A Simple Kit Formulation for Preparation and Exploratory Human Studies of a Novel <sup>99m</sup>Tc-Labeled Fibroblast Activation Protein Inhibitor Tracer for Imaging of the Fibroblast Activation Protein in Cancers
Qing Ruan, Cheng Zhou, Qianna Wang, Fei Kang, Yuhao Jiang, Guoquan Li, Junhong Feng, Shu Zong, Junbo Zhang, Jing Wang
Abstract
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 99m Tc-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 99m Tc-labeled complex ([ 99m Tc]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 [ 99m Tc]Tc-DP-FAPI indicated that it had significantly high target affinity for FAP. Biodistribution and micro-SPECT imaging studies showed that [ 99m Tc]Tc-DP-FAPI exhibited high uptake and high tumor-to-nontargeted ratios. The calculated effective dose for [ 99m Tc]Tc-DP-FAPI was approximately <5 mSv in four healthy volunteers. In three patients with gastrointestinal tumors, [ 99m Tc]Tc-DP-FAPI quantitative SPECT/CT revealed high and reliable uptake. [ 99m Tc]Tc-DP-FAPI exhibited high selectivity and affinity for FAP in vitro . The safety and effectiveness of [ 99m Tc]Tc-DP-FAPI in primary tumor imaging have been confirmed by animal and clinical studies, revealing the potential clinical application value of this tracer.