In Vitro Evaluation of Amide-Linked Coumarin Scaffolds for the Inhibition of α‑Synuclein and Tau Aggregation

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most prevalent neurodegenerative disorders characterized by continuous loss of functional neurons. The numbers of AD and PD patients will likely double by 2060 and 2040, reaching 13.9 and 1.2 million, respectively, in the US alone. Although both AD and PD are multifactorial in origin, the accumulation of misfolded proteins such as α-synuclein (α-syn) and tau contribute to nerve function disruption. Therefore, inhibition of α-syn and tau aggregation via small-molecule disruptors of oligomer and fibril formation presents a promising method for treating AD and PD. Coumarin scaffolds possess a wide range of bioactivities, particularly their antiamyloidogenic potential, which was explored in this study. Our previous work demonstrated that amide linkers and amino indole moieties have antioligomer and antifibrillar effects. This study involves coupling the coumarin scaffold with various aromatic moieties, including aminoindoles, methoxy-substituted phenyl, and polyhydroxy aromatic functionalities, via an amide linker for establishing the structural activity relationship (SAR) for the inhibition of oligomer and fibril formation. In total, 38 coumarin-based amide compounds were prepared to first explore the antifibrillar activity on recombinant α-syn. The best compounds were then tested to assess the antioligomer effects, tau aggregation activity, inclusion inhibition, and dimerization in cells. Biophysical methods such as thioflavin T (ThT) fluorescence assays, photoinduced cross-linking of unmodified proteins (PICUP), survival assays, and electron microscopic observations were used to evaluate the inhibitory effects of analogs on α-syn and tau aggregation. The coumarin-amide-dihydroxybenzene derivatives demonstrated superior effects on the inhibition of α-syn aggregation when compared with the coumarin-amide-indole derivatives. The methoxy (nondemethylated) counterparts of compounds 13 and 17 failed at reducing α-syn fibril formation. The coumarin-amide-dihydroxybenzene derivatives 13 and 17, exhibited different degrees of inhibition on the α-syn oligomer and inclusion formation. Compound 13 inhibited tau (2N4R isoform) oligomer formation and reduced tau dimerization in a cell-based assay. In conclusion, the results presented herein will guide future optimization of molecules with inhibitory effects on prone-to-aggregate proteins and may pave the way for disease-modifying treatments for neurodegenerative disorders.