Development of a Neurotensin-Derived (68)Ga-Labeled PET Ligand with High In Vivo Stability for Imaging of NTS(1) 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 (NTS(1)R) is present in certain malignant tumors, radiolabeled NTS(1)R ligands can serve as molecular tools for tumor imaging. A straightforward approach for developing NTS(1)R 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 NTS(1)R affinity. Applying a new strategy for peptide stabilization, it is possible to develop a peptidic gallium-68 labeled NTS(1)R PET ligand with high in vivo stability and high NTS(1)R affinity. Investigations of the PET ligand in mice with subcutaneous NTS(1)R-positive tumors revealed the NTS(1)R-mediated visualization of the tumor. Future developments, such as NTS(1)R PET ligands with improved biodistribution, will benefit from these results. ABSTRACT: Overexpression of the neurotensin receptor type 1 (NTS(1)R), a peptide receptor located at the plasma membrane, has been reported for a variety of malignant tumors. Thus, targeting the NTS(1)R with (18)F- or (68)Ga-labeled ligands is considered a straightforward approach towards in vivo imaging of NTS(1)R-expressing tumors via positron emission tomography (PET). The development of suitable peptidic NTS(1)R PET ligands derived from neurotensin is challenging due to proteolytic degradation. In this study, we prepared a series of NTS(1)R 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(ω)-carbamoylated arginine side chain. Insertion of Ga(3+) in the DOTA chelator gave potential PET ligands that were evaluated concerning NTS(1)R affinity (range of K(i) values: 1.2–21 nM) and plasma stability. Four candidates were labeled with (68)Ga(3+) and used for biodistribution studies in HT-29 tumor-bearing mice. [(68)Ga]UR-LS130 ([(68)Ga]56), containing an N-terminal methyl group and a β,β-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 [(68)Ga]56 in the tumor was NTS(1)R-mediated, as proven by blocking studies.