Exploiting the 4‑Phenylquinazoline Scaffold for the Development of High Affinity Fluorescent Probes for the Translocator Protein (TSPO)

The
quinazoline class was exploited to search for a new translocator
protein (TSPO) fluorescent probe endowed with improved affinity and
residence time (RT). Computational studies on an “in-house”
collection of quinazoline derivatives, featuring highly steric demanding
groups at the amide nitrogen, suggested that, despite their molecular
extension, these ligands are still easily lodged in the TSPO binding
site. Binding assays supported this hypothesis, highlighting a low
nanomolar/subnanomolar affinity of these ligands, together with a
higher RT of the representative compound 11 with respect
to our previously reported indole-based fluorescent probe. Thanks
to the amenability of the amide nitrogen atom to be substituted with
bulky groups, we developed quinazoline-based imaging tools by fluorescently
labeling the scaffold at this position. Probes with relevant TSPO
affinity, favorable spectroscopic properties, and improved RT were
identified. The results from fluorescence microscopy showed that these
probes specifically labeled the TSPO at the mitochondrial level in
the U343 cell line.