br Advantages of modified proteins
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 , 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 ski or skii and the glomerular basement membrane contribute to an anionic barrier, which partially prevents the passage of negatively-charged plasma macromolecules . 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 . 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. 
Strategies for producing long-acting protein therapeutics Significant effort has been expended to discover different approaches to extend the half-lives of protein drugs, not least by evading or interfering with their common clearance pathway. Modifications to protein drugs that prolong their half-lives include conjugation or fusion to specific moieties and the discovery of variants of the therapeutic protein drugs . These strategies also include chemical coupling of polymers and carbohydrates, post-translational modifications such as N -glycosylation , and fusion to recombinant polymer mimetics [20,21]. (Fig. 1; Table 1).  On the other hand, changes in structure or sequence of protein molecules (e.g. through glycosylation or PEGylation) may cause changes in the pharmacokinetic properties of these compounds. The size of a therapeutic protein may hinder its passage across a biological membrane. Other factors that affect its half-life include its immunogenicity, the level of the corresponding endogenous protein, the period of drug administration, and the rate and site of drug delivery . Gene modification can be used to create therapeutic proteins with altered isoelectric points and protein dynamics . Such mutations can also modulate both enzyme selectivity and the intrinsic activity of the enzyme. In one example, both the activity and the specificity of Neprilysin, a protease that degrades amyloid beta and hence might be of use in the treatment of Alzheimer’s disease, were altered through site-specific mutagenesis. The engineered Neprilysin double mutant G399 V/G714 K showed a ˜20-fold increase in activity on amyloid beta 1–40 but a ˜3,200-fold reduction in activity on other peptides. Further, this therapeutic drug is therefore, a promising candidate for the in vivo treatment of Alzheimer's disease .