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

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 ¹⁸F- or ⁶⁸Ga-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), Arg⁸-Arg⁹-Pro¹⁰-Tyr¹¹-Ile¹²-Leu¹³) 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 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 ⁶⁸Ga³⁺ and used for biodistribution studies in HT-29 tumor-bearing mice. [⁶⁸Ga]UR-LS130 ([⁶⁸Ga]56), containing an N-terminal methyl group and a β,β-dimethylated tyrosine instead of Tyr¹¹, 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 [⁶⁸Ga]56 in the tumor was NTS₁R-mediated, as proven by blocking studies.