Development of a potential high-throughput workflow to characterize sites of bioconjugation by immuno-affinity capture coupled to MALDI-TOF mass spectrometry.

The characterization of conjugation sites in bioconjugates is critical in the early discovery phase because site-specific conjugation improves in vivo stability and drug efficacy. We previously developed an engineered monoclonal antibody (mAb) scaffold which enables site-specific conjugation toward a reactive lysine (Lys) residue on each heavy chain (HC) by using an azetidinone (AZD) linker. In order to explore conjugations in other location which avoids potential interference with target binding, other chemical linkers have been studied and the investigation of N-hydroxysuccinimade (NHS) linker is reported here. The complexity of identifying the sites lies in part to the large number of Lys residues available for conjugation on the mAb scaffold. This has posed technical challenges to standard peptide mapping approaches. Therefore, an alternative strategy intended for a rapid analysis has been investigated by coupling immuno-affinity capture to matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS). In this study, we have employed a novel application of two different capture formats: Surface enhanced laser dissociation/ionization (SELDI) and mass spectrometry immunoassay (MSIA) tips to reduce the analysis time. An antibody against the pharmacophore portion was immobilized to capture the conjugated peptides, and subsequently provide characterization of the conjugation sites on the scaffold. Multiple sites for the AZD and NHS linkers have been easily identified and confirmed by MS2 sequencing. Lysine99 is the predominant site for the AZD linker, and Lysine55 is the primary site for the NHS linker with Lysine193 and Tyrosine37 being minor sites as shown in the abstract figure. We have also demonstrated the use of conjugation mapping to compare the distribution pattern between the AZD and NHS linkers as well as to study the stability of conjugation sites in a rapid way.