Design, synthesis and evaluation of fluorescent probes for the diagnosis of Alzheimer's disease and Parkinson's disease

Alzheimer’s disease (AD) and Parkinson's disease (PD) are two of the most common neurodegenerative disorders. Amyloid-β (Aβ) and tau are the two hallmark proteins in AD, while typical pathological hallmarks of PD are characterized by the formation of fibrillar aggregates composed of α-synuclein (ASN). In AD, the development of brain penetrating probes which could specifically target to Aβ plaques or neurofibrillary tangles (NFTs) is considered to be a significant advance not only to diagnosis but as an enabling tool for drug development. Novel non-invasive imaging may improve the diagnosis of AD. Several compounds have been developed to detect Aβ plaques pathology in the brains of patients with AD, whereas only few compounds have been reported to visualize NFTs. Here we have successfully synthesized and evaluated series of fluorescent probess for the detection of Aβ plaques and NFTs in AD, such as benzothiazoles, bis(styryl) derivatives, spiropyrans, indolium derivatives, trimethine cyanines and N-2-aryl-1,2,3-triazoles. The staining experiments using hippocampus sections from AD brain were visible by fluorescence microscopy indicated that several compounds may bind selectively to Aβ plaques or NFTs in the tissue. Quantification of the binding affinity of several active probes by a new competitive Thiazine Red R displacement assay was carried out. Selected candidates were profiled in the in vitro liver hepatocellular carcinoma cell (HepG2) assay and in the in vivo wild-type zebrafish embryo development assay for toxicological studies. Some probes display no or negligible cytotoxicity at different corresponding concentrations, especially benzothiazoles, spiropyrans, indolium derivatives, trimethine cyanines, which indicates sufficient safety for in vivo evaluation in mouse models. Additionally, selected tau active probes were applied to detect tau deposits in the olfactory epithelium tissues which may give assess to detection method for the diagnosis of AD in the early stages. At the same time, we investigated several imaging agents for the detection of ASN aggregates in postmortem PD brain sections. In addition, these probes may be radiolabeled and applied as PET and SPECT radioligands for noninvasive imaging in vivo