Detection and quantitation of IgG 1 hinge aspartate isomerization: a rapid degradation in stressed stability studies

In biopharmaceutical process development, it is desirable to identify sites of covalent degradations to ensure product consistency. One characterization method used for therapeutic immunoglobulin gamma (IgG) 1 antibodies is limited LysC proteolysis followed by reversed-phase LC/MS. Limited LysC proteolysis leads to high efficiency cleavage at the C-terminal side of the hinge lysine 222 residue, generating Fab and Fc fragments. In this report, we show that IgG 1 samples incubated under mildly acidic conditions at elevated temperatures were partially resistant to LysC cleavage at the hinge and resulted in a species where one of the Fab arms remained connected to the Fc region (Fab-Fc). The growth of the Fab-Fc species was proportional to the duration and storage temperature of the incubation period and correlated with the amount of isomerization of the aspartic acid residue preceding lysine 222, determined by peptide mapping. The isomerization rates of samples stored for up to one year at 4 degrees C, 6 months at 29 or 37 degrees C, or 3 months at 45 degrees C were determined, and the activation energy for this conversion was calculated to be approximately 33 kJ mol(-1). The apparent isomerization rate constant was only 0.02 week(-1) for samples stored at 4 degrees C, which resulted in a modest increase from 5.1 to 6.0% isoD after twenty four weeks of storage and, hence, is not a significant concern under normal storage conditions typically used for monoclonal antibodies. However, when stored at 29 degrees C, the apparent rate constant of this reaction was found to be 0.06 week(-1) and resulted in an increase from 5.1 to 21.1% isoD after twenty four weeks of storage and is a major degradant in stressed IgG 1 antibodies.