucial role in the onset and progression of neurological disorders. This pathological phenomenon has attracted increasing attention from researchers, but the underlying mechanism has not been fully elucidated yet. Researchers are increasingly interested in identifying chemicals or methods that can effectively detect protein aggregation or maintain protein stability to prevent aggregation formation. To date, several methods are available for detecting protein aggregates, including fluorescence correlation spectroscopy, electron microscopy, and molecular detection methods. Unfortunately, there is still a lack of methods to observe protein aggregation in situ under a microscope. This article reviews the two main aspects of protein aggregation: the mechanisms and detection methods of protein aggregation. The aim is to provide clues for the development of new methods to study this pathological phenomenon.

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

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

Repurposing of the PET Probe Prototype PiB for Label and Radiation-Free CEST MRI Molecular Imaging of Amyloid

Positron emission tomography (PET) probes are specific and sensitive while suffering from radiation risk. It is worthwhile to explore the chemical emission saturation transfer (CEST) effects of the probe prototypes and repurpose them for CEST imaging to avoid radiation. In this study, we used 11C-PiB as an example of a PET probe for detecting amyloid and tested the feasibility of repurposing this PET probe prototype, PiB, for CEST imaging. After optimizing the parameters through preliminary phantom experiments, we used APP/PS1 transgenic mice and age-matched C57 mice for in vivo CEST magnetic resonance imaging (MRI) of amyloid. Furthermore, the pathological assessment was conducted on the same brain slices to evaluate the correlation between the CEST MRI signal abnormality and β-amyloid deposition detected by immunohistochemical staining. In our results, the Z-spectra revealed an apparent CEST effect that peaked at approximately 6 ppm. APP/PS1 mice as young as 9 months injected with PiB showed a significantly higher CEST effect compared to the control groups. The hyperintense region was correlated with the Aβ deposition shown by pathological staining. In conclusion, repurposing the PET probe prototype for CEST MRI imaging is feasible and enables label- and radiation-free detection of the amyloid while maintaining the sensitivity and specificity of the ligand. This study opens the door to developing CEST probes based on PET probe prototypes.

Distinct Heterocyclic Moieties Govern the Selectivity of Thiophene-Vinylene-Based Ligands Towards Aβ or Tau Pathology in Alzheime's Disease

Distinct aggregated proteins are correlated with numerous neurodegenerative diseases and the development of ligands that selectively detect these pathological hallmarks is vital. Recently, the synthesis of thiophene-based optical ligands, denoted bi-thiophene-vinyl-benzothiazoles (bTVBTs), that could be utilized for selective assignment of tau pathology in brain tissue with Alzheime's disease (AD) pathology, was reported. Herein, we investigate the ability of these ligands to selectively distinguish tau deposits from aggregated amyloid-β (Aβ), the second AD associated pathological hallmark, when replacing the terminal thiophene moiety with other heterocyclic motifs. The selectivity for tau pathology was reduced when introducing specific heterocyclic motifs, verifying that specific molecular interactions between the ligands and the aggregates are necessary for selective detection of tau deposits. In addition, ligands having certain heterocyclic moieties attached to the central thiophene-vinylene building block displayed selectivity to aggregated Aβ pathology. Our findings provide chemical insights for the development of ligands that can distinguish between aggregated proteinaceous species consisting of different proteins and might also aid in creating novel agents for clinical imaging of tau pathology in AD.

Thiophene-Based Ligands: Design, Synthesis and Their Utilization for Optical Assignment of Polymorphic-Disease-Associated Protein Aggregates

The development of ligands for detecting protein aggregates is of great interest, as these aggregated proteinaceous species are the pathological hallmarks of several devastating diseases, including Alzheimer's disease. In this regard, thiophene-based ligands have emerged as powerful tools for fluorescent assessment of these pathological entities. The intrinsic conformationally sensitive photophysical properties of poly- and oligothiophenes have allowed optical assignment of disease-associated protein aggregates in tissue sections, as well as real-time in vivo imaging of protein deposits. Herein, we recount the chemical evolution of different generations of thiophene-based ligands, and exemplify their use for the optical distinction of polymorphic protein aggregates. Furthermore, the chemical determinants for achieving a superior fluorescent thiophene-based ligand, as well as the next generation of thiophene-based ligands targeting distinct aggregated species are described. Finally, the directions for future research into the chemical design of thiophene-based ligands that can aid in resolving the scientific challenges around protein aggregation diseases are discussed.

Thiophene-Based Ligands for Histological Multiplex Spectral Detection of Distinct Protein Aggregates in Alzheimer's Disease

The aggregation and accumulation of proteins in the brain is the defining feature of many devastating neurodegenerative diseases. The development of fluorescent ligands that bind to these accumulations, or deposits, is essential for the characterization of these neuropathological lesions. We report the synthesis of donor-acceptor-donor (D-A-D) thiophene-based ligands with different emission properties. The D-A-D ligands displayed selectivity towards distinct disease-associated protein deposits in histological sections from postmortem brain tissue of individuals affected by Alzheimer's disease (AD). The ability of the ligands to selectively identify AD-associated pathological alterations, such as deposits composed of aggregates of the amyloid-β (Aβ) peptide or tau, was reduced when the chemical composition of the ligands was altered. When combining the D-A-D ligands with conventional thiophene-based ligands, superior spectral separation of distinct protein aggregates in AD tissue sections was obtained. Our findings provide the structural and functional basis for the development of new fluorescent ligands that can distinguish between aggregated proteinaceous species, as well as offer novel strategies for developing multiplex fluorescence detection of protein aggregates in tissue sections.

