Fluorescent 2-styrylpyridazin-3(2H)-one derivatives as probes targeting amyloid-beta plaques in Alzheimer's disease

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.

Picomolar-sensitive β-amyloid fibril fluorophores by tailoring the hydrophobicity of biannulated π-elongated dioxaborine-dyes

The pathological origin of Alzheimer's disease (AD) is still shrouded in mystery, despite intensive worldwide research efforts. The selective visualization of β-amyloid (Aβ), the most abundant proteinaceous deposit in AD, is pivotal to reveal AD pathology. To date, several small-molecule fluorophores for Aβ species have been developed, with increasing binding affinities. In the current work, two organic small-molecule dioxaborine-derived fluorophores were rationally designed through tailoring the hydrophobicity with the aim to enhance the binding affinity for Aβ1-42 fibrils -while concurrently preventing poor aqueous solubility-via biannulate donor motifs in D-π-A dyes. An unprecedented sub-nanomolar affinity was found (K d = 0.62 ± 0.33 nM) and applied to super-sensitive and red-emissive fluorescent staining of amyloid plaques in cortical brain tissue ex vivo. These fluorophores expand the dioxaborine-curcumin-based family of Aβ-sensitive fluorophores with a promising new imaging agent.

Highly Chemo- and Regioselective Vinylation of N-Heteroarenes with Vinylsulfonium Salts

An efficient chemo- and regioselective N-vinylation of N-heteroarenes has been developed using vinylsulfonium salts. The reaction proceeded under mild and transition-metal-free conditions and consistently provided moderate to high yields of vinylation products with 100% E-stereoselectivity. This reaction is also highly chemoselective, and compatible with a variety of functional groups, such as -NHR, -NH₂, -OH, -COOH, ester, etc.

Chalcone Based Homodimeric PET Agent, 11C-(Chal)2DEA-Me, for Beta Amyloid Imaging: Synthesis and Bioevaluation

Homodimeric chalcone based ¹¹C-PET radiotracer, ¹¹C-(Chal)₂DEA-Me, was synthesized, and binding affinity toward beta amyloid (Aβ) was evaluated. The computational studies revealed multiple binding of the tracer at the recognition sites of Aβ fibrils. The bivalent ligand ¹¹C-(Chal)₂DEA-Me displayed higher binding affinity compared to the corresponding monomer, ¹¹C-Chal-Me, and classical Aβ agents. The radiolabeling yield with carbon-11 was 40-55% (decay corrected) with specific activity of 65-90 GBq/μmol. A significant ( p < 0.0001) improvement in the binding affinity of ¹¹C-(Chal)₂DEA-Me with synthetic Aβ42 aggregates over the monomer, ¹¹C-Chal-Me, demonstrates the utility of the bivalent approach. The PET imaging and biodistribution data displayed suitable brain pharmacokinetics of both ligands with higher brain uptake in the case of the bivalent ligand. Metabolite analysis of healthy ddY mouse brain homogenates exhibited high stability of the radiotracers in the brain with >93% intact tracer at 30 min post injection. Both chalcone derivatives were fluorescent in nature and demonstrated significant changes in the emission properties after binding with Aβ42. The preliminary analysis indicates high potential of ¹¹C-(Chal)₂DEA-Me as in vivo Aβ42 imaging tracer and highlights the significance of the bivalent approach to achieve a higher biological response for detection of early stages of amyloidosis.

A Pyridazine-Based Fluorescent Probe Targeting Aβ Plaques in Alzheimer's Disease

Accumulation of β-amyloid (Aβ) plaques comprising Aβ40 and Aβ42 in the brain is the most significant factor in the pathogenesis of Alzheimer's disease (AD). Thus, the detection of Aβ plaques has increasingly attracted interest in the context of AD diagnosis. In the present study, a fluorescent pyridazine-based dye that can detect and image Aβ plaques was designed and synthesized, and its optical properties in the presence of Aβ aggregates were evaluated. An approximately 34-fold increase in emission intensity was exhibited by the fluorescent probe after binding with Aβ aggregates, for which it showed high affinity (K_D  = 0.35 µM). Moreover, the reasonable hydrophobic properties of the probe (log P = 2.94) allow it to penetrate the blood brain barrier (BBB). In addition, the pyridazine-based probe was used in the histological costaining of transgenic mouse (APP/PS1) brain sections to validate the selective binding of the probe to Aβ plaques. The results suggest that the pyridazine-based compound has the potential to serve as a fluorescent probe for the diagnosis of AD.

