Synthesis and structure-affinity relationships of new 4-(6-iodo-H-imidazo[1,2-a]pyridin-2-yl)-N-dimethylbenzeneamine derivatives as ligands for human beta-amyloid plaques

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.

Synthesized NaA nanozeolite as a catalyst for the preparation of 3-amino imidazo[1,2-a]pyridines under solvent-free conditions

Abstract:

The present study explores a new method for the fabrication of NaA nanozeolite as a simple and efficient catalyst for the production of 3-aminoimidazo [1,2-a] pyridines through the 3-component reaction of aldehydes, 2-aminoperidines and isocyanides under solvent-free conditions. The production of organic template free (OTF) NaA nanozeolite was performed at room temperature. The prepared nanozeolite was identified by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electronic microscopy (FESEM), N2 sorption isotherm and particle size analysis (PSA). The particle sizes of synthesized spherical NaA nanozeolite were under 100 nm via the FESEM method. The BET surface area, total pore volume and mean pore diameter of the created sample were attained to be 362 m2g-1, 0.44 cm3 g-1 and 5.9 nm, respectively. The developed method has some advantages such as OTF production of NaA nanozeolite, simple synthesis method with short reaction time and easy separation using filtration and the ability to recycle and reuse of catalyst several times without reducing its efficiency.

Imidazo[1,2-b]pyridazine as privileged scaffold in medicinal chemistry: An extensive review

Imidazo[1,2-b]pyridazine scaffold represents an important class of heterocyclic nucleus which provides various bioactives molecules. Among them, the successful kinase inhibitor ponatinib led to a resurgence of interest in exploring new imidazo[1,2-b]pyridazine-containing derivatives for their putative therapeutic applications in medicine. This present review intends to provide a state-of-the-art of this framework in medicinal chemistry from 1966 to nowadays, unveiling different aspects of its structure-activity relationships (SAR). This extensive literature surveil may guide medicinal chemists for the quest of novel imidazo[1,2-b]pyridazine compounds with enhanced pharmacokinetics profile and efficiency.

Core Binding Site of a Thioflavin-T-Derived Imaging Probe on Amyloid β Fibrils Predicted by Computational Methods

Development of new diagnostic imaging probes for Alzheimer's disease, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) probes, has been strongly desired. In this study, we investigated the most accessible amyloid β (Aβ) binding site of [¹²³I]IMPY, a Thioflavin-T-derived SPECT probe, using experimental and computational methods. First, we performed a competitive inhibition assay with Orange-G, which recognizes the KLVFFA region in Aβ fibrils, suggesting that IMPY and Orange-G bind to different sites in Aβ fibrils. Next, we precisely predicted the IMPY binding site on a multiple-protofilament Aβ fibril model using computational approaches, consisting of molecular dynamics and docking simulations. We generated possible IMPY-binding structures using docking simulations to identify candidates for probe-binding sites. The binding free energy of IMPY with the Aβ fibril was calculated by a free energy simulation method, MP-CAFEE. These computational results suggest that IMPY preferentially binds to an interfacial pocket located between two protofilaments and is stabilized mainly through hydrophobic interactions. Finally, our computational approach was validated by comparing it with the experimental results. The present study demonstrates the possibility of computational approaches to screen new PET/SPECT probes for Aβ imaging.

Oxyfunctionalization of pyridine derivatives using whole cells of Burkholderia sp. MAK1

Pyridinols and pyridinamines are important intermediates with many applications in chemical industry. The pyridine derivatives are in great demand as synthons for pharmaceutical products. Moreover, pyridines are used either as biologically active substances or as building blocks for polymers with unique physical properties. Application of enzymes or whole cells is an attractive strategy for preparation of hydroxylated pyridines since the methods for chemical synthesis of pyridinols, particularly aminopyridinols, are usually limited or inefficient. Burkholderia sp. MAK1 (DSM102049), capable of using pyridin-2-ol as the sole carbon and energy source, was isolated from soil. Whole cells of Burkholderia sp. MAK1 were confirmed to possess a good ability to convert different pyridin-2-amines and pyridin-2-ones into their 5-hydroxy derivatives. Moreover, several methylpyridines as well as methylated pyrazines were converted to appropriate N-oxides. In conclusion, regioselective oxyfunctionalization of pyridine derivatives using whole cells of Burkholderia sp. MAK1 is a promising method for the preparation of various pyridin-5-ols and pyridin-N-oxides.