Tackling prion diseases: a review of the patent landscape

Introduction: Prion diseases are a class of rare and fatal neurodegenerative diseases for which no cure is currently available. They are characterized by conformational conversion of cellular prion protein (PrP^C) into the disease-associated 'scrapie' isoform (PrP^Sc). Under an etiological point of view, prion diseases can be divided into acquired, genetic, and idiopathic form, the latter of which are the most frequent.Areas covered: Therapeutic approaches targeting prion diseases are based on the use of chemical and nature-based compounds, targeting either PrP^C or PrP^Sc or other putative player in pathogenic mechanism. Other proposed anti-prion treatments include passive and active immunization strategies, peptides, aptamers, and PrP^C-directed RNA interference techniques. The treatment efficacy has been mainly assessed in cell lines or animal models of the disease testing their ability to reduce prion accumulation.Expert opinion: The assessed strategies focussing on the identification of an efficient anti-prion therapy faced various issues, which go from permeation of the blood brain barrier to immunological tolerance of the host. Indeed, the use of combinatory approaches, which could boost a synergistic anti-prion effect and lower the potential side effects of single treatments and may represent an extreme powerful and feasible way to tackle prion disease.

Strategies for the Molecular Imaging of Amyloid and the Value of a Multimodal Approach

Protein aggregation has been widely implicated in neurodegenerative diseases such as Alzheimer's disease, frontotemporal dementia, Parkinson's disease, and Huntington disease, as well as in systemic amyloidoses and conditions associated with localized amyloid deposits, such as type-II diabetes. The pressing need for a better understanding of the factors governing protein assembly has driven research for the development of molecular sensors for amyloidogenic proteins. To date, a number of sensors have been developed that report on the presence of protein aggregates utilizing various modalities, and their utility demonstrated for imaging protein aggregation in vitro and in vivo. Analysis of these sensors highlights the various advantages and disadvantages of the different imaging modalities and makes clear that multimodal sensors with properties amenable to more than one imaging technique need to be developed. This critical review highlights the key molecular scaffolds reported for molecular imaging modalities such as fluorescence, positron emission tomography, single photon emission computed tomography, and magnetic resonance imaging and includes discussion of the advantages and disadvantages of each modality, and future directions for the design of amyloid sensors. We also discuss the recent efforts focused on the design and development of multimodal sensors and the value of cross-validation across multiple modalities.

Molecular Imaging of Fluorinated Probes for Tau Protein and Amyloid-β Detection

Alzheimer’s disease (AD) is the most common form of dementia and results in progressive neurodegeneration. The incidence rate of AD is increasing, creating a major public health issue. AD is characterized by neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein and senile plaques composed of amyloid-β (Aβ). Currently, a definitive diagnosis of AD is accomplished post-mortem. Thus, the use of molecular probes that are able to selectively bind to NFTs or Aβ can be valuable tools for the accurate and early diagnosis of AD. The aim of this review is to summarize and highlight fluorinated molecular probes that can be used for molecular imaging to detect either NFTs or Aβ. Specifically, fluorinated molecular probes used in conjunction with ¹⁹F MRI, PET, and fluorescence imaging will be explored.

Temporal and anatomical distribution of 18F-flutemetamol uptake in canine brain using positron emission tomography

BACKGROUND

Positron emission tomography (PET) is increasingly being used as an imaging modality for clinical and research applications in veterinary medicine. Amyloid PET has become a useful tool for diagnosing Alzheimer's disease (AD) in humans, by accurately identifying amyloid-beta (Aβ) plaques. Cognitive dysfunction syndrome in dogs shows cognitive and pathophysiologic characteristics similar to AD. Therefore, we assessed the physiologic characteristics of uptake of 18F-flutemetamol, an Aβ protein-binding PET tracer in clinical development, in normal dog brains, for distinguishing an abnormal state. Static and dynamic PET images of six adult healthy dogs were acquired after 18F-flutemetamol was administered intravenously at approximately 3.083 MBq/kg. For static images, PET data were acquired at 30, 60, and 90 min after injection. One week later, dynamic images were acquired for 120 min, from the time of tracer injection. PET data were reconstructed using an iterative technique, and corrections for attenuation and scatter were applied. Regions of interest were manually drawn over the frontal, parietal, temporal, occipital, anterior cingulate, posterior cingulate, and cerebellar cortices, cerebral white matter, midbrain, pons, and medulla oblongata. After calculating standardized uptake values with an established formula, standardized uptake value ratios (SUVRs) were obtained, using the cerebellar cortex as a reference region.

RESULTS

Among the six cerebral cortical regions, the cingulate cortices and frontal lobe showed the highest SUVRs. The lowest SUVR was observed in the occipital lobe. The average values of the cortical SUVRs were 1.25, 1.26, and 1.27 at 30, 60, and 90 min post-injection, respectively. Tracer uptake on dynamic scans was rapid, peaking within 4 min post-injection. After reaching this early maximum, cerebral cortical regions showed a curve with a steep descent, whereas cerebral white matter demonstrated a curve with a slow decline, resulting in a large gap between cerebral cortical regions and white matter.

