Design and Development of 99mTc-Labeled FAPI Tracers for SPECT Imaging and 188Re Therapy

Most epithelial tumors recruit fibroblasts and other nonmalignant cells and activate them into cancer-associated fibroblasts. This often leads to overexpression of the membrane serine protease fibroblast-activating protein (FAP). It has already been shown that DOTA-bearing FAP inhibitors (FAPIs) generate high-contrast images with PET/CT scans. Since SPECT is a lower-cost and more widely available alternative to PET, Tc-99m-labeled FAPIs represent attractive tracers for imaging applications in a larger number of patients. Furthermore, the chemically homologous nuclide Re-188 is available from generators, which allows FAP-targeted endoradiotherapy. Methods: For the preparation of Tc-99m-tricarbonyl complexes, a chelator was selected whose carboxylic acids can easily be converted into various derivatives in the finished product, enabling a platform strategy based on the original tracer. The obtained Tc-99m complexes were investigated in vitro by binding and competition experiments on FAP-transfected HT-1080 ( HT-1080-FAP) or on mouse FAP-expressing (HEK-muFAP) and CD26-expressing (HEKCD26) HEK cells and characterized by planar scintigraphy and organ distribution studies in tumor-bearing mice. Furthermore, a first-in-humans application was done on 2 patients with ovarian and pancreatic cancer, respectively. Results: Tc-99m-FAPI-19 showed specific binding to recombinant FAP-expressing cells with high affinity. Unfortunately, liver accumulation, biliary excretion, and no tumor uptake were observed on planar scintigraphy for a HT1080-FAP-xenotransplanted mouse. To improve the pharmacokinetic properties, hydrophilic amino acids were attached to the chelator moiety of the compound. The resulting Tc-99m-labeled FAPI tracers revealed excellent binding properties (#45% binding;.95% internalization), high affinity (half-maximal inhibitory concentration, 6.4-12.7 nM), and significant tumor uptake (#5.4% injected dose per gram of tissue) in biodistribution studies. The lead candidate Tc-99m-FAPI-34 was applied for diagnostic scintigraphy and SPECT of patients with metastasized ovarian and pancreatic cancer for follow-up to therapy with 90Y-FAPI-46. Tc-99m-FAPI-34 accumulated in the tumor lesions, as also shown on PET/CT imaging using 68GaFAPI-46. Conclusion: Tc-99m-FAPI-34 represents a powerful tracer for diagnostic scintigraphy, especially when PET imaging is not available. Additionally, the chelator used in this compound allows labeling with the therapeutic nuclide Re-188, which is planned for the near future.