Identification and Evaluation of Bisquinoline Scaffold as a New Candidate for α-Synuclein-PET Imaging.

α-Synuclein (α-syn) aggregates are pathologically associated with the hallmarks found in brains affected by synucleinopathies such as Parkinson's disease (PD) and multiple system atrophy (MSA). Therefore, the in vivo detection of α-syn aggregates using radiolabeled probes is useful for the comprehension of and medical intervention for synucleinopathies. In the present study, we identified a bisquinoline scaffold as a new promising structure for targeting α-syn aggregates by a screening assay. Then, based on the scaffold, novel bisquinoline derivatives, BQ1 and BQ2, were designed and synthesized, and we evaluated their utilities as α-syn imaging probes. Both compounds showed high affinity for recombinant α-syn aggregates in binding assays in vitro and clearly detected α-syn aggregates in human brain sections. BQ2 showed higher affinity for α-syn aggregates than BQ1, leading to performing 18F-labeling to obtain [18F]BQ2. In a biodistribution study using normal mice, [18F]BQ2 displayed moderate uptake (1.59% ID/g at 2 min postinjection) into but subsequent retention (1.35% ID/g at 60 min postinjection) in the brain. The results of this study suggest that a bisquinoline derivative may be a new candidate as an α-syn-PET imaging probe after appropriate structure modification for further improvement in the pharmacokinetics.