Synthesis and Evaluation of ¹⁸F-Labeled Chalcone Analogue for Detection of α-Synuclein Aggregates in the Brain Using the Mouse Model

Abstract

In the brains of patients with synucleinopathies such as Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy, α-synuclein (α-syn) aggregates deposit abnormally to induce neurodegeneration, although the mechanism is unclear. Thus, in vivo imaging studies targeting α-syn aggregates have attracted much attention to guide medical intervention against synucleinopathy. In our previous study, a chalcone analogue, [125I]­PHNP-3, functioned as a feasible probe in terms of α-syn binding in vitro; however, it did not migrate to the mouse brain, and further improvement of brain uptake was required. In the present study, we designed and synthesized two novel 18F-labeled chalcone analogues, [18F]­FHCL-1 and [18F]­FHCL-2, using a central nervous system multiparameter optimization (CNS MPO) algorithm with the aim of improving blood–brain barrier permeation in the mouse brain. Then, we evaluated their utility for in vivo imaging of α-syn aggregates using a mouse model. In the competitive inhibition assay, both chalcone analogues exhibited high binding affinity for α-syn aggregates (K i = 2.6 and 3.4 nM, respectively), while no marked amyloid β (Aβ)-binding was observed. The 18F-labeling reaction was successfully performed. In a biodistribution experiment, brain uptake of both chalcone analogues in normal mice (2.09 and 2.40% injected dose/gram (% ID/g) at 2 min postinjection, respectively) was higher than that of [125I]­PHNP-3, suggesting that the introduction of 18F into the chalcone analogue led to an improvement in brain uptake in mice while maintaining favorable binding ability for α-syn aggregates. Furthermore, in an ex vivo autoradiography experiment, [18F]­FHCL-2 showed the feasibility of the detection of α-syn aggregates in the mouse brain in vivo. These preclinical studies demonstrated the validity of the design of α-syn-targeting probes based on the CNS MPO score and the possibility of in vivo imaging of α-syn aggregates in a mouse model using 18F-labeled chalcone analogues.

A Fluorescent Probe as a Lead Compound for a Selective α-Synuclein PET Tracer: Development of a Library of 2-Styrylbenzothiazoles and Biological Evaluation of [18F]PFSB and [18F]MFSB

“…The most relevant finding provided by the autoradiography experiment was the ability of both tracers to differentiate between misfolded proteins. Although some scaffolds successfully afforded a moderate αSYN/Aβ selectivity, the complete lack of binding to Aβ plaques has been displayed solely by the 4-nitrophenyl chalcone derivatives developed by Kaide et al However, these compounds displayed insufficient clearance when injected in the healthy brain of ddY mice. , To the best of our knowledge, 2-styrylbenzothiazoles are the only other scaffold exhibiting such selectivity but additionally offer the basis for extensive structural optimization in the perspective of lower lipophilicity.…”

“…These factors highlight the import of high selectivity as a key feature of candidate αSYN tracers. An encouraging αSYN/Aβ selectivity was detected for a chalcone-based scaffold recently proposed by Kaide et al, but its pharmacokinetic profile was unsatisfactory. , General criteria for the successful development of an αSYN PET tracer were outlined by the Michael J. Fox Foundation and are more comprehensively discussed by Korat et al, together with the most relevant scaffolds developed so far …”

A closer look at amyloid ligands, and what they tell us about protein aggregates

“…13 shows the ligands that selectivity bind to aSyn fibrils. 54,55,[57][58][59][60][61] Fig. 14 fibrils, and a screen of several indanones by Nan et al demonstrated that all had a higher affinity for Ab fibrils (e.g.…”

“…We compiled a database of 3457 measurements of ligand–amyloid fibril interactions from the scientific literature. 50–385 The requirements to be included in this database were (1) a known chemical structure, (2) a structure that does not contain metals (metalloligands have been thoroughly reviewed elsewhere in the literature 43,386–391 ), (3) a K d or IC 50 measurement, and (4) a fibril target related to Aβ, tau, or αSyn. Nearly half of the database reports on binding to Aβ fibrils formed in vitro , although recent years has seen a shift in focus toward tau and αSyn fibrils formed in vitro (Table 1).…”

Structure–Activity Relationships of Cyano-substituted Indole Derivatives as Ligands for α-Synuclein Aggregates

“…Among them, the benzothiazole derivative (PP-BTA-4) and the benzofuranone derivative (Tg-52) have been reported as α-syn fluorescent probes and could label α-syn aggregates in in vitro histopathological staining. Besides, indolinone derivative (WC58a), benzimidazole derivative (BI-2), and chalcone derivatives (IDP-3, PHNP-3, and FHCL-2) − have been found to have good affinities to α-syn aggregates. With a fluorine or iodine atom in their scaffolds, they have been converted to radiotracers and might be applied in nuclear medicine imaging with further development (Figure ).…”