Advances in development of fluorescent probes for detecting amyloid-β aggregates

With accumulating evidence suggesting that amyloid-β (Aβ) deposition is a good diagnostic biomarker for Alzheimer's disease (AD), the discovery of active Aβ probes has become an active area of research. Among the existing imaging methods, optical imaging targeting Aβ aggregates (fibrils or oligomers), especially using near-infrared (NIR) fluorescent probes, is increasingly recognized as a promising approach for the early diagnosis of AD due to its real time detection, low cost, lack of radioactive exposure and high-resolution. In the past decade, a variety of fluorescent probes have been developed and tested for efficiency in vitro, and several probes have shown efficacy in AD transgenic mice. This review classifies these representative probes based on their chemical structures and functional modes (dominant solvent-dependent mode and a novel solvent-independent mode). Moreover, the pharmaceutical characteristics of these representative probes are summarized and discussed. This review provides important perspectives for the future development of novel NIR Aβ diagnostic probes.

(68)Ga based probe for Alzheimer's disease: synthesis and preclinical evaluation of homodimeric chalcone in β-amyloid imaging

In an attempt to explore use of PET radioisotope, (68)Ga, in the diagnosis of Alzheimer's disease, a metal-based homodimeric ligand exhibiting high affinity towards Aβ aggregates was designed by conjugating two chalcone units with the chelating system, diethylenetriaminepentaacetic acid. Bischalcone derivative, 5,8-bis(carboxymethyl)-13-(4-((E)-3-(4-(dimethylamino)phenyl)acryloyl)phenoxy)-2-(2-(2-(4-((E)-3-(4-(dimethylamino)phenyl)acryloyl)phenoxy)ethylamino)-2-oxoethyl)-10-oxo-2,5,8,11-tetraazatridecane-1-carboxylic acid, DT(Ch)2 was synthesized in 95% yield with high purity. It was radiolabelled with (68)Ga under mild conditions with 85.4% efficiency and 9.5-10 MBq nmol(-1) specific activity. An in vitro binding assay on Aβ42 aggregates displayed high binding affinity of (68)Ga-DT(Ch)2 and inhibition constant of 4.18 ± 0.62 nM. The fluorescent properties of the ligand with peaks of absorption/emission at 410/540 nm exhibited a blue shift with 5.5-fold increase in emission intensity on binding with Aβ aggregates. Blood kinetics of the complex performed on normal rabbit exhibited fast clearance (t1/2(F) = 24 ± 0.08 min; t1/2(S) = 2 h 40 ± 0.04 min). Ex vivo biodistribution analysis demonstrated blood-brain barrier penetration with brain uptake of 1.24 ± 0.31% ID g(-1) at 2 min p.i. and rapid washout with negligible activity (0.36% ID g(-1)) left at 30 min p.i. These preliminary studies reveal that the bivalent approach of synthesis had minimal effect on binding affinity, signifying that the developed (68)Ga-complex, (68)Ga-DT(Ch)2, may offer a new perspective in generator produced PET imaging probes for Alzheimer's disease.

Development of fluorescent probes that bind and stain amyloid plaques in Alzheimer's disease

β-amyloid (Aβ) plaques in the brain are composed of Aβ40 and Aβ42 peptides, and are the defining pathological feature of Alzheimer's disease (AD). Fluorescent probes that can detect Aβ plaques have gained increasing interest as potential tools for in vitro and in vivo monitoring of the progression of AD. In this study, chalcone-mimic fluorescent probe 5 was designed and prepared. Probe 5 exhibited an approximately 50-fold increase in emission intensity after mixing with Aβ42 aggregates, a high affinity for Aβ42 aggregates (K D = 1.59 μM), and reasonable lipophilicity (log P value = 2.55). Probe 5 also exhibited specific staining of Aβ plaques in the transgenic mice (APP/PS1) brain sections. Ex vivo fluorescence imaging of the brain from normal and TG mice revealed that probe 5 was able to penetrate the BBB and stain the Aβ plaques. These results suggest that chalcone-mimic probe 5 possessed the requirements of a fluorescent probe for Aβ plaques and may be useful in AD research.