Glycosylation, an effective synthetic strategy to improve the bioavailability of therapeutic peptides

Abstract

Glycosylation of peptides is a promising strategy for modulating the physicochemical properties of peptide drugs and for improving their absorption through biological membranes. This review highlights various methods for the synthesis of glycoconjugates and recent progress in the development of glycosylated peptide therapeutics. Furthermore, the impacts of glycosylation in overcoming the existing barriers that restrict oral and brain delivery of peptides are described herein.

Fig. 1. (A) Examples of O-linked and N-linked glycosylated amino acids, (B) direct and convergent strategies for glycopeptide synthesis.

Fig. 2. Synthesis of tetra-acetylated galactose ethanoic acid building block via the formation of the two intermediates, allyl and aldehyde derivatives. Galactose was conjugated to a peptide through anomeric carbon modified by ethanoic acid., The attachment of galactose building block (2 eq.) to the N-terminus of NAPamide peptide on resin was performed on SPPS by using 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU) and N,N-diisopropylethylamine (DIPEA) in dimethylformamide (DMF). Removal of acetyl groups from sugar was achieved on solid phase by using hydrazine hydrate/DMF. All-TMS, allyltrimethylsilane; BF₃·Et₂O, boron trifluoride diethyl etherate.

Fig. 3. Succinamic acid derivative building block was synthesized through the reduction of azide, followed by treating the resulting product with succinic anhydride., Sugar was attached to the peptide through a succinamic acid linker as shown in galactose derivative of luteinizing hormone-releasing hormone (LHRH). DMAP, 4-dimethylaminopyridine; Pd–C, palladium carbon.

Fig. 4. Ligation of oxazoline hexasaccharide (donor) with N-acetyl glucose derivative (acceptor) using Endo-A enzyme.

Fig. 5. Enzymatic glycosylation of lactose derivative of lipo-enkephalin (acceptor) using UDP-galactose derivative (donor) and LgtC enzyme. UDP, uridine-5′-diphosphate.

Fig. 6. Analogues of Met-enkephalin peptide with β-d-glucose attached to different positions.

Fig. 7. Binding of Gal–Leu–enkephalin to ASGPR displayed by surface plasmon resonance and molecular modelling. Gal–Lac–Enk, galactose–lactose–enkephalin.

Fig. 8. Sialyl N-acetyllactosamine derivative of GLP-1. GLP-1 peptide sequence (7–36) was modified by replacing the Lys34 with sialyl N-acetyllactosamine Asn residue. This analogue was synthesised by enzymatic carbohydrate elongation using galactosyltransferase and sialyltransferase.

Fig. 9. (a) Structure of the lactose (Lac)–succinate and Lac-endomorphin-1 and (b) time course of the antinociceptive bacterial effects of Lac-endomorphin-1, morphine, and vehicle in CCI-rats after oral administration. A single oral dose of Lac-endomorphin-1 produced dose-dependent analgesic activity in the ipsilateral hindpaws of a CCI-rat model of neuropathic pain. Lac-Endo-1, lactose-endomorphin-1; ΔPWT/g, normalised Δ paw withdrawal thresholds.

Fig. 10. Glycated [^99mTc(CO)₃]-labeled bombesin analogue. This compound was synthesised via a click reaction between azide derivative of glucose and a stabilised bombesin (7–14) sequence bearing the (N^αHis)Ac-chelator that modified with amino acid linkers containing propargylglycine residue.