Fibroblast
activation protein (FAP) is overexpressed in cancer-associated
fibroblasts (CAFs) in a majority of human epithelial cancers. With
low expression in normal organs, FAP has become a promising molecular
target for tumor theranostics. To develop a lower cost and more widely
available alternative to positron emission tomography (PET), two isocyanide-containing
FAP inhibitors (CN-C5-FAPI and CN-PEG4-FAPI)
were synthesized and radiolabeled with 99mTc to obtain
[99mTc][Tc-(CN-C5-FAPI)6]+ and [99mTc][Tc-(CN-PEG4-FAPI)6]+ in high yields (>95%). They showed good stability in saline
and mouse serum. The partition coefficient (log P) values of [99mTc][Tc-(CN-C5-FAPI)6]+ and [99mTc][Tc-(CN-PEG4-FAPI)6]+ were −0.86 ± 0.03 and
−2.38 ± 0.07, respectively, indicating that they were
good hydrophilic complexes. The low nanomolar IC50 values
of CN-C5-FAPI and CN-PEG4-FAPI indicated that
they had specificity to FAP. In vitro cellular uptake
and blocking experiments implied a FAP-targeted uptake mechanism.
The nanomolar Kd values from the saturation
binding assay indicated that they had significantly high target affinity
to FAP. The biodistribution and blocking study in BALB/c nude mice
bearing U87MG tumors showed that both exhibited specific tumor uptake.
[99mTc][Tc-(CN-PEG4-FAPI)6]+ showed a higher tumor uptake and a higher tumor/nontarget ratio
than [99mTc][Tc-(CN-C5-FAPI)6]+. The results of micro-single-photon emission computed tomography
(SPECT) imaging studies of [99mTc][Tc-(CN-C5-FAPI)6]+ and [99mTc][Tc-(CN-PEG4-FAPI)6]+ were in accordance
with the biodistribution results, suggesting that [99mTc][Tc-(CN-PEG4-FAPI)6]+ is a promising tumor imaging
agent for targeting FAP.