7. Mass Spectrometry of Antibody–Drug Conjugates in Plasma and Tissue in Drug Development

The discovery of “soft” ionization techniques in mass spectrometry (MS) such as electrospray ionization (ESI) [1] and matrix-assisted laser desorption/ionization (MALDI) [2] for measuring molecular masses of intact proteins was a significant breakthrough in the analysis of proteins. The work was recognized by a Nobel Prize in Chemistry in 2002 and led to the widespread use of ESI–MS to characterize intact protein molecular masses in protein therapeutics discovery and development in biotechnology. Previously, it was only possible to measure peptide molecular masses. In contrast to the analysis of intact purified proteins in simple buffers by ESI–MS, the ability to analyze biotherapeutic proteins in plasma or other tissues is significantly more challenging due to interference from the background plasma/tissue proteome and has only recently been reported for plasma [3]. The intact molecular mass measurement of biotherapeutic proteins in plasma by ESI–MS required isolation of the proteins from plasma using affinity capture followed by elution of the isolated intact biotherapeutics of interest and liquid chromatography (LC)–ESI–MS characterization to determine their intact molecular masses. The ability to obtain intact molecular masses and thereby characterize structural changes in biotherapeutics in plasma for in vivo studies provides key insights for large molecule drug development. Additional information can be obtained by enzymatic digestion followed by peptide analysis using LC-tandem MS (MS/MS) methodology. Understanding biotransformation and molecular changes of biotherapeutics in vivo is particularly valuable for the development of antibody–drug conjugates (ADCs) where efficacy and safety may be affected. It also provides essential structural characterization information for the ADCs in vivo, necessary for designing appropriate quantitative assays for measuring pharmacokinetics (PK) and toxicokinetics (TK).