CONCLUSION

This study provides normal baseline data of 18F-flutemetamol PET that can facilitate an objective diagnosis of cognitive dysfunction syndrome in dogs in future.

Congo Red and amyloids: history and relationship

Staining with Congo Red (CR) is a qualitative method used for the identification of amyloids in vitro and in tissue sections. However, the drawbacks and artefacts obtained when using this dye can be found both in vitro and in vivo. Analysis of scientific data from previous studies shows that CR staining alone is not sufficient for confirmation of the amyloid nature of protein aggregates in vitro or for diagnosis of amyloidosis in tissue sections. In the present paper, we describe the characteristics and limitations of other methods used for amyloid studies. Our historical review on the use of CR staining for amyloid studies may provide insight into the pitfalls and caveats related to this technique for researchers considering using this dye.

Amyloid from a histochemical perspective. A review of the structure, properties and types of amyloid, and a proposed staining mechanism for Congo red staining

Amyloid is a diverse group of unrelated peptides or proteins that have positive functionality or are associated with various pathologies. Despite vast differences, all amyloids share several features that together uniquely define the group. 1) All amyloids possess a characteristic cross-ß pattern with X-ray diffraction typical of ß-sheet secondary protein structures. 2) All amyloids are birefringent and dichroic under polarizing microscopy after staining with Congo red, which indicates a crystalline-like (ordered) structure. 3) All amyloids cause a spectral shift in the peak wavelength of Congo red with conventional light microscopy due to perturbation of π electrons of the dye. 4) All amyloids show heightened intensity of fluorescence with Congo red, which suggests an unusual degree of packing of the dye onto the substrate. The ß portion of amyloid molecules, the only logical substrate for specific Congo red staining under histochemical conditions, consists of a stack of ß-sheets laminated by hydrophilic and hydrophobic interactions between adjacent pairs. Only the first and last ß-sheets are accessible to dyes. Each sheet is composed of numerous identical peptides running across the width of the sheet and arranged in parallel with side chains in register over the length of the fibril. Two sets of grooves are bordered by side chains. X grooves run perpendicular to the long axis of the fibril; these grooves are short (the width of the sheet) and number in the hundreds or thousands. Y grooves are parallel with the long axis. Each groove runs the entire length of the fibril, but there are very few of them. While Congo red is capable of ionic bonding with proteins via two sulfonic acid groups, physical constraints on the staining solution preclude ionic interactions. Hydrogen bonding between dye amine groups and peptide carbonyls is the most likely primary bonding mechanism, because all ß-sheets possess backbone carbonyls. Various amino acid residues may form secondary bonds to the dye via any of three van der Waals forces. It is possible that Congo red binds within the Y grooves, but that would not produce the characteristic staining features that are the diagnostic hallmarks of amyloid. Binding in the X grooves would produce a tightly packed series of dye molecules over the entire length of the fibril. This would account for the signature staining of amyloid by Congo red: dichroic birefringence, enhanced intensity of fluorescence and a shift in visible absorption wavelength.

Synergistic Inhibitory Effect of GQDs-Tramiprosate Covalent Binding on Amyloid Aggregation

Inhibiting the amyloid aggregation is considered to be an effective strategy to explore possible treatment of amyloid-related diseases including Alzheimer's disease, Parkinson's disease, and type II diabetes. Herein, a new high-efficiency and low-cytotoxicity Aβ aggregation inhibitors, GQD-T, was designed through the combination of two Aβ aggregation inhibitors, graphene quantum dots (GQDs) and tramiprosate. GQD-T showed the capability of efficiently inhibiting the aggregation of Aβ peptides and rescuing Aβ-induced cytotoxicity due to the synergistic effect of the GQDs and tramiprosate. In addition, the GQD-T has the characteristics of low toxicity and great biocompatibility. It is believed that GQD-T may be a potential candidate for an Alzheimer's drug and this work provides a new strategy for exploring Aβ peptide aggregation inhibitors.

Interactions between Microtubule-Associated Protein Tau (MAPT) and Small Molecules

Tau aggregation is linked to multiple neurodegenerative disorders that are collectively termed tauopathies. Small molecules are powerful probes of the aggregation process, helping to reveal the key steps and serving as diagnostics and reporters. Moreover, some of these small molecules may have potential as therapeutics. This review details how small molecules and chemical biology have helped to elucidate the mechanisms of tau aggregation and how they are being used to detect and prevent tau aggregation. In addition, we comment on how new insights into tau prions are changing the approach to small molecule discovery.

Exploring Anti-Prion Glyco-Based and Aromatic Scaffolds: A Chemical Strategy for the Quality of Life

Prion diseases are fatal neurodegenerative disorders caused by protein misfolding and aggregation, affecting the brain progressively and consequently the quality of life. Alzheimer’s is also a protein misfolding disease, causing dementia in over 40 million people worldwide. There are no therapeutics able to cure these diseases. Cellular prion protein is a high-affinity binding partner of amyloid β (Aβ) oligomers, the most toxic species in Alzheimer’s pathology. These findings motivate the development of new chemicals for a better understanding of the events involved. Disease control is far from being reached by the presently known therapeutics. In this review we describe the synthesis and mode of action of molecular entities with intervention in prion diseases’ biological processes and, if known, their role in Alzheimer’s. A diversity of structures is covered, based on glycans, steroids and terpenes, heterocycles, polyphenols, most of them embodying aromatics and a structural complexity. These molecules may be regarded as chemical tools to foster the understanding of the complex mechanisms involved, and to encourage the scientific community towards further developments for the cure of these devastating diseases.

