Design, Synthesis, and Biological Evaluation of a Novel [(18)F]AlF-H(3)RESCA-FAPI Radiotracer Targeting Fibroblast Activation Protein.

Background: Cancer-associated fibroblasts (CAFs) are key contributors to the tumorigenic process, with fibroblast activation protein (FAP) overexpressed on CAFs in numerous epithelial carcinomas. FAP represents a promising target for tumor imaging and therapy. We aimed to develop a novel [(18)F]AlF-H(3)RESCA-FAPI radiotracer with a high labeling yield at room temperature for positron emission tomography (PET) imaging of FAP-expressing tumors. Methods: The H(3)RESCA-FAPI chelator was synthesized and radiolabeled with [(18)F]AlF. Its radiotracer binding affinity to FAP was assessed using surface plasmon resonance (SPR). Its in vitro stability, plasma clearance, and biodistribution were evaluated. PET imaging was performed in U87MG tumor-bearing mice, with a blocking study to assess tracer specificity. Results: The [(18)F]AlF-H(3)RESCA-FAPI radiotracer demonstrated a high binding affinity to FAP (K(D) < 10.09 pM) and favorable radiochemical yields (92.4 ± 2.4%) with >95% radiochemical purity. In vitro and in vivo studies showed good stability and rapid clearance from non-target tissues. PET imaging revealed specific tumor uptake, which was significantly reduced by co-injection with unlabeled DOTA-FAPI-04. Conclusions: [(18)F]AlF-H(3)RESCA-FAPI is a promising radiotracer for PET imaging of FAP-expressing tumors. Further optimization of its pharmacokinetics could make it a potential candidate for clinical translation.