The physical degradation involves denaturation, precipitation, aggregation, changes in or unfolding of the secondary, tertiary, and quaternary structure of proteins (32-34), and also changes in the primary structure; for example, chemical degradation can lead to perturbation of the protein structure (33,35). However, some changes do not lead to any alteration in protein activity and might occur in areas of the protein that are not directly involved in its functional properties (23).

From a drug delivery aspect, the chemical degradation and the biological activity are, nevertheless, major points to consider, and a drug delivery system with a protein drug cannot be applied fully until the effects on the protein stability are studied. Whether the changes (physical or chemical) are reversible or not depends on the nature of the changes (25).

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