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