For these reasons, the native structures of proteins are designed to adapt to environmental changes, e.g., pH, ionic strength, hydrophobicity, surface, ligands, other biological molecules including proteins, etc (Shahrokh et al, 1997; Carpenter and Manning, 2000; McNalley, 2000). While the flexibility of structure is required for necessary biological function of proteins, it also presents unique challenges in the development of proteins for pharmaceutical applications by causing conformational changes, aggregation, and precipitation, which often are related to loss of biological activity as well as immunogenicity of the proteins (Schellekens, 2008; Rosenberg, 2006; Bennett et al., 2004; Hermeling et al., 2006, 2004). In order to address this particular issue, various platforms for manufacturing process and formulation development have been introduced to the pharmaceutical industry. In practice, research focuses on comprehending the nature of protein structures, understanding the conditions that can cause the alteration from native structures and their contribution to the degradation of proteins, developing novel analytical tools that examine these processes and end results of degradation pathways, and designing approaches to utilize this information for maintaining the nativity of each protein for its safe pharmaceutical application (for related review articles, see Carpenter et al., 1996; Carpenter et al., 1999; Wang, 1999; Carpenter et al, 2002; Chang and Hershenson, 2002; Kendrick et al., 2002; Patro et al, 2002; Chi et al., 2003; Frokjaer and Otzen, 2005).
Although fully understanding all the possible combinations of structural variants, especially under stressed conditions, would be virtually impossible, several decades worth of research by both academic and pharmaceutical institutions has established general approaches that can be applied for the successful development of most biopharmaceutical products (Cleland et al, 1993; Nail and Akers, 2002; Cromwell et al., 2006; Patro et al., 2002; Chang and Hershenson, 2002). Among these, key accomplishments relevant for pharmaceutical development, e.g., identification of key stress factors, understanding major degradation products, and developing analytical methods to determine the degradation products, will be discussed in this chapter.
This chapter will specifically focus on the physical changes that occur in proteins and their contribution to the overall stability relevant to product development, so readers are referred to other chapters in this book for chemical degradations and the like.