Synergistic Inhibitory Effect of Peptide-Organic Coassemblies on Amyloid Aggregation

Inhibition of amyloid aggregation is important for developing potential therapeutic strategies of amyloid-related diseases. Herein, we report that the inhibition effect of a pristine peptide motif (KLVFF) can be significantly improved by introducing a terminal regulatory moiety (terpyridine). The molecular-level observations by using scanning tunneling microscopy reveal stoichiometry-dependent polymorphism of the coassembly structures, which originates from the terminal interactions of peptide with organic modulator moieties and can be attributed to the secondary structures of peptides and conformations of the organic molecules. Furthermore, the polymorphism of the peptide-organic coassemblies is shown to be correlated to distinctively different inhibition effects on amyloid-β 42 (Aβ42) aggregations and cytotoxicity.

Quantitative thermophoretic study of disease-related protein aggregates

Amyloid fibrils are a hallmark of a range of neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. A detailed understanding of the physico-chemical properties of the different aggregated forms of proteins, and of their interactions with other compounds of diagnostic or therapeutic interest, is crucial for devising effective strategies against such diseases. Protein aggregates are situated at the boundary between soluble and insoluble structures, and are challenging to study because classical biophysical techniques, such as scattering, spectroscopic and calorimetric methods, are not well adapted for their study. Here we present a detailed characterization of the thermophoretic behavior of different forms of the protein α-synuclein, whose aggregation is associated with Parkinson's disease. Thermophoresis is the directed net diffusional flux of molecules and colloidal particles in a temperature gradient. Because of their low volume requirements and rapidity, analytical methods based on this effect have considerable potential for high throughput screening for drug discovery. In this paper we rationalize and describe in quantitative terms the thermophoretic behavior of monomeric, oligomeric and fibrillar forms of α-synuclein. Furthermore, we demonstrate that microscale thermophoresis (MST) is a valuable method for screening for ligands and binding partners of even such highly challenging samples as supramolecular protein aggregates.

Compounds for imaging amyloid-β deposits in an Alzheimer's brain: a patent review

INTRODUCTION

β-amyloid (Aβ) plaques in the brain are regarded as a hallmark of Alzheimer's disease (AD), and the imaging of Aβ is a critical step for early diagnosis. Extensive research has been done to develop probes for targeting Aβ with available imaging modalities.

AREAS COVERED

In this review, the authors give an overview of published patents and papers about the discovery and development of compounds possessing potential utilization in imaging Aβ for the diagnosis of AD. SciFinder is the main electronic database for patent study in this review.

EXPERT OPINION

Despite achievements in Aβ imaging, there is still a need to develop innovative compounds with selectivity and high affinity to Aβ. Positron emission tomography imaging agents will still be the trend in the field in the short term. Due to the low costs for single-photon emission computed tomography (SPECT) and the excellent nuclear properties of (99m)Tc, substantial research should be conducted on the development of the probes for SPECT. Refining the current imaging techniques and in the meantime developing new efficient imaging multimodality and compounds would be a promising approach to imaging Aβ.

Thioflavin-based molecular probes for application in Alzheimer's disease: from in silico to in vitro models

Alzheimer's disease (AD) is a neurological disease of confusing causation with no cure or prevention available. The definitive diagnosis is made postmortem, in part through the presence of amyloid-beta plaques in the brain tissue, which can be done with the small molecule thioflavin-T (ThT). Plaques are also found to contain elevated amounts of metal ions Cu(ii) and Zn(ii) that contribute to the neurotoxicity of amyloid-beta (Aβ). In this paper, we report in silico, in vitro, and ex vivo studies with ThT-derived metal binders 2-(2-hydroxyphenyl)benzoxazole (HBX), 2-(2-hydroxyphenyl)benzothiazole (HBT) and their respective iodinated counterparts, HBXI and HBTI. They exhibit low cytotoxicity in a neuronal cell line, potential blood-brain barrier penetration, and interaction with Aβ fibrils from senile plaques present in human and transgenic mice AD models. Molecular modelling studies have also been undertaken to understand the prospective ligand-Aβ complexes as well as to rationalize the experimental findings. Overall, our studies demonstrate that HBX, HBT, HBXI, and HBTI are excellent agents for future use in in vivo models of AD, as they show in vitro efficacy and biological compatibility. In addition to this, we present the glycosylated form of HBX (GBX), which has been prepared to take advantage of the benefits of the prodrug approach. Overall, the in vitro and ex vivo assays presented in this work validate the use of the proposed ThT-based drug candidate series as chemical tools for further in vivo development.

Molecular tethering effect of C-terminus of amyloid peptide aβ42

Amyloid peptides are considered to be the main contributor for the membrane disruption related to the pathogenesis of degenerative diseases. The variation of amino acids at the carboxylic terminus of amyloid peptide has revealed significant effects on the modulation of abnormal assemblies of amyloid peptides. In this work, molecular binding agents were tethered to the C-terminus of β-amyloid peptide 1-42 (Aβ42). The molecular interaction between Aβ42 and molecule tethers was identified at single molecule level by using scanning tunneling microscopy (STM). The mechanistic insight into the feature variation of the self-assembly of Aβ42 peptide caused by molecular tethering at C-terminus was clearly revealed, which could appreciably affect the nucleation of amyloid peptide, thus reducing the membrane disruptions.

