No abstract

Fibroblast activation protein (FAP) is a potential target for tumor diagnosis and treatment due to its selective expression on cancer-associated fibroblasts (CAFs) in most solid tumor stroma. Two FAP inhibitor (FAPI) derived ligands (L1 and L2) containing different lengths of D Pro-Gly (PG) repeat units as linkers were designed and synthesized with high affinity for FAP. Two stable hydrophilic 99m Tc-labeled complexes ([ 99m Tc]Tc-L1 and [ 99m Tc]Tc-L2) were obtained. In vitro cellular studies show that the uptake mechanism is correlated with FAP uptake, and [ 99m Tc]Tc-L1 shows a higher cell uptake and specific binding to FAP. A nanomolar K d value for [ 99m Tc]Tc-L1 indicates its significantly high target affinity for FAP. The biodistribution and microSPECT/CT images obtained for U87MG tumor mice show that [ 99m Tc]Tc-L1 has high tumor uptake with specificity to FAP and high tumor-to-nontarget ratios. As an inexpensive, easily made, and widely available tracer, [ 99m Tc]Tc-L1 holds great promise for clinical applications.

Fibroblast activation protein (FAP) is a potential target for tumor diagnosis and treatment because it is selectively expressed on the cell membrane of cancer-associated fibroblasts in most solid tumor stroma. The aim of this study was to develop a 99mTc-labeled fibroblast activation protein inhibitor (FAPI) tracer, evaluate its imaging efficacy in nude mice, and further explore its biodistribution in healthy volunteers and uptake in tumor patients. An FAPI-derived ligand (DP-FAPI) containing d-proline was designed and synthesized as a linker, and a stable hydrophilic 99mTc-labeled complex ([99mTc]Tc-DP-FAPI) was obtained by kit formulation. In vitro cellular uptake and saturation binding assays were performed in FAP-transfected HT-1080 cells (FAP-HT-1080). The biodistribution was characterized, and micro-single-photon emission computed tomography (SPECT) imaging was performed in BALB/c nude mice bearing U87 MG tumors. Furthermore, a first-in-man application was performed in four healthy volunteers and three patients with gastrointestinal tumors. In vitro, the nanomolar K d values of [99mTc]Tc-DP-FAPI indicated that it had significantly high target affinity for FAP. Biodistribution and micro-SPECT imaging studies showed that [99mTc]Tc-DP-FAPI exhibited high uptake and high tumor-to-nontargeted ratios. The calculated effective dose for [99mTc]Tc-DP-FAPI was approximately <5 mSv in four healthy volunteers. In three patients with gastrointestinal tumors, [99mTc]Tc-DP-FAPI quantitative SPECT/CT revealed high and reliable uptake. [99mTc]Tc-DP-FAPI exhibited high selectivity and affinity for FAP in vitro. The safety and effectiveness of [99mTc]Tc-DP-FAPI in primary tumor imaging have been confirmed by animal and clinical studies, revealing the potential clinical application value of this tracer.