Abstract
Labeled ligands for the neurotensin receptor 1 (NTS(1)R), which is expressed in the CNS, the gastrointestinal tract, and in malignant tumors, are needed to investigate NTS(1)R-ligand binding and NTS(1)R expression. Aiming for fluorescence-labeled neurotensin(8-13)-derived NTS(1)R ligands with high affinity and proteolytic stability, several previous approaches were combined: (1) replacement of Arg(8) by an amino-functionalized carbamoylated arginine, allowing conjugation to a fluorophore, (2) N(α)-methylation of Arg(8) and replacement of Tyr by β,β-dimethyl-l-Tyr(11), conferring proteolytic stability, and (3) replacement of Leu(13) by trimethylsilyl-Ala, boosting binding affinity. This strategy gave fluorescent NTS(1)R ligands with unprecedented NTS(1)R binding affinity (5-TAMRA-labeled ligand 19: K(i) 0.14 nM, sulfo-Cy5-labeled probe 21: K(i) 0.094 nM) and high stability in human plasma (t(1/2) ≫ 48 h). Their suitability for competition binding studies (flow cytometry; 19, 21) and the imaging of NTS(1)R expression in living cells (confocal microscopy, biomolecular imaging; 19, 21) and tumor tissue (biomolecular imaging; 21) is demonstrated.