Preclinical Evaluation and Pilot Clinical Study of 18F-AlF-labeled FAPI-Tracers for PET Imaging of Cancer Associated Fibroblasts

Abstract Background The recent development of many different types of radiotracers that target the fibroblast activation protein (FAP) has been promising for tumor diagnosis. Here, we set out to develop 18F-labeled FAP tracers for cancer-associated fibroblast imaging and evaluated the potential of these tracers for clinical application. Methods The non-radioactive reference compounds and labeling precursors were synthesized using organic chemistry and the binding affinities were identified using surface plasmon resonance. Both radioligands (18F-P-FAPI and 18F-FAPI-42) were produced in an automated manner via complexation of Al18F. For in vitro characterization of 18F-P-FAPI and 18F-FAPI-42, we conducted studies to determine the partition coefficients, stability, cellular uptake, internalization, and efflux. In vivo biodistribution studies and microPET imaging of 18F-P-FAPI and 18F-FAPI-42, in comparison to 68Ga-FAPI-04, were conducted on the A549-FAP tumor bearing mice. Finally, one nasopharyngeal cancer patient underwent whole-body PET/CT after being injected with 18F-P-FAPI. Results Both 18F-P-FAPI and 18F-FAPI-42 were successful prepared with high specificity, rapid internalization, and potent affinity binding toward FAP. Compared to 18F-FAPI-42, 18F-P-FAPI exhibited lower levels of cellular efflux in the A549-FAP cells and higher stability in vivo. Furthermore, in vivo studies of 18F-P-FAPI in the A549-FAP tumor model indicated a higher tumor uptake than 18F-FAPI-42 and 68Ga-FAPI-04. An initial diagnostic application in patient with nasopharyngeal cancer, 18F-P-FAPI and 18F-FDG PET/CT showed comparable results for both primary tumors and lymph node metastases. Conclusion The radiofluorinated FAP-ligands demonstrated promising characteristics in their preclinical evaluation, and the feasibility of cancer-associated fibroblasts imaging was demonstrated using PET studies.