Synthesis and Preclinical Evaluation of ⁶⁸Ga-Labeled Radiotracers for Monitoring PD-L1 Expression in Tumors

Programmed death-ligand 1 (PD-L1) plays a crucial role in tumor immune evasion, making it an important biomarker and a validated target for cancer immunotherapy. In this study, we designed, synthesized, and preclinically evaluated a series of novel PD-L1-targeted radiotracers based on a phenoxymethyl-biphenyl scaffold. Our design strategy was to incorporate different functional amino acid residues and flexible linkers between the phenoxymethyl-biphenyl scaffold and the DOTA chelator. Among the six radiotracers, [⁶⁸Ga]Ga-PEG-PRO-ZB exhibited high stability, strong binding affinity to PD-L1 (K_D = 19.3 ± 0.6 nM), and low nonspecific uptake. Micro-PET/CT imaging confirmed its ability to detect PD-L1 expression in multiple tumor models. Notably, [⁶⁸Ga]Ga-PEG-PRO-ZB also enabled the dynamic monitoring of PD-L1 expression following immunotherapy. These results demonstrate that [⁶⁸Ga]Ga-PEG-PRO-ZB can effectively visualize PD-L1 expression in vivo, offering valuable insights into the rational design and optimization of small-molecule based PD-L1 radiotracers.