Small-molecule theranostic probes: a promising future in neurodegenerative diseases

Prion diseases are fatal neurodegenerative illnesses, which include Creutzfeldt-Jakob disease in humans and scrapie, chronic wasting disease, and bovine spongiform encephalopathy in animals. They are caused by unconventional infectious agents consisting primarily of misfolded, aggregated, β -sheet-rich isoforms, denoted prions, of the physiological cellular prion protein (PrP(C)). Many lines of evidence suggest that prions (PrP(Sc)) act both as a template for this conversion and as a neurotoxic agent causing neuronal dysfunction and cell death. As such, PrP(Sc) may be considered as both a neuropathological hallmark of the disease and a therapeutic target. Several diagnostic imaging probes have been developed to monitor cerebral amyloid lesions in patients with neurodegenerative disorders (such as Alzheimer's disease, Parkinson's disease, and prion disease). Examples of these probes are Congo red, thioflavin T, and their derivatives. We synthesized a series of styryl derivatives, denoted theranostics, and studied their therapeutic and/or diagnostic potentials. Here we review the salient traits of these small molecules that are able to detect and modulate aggregated forms of several proteins involved in protein misfolding diseases. We then highlight the importance of further studies for their practical implications in therapy and diagnostics.

Progress and developments in tau aggregation inhibitors for Alzheimer disease

Pharmacological approaches directed toward Alzheimer disease are diversifying in parallel with a growing number of promising targets. Investigations on the microtubule-associated protein tau yielded innovative targets backed by recent findings about the central role of tau in numerous neurodegenerative diseases. In this review, we summarize the recent evolution in the development of nonpeptidic small molecules tau aggregation inhibitors (TAGIs) and their advancement toward clinical trials. The compounds are classified according to their chemical structures, providing correlative insights into their pharmacology. Overall, shared structure-activity traits are emerging, as well as specific binding modes related to their ability to engage in hydrogen bonding. Medicinal chemistry efforts on TAGIs together with encouraging in vivo data argue for successful translation to the clinic.

Development of positron emission tomography β-amyloid plaque imaging agents

For 100 years, β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) have been recognized as the neuropathological hallmarks of Alzheimer's disease (AD), and their presence or absence could only be assessed postmortem using stains and dyes that identified these microscopic structures. Approximately 10 years ago, the first successful Aβ plaque-specific positron emission tomography (PET) imaging study was conducted in a living human subject clinically diagnosed with probable AD using the (11)C-labeled radiopharmaceutical Pittsburgh Compound B (PiB). Laboratory studies and preclinical evaluations to design PiB began a decade earlier than the first human PiB PET study and involved chemical modifications of different well-known dyes that bound specifically to the extended β-pleated sheets that comprise the fibrils of amyloid proteins such as Aβ plaques, NFTs, α-synuclein deposits, and prions. These preclinical studies were conducted in our laboratories at the University of Pittsburgh, starting with Congo red derivatives, followed by Chrysamine G derivatives, followed by X-series compounds, and finally with neutral derivatives of thioflavin-T. The in vitro and in vivo evaluations of the different derivatives as candidate PET radioligands for imaging Aβ plaques and neurofibrillary tangles in human brain are described in this review, along with the specific evaluation criteria by which the candidate radioligands were judged. Out of these studies came PiB, a PET radioligand that binds selectively and with high affinity to only fibrillar forms of Aβ. PiB has been used in many different human research protocols throughout the world and has demonstrated the usefulness of assessing the Aβ plaque status of subjects many years before the clinical diagnosis of probable AD. Recently, longer-lived (18)F-radiolabeled Aβ-selective radiopharmaceuticals have been developed. It is likely that the full clinical impact of these imaging agents will be realized by identifying presymptomatic subjects who would benefit from early drug treatments with future disease-modifying AD therapeutics.

Small-molecule inhibitors/modulators of amyloid-β peptide aggregation and toxicity for the treatment of Alzheimer's disease: a patent review (2010 - 2012)

INTRODUCTION

Genetic, physiological, and biochemical data indicate that agglomerates of the 42-amino acid form of the amyloid-β (Aβ(42)) peptide are strongly linked to Alzheimer's disease (AD) etiology and thus represent a particularly attractive target for the development of an effective disease-modifying approach for AD treatment. A plethora of chemical entities able to modulate Aβ(42) self-assembly have been developed in recent years, among them, several are in clinical or preclinical development.

AREAS COVERED

This review accounts for small-molecule inhibitors of Aβ peptide polymerization and toxicity, reported in the patent literature during the 2010 - 2012 period, and their potential use as disease-modifying therapeutics for AD cure.

EXPERT OPINION

The earliest pathogenic event is the formation of soluble Aβ oligomers that disrupt synaptic communication. Drug design strategies targeting these primary toxic agents could hold considerable promises for obtaining effective anti-AD drugs candidate. The heterogeneous aggregation of Aβ and the resulting difficulty to structurally characterize the peptide represent important drawbacks.

