Indolizine-Based Small-Molecule Fluorescent Probes for Ex Vivo Imaging of Amyloid-β Plaques

Abstract Fluorescence probes for visualizing amyloid-β (Aβ) plaques are important tools in Alzheimer’s disease (AD) research. We synthesized a library of 49 indolizine derivatives using a modular synthetic approach and evaluated their fluorescence labeling behavior directly in ex vivo brain tissue sections from 5XFAD transgenic mice. Through an ex vivo tissue-based screening workflow using anti-Aβ antibody 6E10 as a reference standard for plaque-containing regions, five hit compounds (YIZ-20, YIZ-32, YIZ-36, YIZ-45, and YIZ-47) displaying plaque-associated fluorescence patterns were identified. Costaining experiments demonstrated spatial correspondence between probe fluorescence and antibody-defined plaque regions. Spectral analysis showed that the selected YIZ probes exhibited emission profiles with somewhat reduced overlap with the Hoechst 33342 channel compared with thioflavin S (ThS), providing a modest but practical advantage for multichannel imaging applications. These findings highlight the utility of ex vivo tissue-based screening for fluorescent-probe discovery and establish indolizine as a promising scaffold for the development of complementary fluorescence imaging tools for AD-related ex vivo tissue studies.