1 research outputs found
Optimized Translocator Protein Ligand for Optical Molecular Imaging and Screening
Translocator
protein (TSPO) is a validated target for molecular
imaging of a variety of human diseases and disorders. Given its involvement
in cholesterol metabolism, TSPO expression is commonly elevated in
solid tumors, including glioma, colorectal cancer, and breast cancer.
TSPO ligands capable of detection by optical imaging are useful molecular
tracers for a variety of purposes that range from quantitative biology
to drug discovery. Leveraging our prior optimization of the pyrazolopyrimidine
TSPO ligand scaffold for cancer imaging, we report herein a new generation
of TSPO tracers with superior binding affinity and suitability for
optical imaging and screening. In total, seven candidate TSPO tracers
were synthesized and vetted in this study; the most promising tracer
identified (<b>29</b>, <i>K</i><sub>d</sub> = 0.19
nM) was the result of conjugating a high-affinity TSPO ligand to a
fluorophore used routinely in biological sciences (FITC) via a functional
carbon linker of optimal length. Computational modeling suggested
that an <i>n</i>-alkyl linker of eight carbons in length
allows for positioning of the bulky fluorophore distal to the ligand
binding domain and toward the solvent interface, minimizing potential
ligand–protein interference. Probe <b>29</b> was found
to be highly suitable for in vitro imaging of live TSPO-expressing
cells and could be deployed as a ligand screening and discovery tool.
Competitive inhibition of probe <b>29</b> quantified by fluorescence
and <sup>3</sup>H-PK11195 quantified by traditional radiometric detection
resulted in equivalent affinity data for two previously reported TSPO
ligands. This study introduces the utility of TSPO ligand <b>29</b> for in vitro imaging and screening and provides a structural basis
for the development of future TSPO imaging ligands bearing bulky signaling
moieties