NTS1 affinities of the precursor peptides 7–9, 14, 16, 18 and 19 as well as of the potential PET ligands 11–13, 20 and 21 were determined by competitive receptor binding experiments on human HT-29 colon carcinoma cells stably expressing NTS1, using the previously reported tritium-labeled NT(8–13) derivative [3H]UR-MK30023 as radioligand. Here, NTS is linked to colon carcinoma.