Stability Determination

In a formulation study, alterations caused by either the process or the excipients are of interest. These alterations can be either chemical or physical in nature. The physical stability of formulations is assessed by determining the physical structure of a protein in the formulation, that is, by determining the changes in secondary, tertiary, and quaternary structure. This is often done by FTIR and CD, but differential scanning calorimetry (DSC) can also determine changes in the stability of the protein. DSC is useful for obtaining information on the folding thermodynamics of globular proteins (148,160), for example, the transition from native to denatured conformation and the unfolding of the different domains comprising the globular protein (161,162). It is also a frequently used technique for screening of the effectiveness of excipients as regards the thermal stress that the protein solution is exposed to in the DSC experiment (163). However, for accurate determination of the effects of excipients, a combination of methods is useful. For example, when Tween was added to a protein, the Tm value fell, which usually indicates a destabilization, where other tests did not indicate this destabilization (163,164). The a-helical structure is generally disrupted by the thermal denaturation, which also reflects change in the overall protein structure (165). In addition, methods like SDS-PAGE, light scattering, and others can also be used to assess physical changes such as aggregation. Chemical stability is often determined by, for example, HPLC or MS (150).

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