Structural characterization was performed on an antibody-drug conjugate (ADC), composed of an IgG1 monoclonal antibody (mAb), mertansine drug (DM1), and a noncleavable linker. The DM1 molecules were conjugated through nonspecific modification of the mAb at solvent-exposed lysine residues. Due to the nature of the lysine conjugation process, the ADC molecules are heterogeneous, containing a range of species that differ with respect to the number of DM1 per antibody molecule. The DM1 distribution profile of the ADC was characterized by electrospray ionization mass spectrometry (ESI-MS) and capillary isoelectric focusing (cIEF), which showed that 0-8 DM1s were conjugated to an antibody molecule. By taking advantage of the high-quality MS/MS spectra and the accurate mass detection of diagnostic DM1 fragment ions generated from the higher-energy collisional dissociation (HCD) approach, we were able to identify 76 conjugation sites in the ADC, which covered approximately 83% of all the putative conjugation sites. The diagnostic DM1 fragment ions discovered in this study can be readily used for the characterization of other ADCs with maytansinoid derivatives as payload. Differential scanning calorimetric (DSC) analysis of the ADC indicated that the conjugation of DM1 destabilized the C(H)2 domain of the molecule, which is likely due to conjugation of DM1 on lysine residues in the C(H)2 domain. As a result, methionine at position 258 of the heavy chain, which is located in the C(H)2 domain of the antibody, is more susceptible to oxidation in thermally stressed ADC samples when compared to that of the naked antibody.

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Monoclonal antibodies (mAbs) are composed of two heavy chain (HC) and two light chain (LC) polypeptides. The proper folding and assembly of HC and LC is critical for antibody production. Current dogma indicates that the free HCs are retained in the endoplasmic reticulum (ER) unless assembled with LCs into antibodies, while the LCs on the other hand can be secreted as free monomer or dimer molecules. In this study, high levels of extracellular HC homodimers (7%-45%) were observed in the cell culture media during cell line development for mAb1. Excellent correlation (R > 0.9) between the level of free HC homodimers and the percentage of high molecular weight species indicates that the free HC homodimers might be causative of unwanted aggregation. Due to the different surface charge of HC homodimer and fully assembled antibodies, the unwanted extracellular HC homodimers were successfully removed by downstream processing, through a cation exchange chromatography step. Reduced capillary electrophoresis-sodium dodecyl sulfate (rCE-SDS) analysis of the cell culture media from different MTX-amplified pools indicated that insufficient expression of LC is one potential root cause for the high level of free HC homodimers. The level of free HC homodimers decreased significantly (3%-25%) after retransfecting the MTX amplified pools with additional LC gene. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:738-745, 2018.