Novel Pyridine-Based Fibroblast Activation Protein-Targeted Tracers: Design, Synthesis, and PET Imaging Evaluation for Tumors

To overcome the short tumor retention and nonspecific uptake of quinoline-based FAP inhibitors (e.g., FAP-46), we employed scaffold hopping to design novel phenylalanine-pyridyl-glycyl-prolamide derivatives. Among synthesized compounds (1a–i), multimeric derivatives 1h and 1i showed enhanced FAP affinity (IC50: 0.51 and 0.27 nM). In A549-FAP tumor models, [68Ga]­1b exhibited higher tumor uptake (7.02% ID/g) and lower organ uptake than [68Ga]­FAP-46, while [68Ga]­1f demonstrated improved tumor retention (7.84% ID/g at 4 h) and high target-to-background ratios. [68Ga]­1h/1i showed higher tumor uptake but increased nontarget accumulation. Specific FAP targeting was confirmed, and [68Ga]­1f exhibited favorable hydrophilicity and stability. This work shows the phenylalanine-pyridyl-glycyl-prolamide scaffold is a viable alternative to quinoline-based ones, with [68Ga]­1f promising for clinical PET imaging and [68Ga]­1h/1i for radioligand therapy, providing a new direction for FAP-targeted radiopharmaceuticals.