Direct labeling of peptides with [¹⁸F]FDG

Objectives: The routine ¹⁸F labeling of biomacromolecules like peptides and proteins mainly exploits the use of bifunctional labeling precursors, also referred to as prosthetic groups. 2-[¹⁸F]Fluoro-2-deoxy-D-glucose ([¹⁸F]FDG) is the most important clinical PET radiotracer, but only very few examples using readily available [¹⁸F]FDG as a building block for the radiosynthesis of ¹⁸F-labeled compounds. The present study describes the use of [¹⁸F]FDG as a ¹⁸F building block for the direct labeling of various aminooxy-functionalized peptides via chemoselective oxime formation. The potential of this novel peptide labeling reaction was expemplified by means of various neurotensin NT(8-13) derivatives.
Methods: The labeling reaction was performed using a 0.9% NaCl solution of [¹⁸F]FDG and aminooxy-functionalized peptide at different concentrations in a mixture of MeOH/H²O at 80 °C for 30 min. The reaction mixture was analyzed by radio-HPLC to determine the radiochemical yield of the conjugation reaction (Fig. 1).
Results: Direct labeling of aminooxy-functionalized peptides with [¹⁸F]FDG was strongly dependent on the amount of used peptide. Monomeric NT(8-13) derivative gave best radiochemical yields of up to 80% based upon [¹⁸F]FDG. More complex dimeric and tetrameric neurotensin derivatives gave lower radiochemical yields at comparable peptide concentrations. Increase of [¹⁸F]FDG activity also lowered radiochemical yield due to an increasing competitive reaction with glucose originating from the [¹⁸F]FDG solution. Depending on the size of the used peptide, separation of [¹⁸F]FDG-labeled peptide from glucose-labeled peptide is possible by semi-preparative HPLC. The formation of isomers during the aminooxy-aldehyde conjugation reaction between [¹⁸F]FDG and aminooxy-functionalized peptides in aqueous media leads to the formation of isomers according to literature reports.
Conclusions: For the first time, readily available PET radiotracer [¹⁸F]FDG was shown to be a suitable prosthetic group for direct labeling of aminooxy-functionalized peptides with fluorine-18 under mild conditions. The reaction is especially suitable for small peptides. However, application of larger peptides seems to be limited by increasing separation difficulties of the corresponding glucose-peptide conjugate.