Dye-binding assays for evaluation of the effects of small molecule inhibitors on amyloid (aβ) self-assembly

Dye-binding assays, such as those utilizing Congo red and thioflavin T, are among the most widely used tools to probe the aggregation of amyloidogenic biomolecules and for the evaluation of small molecule inhibitors of amyloid aggregation and fibrillization. A number of recent reports have indicated that these dye-binding assays could be prone to false positive effects when assessing inhibitors' potential toward Aβ peptides, species involved in Alzheimer's disease. Specifically, this review focuses on the application of thioflavin T for determining the efficiency of small molecule inhibitors of Aβ aggregation and addresses potential reasons that might be associated with the false positive effects in an effort to increase reliability of dye-binding assays.

Preclinical characterization of a novel class of 18F-labeled PET tracers for amyloid-β

Imaging of amyloid-β (Aβ) plaques by PET is more and more integrated into concepts for Alzheimer disease (AD) diagnosis and drug development. The objective of this study was to find novel chemical entities that can be transformed into (18)F-labeled Aβ tracers with favorable brain washout kinetics and low background signal.

METHODS

High-throughput screening of a large chemical library was used to identify new ligands for fibrillar aggregates of Aβ(1-42) peptide. Thirty-two fluorinated derivatives were synthesized and tested for their affinity toward AD brain homogenate. Twelve ligands have been radiolabeled with (18)F. The pharmacokinetic properties of the radioligands were investigated in mouse and monkey biodistribution studies. Binding characteristics were determined by autoradiography of AD brain sections in vitro and using amyloid precursor protein transgenic mice in vivo.

RESULTS

The systematic search for Aβ imaging agents revealed several fluorinated derivatives with nanomolar affinity for Aβ. The fluoropyridyl derivative BAY 1008472 showed a high initial brain uptake (6.45 percentage injected dose per gram at 2 min) and rapid brain washout (ratio of percentage of injected dose per gram of tissue at 2 and 30 min after injection, 9.2) in mice. PET studies of healthy rhesus monkeys confirmed the high initial brain uptake of BAY 1008472 (2.52 standardized uptake value at peak) and a fast elimination of total radioactivity from gray and white matter areas (ratio of standardized uptake value at peak uptake and 60 min 11.0). In autoradiographic analysis, BAY 1008472 selectively detected Aβ deposits in human AD brain sections with high contrast and did not bind to τ- or α-synuclein pathologies. Finally, ex vivo autoradiography of brain sections from amyloid precursor protein-transgenic mice confirmed that BAY 1008472 is indeed suitable for the in vivo detection of Aβ plaques.

CONCLUSION

A new chemical class of Aβ tracers has been identified by high-throughput screening. The fluoropyridyl derivative BAY 1008472 shows a favorable preclinical profile including low background binding in gray and white matter. These properties might qualify this new tracer, in particular, to detect subtle amounts or changes of Aβ burden in presymptomatic AD and during therapy.

Ligand electronic properties modulate tau filament binding site density

Small molecules that bind tau-bearing neurofibrillary lesions are being sought for premortem diagnosis, staging, and treatment of Alzheimer's disease and other tauopathic neurodegenerative diseases. The utility of these agents will depend on both their binding affinity and binding site density (B(max)). Previously we identified polarizability as a descriptor of protein aggregate binding affinity. To examine its contribution to binding site density, we investigated the ability of two closely related benzothiazole derivatives ((E)-2-[[4-(dimethylamino)phenyl]azo]-6-methoxybenzothiazole) and ((E)-2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methoxybenzothiazole) that differed in polarizability to displace probes of high (Thioflavin S) and low (radiolabeled (E,E)-1-iodo-2,5-bis(3-hydroxycarbonyl-4-methoxy)styrylbenzene; IMSB) density sites. Consistent with their site densities, Thioflavin S completely displaced radiolabeled IMSB, but IMSB was incapable of displacing Thioflavin S. Although both benzothiazoles displaced the low B(max) IMSB probe, only the highly polarizable analog displaced near saturating concentrations of the Thioflavin S probe. Quantum calculations showed that high polarizability reflected extensive pi-electron delocalization fostered by the presence of electron donating and accepting groups. These data suggest that electron delocalization promotes ligand binding at a subset of sites on tau aggregates that are present at high density, and that optimizing this aspect of ligand structure can yield tau-directed agents with superior diagnostic and therapeutic performance.

A chemical analog of curcumin as an improved inhibitor of amyloid Abeta oligomerization

Amyloid-like plaques are characteristic lesions defining the neuropathology of Alzheimer's disease (AD). The size and density of these plaques are closely associated with cognitive decline. To combat this disease, the few therapies that are available rely on drugs that increase neurotransmission; however, this approach has had limited success as it has simply slowed an imminent decline and failed to target the root cause of AD. Amyloid-like deposits result from aggregation of the Aβ peptide, and thus, reducing amyloid burden by preventing Aβ aggregation represents an attractive approach to improve the therapeutic arsenal for AD. Recent studies have shown that the natural product curcumin is capable of crossing the blood-brain barrier in the CNS in sufficient quantities so as to reduce amyloid plaque burden. Based upon this bioactivity, we hypothesized that curcumin presents molecular features that make it an excellent lead compound for the development of more effective inhibitors of Aβ aggregation. To explore this hypothesis, we screened a library of curcumin analogs and identified structural features that contribute to the anti-oligomerization activity of curcumin and its analogs. First, at least one enone group in the spacer between aryl rings is necessary for measureable anti-Aβ aggregation activity. Second, an unsaturated carbon spacer between aryl rings is essential for inhibitory activity, as none of the saturated carbon spacers showed any margin of improvement over that of native curcumin. Third, methoxyl and hydroxyl substitutions in the meta- and para-positions on the aryl rings appear necessary for some measure of improved inhibitory activity. The best lead inhibitors have either their meta- and para-substituted methoxyl and hydroxyl groups reversed from that of curcumin or methoxyl or hydroxyl groups placed in both positions. The simple substitution of the para-hydroxy group on curcumin with a methoxy substitution improved inhibitor function by 6-7-fold over that measured for curcumin.

