Advancing Parkinson’s Disease Diagnostics: The Potential of Arylpyrazolethiazole Derivatives for Imaging α-Synuclein Aggregates

The development of positron emission tomography (PET) tracers capable of detecting α-synuclein (α-syn) aggregates in vivo would represent a breakthrough for advancing the understanding and enabling the early diagnosis of Parkinson’s disease and related disorders. It also holds the potential to assess the efficacy of therapeutic interventions. However, this remains challenging due to different structures of α-syn aggregates, the need for selectivity over other structurally similar amyloid proteins, like amyloid-β (Aβ), which frequently coexist with α-syn pathology, and the low abundance of the target in the brain that requires the development of a high-affinity ligand. To develop a successful PET tracer for the central nervous system (CNS), stringent criteria in terms of polarity and molecular size must also be considered, as the tracer must penetrate the blood–brain barrier and have low nonspecific binding to brain tissue. Here, we report a series of arylpyrazolethiazole (APT) derivatives, rationally designed from a structure–activity relationship study centered on existing ligands for α-syn fibrils, with a particular focus on the selectivity toward α-syn fibrils and control of physicochemical properties suitable for a CNS PET tracer. In vitro competition binding assays performed against [3H]MODAG-001 using recombinant α-syn and Aβ1–42 fibrils revealed APT-13 with an inhibition constant of 27.8 ± 9.7 nM and a selectivity of more than 3.3 fold over Aβ. Radiolabeled [11C]APT-13 demonstrated excellent brain penetration in healthy mice with a peak standardized uptake value of 1.94 ± 0.29 and fast washout from the brain (t1/2 = 9 ± 1 min). This study highlights the potential of APT-13 as a lead compound for developing PET tracers to detect α-syn aggregates in vivo.


Table of Contents
Table of Contents             The NMR spectra of compounds were analysed using MestreNova.
Figure S8. 1 H and 13 C NMR spectra of 1.

Figure
Figure S9. 1 H and 13 C NMR spectra of 2.

Figure
Figure S33. 1 H and 13 C NMR spectra of 26.

Figure S35 .
Figure S35.Chromatogram of APT-2 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S36 .
Figure S36.Chromatogram of APT-3 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S37 .
Figure S37.Chromatogram of APT-4 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S38 .
Figure S38.Chromatogram of APT-5 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S39 .
Figure S39.Chromatogram of APT-6 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S40 .
Figure S40.Chromatogram of APT-7 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S41 .
Figure S41.Chromatogram of APT-8 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S42 .
Figure S42.Chromatogram of APT-9 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S43 .
Figure S43.Chromatogram of APT-10 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S44 .
Figure S44.Chromatogram of APT-11 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S45 .
Figure S45.Chromatogram of APT-12 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Figure S46 .
Figure S46.Chromatogram of APT-13 at Sig. 254 nm and mass spectrum of the main peak [M+H] + .

Table S1 .
Chemical properties, BBB score and CNS MPO for all APT-compounds calculated with Chemicalize.n.d.

Table S2 .
K i value for APT library on α-syn and Aβ.K i data reported in MEAN K i + SEM, three data points (dp) for α-syn, two dp for Aβ; a dp = 4, b dp =2; n.d.= not determined

Table S3 .
Results from LogD determination assay.