Structure-Activity Relationships of Styrylquinoline and Styrylquinoxaline Derivatives as α-Synuclein Imaging Probes

Synthesis and identification of azocoumarin derivatives toward imaging of α-synuclein aggregates in the brain

To identify α-synuclein aggregation in synucleinopathies is still challenging, due to the lack of specific probes for α-synuclein aggregates with efficient brain uptake. In this work, compact molecules based on coumarin scaffold were synthesized and evaluated for detection and bioimaging of α-synuclein aggregates in the brain. Among the developed compounds, azocoumarin 5 containing push-pull electronic architecture featured selective fluorescence enhancement towards α-synuclein aggregates in comparison to other β-sheet protein species (β-amyloid, tau). In addition, azocoumarin [18F]Cou-NNF was succesfully developed, and demonstrated its potential as radiotracer for imaging brain α-synuclein aggregates, owing to its favorable affinity for α-synuclein aggregates accompanied with efficient brain uptake and little defluorination in vivo. Overall, compact azocoumarin provides an effective lead structure for developing α-synuclein probes, and N=N bond shows promise in enhancing selective affinity for α-synuclein aggregates.

Ligands for Protein Fibrils of Amyloid-β, α-Synuclein, and Tau

Amyloid fibrils are characteristic features of many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. The use of small molecule ligands that bind to amyloid fibrils underpins both fundamental research aiming to better understand the pathology of neurodegenerative disease, and clinical research aiming to develop diagnostic tools for these diseases. To date, a large number of amyloid-binding ligands have been reported in the literature, predominantly targeting protein fibrils composed of amyloid-β (Aβ), tau, and α-synuclein (αSyn) fibrils. Fibrils formed by a particular protein can adopt a range of possible morphologies, but protein fibrils formed in vivo possess disease-specific morphologies, highlighting the need for morphology-specific amyloid-binding ligands. This review details the morphologies of Aβ, tau, and αSyn fibril polymorphs that have been reported as a result of structural work and describes a database of amyloid-binding ligands containing 4,288 binding measurements for 2,404 unique compounds targeting Aβ, tau, or αSyn fibrils.

Development of an 18F-labeled azobenzothiazole tracer for α-synuclein aggregates in the brain

Nuclear imaging of aggregated α-synuclein pathology is an urgent clinical need for Parkinson's disease, yet promising tracers for brain α-synuclein aggregates are still rare. In this work, a class of compact benzothiazole derivatives was synthesized and evaluated for α-synuclein aggregates. Among them, azobenzothiazoles exhibited specific and selective detection of α-synuclein aggregates under physiological conditions. Fluoro-pegylated azobenzothiazole NN-F further demonstrated high-affinity binding to α-synuclein aggregates and efficient 18F-radiolabeling via nucleophilic displacement of a tosyl precursor. [18F]NN-F was stable in plasma in vitro and showed efficient brain uptake with little defluorination in vivo.

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.

In Vivo Near-Infrared Fluorescence Imaging Selective for Soluble Amyloid β Aggregates Using y-Shaped BODIPY Derivative

Soluble amyloid β (Aβ) aggregates, suggested to be the most toxic forms of Aβ, draw attention as therapeutic targets and biomarkers of Alzheimer's disease (AD). As soluble Aβ aggregates are transient and diverse, imaging their diverse forms in vivo is expected to have a marked impact on research and diagnosis of AD. Herein, we report a near-infrared fluorescent (NIRF) probe, BAOP-16, targeting diverse soluble Aβ aggregates. BAOP-16, whose molecular shape resembles "y", showed a marked selective increase in fluorescence intensity upon binding to soluble Aβ aggregates in the near-infrared region and a high binding affinity for them. Additionally, BAOP-16 could detect Aβ oligomers in the brains of Aβ-inoculated model mice. In an in vivo fluorescence imaging study of BAOP-16, brains of AD model mice displayed significantly higher fluorescence signals than those of wild-type mice. These results indicate that BAOP-16 could be useful for the in vivo NIRF imaging of diverse soluble Aβ aggregates.