Synthesis and Evaluation of [¹⁸F]SiFA-Conjugated Ligands for Fibroblast Activation Protein Imaging

In recent years, fibroblast activation protein (FAP) has emerged as an important target for the diagnosis and therapy of various tumors due to its high expression on the cell surface of cancer-associated fibroblasts, which are the major components of the tumor stroma. In this study, we synthesized and evaluated ¹⁸F-labeled FAP inhibitors (FAPIs) for FAP imaging. Two silicon fluoride acceptor (SiFA)-conjugated FAPIs were synthesized: one containing a γ-carboxy-l-glutamic acid (Gla) residue (1) and another containing two Gla residues (2). Both ligands exhibited high binding affinities for FAP. ¹⁸F/¹⁹F exchange reactions on both ligands were performed in the presence of 2% water. This resulted in the formation of radioligands [¹⁸F]1 and [¹⁸F]2 in high radiochemical yields. Radioligand [¹⁸F]2, with a more favorable partition coefficient, was selected for the U87MG cell binding study, and the results showed FAP-specific binding of the radioligand to the cells. An ex vivo biodistribution study in U87MG tumor-bearing mice 60 min after injection demonstrated a 5.8-fold higher tumor accumulation of [¹⁸F]2 than that of [¹⁸F]1. Furthermore, PET and ex vivo biodistribution studies of [¹⁸F]2 in U87MG tumor-bearing mice showed high and persistent tumor uptake over time, which was significantly blocked by the preinjection of FAPI-04. Our results indicate that [¹⁸F]SiFA-(Gla)₂-conjugated FAPI ([¹⁸F]2) has the potential for FAP imaging.