Method to differentiate asn deamidation that occurred prior to and during sample preparation of a monoclonal antibody

Asparagine (Asn) deamidation is a major source of antibody instability and micro heterogeneity. For this reason, it is critical to accurately characterize both the levels and the sites of Asn deamidation in therapeutic antibodies. Asn deamidation is normally quantified by analyzing antibodies at the peptide level by liquid chromatography-mass spectrometry. This requires denaturation, reduction, alkylation, and enzyme digestion of the antibody prior to analysis. These steps in sample preparation may directly contribute to the total levels of Asn deamidation detected. Therefore, to obtain accurate levels and sites of Asn deamidation, it is important to determine if any deamidation occurred during the sample preparation steps. However, this could be challenging because deamidation that occurred prior to and during sample preparation resulted in peptides with the same retention times and the same molecular weight increase of 1 Da. Sample preparation was carried out in (18)O-water in the current study to differentiate between the two events of Asn deamidation. Using this method, deamidation that occurred during sample preparation resulted in a molecular weight increase of 3 Da instead of 1 Da. This molecular weight difference was readily detected by inspection of the isotopic peak cluster of the peptides containing the deamidation products, isoAsp and Asp residues. It enabled discrimination of deamidation that was due to analytical artifacts and thus determination of the level of deamidation that was present in the samples.