F-18 peptide labelling: Neurotensin derivatives

The tridecapeptide Neurotensin (NT) exhibits a high specific affinity to receptors located on various tumours like small cell lung carcinoma or human colon carcinoma. The purpose of this study was to evaluate the potential of using succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) for the specific radiolabelling of a non-lysine containing oligopeptide. This was demonstrated using the C-terminal bioactive hexapeptide NT(8-13) as well as its partially reduced stabilised congeners [Arg⁸ps(CH₂NH)Arg⁹]NT(8-13), [Arg⁸ps(CH₂NH)Arg⁹, Tle¹²]NT(8-13). The radiosynthesis of [¹⁸F]SFB started from ethyl trimethylammonium benzoate via a standard three-step procedure. Finally O-(N-succinimidyl) N,N,N',N'-tetramethyluronium tetrafluoroborate was used as activating agent to give [¹⁸F]SFB in 40 % overall radiochemical yield related to [¹⁸F]F^- within 100 min (Wester et al. Nucl. Med. Biol. 1996, 23, 365).
Our labelling experiments revealed that [¹⁸F]SFB reacts with selected N-terminal Arg-peptides with moderate to good chemoselectivities in aqueous buffered solutions (phosphate-borate buffer; pH 7.1 - 8.3). To shorten the period of time necessary for a complete consumption of [¹⁸F]SFB we found it advisable to heat the mixture to 40 - 50 °C for 20 min. The desired products were obtained after HPLC purification in radiochemical yields of 10 to 43 % (80 - 120 min) related to [¹⁸F]SFB. The conditions mentioned above allow the active ester to react specifically with the a-amino group of the N-terminal arginine unit. The radioactive product was identified by chromatographic comparison with a reference compound. The reactions carried out with nonradioactive SFB were shown to give the corresponding 4-FB peptides of which their structural identity was confirmed by proton NMR and MS studies.

The labelled peptides display in vitro affinities to the Neurotensin receptor NTR1 in the nanomolar range (K_D values: 1-9 nM). The accumulation of radioactivity in tumour tissue of tumour bearing mice in vivo is still moderate, demonstrated by tumour/blood and tumour/muscle relations of 2-8 after 30 min (calculated from % D/g tissue).

This work is supported by the European Community (BMH4-CT98-3198)