Comparative evaluation of three 18F-fluorinated FAP ligands in rodent tumor models

Fibroblast activation protein (FAP) is almost exclusively expressed on cancer-associated stromal cells, making it a promising target for tumor imaging by positron emission tomography (PET). While 68 Ga- or Al 18 FF-labeled FAP inhibitors (FAPIs) have been characterized in detail, the potential advantages of FAPIs containing a covalently bound 18 F-label remain largely unknown. The aim of the present work was to address this gap by comparing two FAPIs with a covalently bound 18 F-label and the chelator-based radioligand Al 18 FF-FAPI-42. The 18 F-labeled FAPIs were prepared by direct (6- 18 FF-FAPI) or indirect ( 18 FAFA-FAPI) radiofluorination, or by the Al 18 FF chelation method (Al 18 FF-FAPI-42), which afforded the tracers in activity yields of 11–57 % and with molar activities of 5–170 GBq/μmol. Cellular uptake studies revealed significantly higher accumulation of all three candidates in HT1080-FAP compared to HT1080-WT cells. 6- 18 FF-FAPI and Al 18 FF-FAPI-42 showed comparable FAP-selectivity and tumor uptake in mice inoculated with the two cell lines and rats bearing subcutaneous DSL-6A/C1 tumors, while no in vivo FAP-selectivity was observed for 18 FAFA-FAPI. Al 18 FF-FAPI-42 exhibited lower hepatobiliary excretion and faster clearance from FAP-negative tissues in the subcutaneous tumor models. In contrast, 6- 18 FF-FAPI showed higher tumor uptake and better tumor retention in an intracerebral U87 glioma tumor model. When compared to the established glioma tracer 18 FFET, both FAP-targeting tracers visualized intracerebral tumors with more than two-fold higher tumor-to-background ratios. In conclusion, while the chelator-based radioligand Al 18 FF-FAPI-42 is well-suited for visualization of peripheral tumors, 6- 18 FF-FAPI with a covalently bound 18 F-label shows more favorable properties for brain tumor imaging. • μPET-Studies with three radiofluorinated FAP-specific probes in rodents. • Al 18 FF-FAPI-42 demonstrated best performance in peripheral tumor models. • 6- 18 FF-FAPI showed superior uptake and retention in intracerebral tumor models. • Both tracers outperformed 18 FFET with higher tumor-to-background ratios.