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 ⁶⁸Ga- or Al[¹⁸F]F-labeled FAP inhibitors (FAPIs) have been characterized in detail, the potential advantages of FAPIs containing a covalently bound ¹⁸F-label remain largely unknown. The aim of the present work was to address this gap by comparing two FAPIs with a covalently bound ¹⁸F-label and the chelator-based radioligand Al[¹⁸F]F-FAPI-42. The ¹⁸F-labeled FAPIs were prepared by direct (6-[¹⁸F]F-FAPI) or indirect ([¹⁸F]AFA-FAPI) radiofluorination, or by the Al[¹⁸F]F chelation method (Al[¹⁸F]F-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-[¹⁸F]F-FAPI and Al[¹⁸F]F-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 [¹⁸F]AFA-FAPI. Al[¹⁸F]F-FAPI-42 exhibited lower hepatobiliary excretion and faster clearance from FAP-negative tissues in the subcutaneous tumor models. In contrast, 6-[¹⁸F]F-FAPI showed higher tumor uptake and better tumor retention in an intracerebral U87 glioma tumor model. When compared to the established glioma tracer [¹⁸F]FET, 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[¹⁸F]F-FAPI-42 is well-suited for visualization of peripheral tumors, 6-[¹⁸F]F-FAPI with a covalently bound ¹⁸F-label shows more favorable properties for brain tumor imaging.