Synthesis and evaluation of 6-(3-[(18)F]fluoro-2-hydroxypropyl)-substituted 2-pyridylbenzothiophenes and 2-pyridylbenzothiazoles as potential PET tracers for imaging Aβ plaques

3-[(18)F]Fluoro-2-hydroxypropyl substituted compounds were synthesized and evaluated as novel (18)F-labeled PET tracers for imaging Aβ plaque in a living brain. All compounds exhibited high binding affinities toward the synthetic Aβ1-42 aggregate and/or Alzheimer's disease brain homogenate. In the microPET study with normal mice, the 3-[(18)F]fluoro-2-hydroxypropyl substituted compounds resulted in fast brain washout by reducing the lipophilicities of the compounds. Intriguingly, (S)-configured PET tracers, (S)-[(18)F]1b and (S)-[(18)F]1c, exhibited a 2.8 and 4.0-fold faster brain washout rate at a peak/30 min in the mouse brain than the corresponding (R)-configured PET tracers despite there being no meaningful difference in binding affinities toward Aβ plaque. A further evaluation of (S)-[(18)F]1c with healthy rhesus monkeys also revealed excellent clearance from the frontal cortex with ratios of 7.0, 16.0, 30.0 and 49.0 at a peak/30, 60, 90, and 120 min, respectively. These results suggest that (S)-[(18)F]1c may be a potential PET tracer for imaging Aβ plaque in a living brain.

Dimeric Cinchona alkaloids

Nature is full of dimeric alkaloids of various types from many plant families, some of them with interesting biological properties. However, dimeric Cinchona alkaloids were not isolated from any species but were products of designed partial chemical synthesis. Although the Cinchona bark is amongst the sources of oldest efficient medicines, the synthetic dimers found most use in the field of asymmetric synthesis. Prominent examples include the Sharpless dihydroxylation and aminohydroxylation ligands, and dimeric phase transfer catalysts. In this article the syntheses of Cinchona alkaloid dimers and oligomers are reviewed, and their structure and applications are outlined. Various synthetic routes exploit reactivity of the alkaloids at the central 9-hydroxyl group, quinuclidine, and quinoline rings, as well as 3-vinyl group. This availability of reactive sites, in combination with a plethora of linker molecules, contributes to the diversity of the products obtained.

Rational Design of Novel Pyridinol-Fused Ring Acetaminophen Analogues

Acetaminophen (ApAP) is an electron donor capable of reducing radicals generated by redox cycling of hemeproteins. It acts on the prostaglandin H synthases (cyclooxygenases; COXs) to reduce the protoporphyrin radical cation in the peroxidase site of the enzyme, thus preventing the intra-molecular electron transfer that generates the Tyr385 radical required for abstraction of a hydrogen from arachidonic acid to initiate prostaglandin synthesis. Unrelated to this pharmacological action, metabolism of ApAP by CYPs yields an iminoquinone electrophile that is responsible for the hepatotoxicity, which results from high doses of the drug. We synthesized novel heterocyclic phenols predicted to be electron donors. Two of these inhibited the oxygenation of arachidonic acid by PGHS-1 and myoglobin and also were shown to be more metabolically stable and exhibited less direct cytotoxicity than acetaminophen. They are leading candidates for studies to determine whether they are free of the metabolism-based hepatotoxicity produced by acetaminophen.

Amyloid-β probes: Review of structure-activity and brain-kinetics relationships

The number of people suffering from Alzheimer's disease (AD) is expected to increase dramatically in the coming years, placing a huge burden on society. Current treatments for AD leave much to be desired, and numerous research efforts around the globe are focused on developing improved therapeutics. In addition, current diagnostic tools for AD rely largely on subjective cognitive assessment rather than on identification of pathophysiological changes associated with disease onset and progression. These facts have led to numerous efforts to develop chemical probes to detect pathophysiological hallmarks of AD, such as amyloid-β plaques, for diagnosis and monitoring of therapeutic efficacy. This review provides a survey of chemical probes developed to date for AD with emphasis on synthetic methodologies and structure-activity relationships with regards to affinity for target and brain kinetics. Several probes discussed herein show particularly promising results and will be of immense value moving forward in the fight against AD.

Recent Development of Bifunctional Small Molecules to Study Metal-Amyloid-β Species in Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease related to the deposition of aggregated amyloid-β (Aβ) peptides in the brain. It has been proposed that metal ion dyshomeostasis and miscompartmentalization contribute to AD progression, especially as metal ions (e.g., Cu(II) and Zn(II)) found in Aβ plaques of the diseased brain can bind to Aβ and be linked to aggregation and neurotoxicity. The role of metal ions in AD pathogenesis, however, is uncertain. To accelerate understanding in this area and contribute to therapeutic development, recent efforts to devise suitable chemical reagents that can target metal ions associated with Aβ have been made using rational structure-based design that combines two functions (metal chelation and Aβ interaction) in the same molecule. This paper presents bifunctional compounds developed by two different design strategies (linkage or incorporation) and discusses progress in their applications as chemical tools and/or potential therapeutics.

