A Novel Tetrabranched Neurotensin(8-13) Cyclam Derivative: Synthesis, ⁶⁴Cu-Labeling and Biological Evaluation

New macrocyclic 1,4,8,11-tetraazacyclotetradecane (cyclam) derivatives with 1, 2 and 4 neurotensin(8-13) units 4, 5 and 7 have been synthesized. Compounds 4 and 5 were prepared by the reaction of non-stabilized neurotensin(8-13) and cyclam tetrapropionic acid 2 using 1-ethyl-3-(3-dimethylaminocarbonyl)carbodiimide hydrochloride and N-hydroxysulfosuccinimide. The tetrameric compound 7 was synthesized by Michael addition of neurotensin(8-13) acrylamide 6 and cyclam 1. The copper(II) complexation behavior of 4, 5 and 7 was investigated by UV/visible spectrophotometry and shows that the metal center resides inside the N4 chromophore with additional apical interactions established with pendant arms. The novel tetrabranched neurotensin(8-13) cyclam 7 with nanomolar binding affinity to the neurotensin receptor 1 was efficiently radiolabeled with ⁶⁴Cu under mild conditions. ⁶⁴Cu⊂7 showed slow transchelation in the presence of a large amount of cyclam as competing ligand, while it completely remains intact in the presence of EDTA. The in vivo behavior of ⁶⁴Cu⊂7 was studied in rats and mice. The metabolic stability in rodent models was high with a half-life of intact ⁶⁴Cu⊂7 in plasma of 34 min in rats and 60 min in the mice, respectively. The binding affinity was high enough to demonstrate in vivo binding of ⁶⁴Cu⊂7 to NTR1 overexpressing HT-29 tumor xenotransplants in nude mice. Regarding elimination, ⁶⁴Cu⊂7 showed a substantial renal and reticuloendothelial accumulation. On the other hand, metabolization of the compound in vivo with a resulting metabolite – postulated to be the ⁶⁴Cu-cyclam-tetraarginine complex – also showed long retention in the circulating blood, preventing a better contrast of tumor imaging.