Synthesis and Evaluation of Novel 99m Tc-Labeled Fibroblast Activation Protein Inhibitors Containing Cyclohexane as Promising Tumor Tracers

Fibroblast activation protein (FAP) is highly expressed in cancer-associated fibroblasts (CAFs) of most solid tumors, and this protein has been a promising target for tumor diagnosis and treatment. Nuclear medicine diagnostic technology has important clinical value for early detection, precise localization and evaluation of treatment effects. To reduce the cost, improve the effectiveness, and increase the scope of use of SPECT radiotracers, a ligand containing cyclohexane as a linker was synthesized, and four 99mTc-labeled FAP inhibitor complexes with different coligands were prepared ([99mTc]Tc-(tricine/TPPTS)-L, [99mTc]Tc-(tricine/TPPMS)-L, [99mTc]Tc-(tricine/PDA)-L, and [99mTc]Tc-(tricine/NIC)-L). In vitro cell experiments revealed that the four complexes specifically bound to cells, and the binding was related to the uptake pathway of FAP; among them, [99mTc]Tc-(tricine/TPPTS)-L showed the best specific uptake. Biodistribution experiments in normal and tumor-bearing mice revealed that [99mTc]Tc-(tricine/TPPTS)-L had relatively low uptake in various nontarget organs and high uptake in U87MG tumors. SPECT/CT imaging revealed obvious uptake in tumors with high FAP expression (U87MG, HT-1080-FAP and HT-29) with a clean background, and no visualization was observed in the tumors of the blocking group (U87MG block, HT-1080-FAP block and HT-29 block) or the tumors with low FAP expression (HT-1080), indicating that the uptake of [99mTc]Tc-(tricine/TPPTS)-L in the tumors had high FAP specificity. Therefore, [99mTc]Tc-(tricine/TPPTS)-L is expected to become a potential tumor molecular probe for clinical application.