Biodistribution and catabolism of ¹⁸F-labeled neurotensin(8-13) analogs

4-([¹⁸F]fluoro)benzoyl-neurotensin(8-13) (¹⁸FB-Arg⁸-Arg⁹-Pro¹⁰-Tyr¹¹- Ile¹²-Leu¹³-OH, 1) and two analogs stabilized in one and two positions (¹⁸FB-Arg⁸Psi(CH₂NH)Arg⁹-Pro¹⁰-Tyr¹¹- Ile¹²-Leu¹³-OH, 2, ¹⁸FB-Arg⁸Psi(CH₂NH)Arg⁹-Pro¹⁰-Tyr¹¹-Tle¹²-Leu¹³-OH, 3) 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 tumors, overexpressing neurotensin receptor 1 (NTR1) by positron emission tomography (PET). All analogs exhibited in vitro binding affinity in the low nanomolar range to NTR1-expressing human tumors, measured by quantitative receptor autoradiography, HT-29 and WiDr cells, and to sections of tumors derived from these cell lines in mice. The radiotracers were internalized 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 tumors, only a moderate uptake of the radioligands into the studied tumors was observed, presumingly due to degradation in vivo and fast elimination by the kidneys. In comparison with the other analogs, the specific tumor uptake expressed as tumor-to-muscle relation was highest for the radioligand 3. The blood clearance of 3 was reduced by co-injection of peptidase inhibitors. The catabolic pathways of the radiofluorinated peptides were elucidated. The results suggest that the high binding affinity to NTR1 and the stabilization against proteolytic degradation are not yet sufficient for tumor imaging by PET.