Development of a Neurotensin-Derived Ga-68-Labeled PET Ligand with High In Vivo Stability for Imaging of NTS1 Receptor-Expressing Tumors

Simple Summary Cancer diagnostics based on molecular imaging techniques such as positron emission tomography (PET) requires radiolabeled tracers, which are taken up by tumors. As the neurotensin receptor type 1 (NTS1R) is present in certain malignant tumors, radiolabeled NTS1R ligands can serve as molecular tools for tumor imaging. A straightforward approach for developing NTS1R PET ligands would be the preparation of fluorine-18 or gallium-68 labeled analogs of the peptide neurotensin. However, as neurotensin derivatives are prone to enzymatic cleavage, structural modifications are needed to prevent peptide degradation while retaining NTS1R affinity. Applying a new strategy for peptide stabilization, it is possible to develop a peptidic gallium-68 labeled NTS1R PET ligand with high in vivo stability and high NTS1R affinity. Investigations of the PET ligand in mice with subcutaneous NTS1R-positive tumors revealed the NTS1R-mediated visualization of the tumor. Future developments, such as NTS1R PET ligands with improved biodistribution, will benefit from these results. Overexpression of the neurotensin receptor type 1 (NTS1R), a peptide receptor located at the plasma membrane, has been reported for a variety of malignant tumors. Thus, targeting the NTS1R with F-18- or Ga-68-labeled ligands is considered a straightforward approach towards in vivo imaging of NTS1R-expressing tumors via positron emission tomography (PET). The development of suitable peptidic NTS1R PET ligands derived from neurotensin is challenging due to proteolytic degradation. In this study, we prepared a series of NTS1R PET ligands based on the C-terminal fragment of neurotensin (NT(8-13), Arg(8)-Arg(9)-Pro(10)-Tyr(11)-Ile(12)-Leu(13)) by attachment of the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) via an N-omega-carbamoylated arginine side chain. Insertion of Ga3+ in the DOTA chelator gave potential PET ligands that were evaluated concerning NTS1R affinity (range of K-i values: 1.2-21 nM) and plasma stability. Four candidates were labeled with Ga-68(3+) and used for biodistribution studies in HT-29 tumor-bearing mice. [Ga-68]UR-LS130 ([Ga-68]56), containing an N-terminal methyl group and a beta,beta-dimethylated tyrosine instead of Tyr(11), showed the highest in vivo stability and afforded a tumor-to-muscle ratio of 16 at 45 min p.i. Likewise, dynamic PET scans enabled a clear tumor visualization. The accumulation of [Ga-68]56 in the tumor was NTS1R-mediated, as proven by blocking studies.