Design, Synthesis, and Biological Evaluation of Small-Molecule-Based Radioligands with Improved Pharmacokinetic Properties for Imaging of Programmed Death Ligand 1

Small molecules offer some advantages for developing positron emission tomography (PET) tracers and are therefore a promising approach for imaging and therapy monitoring of programmed death ligand 1 (PD-L1) positive tumors. Here, we report six biphenyl PD-L1 radioligands using the NODA-GA-chelator for efficient copper-64 complexation. These radioligands contain varying numbers of sulfonic and/or phosphonic acid groups, serving as hydrophilizing units to lower the log D-7.4 value down to -4.28. The binding affinities of compounds were evaluated using saturation binding and a real-time binding assay, with a highest binding affinity of 21 nM. Small-animal PET imaging revealed vastly different pharmacokinetic profiles depending on the quantity and type of hydrophilizing units. Of the investigated radioligands, [Cu-64]Cu-3 showed the most favorable kinetics in vitro. This was also found in vivo, with a predominantly renal clearance and a specific uptake in the PD-L1-overexpressing tumor. With further modifications, this compound could be a promising candidate for the imaging of PD-L1 in the clinical setting.