Synthesis, radiolabeling, and evaluation of a (4-quinolinoyl)glycyl-2-cyanopyrrolidine analogue for fibroblast activation protein (FAP) PET imaging

Fibroblast activation protein (FAP) is regarded as a promising target for the diagnosis and treatment of tumors as it was overexpressed in cancer-associated fibroblasts. FAP inhibitors bearing a quinoline scaffold have been proven to show high affinity against FAP in vitro and in vivo, and the scaffold has been radio-labeled for the imaging and treatment of FAP-positive tumors. However, currently available FAP imaging agents both contain chelator groups to enable radio-metal labeling, making those tracers more hydrophilic and not suitable for the imaging of lesions in the brain. Herein, we report the synthesis, radio-labeling, and evaluation of a (18)F-labeled quinoline analogue ([(18)F]3) as a potential FAP-targeted PET tracer, which holds the potential to be blood–brain barrier-permeable. [(18)F]3 was obtained by one-step radio-synthesis via a copper-mediated S(N)A(R) reaction from a corresponding boronic ester precursor. [(18)F]3 showed moderate lipophilicity with a log D ( 7.4 ) value of 1.11. In cell experiments, [(18)F]3 showed selective accumulation in A549-FAP and U87 cell lines and can be effectively blocked by the pre-treatment of a cold reference standard. Biodistribution studies indicated that [(18)F]3 was mainly excreted by hepatic clearance and urinary excretion, and it may be due to its moderate lipophilicity. In vivo PET imaging studies indicated [(18)F]3 showed selective accumulation in FAP-positive tumors, and specific binding was confirmed by blocking studies. However, low brain uptake was observed in biodistribution and PET imaging studies. Although our preliminary data indicated that [(18)F]3 holds the potential to be developed as a blood–brain barrier penetrable FAP-targeted PET tracer, its low brain uptake limits its application in the detection of brain lesions. Herein, we report the synthesis and evaluation of [(18)F]3 as a novel small-molecule FAPI-targeted PET tracer, and our results suggest further structural optimizations would be needed to develop a BBB-permeable PET tracer with this scaffold.