3,4,6-Tri-O-acetyl-2-deoxy-2-[¹⁸F]fluoroglucopyranosyl phenylthiosulfonate: A thiol-reactive agent for the chemoselective ¹⁸F-glycosylation of peptides

3,4,5-Tri-O-acetyl-2-[¹⁸F]fluoro-2-deoxy-D-glucopyranosyl 1-phenylthiosulfonate (Ac₃-[¹⁸F]FGlc-PTS) was developed as a thiol-reactive labeling reagent for the site-specific ¹⁸F- glycosylation of peptides. Taking advantage of highly accessible 1,3,4,6-tetra-O-acetyl-2-deoxy-2-[¹⁸F]fluoroglucopyranose, a three-step radiochemical pathway was investigated and optimized, providing Ac₃-[¹⁸F]FGlc-PTS in a radiochemical yield of about 33% in 90 min (decay-corrected and based on starting [¹⁸F]fluoride). Ac₃-[¹⁸F]FGlc-PTS was reacted with the model pentapeptide CAKAY, confirming chemoselectivity and excellent conjugation yields of >90% under mild reaction conditions. The optimized method was adopted to the ¹⁸F-glycosylation of the α_vβ₃-affine peptide c(RGDfC), achieving high conjugation yields (95%, decay-corrected). The α_vβ₃ binding affinity of the glycosylated c(RGDfC) remained uninfluenced as determined by competition binding studies versus ¹²⁵I-echistatin using both isolated α_vβ ₃ and human umbilical vein endothelial cells (K_i = 68 ± 10 nM (α_vβ₃) versus K_i = 77 ± 4 nM (HUVEC)). The whole radiosynthetic procedure, including the preparation of the ¹⁸F-glycosylating reagent Ac₃-[ ¹⁸F]FGlc-PTS, peptide ligation, and final HPLC purification, provided a decay-uncorrected radiochemical yield of 13% after a total synthesis time of 130 min. Ac₃-[¹⁸F]FGlc-PTS represents a novel ¹⁸F-labeling reagent for the mild chemoselective ¹⁸F- glycosylation of peptides indicating its potential for the design and development of ¹⁸F-labeled bioactive S-glycopeptides suitable to study their pharmacokinetics in vivo by positron emission tomography (PET).