Using 5-deoxy-5-[18F]fluororibose to glycosylate peptides for positron emission tomography

Abstract

So far seven peptide-based (18)F-radiopharmaceuticals for diagnostic applications with positron emission tomography (PET) have entered into clinical trials. Three candidates out of these seven are glycosylated peptides, which may be explained by the beneficial influence of glycosylation on in vivo pharmacokinetics of peptide tracers. This protocol describes the method for labeling peptides with 5-deoxy-5-[(18)F]fluororibose ([(18)F]FDR) as a prosthetic group. The synthesis of [(18)F]FDR is effected by a nucleophilic fluorination step by using dried Kryptofix 2.2.2-K2CO3-K(18)F complex and a subsequent HCl-catalyzed hydrolysis. The conjugation of [(18)F]FDR to the N-terminus aminooxy (-ONH2)-functionalized peptides is carried out in anilinium buffer at pH 4.6 and at room temperature (RT, 21-23 °C), with the concentration of peptide precursors being 0.3 mM. The procedure takes about 120 min and includes two cartridge isolation steps and two reversed-phase (RP) HPLC purification steps. The quaternary methyl amine (QMA) anion exchange cartridge and the hydrophilic-lipophilic balanced (HLB) cartridge are used for the isolation of (18)F-fluoride and [(18)F]FDR-conjugated peptides, respectively. The first HPLC purification provides the (18)F-fluorinated precursor of [(18)F]FDR and the second HPLC purification is to separate labeled peptides from their unlabeled precursors. The final product is formulated in PBS ready for injection, with a radiochemical purity of >98% and a radiochemical yield (RCY) of 27-37% starting from the end of bombardment (EOB). The carbohydrate nature of [(18)F]FDR and the operational convenience of this protocol should facilitate its general use.