Imaging of Fibroblast Activation Protein in the Microenvironment of Tumors by 18F‑Labeled Positron Emission Tomography Tracers with Increased Polarity.

Fibroblast activation protein (FAP) is a serine protease that is highly expressed by fibroblasts in the microenvironment of tumors. Therefore, it represents a promising target for molecular imaging and therapy of tumors. 68Ga-chelator-based PET tracers addressing FAP have emerged as promising tools for imaging of a variety of tumors. To further improve the imaging quality and to expand the application range, we developed 18F-fluorinated FAP inhibitors for tumor imaging. For this purpose, the fluorinated anionic quinoline derivative 17a was synthesized and biologically evaluated. The synthesis consisted of nine linear steps and 15 steps in total, including a Pd-catalyzed Buchwald-Hartwig coupling and two Cu-catalyzed 1,3-dipolar cycloadditions (click reactions) as key steps. The fluorinated quinoline derivative 17a showed high inhibitory activity against FAP (IC50 = 20 nM) and high selectivity toward related enzymes. The FAP inhibitor 17a is highly hydrophilic (log D 7.4 = -1.10) and metabolically stable (93% intact compound after incubation with mouse liver microsomes for 90 min) and was therefore synthesized as an 18F-labeled PET tracer. The two-step radiosynthesis with 1-azido-2-[18F]-fluoroethane provided the PET tracer [18F]17a in acceptable radiochemical yield (21.3%), high radiochemical purity (98.2%), and molar activities in the range of 0.8-3.0 GBq/μmol within a total synthesis time of 165 min. [18F]17a was stable in human and murine serum. In biodistribution studies, [18F]17a showed relatively fast renal and hepatobiliary elimination. In a mouse xenograft model with an FAP-expressing tumor, [18F]17a displayed high accumulation in the tumor tissue with very high tumor-to-background ratios. This PET tracer shows promising imaging characteristics and could serve as a lead compound for next-generation imaging probes targeting FAP.