Preclinical studies of potential amyloid binding PET/SPECT ligands in Alzheimer's disease

Visualizing the neuropathological hallmarks amyloid plaques and neurofibrillary tangles of Alzheimer's disease in vivo using positron emission tomography (PET) or single photon emission computed tomography will be of great value in diagnosing the individual patient and will also help in our understanding of the disease. The successful introduction of [(11)C]PIB as a PET tracer for the amyloid plaques less than 10 years ago started an intensive research, and numerous new compounds for use in molecular imaging of the amyloid plaques have been developed. The candidates are based on dyes like thioflavin T, Congo red and chrysamine G, but also on other types such as benzoxazoles, curcumin and stilbenes. In the present review, we present methods of the radiochemistry and preclinical evaluation as well as the main properties of some of these compounds.

Observation of reduced cytotoxicity of aggregated amyloidogenic peptides with chaperone-like molecules

The pathogenesis of many neurodegenerative diseases is associated with different types of aggregates of amyloidogenic peptides, including senile plaques, fibrils, protofibrils, and oligomers. It is therefore valuable to explore diversity of approaches toward reducing the cytotoxicity of amyloidogenic peptides by modulating aggregation behaviors. Herein we report an approach toward reducing the neuronal cytotoxicity of amyloidogenic peptides by accelerating the aggregation process, which is different from prevalent methods via inhibiting the aggregation of peptides. The pyridyl derivatives behave like chaperones to dramatically change the assembling characteristics of the peptides via strong hydrogen bond formation with C-termini of amyloid β (Aβ) peptides, which is revealed by using scanning probe microscopy. The light scattering experiments demonstrated the effect of the chaperone-like molecules on accelerating the aggregation process of Aβ peptides, accompanied by the reduced neuronal cytotoxicity of amyloidogenic peptides. These results would give rise to a complementary approach for modulating biological effects of the aggregates of amyloidogenic peptides.

Insight into amyloid structure using chemical probes

Alzheimer's disease (AD) is a common neurodegenerative disorder characterized by the deposition of amyloids in the brain. One prominent form of amyloid is composed of repeating units of the amyloid-β (Aβ) peptide. Over the past decade, it has become clear that these Aβ amyloids are not homogeneous; rather, they are composed of a series of structures varying in their overall size and shape and the number of Aβ peptides they contain. Recent theories suggest that these different amyloid conformations may play distinct roles in disease, although their relative contributions are still being discovered. Here, we review how chemical probes, such as Congo red, thioflavin T and their derivatives, have been powerful tools for the better understanding of amyloid structure and function. Moreover, we discuss how design and deployment of conformationally selective probes might be used to test emerging models of AD.

Amyloid PET Ligands for Dementia

The progressive nature of neurodegeneration suggests an age-dependent process that ultimately leads to synaptic failure and neuronal damage in cortical areas of the brain critical for memory and higher mental functions. The increasing age of the population in developed countries suggests that, if unchecked, these disorders will become increasingly prevalent. In the absence of specific biologic markers, direct pathologic examination of brain tissue still is the only definitive method for establishing a diagnosis of Alzheimer disease (AD) and other types of dementia. Pathologic hallmarks of AD are intracellular neurofibrillary tangles (NFT) and extracellular amyloid plaques. NFT are intraneuronal bundles of paired helical filaments mainly composed of the aggregates of an abnormally phosphorylated form of tau protein; neuritic plaques consist of dense extracellular aggregates of β-amyloid (Aβ), surrounded by reactive gliosis and dystrophic neurites. To date, all available evidence strongly supports the notion that an imbalance between the production and removal of Aβ leading to its progressive accumulation is central to the pathogenesis of AD. A growing understanding of the molecular mechanisms of Aβ formation, degradation, and neurotoxicity is being translated into new therapeutic approaches. Whereas AD is the most common cause of dementia in the elderly, postmortem studies have found dementia with Lewy Bodies and frontotemporal lobe degeneration each to account for about 20% of cases. Molecular neuroimaging techniques such as PET have been used for the in vivo assessment of molecular processes at their sites of action, permitting detection of subtle pathophysiological changes in the brain at asymptomatic stages The development of molecular imaging methods for noninvasively assessing disease-specific traits such as Aβ burden in AD is allowing early diagnosis at presymptomatic stages, more accurate differential diagnosis and, when available, the evaluation and monitoring of disease-modifying therapy.

