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