Custom-Designed Affinity Capture LC-MS F(ab′)2 Assay for Biotransformation Assessment of Site-Specific Antibody Drug Conjugates

Affinity capture liquid chromatography–mass spectrometry (LC-MS) intact antibody assay has been widely used for direct drug-to-antibody ratio (DAR) and catabolite characterization of antibody-drug conjugates (ADCs). However, the intact mass spectra of new ADCs, which incorporate new types of linkers and payloads other than maytansines and auristatins, are more complex than those examined previously. The current method has showed some limitations in elucidating certain structural modifications. Herein, we report an alternative analytical approach for ADCs, such as THIOMAB antibody-drug conjugates (TDCs), where the linker drugs are site-specifically conjugated in the Fab region. The newly developed affinity capture LC-MS F­(ab′)­2 assay incorporates affinity capture of human IgGs via binding to the Fab region, followed by on-bead IdeS digestion to remove the Fc domain specifically and uniformly. The resulting F­(ab′)­2 (∼100 kDa) fragments contain the key ADC biotransformation information, such as drug-to-antibody ratio and drug metabolism and are more readily analyzed by electrospray ionization LC-MS than the intact ADC (∼150 kDa). The reduced size of analytes results in improved mass spectral sensitivity and resolution. In addition, the reduced and optimized sample preparation time, for example, rapid removal of the Fc fragment by IdeS digestion, minimizes assay artifacts of drug metabolism and skewed DAR profiles that may result from the prolonged incubation times (e.g., overnight enzymatic treatment for Fc deglycosylation). The affinity capture LC-MS F­(ab′)­2 assay provides more detailed and accurate information on ADC biotransformations in vivo, enabling analysis of low-dose, labile, and complex site-specific ADCs with linker-drug conjugated in the Fab region