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

A method to detect and quantify aggregated & alpha;-synuclein(& alpha;SYN)fibrils in vivo would drastically impact the currentunderstanding of multiple neurodegenerative diseases, revolutionizingtheir diagnosis and treatment. Several efforts have produced promisingscaffolds, but a notable challenge has hampered the establishmentof a clinically successful & alpha;SYN positron emission tomography(PET) tracer: the requirement of high selectivity over the other misfoldedproteins amyloid & beta; (A & beta;) and tau. By designing and screeninga library of 2-styrylbenzothiazoles based on the selective fluorescentprobe RB1, this study aimed at developing a selective & alpha;SYN PET tracer. [H-3]PiB competition binding assaysidentified PFSB (K-i = 25.4 & PLUSMN; 2.3 nM) and its less lipophilic analogue MFSB,which exhibited enhanced affinity to & alpha;SYN (K-i = 10.3 & PLUSMN; 4.7 nM) and preserved selectivity over A & beta;.The two lead compounds were labeled with fluorine-18 and evaluatedusing in vitro autoradiography on human brain slices,where they demonstrated up to 4-fold increased specific binding inMSA cases compared to the corresponding control, reasonably reflectingselective binding to & alpha;SYN pathology. In vivo PET imaging showed [F-18]MFSB successfullycrosses the blood-brain barrier (BBB) and is taken up in thebrain (SUV = 1.79 & PLUSMN; 0.02). Although its pharmacokinetic profileraises the need for additional structural optimization, [F-18]MFSB represents a critical step forward in the developmentof a successful & alpha;SYN PET tracer by overcoming the major challengeof & alpha;SYN/A & beta; selectivity.

Automated summary

This study aimed to develop a selective αSYN PET tracer by designing and screening a library of 2-styrylbenzothiazoles based on the selective fluorescent probe RB1. The lead compounds PFSB and MFSB showed enhanced affinity to αSYN and preserved selectivity over Aβ. In vitro and in vivo experiments demonstrated the successful crossing of the blood-brain barrier and uptake in the brain by [F-18]MFSB.