Fibroblast activation protein (FAP) is overexpressed in cancer-associated fibroblasts and is involved in a variety of tumor-promoting activities such as matrix remodeling, angiogenesis, chemotherapy resistance, and immunosuppression. Because FAP shows low expression in most normal organs, it presents an interesting target for imaging and endoradiotherapy. In this investigation, FAP inhibitors (FAPIs) were modified and optimized for use as theranostic tracers. FAPIs based on a quinoline structure were synthesized and characterized with respect to binding, internalization, and efflux in cells expressing human and murine FAP as well as CD26. Preclinical pharmacokinetics were determined in tumor-bearing animals with biodistribution experiments and small-animal PET. Finally, a proof-of-concept approach toward imaging and therapy was chosen for 2 patients with metastasized breast cancer. Of 15 synthesized FAPIs, FAPI-04 was identified as the most promising tracer for clinical application. Compared with the previously published ligand, FAPI-02, FAPI-04 showed excellent stability in human serum, higher affinity for FAP as opposed to CD26, and slower excretion in vitro. In vivo, a higher SUV was reached in tumor-bearing animals, leading to larger areas under the curve as calculated from biodistribution experiments. Finally, PET/CT scans with Ga-FAPI-04 in 2 patients with metastasized breast cancer revealed high tracer uptake in metastases and a reduction in pain symptoms after therapy with a considerably low dose ofY-FAPI-04. FAPI-04 represents a promising tracer for both diagnostic imaging and, possibly, targeted therapy of malignant tumors with a high content of activated fibroblasts, such as breast cancer.

Fibroblast activation protein (FAP) is overexpressed in cancer associated fibroblasts of several tumor entities. Recent development of quinoline based positron-emission-tomography (PET)-tracers that act as FAP-Inhibitors (FAPI) demonstrated promising results preclinically and already also in few clinical cases. Consecutively this novel tracer is now applied in our hospital to amend the diagnostics of cancer patients facing limitations of standard exams. Here we analyze the tissue biodistribution and preliminary dosimetry of two members of this new class of PET-radiopharmaceuticals. A preliminary dosimetry estimate for FAPI-02 and FAPI-04 was based on two patients examined at 0.2h, 1h and 3h after tracer injection using the QDOSE dosimetry software suit. Further PET/CT scans of tumor patients were acquired 1 h after injection of either FAPI-02 ( = 25) or FAPI-04 ( = 25); for 6 patients an intra-individual related FDG-scan (also acquired 1h p.i.) was available. For the normal tissue of 16 organs, a 2 cm Spheric-VOI was placed in the parenchyma, for tumor lesions a threshold segmented VOI was used to quantify SUVmean/max. Very similar to literature values forF-FDG, Ga-DOTATATE orGa-PSMA-11, an exam with 200 MBq Ga-FAPI-2/4 corresponds to an equivalent dose of approx. 3-4 mSv. After a fast clearance via the kidneys the normal organs showed a low tracer uptake with only minimal changes between 10 min and 3 h p.i.. In FAPI-02 the tumor uptake from 1h to 3h p.i. decreased by 75%, whereas the tumor retention was prolonged with FAPI-04 (25% washout). Regarding tumor-to-background ratios, at 1h p.i. both FAPI-tracers performed equally. In comparison to FDG the tumor uptake was almost equal (average SUV-FDG 7.41; SUV-FAPI-2 7.37; n.s.); the background uptake in brain (11.01 vs 0.32), liver (2.77 vs 1.69) and oral/pharyngeal mucosa (4.88 vs 2.57) was significantly lower with FAPI; other organs were not relevantly different between FDG and FAPI. FAPI-PET/CT is a new diagnostic method in imaging cancer patients. In contrast to FDG no diet/fasting in preparation of the exam is necessary and image acquisition can potentially be started few minutes after tracer application. Tumor-to-background contrast ratios were equal or even improved in comparison to FDG.