N-Methyl-N-(2-methylphenyl)acetamide

In the title compound, C₁₀H₁₃NO, the N atom and the methyl group are almost coplanar with the benzene ring to which they are bonded [deviations of 0.131 (1) and 0.038 (1) Å, respectively, from the ring plane]. In the crystal structure, inter­molecular C—H⋯O hydrogen bonds form a three-dimensional network. Mol­ecules are stacked parallel to the b-axis direction.

Synthesis and In Vitro Evaluation of Imidazo[1,2-b]pyridazines as Ligands for β-Amyloid Plaques

A series of imidazo[1,2-b]pyridazine derivatives were synthesized and evaluated for binding to amyloid plaques in vitro using synthetic aggregates of Aβ1-40. Binding affinities of these compounds were found to range from 11.0 to >1000 nM, depending on the various substitution patterns in the 6-position and 2-position. 2-(4'-Dimethylaminophenyl)-6-(methylthio)imidazo[1,2-b]pyridazine (4) showed high binding affinity (K i = 11.0 nM) and might be useful for the development of novel positron emission tomography radiotracers for imaging Aβ plaques.

Synthesis and characterization of novel phenylindoles as potential probes for imaging of β-amyloid plaques in the brain

We synthesized a novel series of phenylindole (PI) derivatives and evaluated their biological activities as probes for imaging Aβ plaques in vivo. The affinity for Aβ plaques was assessed by an in vitro-binding assay using pre-formed synthetic Aβ aggregates. 2-phenyl-1H-indole (2-PI) derivatives showed high affinity for Aβ42 aggregates with K(i) values ranging from 4 to 32 nM. 2-PI derivatives clearly stained Aβ plaques in an animal model of AD. In biodistribution experiments using normal mice, 2-PI derivatives displayed sufficient uptake for imaging, ranging from 1.1% to 2.6% ID/g. Although additional modifications are necessary to improve uptake by and clearance from the brain, 2-PI derivatives may be useful as a backbone structure to develop novel Aβ imaging agents.

Synthesis and evaluation of indolinyl- and indolylphenylacetylenes as PET imaging agents for beta-amyloid plaques

Two new phenylacetylene derivatives, 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)indoline 8 and 5-((4-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)phenyl)ethynyl)-1H-indole 14, targeting beta-amyloid (Abeta) plaques have been prepared. In vitro binding carried out in tissue homogenates prepared from postmortem AD brains with [(125)I]IMPY (6-iodo-2-(4'-dimethylamino-)phenyl-imidazo[1,2-a]pyridine) as the radioligand indicated good binding affinities (K(i)=4.0 and 1.5nM for 8 and 14, respectively). Brain penetration of the corresponding radiofluorinated ligands, evaluated in the normal mice, showed good initial brain penetration (4.50 and 2.43% ID/g (injected dose/gram) for [(18)F]8 and [(18)F]14 at 2min after injection) with moderate to low washout rates from the brain (1.71% ID/g at 2h and 2.10% ID/g at 3h, respectively). Autoradiography and homogenate binding studies demonstrated the high specific binding of [(18)F]14 to the Abeta plaques; however, [(18)F]8 showed low specific binding. These preliminary results identified that indolylphenylacetylene, 14, may be a good lead for further structural modification to develop a useful Abeta plaque imaging agent.

Synthesis of 5- and 6-substituted 2-(4-dimethylaminophenyl)-1,3-benzoxazoles and their in vitro and in vivo evaluation as imaging agents for amyloid plaque

A series of novel 5- and 6-substituted 2-(4-dimethylaminophenyl)-1,3-benzoxazoles was synthesized and their potential as imaging probes for Alzheimer's Disease (AD)-related amyloid plaque was evaluated in vitro and in vivo. In vitro binding affinities for Abeta1-40 peptide of several of these compounds were in the low-nanomolar range . The lowest K(i) of 9.3nM was found for N-(2-(4-(dimethylamino)phenyl)-1,3-benzoxazol-5-yl)-4-iodobenzamide (1e). Its (123)I-radiolabeled form ([(123)I]1e) was subsequently prepared by iododestannylation of the corresponding tributylstannyl precursor and evaluated in vivo in a baboon model using SPECT imaging. Contrary to our expectations, 1e did not cross the blood-brain barrier (BBB) to any significant extent.