Pegylation of Peptides---Chemical Approaches and Biological Applications
Peptides play an important role in pharmaceutical chemistry; however, their in vivo applications are often limited for reasons of fast degradation by protease,
Peptides play an important role in pharmaceutical chemistry; however, their in vivo applications are often limited for reasons of fast degradation by protease, antigenic response and glomerular filtration. These limitations are broken through via bonding peptides with macromolecules. Peptides’ effective mass are increased and the corresponding renal clearance are reduced. It will also shield the surface of peptide from proteolysis. In this respect, PEG acts as the most suitable macromolecule for requirements. Covalent bonding linear PEG chain with peptide is named as Pegylation, which improves original peptide’s solubility, stability, applicability and biocompatibility.
PEG itself is chemically inert and has a low immunogenicity. Upon bonding with PEG, a peptide is entangled with flexible PEG via hydrophobic interactions. At the same time, hydrogen bonds between PEG and surrounding H2O molecules are formed immediately. Consequently the original peptide’s mass and solubility in aqueous media are increased, and its accessible surface area is decreased. Research indicated that bonding with PEG with molecular weight of 30K or more will greatly decrease the renal clearance. The increase in antigen presentation of the host and activation of immune cells after pegylation ofα-2a (an antiviral cytokine for the treatment of hepatitis C and B) well demonstrated the shielding effect. The circulation in vivo half-life of peptides can also be increased to many folds after Pegylation. For example, after bonding IFN-α2b with PEG 40K, its half life in serum of Sprague Dawley rats will be increased to 330 folds. What’s more, a correlation between its in vivo half life with the size and number of bonded PEG chain is verified by Pharmacologist.
Disadvantages of Pegylation should also be mentioned. Scientists are always in dilemmas during Pegylation of a peptide for the maximal shielding and keeping its active site from capping. The problem can be resolved through more rational design of pegylated peptide species. In this respect, site specific Pegylation is crucial for a peptide. Another drawback of PEG is its polydispersity, which leads to analytical problems. The PEG is also nondegradable, which may cause side effects in in vivo application.
A peptide can be pegylated in various sites. The N terminal Pegylation can be realized through direct PEG carboxylic acid coupling or native chemical ligation with PEG thioester (if Cys is present). The C terminal Pegylation is more complicated. A thioacid modification of a peptide should be made at first and then it could be pegylated with sulfone-azide PEG reagent. Another approach is C terminal hydrazide modification and then reaction with pyruvoyl PEG reagent. Besides N terminal and C terminal Pegylation, peptide can also be pegylated almost in any sites as long as the unnatural amino acid (containing corresponding function group) is incorporated in peptide chain. In this context, three chemical reactions are frequently used:
1) The application of Click Chemistry, which takes place between azide group of PEG reagent with the alkyne group of peptide, or vice versa.
2) The application of Sonogashira Coupling, which takes place between iodophenyl group of PEG reagent with the alkyne group of peptide, or vice versa.
3) The application of Suzuki-Miyaura Coupling, with takes place between iodophenyl group of PEG reagent with aryl borinic acid group of peptide, or vice versa.
All these Pegylation approaches are well applied in Chinese Peptide Company according to customers’ different biological application requirements.