Anastasia Loktev, Thomas Lindner, Walter Mier, Jürgen Debus, Annette Altmann, Dirk Jäger, Frederik Giesel, Clemens Kratochwil, Philippe Barthe, Christian Roumestand, Uwe Haberkorn

The tumor stroma, which accounts for a large part of the tumor mass, represents an attractive target for the delivery of diagnostic and therapeutic compounds. Here, the focus is notably on a subpopulation of stromal cells, known as cancer-associated fibroblasts (CAFs), which are present in more than 90% of epithelial carcinomas including pancreatic, colon and breast cancer. CAFs feature a high expression of fibroblast activation protein (FAP), which is not detectable in adult normal tissue but associated with a poor prognosis in cancer patients. Methods: Based on a FAP specific enzyme inhibitor (FAPI) we developed an iodinated and a DOTA-coupled radiotracer which were evaluated in vitro using uptake, competition and efflux studies as well as confocal microscopy of a fluorescence labeled variant. Furthermore, imaging and biodistribution studies were done in tumor-bearing animals. Finally, proof of concept was realized by imaging patients with 68Ga-labeled FAPI. Results: Both FAPIs showed a high specificity, affinity and rapid internalization into FAP expressing cells both in vitro and in vivo. Biodistribution studies in tumor-bearing mice as well as in first cancer patients demonstrated high intratumoral uptake of the tracer and a fast body clearance, resulting in high-contrast images and negligible radiation exposure of healthy tissue. A first comparison with the commonly used radiotracer 18F-FDG revealed a clear superiority of the new FAP ligand in one patient with locally advanced lung adenocarcinoma. Conclusion: Radiolabeled FAPIs allow fast imaging with very high contrast in tumors having a high stromal content and may, therefore, serve as pantumor agents. The coupling of these molecules to DOTA or other chelators allows labeling not only with 68Ga, but also with therapeutic isotopes such as 177Lu or 90Y.