In general, peptides can induce a rapid and specific beneficial response at very low concentrations when they bind to specific cell surface receptors or ion channels to trigger intracellular effects. In addition, they are recognized for being relatively safe and well tolerated. Consequently, peptides that possess attractive pharmacological profiles and intrinsic properties are potential therapeutic candidates. However, parenterally-administered bioactive peptides often have limited pharmacological efficacy because natural peptides have a relatively short circulating plasma half-life. Several measures for half-life extension have been investigated, e.g., acylation, PEGylation, protein fusion and glycosylation. Among them, glycosylation is the modification technology that most closely mimics nature. Glycosylation of peptides/proteins with conserved mammalian sugars generally enhances product solubility and diminishes product aggregation and immunogenicity. Thus, we have been focusing on the glycosylation of bioactive peptides to improve not only their pharmacokinetic properties but also their solubility.
Asparagine linked biantennary complex-type sialylundecasacchride obtained from hen egg yolk can be applied in SPPS as a building block without needing protecting groups on its hydroxyl groups, except for that on the carboxyl group of the NucAc residues. This technique is compatible with Fmoc and Boc chemistry when the carboxyl groups of the NucAc residues are protected by benzyl and phenacyl groups, respectively, during the peptide synthesis. Furthermore, haloacetamidyl glycans derived from our established Asn-linked glycan library can be readily coupled with SH groups on free peptides to produce glycopeptide analogs having a native complex-type glycan structure. This facilitates investigation of the function of oligosaccharides by Cys-scanning, as the product possesses a single glycosylation profile in terms of glycan structure, number and position. This glycosylation method enables us to develop peptide/protein therapeutics with improved pharmacokinetic properties and solubility.