Preparation of ¹⁸F-Labeled Tracers Targeting Fibroblast Activation Protein via Sulfur [¹⁸F]Fluoride Exchange Reaction

Early detection and treatment of cancers can significantly increase patient prognosis and enhance the quality of life of affected patients. The emerging significance of the tumor microenvironment (TME) as a new frontier for cancer diagnosis and therapy may be exploited by radiolabeled tracers for diagnostic imaging techniques such as positron emission tomography (PET). Cancer-associated fibroblasts (CAFs) within the TME are identified by biomarkers such as fibroblast activation protein alpha (FAP alpha), which are expressed on their surfaces. Targeting FAP alpha using small-molecule F-18-labeled inhibitors (FAPIs) has recently garnered significant attention for non-invasive tumor visualization using PET. Herein, two potent aryl-fluorosulfate-based FAPIs, 12 and 13, were synthetically prepared, and their inhibition potency was determined using a fluorimetric FAP assay to be IC50 9.63 and 4.17 nM, respectively. Radiofluorination was performed via the sulfur [F-18]fluoride exchange ([F-18]SuFEx) reaction to furnish [F-18]12 and [F-18]13 in high activity yields (AY) of 39-56% and molar activities (A(m)) between 20-55 GBq/mu mol. In vitro experiments focused on the stability of the radiolabeled FAPIs after incubation with human serum, liver microsomes and liver cytosol. Preliminary PET studies of the radioligands were performed in healthy mice to investigate the in vivo biodistribution and F-18 defluorination rate. Fast pharmacokinetics for the FAP-targeting tracers were retained and considerable bone uptake, caused by either F-18 defluorination or radioligand accumulation, was observed. In summary, our findings demonstrate the efficiency of [F-18]SuFEx as a radiolabeling method as well as its advantages and limitations with respect to PET tracer development.