18F-labeled tracers targeting fibroblast activation protein

Abstract

Background: Cancer-associated fibroblasts are found in the stroma of epithelial tumors. They are characterized by overexpression of the fibroblast activation protein (FAP), a serine protease which was already proven as attractive target for chelator-based theranostics. Unfortunately, the value of gallium-68 labeled tracers is limited by their batch size and the short nuclide half-life. To overcome this drawback, radiolabeling with aluminum fluoride complexes and 6-fluoronicotinamide derivatives of the longer-lived nuclide fluorine-18 was established. The novel compounds were tested for their FAP-specific binding affinity. Uptake and binding competition were studied in vitro using FAP expressing HT-1080 cells. HEK cells transfected with the closely related dipeptidyl peptidase-4 (HEK-CD26) were used as negative control. Small animal positron emission tomography imaging and biodistribution experiments were performed in HT-1080-FAP xenografted nude mice. [18F]AlF-FAPI-74 was selected for PET/CT imaging in a non-small cell lung cancer (NSCLC) patient.

Results: In vitro, 18F-labeled FAPI-derivatives demonstrated high affinity (EC50 = < 1 nm to 4.2 nm) and binding of up to 80% to the FAP-expressing HT1080 cells while no binding to HEK-CD26 cells was observed. While small animal PET imaging revealed unfavorable biliary excretion of most of the 18F-labeled compounds, the NOTA bearing compounds [18F]AlF-FAPI-74 and -75 achieved good tumor-to-background ratios, as a result of their preferred renal excretion. These two compounds showed the highest tumor accumulation in PET imaging. The organ distribution values of [18F]AlF-FAPI-74 were in accordance with the small animal PET imaging results. Due to its less complex synthesis, fast clearance and low background values, [18F]AlF-FAPI-74 was chosen for clinical imaging. PET/CT of a patient with metastasized non-small cell lung cancer (NSCLC), enabled visualization of the primary tumor and its metastases at the hepatic portal and in several bones. This was accompanied by a rapid clearance from the blood pool and low background in healthy organs.

Conclusion: [18F]AlF-labeled FAPI derivatives represent powerful tracers for PET. Owing to an excellent performance in PET imaging, FAPI-74 can be regarded as a promising precursor for [18F]AlF-based FAP-imaging.

Keywords: 18F-AlF-FAPI-74; Fibroblast activating protein; Positron Emission Tomography.

Conflict of interest statement

Patent application (EP 18155420.5) for quinoline based FAP targeting agents for imaging and therapy in nuclear medicine (UH, TL, FG, CK and WM).

© 2021. The Author(s).

Figures

Fig. 1

Overview of the synthesized precursors discussed in this study. The corresponding log D values were determined by detection of the radiolabeled tracer in 1-octanol and phosphate buffered saline (pH 7). For values labeled with an asterix, the log D was calculated from the relative content of the non-radioactive reference standards in the 1-octanol and the water phase by HPLC analysis and peak integration

Fig. 2

Small animal PET imaging. The white arrow indicates the site of the xenotransplanted tumor. Shown are the maximum intensity projections (A, C) and time-activity curves (B, D) of selected organs in case of [18F]AlF-FAPI-74 (A, B) and [18F]AlF-FAPI-75 (C, D). Experiments were conducted in nude mice transplanted with HT-1080-FAP cells and images were acquired in the indicated time periods after tracer administration

Fig. 3

Biodistribution study of [18F]AlF-FAPI-74. Accumulation in different tissues (A) and tumor-to-organ ratios (B) in HT-1080-FAP bearing Balb/c nu/nu mice 30, 60, 120, and 240 min after injection

Fig. 4

Whole body PET/CT images of a 68y old patient with NSCLC in the left upper lobe. Shown are the maximum intensity projections and transaxial cuts at A 1 h and B 3 h after administration of 323 MBq [18F]AlF-FAPI-74