Absolute and Relative Quantification of Glycation on Biotherapeutic IgGs Using Heavy Isotope–Labeled Proteins

Post-translational modifications (PTMs), such as glycation, can be introduced during the manufacturing and storage of therapeutic proteins, including monoclonal antibodies (mAbs). Glycation modification may alter the safety and efficacy of therapeutic drugs, thus necessitating their accurate quantification. Relative quantitation is typically performed by comparing the peak area of the glycated peptide with that of its unmodified counterparts. The accuracy of this method depends on the unmodified and modified peptides having similar ionization efficiencies, as well as on the selection of the native peptide used for quantitation. We propose using non-glycated, heavy-isotope-labeled proteins to quantify glycation. The heavy isotope-labeled proteins can provide both the relative and absolute level of glycation by comparing the peak area of the unmodified tryptic peptides to that of the heavy isotope-labeled variants. To develop this approach, glycation was intentionally induced in an immunoglobulin (IgG) to have detectable glycation. A heavy isotope-labeled variant of the IgG (containing 13 C and 15 N lysine and arginine residues) was added to serve as an internal standard. Further, this mixture was subjected to trypsin digestion, and a bottom-up approach using Hydrophilic Interaction Liquid Chromatography (HILIC)-MS was performed. The light/heavy ratio of tryptic peptides with and without a glycation site in the forced glycation sample was monitored. Any decrease in the light/heavy ratio of peptides containing a glycation site relative to the expected ratio was attributed to the production of glycated variants. Glycation quantitation in IgGs such as Adalimumab (IgG1) and Natalizumab (IgG4) was performed. We expect this approach will apply to other PTMs.