Molecular imaging probes such as PET-tracers have the potential to improve the accuracy of tumor characterization by directly\nvisualizing the biochemical situation. Thus, molecular changes can be detected early before morphological manifestation. The\nA3 adenosine receptor (A3AR) is described to be highly expressed in colon cancer cell lines and human colorectal cancer\n(CRC), suggesting this receptor as a tumor marker. The aim of this preclinical study was the evaluation of [18F]FE@SUPPY\nas a PET-tracer for CRC using in vitro imaging and in vivo PET imaging. First, affinity and selectivity of FE@SUPPY and its\nmetabolites were determined, proving the favorable binding profile of FE@SUPPY. The human adenocarcinoma cell line HT-29\nwas characterized regarding its hA3AR expression and was subsequently chosen as tumor graft. Promising results regarding the\npotential of [18F]FE@SUPPY as a PET-tracer for CRC imaging were obtained by autoradiography as �2.3-fold higher accumulation\nof [18F]FE@SUPPY was found in CRC tissue compared to adjacent healthy colon tissue fromthe same patient. Nevertheless, first in\nvivo studies usingHT-29 xenografts showed insufficient tumor uptake due to (1) poor conservation of target expression in xenografts\nand (2) unfavorable pharmacokinetics of [18F]FE@SUPPY in mice.We therefore conclude that HT-29 xenografts are not adequate\nto visualize hA3ARs using [18F]FE@SUPPY.
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