Radiolabelling of isopeptide N^Epsilon-(Gamma-glutamyl)-L-lysine by conjugation with N-succinimidyl-4-[¹⁸F]fluorobenzoate

The isopeptide N^Epsilon-(Gamma-glutamyl)-L-lysine 4 was labelled with ¹⁸F via N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB). A modified approach for the convenient synthesis of [¹⁸F]SFB was used, and [¹⁸F]SFB could be obtained in decay-corrected radiochemical yields of 44-53% (n=20) and radiochemical purity >95% within 40 min after EOB. For labelling N^Epsilon-(Gamma-glutamyl)-L-lysine with [¹⁸F]SFB the effects of isopeptide concentration, temperature, and pH were studied to determine the optimum reaction conditions. The coupling reaction was shown to be temperature and pH independent while being strongly affected by the isopeptide concentration. Using the optimized labelling conditions, in a typical experiment 1.3 GBq of [¹⁸F]SFB could be converted into 447 MBq (46%, decay-corrected) of [¹⁸F]fluorobenzoylated isopeptide within 45 min, including HPLC purification.