Ga-68 labeling of stable neurotensin(8-13) analogs via carbamoylated arginine residues gives NTS ₁ R PET ligands with promising in vitro profile

Ziel/Aim The neurotensin receptor 1 (NTS₁ R) is considered an interesting target for cancer diagnosis and therapy as it is overexpressed in various tumors (e.g. colorectal and pancreatic carcinoma). Applying the concept of functionalized carbamoylated arginines (1), we aimed at the synthesis and in vitro characterization of stable Ga-68 labeled NTS₁ R PET ligands derived from neurotensin(8–13) (NT(8–13)).

Methodik/Methods Analogs of NT(8–13) (sequence: Arg⁸-Arg⁹-Pro¹⁰-Tyr¹¹-Ile¹²-Leu¹²) were prepared by solid-phase peptide synthesis. To stabilize the peptides against enzymatic degradation, Arg⁸ or Arg⁹ was replaced by N^α-methylated Arg and Ile¹² by tert-butylglycine (2). Additionally, an amino-functionalized N^ω-carbamoylated Arg was incorporated instead of Arg⁸ or Arg⁹. Conjugation of the peptides to DOTA and insertion of Ga³⁺ (100 °C, 10 min) gave the potential PET ligands UR-LS029 and UR-LS051. NTS₁ R affinities were determined by radioligand competition binding at HT-29 cells using [³ H]UR-MK300 (1). The PET ligands [⁶⁸ Ga]UR-LS029 and [⁶⁸ Ga]UR-LS051 (synthesis see contribution by Moosbauer et al.) were characterized with respect to stability in human plasma (37 °C) by HPLC, logD (shake flask assay) and by autoradiography at cryo-sections of HT-29 xenografts.

Ergebnisse/Results UR-LS029 and UR-LS051 showed high NTS₁ R affinities (K_i < 16 nM). [⁶⁸ Ga]UR-LS029 and [⁶⁸ Ga]UR-LS051 (logD: −2.6 and −3.2, respectively) exhibited high radiochemical purities (> 99 %), proved to be stable in plasma over 3 h (> 98 % intact tracer), and allowed the detection of NTS₁ Rs in HT-29 xenografts.

Schlussfolgerungen/Conclusions Incorporation of functionalized N^ω-carbamoylated arginines in backbone stabilized NT(8–13) analogs gave access to stable Ga-68 labeled NTS₁ R PET ligands. The in vitro results justify in vivo studies, e.g. in tumor bearing nude mice.

• Literatur/References

• 1 Keller M. et al. J. Med. Chem. 2016; , 59: 1925-1945.
• 2 Schindler L. et al. ACS Med. Chem. Lett. 2019; , 10 , 960-965.