MedicalRF.com/Getty Images
|
Proteins are involved in many biological processes in the human body, as well as in micro-organisms such as bacteria and viruses,
and, as such, deserve to be regarded as the 'work horses of the cell'. The production of peptides and proteins through recombinant
DNA technology offers a formidable alternative to small drug entities. Therapeutically, proteins such as antibodies, cytokines,
growth factors and enzymes are important in treating viral, cancer and many other autoimmune diseases. Their high specificity,
reduced toxicity, and high activity at low concentrations render protein and peptide drugs indispensable in the treatment
of various conditions. The importance of biological drugs, notably protein drugs, has increased during the last few years
and is expected to increase further as a result of huge efforts on human genomics and proteomics. Polypeptides and small peptidases
are slowly becoming popular for their ease of production and suitability of genetic manipulation. Nevertheless, their poor
stability inside the human body has always been a bottleneck. Proteins and other types of biological macromolecules are easily
degradable both chemically and enzymatically, and may lose their biological activity through conformational changes and aggregation.
The development of modern pharmaceuticals requires not only identification of new therapeutic drugs, but also safe and efficient
ways to ensure the drug is retained in the circulation for longer periods.
Constraints in using proteins and peptides
Protein/peptide therapeutics are associated with some inherent problems, including:
- Limited stability towards proteolysis by peptidases in the gastrointestinal tract and in serum (half-life on the order of
minutes).
- Poor transport properties from the intestines to the blood and across the blood–brain barrier because of a high molecular
weight (Mw) and lack of specific transport systems.
- Rapid excretion through the liver and/or kidneys.
- Inherent flexibility enables interaction with multiple receptors besides the target and could result in undesired side-effects.
Table 1 Examples of FDA-approved PEGylated proteins.
|
The problem is that the human body has natural enzymatic processes for breaking down proteins and peptides. As peptides are
often made up of natural amino acids, they are substrates for many proteases and peptidases; consequently, they tend to have
a very short half-life. There are several enzymes that selectively degrade polypetides or peptides. Generally, most peptides
are broken down inside the human body within a few minutes because of the action of an enzyme called peptidase. In addition,
some peptidases are peptide specific, making their degradation even more rapid. Thus, if a peptide is used as a therapeutic
agent, its activity is generally reduced as the peptide quickly degrades in the body because of the action of peptidases.
Modification is required to maintain biological stability and, hence, longer half-life in the human body after administration.
One way to overcome this is to administer large dosages of the therapeutic peptide of interest to the patient so that, even
if some of the peptide is degraded, enough remains to be therapeutically effective. However, this method is quite uncomfortable
for the patient. As most therapeutic peptides cannot be administered orally, the peptide would have to be constantly infused,
frequently administered by intravenous injections or by the inconvenient route of subcutaneous injections. The presence of
large amounts of degraded peptide may also generate undesired side-effects.
An alternative method is to block the action of peptidases to prevent degradation of the therapeutic peptide or to modify
the therapeutic peptides in such a way that their degradation is slowed down while still maintaining biological activity.
Such methods include conjugation with polymeric materials, such as dextrans, polyvinylalcohol, carboxymethyl cellulose, polyvinyl
pyrolidine, heparin, polyethylene glycol (PEG) and polyamino acids. Conjugation of a protein/peptide to PEG is called PEGylation.
Conjugation to PEG protects the therapeutic peptides from peptidase activity and allows for a longer duration of action in
vivo, while maintaining low toxicity and retaining the therapeutic advantages of the modified peptides.
