¹⁸F-Labeled Neurotensin(8-13) Analogues for Tumour Imaging in vivo

4-([¹⁸F]fluoro)benzoyl-neurotensin(8-13) (¹⁸FB-Arg⁸-Arg⁹-Pro¹⁰-Tyr¹¹- Ile¹²-Leu¹³-OH, 1) and three analogues stabilized in one, two and three positions (¹⁸FB-Arg⁸Psi(CH₂NH)Arg⁹-Pro¹⁰-Tyr¹¹- Ile¹²-Leu¹³-OH, 2, ¹⁸FB-Arg⁸Psi(CH₂NH)Arg⁹-Pro¹⁰-Tyr¹¹-Tle¹²-Leu¹³-OH, 3, ¹⁸FB-Arg⁸Psi(CH₂NH)Arg⁹-Pro¹⁰-DmTyr¹¹-Tle¹²-Leu¹³-OH, 4) were synthesized in a radiochemical yield of 25 - 36% and a specific activity of 5 - 15 GBq/mmol. The peptides were evaluated in vitro and in vivo for their potential to image tumours, overexpressing neurotensin receptor 1 (NTR1) by positron emission tomography (PET). The analogues exhibited in vitro binding affinity in the low nanomolar range to NTR1-expressing human tumours, measured by quantitative receptor autoradiography (IC₅₀: 1 0.42±0.05 nM, 2 0.91±0.20 nM, 3 4.1±0.60 nM) , HT-29 (IC₅₀: 24.0±4.2 nM) and WiDr cells and to sections of tumours derived from these cell lines in mice. The radiotracers were internalised in the cells in vitro, and the fluorinated peptides were able to mobilize intracellular Ca²⁺ of WiDr cells. In in vivo studies in rats and in mice bearing HT-29 cell tumours, only a moderate uptake of the radioligands into the studied tumours was observed, presumingly due to degradation in vivo and fast elimination by the kidneys of rats and by the liver of mice. The pathway of the fast enzymatic degradation of the non-stabilized or not sufficiently stabilized ¹⁸F-labeled peptides was elucidated. The main catabolic product is the ¹⁸FB-Arg⁸Psi(CH₂NH)Arg⁹-Pro¹⁰. In vivo, in comparison with the other analogues, the specific tumour uptake expressed as tumour-to-muscle relation was highest for the radioligands 3 and 4. The blood clearance of the analogues was reduced by co-injection of peptidase inhibitors.
The 4-[¹⁸F]fluorobenzoyl label on the N-terminal arginine, the introduction of one reduced peptide bond and two bulky amino acid residues (Tle, DmTyr), are tolerable for a high in vitro affinity to the neurotensin receptor 1 (NTR1) and the ability to be internalised in receptor-positive tumour cells. Only the double and threefold-stabilized (3, 4) exhibited a remarkable specific tumour uptake which was, however, not sufficient for tumour imaging in the abdomen. Additional efforts are required to improve molecule stabilization against enzymatic attack and to enable receptor-based tumour imaging.
This work was partially supported by the BIOMED grant BMH4-CT98-3198 of the EC.