PEGylation is an established drug delivery technology that chemically bonds the synthetic polymer polyethylene glycol (PEG) to another molecule, usually a protein, peptide, or nucleotide. The combination is often called a conjugate.
PEGylation may impart some of the following very desirable properties:
- Reduced toxicity
- Modified and inhibited immunogenicity
- Improved therapeutic index
- Enhanced bioavailability
- Altered drug biodistribution
- Increased chemical stability
- Reduced proteolysis
- Attenuated aggregation
- Improved solubility
PEG is a linear (unbranched) polymer with a regular repeat unit of the general structure -(CH2CH2O)n-. The most useful kind of PEG has a methoxy group on one end and a hydroxyl group on the other. They usually have an average molecular weight of 5kDa to 40kDa. To look at it another way, that’s about 100 to 1,000 -(CH2CH2O)- repeat units, all in a line. However, the nature of synthetic polymers is that the polymerization of monomer to polymer almost always results in a distribution of actual chain lengths. This distribution is called polydispersity or molecular weight distribution. In addition, polymerization processes result in other imperfections such as diols. These are PEG chains that have hydroxyl groups on both ends and are typically almost double the average molecular weight in size. Finally, very small concentrations of impurities in the PEGs can result in decreased stability and other issues.
PEGs are typically available as “activated” polymers having an inert group at one end (e.g., methoxy) and a reactive functional group at the other. Typically, the functional group is selected such that it will react with one of a protein’s amine or sulfhydryl groups. Also, heterobifunctional PEGs can be useful as a flexible linker molecule.
PEGylated proteins can be difficult to characterize by standard methods. Protein structure can be complex with several levels of order consider. In addition, the polydispersity and impurities from PEGs complicate matters. The PEG activation chemistry/purification can result in additional impurities. Therefore product specification and in-process testing are complicated by the presence of the PEG polymer. Process development and stability studies are difficult to carry out if the analytical methods are not up to the task.
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