Enabling Irreversible FAP Inhibitors for Routine Clinical Use: Process and Quality Control Validation for [68Ga]Ga-FAPi-mFS and [177Lu]Lu-FAPi-mFS

Background/Objectives: Fibroblast activation protein (FAP) represents an attractive target for both cancer imaging and radionuclide therapy. Nevertheless, the prolonged retention of many FAP-targeting inhibitors within tumor tissue remains challenging. Therefore, we investigated FAPi-mFS, an FAP inhibitor derived from FAPi-04 that carries a fluorosulfate warhead in meta position. This structural feature was designed to enable a predominantly irreversible covalent binding to FAP, thereby improving tumor retention time. Methods: [68Ga]Ga-FAPi-mFS and [177Lu]Lu-FAPi-mFS were successfully synthesized for large-scale clinical use by radiolabeling FAPi-mFS under validated conditions. The radiochemical conversion, yield, purity and stability were evaluated using radio-thin-layer chromatography (TLC), radio-high-performance liquid chromatography (HPLC), and in vitro human serum stability tests. Results: In the radiolabeling experiments, reproducibly high radiochemical purities of over 96% were achieved for both nuclides, along with decay-corrected yields of over 90% for gallium-68 (Ga-68) and over 80% for lutetium-177 (Lu-177) for batch sizes for Lu-177 up to 50 GBq. Both radiopharmaceuticals exhibited high in vitro stability with minimal release of Ga-68 (<1%) over 90 min and Lu-177 (<11%) over two physical half-lives. Conclusions: FAPi-mFS can be efficiently labeled with both Ga-68 and Lu-177 with high purity and stability using clinically applicable procedures. As a covalent FAP inhibitor, FAPi-mFS represents a promising strategy for prolonging retention time in tumors and potentially enhancing the therapeutic efficacy of FAP-specific radiopharmaceuticals.