br The need for modified therapeutic proteins and why

The need for modified therapeutic proteins and why they need to last longer in the body Chemical and structural changes in therapeutic proteins are possible and are carried out frequently to accomplish pharmacological or clinical benefit. Such modifications are essential as the drug needs to pass through various membrane barriers, e.g. to reach a tumor. Active targeting of a drug is typically achieved by conjugating it to a target entity that improves bioavailability and reduces systemic toxicity [8]. Modified therapeutic proteins can also be applied in a technique called the Antibody Directed Enzyme Prodrug Therapy (ADEPT) for cancer targeted therapy. ADEPT therapies are designed to generate toxic chemotherapeutics at the site of malignancy, potentially improving efficacy and reducing side effects [9,10]. The design of the modified therapeutic proteins aims to produce enzyme variants with good catalytic efficiency, high-levels of stability and reduced immunogenicity. Such extra features will often increase the protein’s circulatory half-life, i.e. the time that the protein will circulate in the blood. This lead to the decrease of the number of doses required to be given to the patient, thereby reducing the possibility that the patient will generate SCH 727965 to the modified protein and limiting the time available for the targeted cancer cells to mutate and hence avoid or resist the treatment as in case of glucarpidase. It has been shown that protein modification using PEGylation or HSA gene fusion of glucarpidase produces forms of the enzyme with a much longer half-life and more resistant to proteases [11].
Advantages of modified proteins over unmodified ones In contrast to small-molecule drugs, proteins are readily amenable to site-specific alterations through genetic engineering. In principle, therefore, it is possible to build in features that allow them to remain active for longer in the body and or to improve their tolerance. These features include: resistance to proteolysis; delayed clearance; reduced capacity to cause local irritation; increased half-life; lower toxicity; increased stability and solubility, and decreased immunogenicity [12,13]. Many of protein therapeutic drugs have now been developed and approved. Many exhibit short half-lives in plasma and hence strategies to improve their pharmacokinetic properties, which influence distribution and excretion [13], are becoming increasingly important. Increasing the size and hydrodynamic radius of the protein, or peptide aims to decrease kidney filtration and to increase the net negative charge of the target protein or peptide has a similar effect, as the net charge of the protein contributes to renal filtration. It has been suggested that the proteoglycans of the endothelial cells and the glomerular basement membrane contribute to an anionic barrier, which partially prevents the passage of negatively-charged plasma macromolecules [14]. Another approach is to increase the degree to which the therapeutic peptide or protein interacts with serum components, e.g. albumin or immunoglobulins, which tends to increase the half-life of the circulating targeted protein. [15,16] Both serum albumin and immunoglobulins (particularly IgG1, IgG2 and IgG4) have extraordinarily long half-lives – around 19 days - in humans [17]. Use of neonatal Fc receptor is another approach that can be used to promote interactions with albumin or with the Fc region of IgG in a pH-dependent manner. FcRn binding can protect albumin and IgG from degradation in the lysosomal compartment and redirects them to the plasma membrane. Thus, such binding can extend or modulate the half-life of the protein that is attached to it. [18]