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 (29, Kd = 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 ...