Proteomics - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 413

8.7.1.4 Biological Properties
�.7.1.4.1 Antigenicity and �mmunogenicity
The biological response elicited by the immune system on coming into contact with
exogeneously administered protein is attributed to its antigenicity. Exposure to such
foreign protein triggers the synthesis of antibodies and is called immunogenicity.
Structurally, proteins have biologically active sites that triggers formation of antibod-
ies in the body. Such potent sites are referred to as epitopes. Proteins that share a
similar structure and function to native protein do not trigger such immune response.
Some of the factors contributing to immunogenicity are self-association of the pro-
tein, its molecular weight, and its antigen density. The presence of sugars in side
chains also plays a pivotal role in determining immunogenicity of a protein.
�.7.1.4.2 Biological Activity
Activity is often assessed in vivo using a suitable animal model with activity given
as per official global standards. To cite a few examples, the USP rabbit hypoglyce-
mic assay for insulin and the rat weight gain bioassay for human growth hormone
are well documented in literature. However, considering the importance of the struc-
ture activity relationship, many in vitro assays have also been developed as potential
substitutes like the radio immune assay or the radio receptor assay to relate binding
of a protein hormone to a cellular receptor and study the activity of protein frag-
ments. However, it should be remembered that though biological activity of a pro-
tein depends on native three-dimensional structures, some minor but subtle physical
alterations may lead to a sea change in protein activity.
Therefore, it is mandatory to use a variety of physiochemical and biological tech-
niques to assure safety and efficacy prior to using proteins as pharmaceuticals.
8.7.2 Protein Stability and Formulation
A protein is subjected to a varied number of physicochemical changes with time,
depending on storage conditions. However, changes that adversely affect the safety
and efficacy of a protein should be extensively studied.
8.7.2.1 Chemical Alterations
�.7.2.1.1 Deamidation
Amide-containing amino acids like asparagines and glutamine readily deamidate to
aspartate and glutamate derivatives. This deamidation has been found to affect the
biological half-life of the protein.
�.7.2.1.2 Oxidation and Reduction
This type of reaction mainly occurs in sulfur-containing amino acids like cystine,
cysteine, and methionine, where sulfur acts as a reducing agent and undergoes rapid
oxidation. The thiol group also undergoes nucleophilic alkylation and complexation
with heavy metals.