However, the use of protein-based therapies is not without its challenges-one of the disadvantages is that their half-life in the body may be short and therefore require frequent dosing, which in turn increases the chance of immunological response and increases the cost of treatment. Another potential disadvantage of some protein therapeutics is that there may be dose-limiting solubility issues, which may prevent proteins from being used as therapeutic drugs. One of the main strategies to alleviate these problems is to attach a polyethylene glycol (PEG) group to the target protein. This process, called PEGylation, has evolved tremendously in recent years, resulting in several approved drugs. The application of PEGylation can be extended to peptides, enzymes, antibody fragments, nucleotides, and even small organic molecules.
Protein PEGylation Advantages
Protein PEGylation prolongs the cycle time of the protein because the PEG-protein conjugation results in an increase in solution size and reduces the kidney clearance time of the conjugate compared to unmodified proteins. PEG has been approved by the US Food and Drug Administration (FDA) as " generally recognized as safe". It is a flexible, randomly coiled macromolecule that exhibits an extended conformation in water, which is maintained when one terminus of PEG is conjugated to a protein. Linear PEG is derived from ethylene oxide repeating units (HO- (CH2CH2O) n-H). PEG is usually activated at one end for protein conjugation with a non-reactive methyl group at the other end. The molecular weight of PEG for protein conjugation is well dissolved in water under physiological conditions.
Currently, more than 17 different PEGylated drugs have been approved by the FDA, and more potential products are under development. Although the total number of PEGylated drugs is small compared to the total number of protein-based therapeutics on the market, many of these PEGylated drugs are still considered "blockbuster drugs." Overall, this indicates a great potential for growth in commercial PEGylated therapeutics.
Main advantages of PEGylated proteins. (Image Source: Drug discovery today 2005, 10 (21), 1451-1458)
Protein PEGylation Disadvantages
The main disadvantage of PEGylation is usually reduced biological activity in vitro, which can be compensated in vivo by significantly improved PK behavior. Generally, the longer the PEG chain, the longer the elimination half-life of the PEG-protein conjugate. Moreover, the polydispersity of the polymer is one of the factors that aggravate the final characterization of the PEG-protein conjugate. Current practice uses linear and branched-chain PEGs with molecular weights up to 40 kDa, resulting in the desired improvement of PK properties. However, new PEG formats (such as forked, multi-arm, and comb-shaped PEGs) show great promise for the future. The macromolecular structure of the conjugated PEG polymer appears to be critical for the improved performance of the conjugate. In this sense, comb-shaped PEG with many short PEG chains attached to the polymer backbone can be prepared through transition metal-mediated living radical polymerization, providing a relatively tightly controlled molecular weight and architecture of the polymer. One promising approach is releasable PEGylation - attachment of a PEG reagent with a releasable linker to a protein. This overcomes drug inactivation by conjugation and enables the release of the full-potency drug, increases the solubility of poorly soluble drugs and deposits such drugs at the target, allows random PEGylation, and by appropriate selection of the linker also control of PK parameters.
PEG reagents are commercially available in different lengths, shapes and chemistries, allowing them to react with particular functional groups of proteins for their covalent attachment. Biochempeg provides a variety of PEG product, or activated PEG derivatives, that are crucial ingredients in the art of PEGylation. The GMP manufacturing facility for the manufacturing of PEG products is under construction and will be in operation this year.
Nearly five decades of development in PEGylation technology has proven its pharmacological advantages and acceptability but the technology still lags in providing a commercially attractive, generic process to produce highly specific PEGylated therapeutic products at high yield. As a multi-million dollar annual business with the growing interest from both emerging biotechnology and established multinational pharmaceutical companies, there is great scientific and commercial interest in improving present methodologies and in introducing innovative process variations.