18F- and 68Ga-Labeled Neurotensin Peptides for PET Imaging of Neurotensin Receptor 1

The neurotensin (NT) receptor-1 (NTS1) is overexpressed in a variety of carcinomas and is therefore an interesting target for imaging with positron emission tomography (PET). The aim of this study was the development of new NT derivatives based on the metabolically stable peptide sequence NLys-Lys-Pro-Tyr-Tle-Leu suitable for PET imaging. The NT peptides were synthesized by solid-phase supported peptide synthesis and elongated with respective chelators (NODA-GA, DOTA) for Ga-68-labeling or propargylglycine for F-18-labeling via copper-catalyzed azide-alkyne cycloaddition. Receptor affinities of the peptides for NTS1 were in the range of 19-110 nM. Biodistribution studies using HT29 tumor-bearing mice showed highest tumor uptake for [Ga-68]6 and [Ga-68]8 and specific binding in small-animal PET studies. The tumor uptake of Ga-68-labeled peptides in vivo significantly correlated with the in-vitro Ki values for NTS1. [Ga-68]8 displayed an excellent tumor-to-background ratio and could therefore be considered as an appropriate molecular probe for NTS1 imaging by PET.