Effects of Congo red on aβ(1-40) fibril formation process and morphology

Alzheimer's disease (AD), an age-related neurodegenerative disorder, is the most common form of dementia, and the seventh-leading cause of death in the United States. Current treatments offer only symptomatic relief; thus, there is a great need for new treatments with disease-modifying potential. One pathological hallmark of AD is so-called senile plaques, mainly made up of β-sheet-rich assemblies of 40- or 42-residue amyloid β-peptides (Aβ). Hence, inhibition of Aβ aggregation is actively explored as an option to prevent or treat AD. Congo red (CR) has been widely used as a model antiamyloid agent to prevent Aβ aggregation. Herein, we report detailed morphological studies on the effect of CR as an antiamyloid agent, by circular dichroism spectroscopy, photo-induced cross-linking reactions, and atomic force microscopy. We also demonstrate the effect of CR on a preaggregated sample of Aβ(1-40). Our result suggests that Aβ(1-40) follows a different path for aggregation in the presence of CR.

Binding mode of Thioflavin T and other molecular probes in the context of amyloid fibrils-current status

Because understanding amyloid fibrillation in molecular detail is essential for development of strategies to control amyloid formation and overcome neurodegenerative disorders, increased understanding of present molecular probes as well as development of new probes are of utmost importance. To date, the binding modes of these molecular probes to amyloid fibrils are by no means adequately described or understood, and the large number of studies on Thioflavin T (ThT) and Congo Red (CR) binding have resulted in models that are incomplete and conflicting. Different types of binding sites are likely to be present in amyloid fibrils with differences in binding modes. ThT may bind in channels running parallel to the long axis of the fibril. In the channels, ThT may bind in either a monomeric or dimeric form of which the molecular conformation is likely to be planar. CR may bind in grooves formed along the β-sheets as a planar molecule in either a monomeric or supramolecular form.

An enzyme-linked immunosorbent assay to compare the affinity of chemical compounds for β-amyloid peptide as a monomer

Aβ(1-42) is the proteolytic cleavage product of cleavage of the amyloid precursor protein by β- and γ-secretases. The aggregation of Aβ(1-42) plays a causative role in the development of Alzheimer's disease. To lock Aβ(1-42) in a homogenous state, we embedded the Aβ(1-42) sequence in an unstructured region of Bcl-x(L). Both the N-terminus and the C-terminus of Aβ(1-42) were constrained in the disordered region, whereas the conjunction did not introduce any folding to Aβ(1-42) but maintained the sequence as a monomer in solution. With Bcl-x(L)-Aβ(42), we developed an enzyme-linked immunosorbent assay to compare the affinity of compounds for monomeric Aβ(1-42). Bcl-x(L)-Aβ(42) was coated on a microplate and this was followed by incubation with different concentrations of compounds. Compounds binding to Leu17-Val24 of Aβ(1-42) inhibited the interaction between Bcl-x(L)-Aβ(42) and antibody 4G8. The method can not only reproduce the activities of the reported Aβ(1-42) inhibitors such as dopamine, tannin, and morin but can also differentiate decoy compounds that do not bind to Aβ(1-42). Remarkably, using this method, we discovered a new inhibitor that binds to monomeric Aβ(1-42) and inhibits Aβ(1-42) fibril formation. As the structure of Bcl-x(L)-Aβ(42) monomer is stable in solution, the assay could be adapted for high-throughput screening with a series of antibodies that bind the different epitopes of Aβ(1-42). In addition, the monomeric form of the Aβ(1-42) sequence in Bcl-x(L)-Aβ(42) would also facilitate the identification of Aβ(1-42) binding partners by coimmunoprecipitation, cocrystallization, surface plasmon resonance technology, or the assay as described here.

Anti-Amyloid Effects of Small Molecule Aβ-Binding Agents in PS1/APP Mice

AIMS: One promising approach for treatment of Alzheimer's disease (AD) is use of anti-amyloid therapies, based on the hypothesis that increases in amyloid-beta (Aβ) deposits in brain are a major cause of AD. Several groups have focused on Aβ immunotherapy with some success. Small molecules derivatives of Congo red have been shown to inhibit Aβ aggregation and protect against Aβ neurotoxicity in vitro. The agents described here are all small molecule Aβ-binding agents (SMAβBA's) derivatives of Congo red. MAIN METHODS: Here, we have explored the anti-amyloid properties of these SMAβBA's in mice doubly transgenic for human prensenilin-1 (PS1) and APP gene mutations that cause early-onset AD. Mice were treated with either methoxy-X04, X:EE:B34 and X:034-3-OMe1. After treatment, brains were examined for Aβ-deposition, using histochemistry, and soluble and insoluble Aβ levels were determined using ELISA. KEY FINDINGS: A range of anti-amyloid activity was observed with these three compounds. PS1/APP mice treated with methoxy-X04 and X:EE:B34 showed decrease in total Aβ load, a decrease in Aβ fibril load, and a decrease in average plaque size. Treatment with methoxy-X04 also resulted in a decrease in insoluble Aβ levels. The structurally similar compound, X:034:3-OMe1, showed no significant effect on any of these measures. The effectiveness of the SMAβBA's may be related to a combination of binding affinity for Aβ and entry into brain, but other factors appear to apply as well. SIGNIFICANCE: These data suggest that SMAβBA's may significantly decrease amyloid burden in brain during the pathogenesis of AD and could be useful therapeutics alone, or in combination with immunotherapy.