Applications of circular dichroism (CD) for structural analysis of proteins: qualification of near- and far-UV CD for protein higher order structural analysis.

Circular dichroism (CD) spectroscopy is routinely used in the biopharmaceutical industry to study the effects of manufacturing, formulation, and storage conditions on protein conformation and stability, and these results are often included in regulatory filings. In this context, the purpose of CD spectroscopy is often to verify that a change in the formulation or manufacturing process of a product has not produced a change in the conformation of a protein. A comparison of two or more spectra is often required to confirm that the protein's structure has been maintained. Traditionally, such comparisons have been qualitative in nature, based on visually inspecting the overlaid spectra. However, visual assessment is inherently subjective and therefore prone to error. Furthermore, recent requests from regulatory agencies to demonstrate the suitability of the CD spectroscopic method for the purpose of comparing spectra have highlighted the need to appropriately qualify CD spectroscopy for characterization of biopharmaceutical protein products. In this study, we use a numerical spectral comparison approach to establish the precision of the CD spectroscopic method and to demonstrate that it is suitable for protein structural characterization in numerous biopharmaceutical applications.

Analytical characterization of a novel degradation product in a PEGylated recombinant protein.

We report the identification and characterization of a novel degradation product associated with PEGylation of a recombinant protein. After several months of storage at 2°C-8°C, an unexpected increase was observed in the proportion of an impurity that eluted with the native unPEGylated protein by size exclusion chromatography--from less than 0.01% at the start of storage to more than 0.25% at 12 months. An investigation into the nature of the impurity determined the presence of an N-terminal adduction with a mass increase of +58 Da over the native unPEGylated protein species, demonstrating that this impurity was the result of degradation. The impurity was subjected to thorough analytical characterization using orthogonal methods to establish its identity, and a mechanistic model proposed for its formation. The data implicate the presence of a monomethoxy polyethylene glycol (mPEG)-acetal aldehyde impurity in the mPEG-aldehyde raw material, indicating the need for diligent raw material testing prior to use.