CD3 Target Affinity Chromatography Mass Spectrometry as a New Tool for Function–Structure Characterization of T-Cell Engaging Bispecific Antibody Proteoforms and Product-Related Variants

T-cell engaging bispecific antibodies (TCBs) targeting CD3 and tumor-specific antigens are very promising therapeutic modalities. Since CD3 binding is crucial for the potency of TCBs, understanding the functional impact of CD3 antigen-binding fragment modifications is of utmost importance for defining critical quality attributes (CQA). The current CQA assessment strategy requires the integration of structure-based physicochemical separation and functional cell-based potency assays. However, this strategy is tedious, and coexisting proteoforms with potentially different functionalities may not be individually assessed. This increases the degree of ambiguities for defining meaningful CQAs, particularly for complex bispecific antibody formats such as TCBs. Here, we report for the first time a proof-of-concept study to separate and identify critically modified proteoforms of TCBs using functional CD3 target affinity chromatography (AC) coupled with online mass spectrometry (MS). Our method enabled functional distinction of relevant deamidated and glycosylated proteoforms and the simultaneous assessment of product-related variants such as TCB mispairings. For example, CD3 AC-MS allowed us to separate TCB mispairings with increased CD3 binding (i.e., knob-knob homodimers) within the bound fraction. The functional separation of proteoforms was validated using an established workflow for CQA identification based on thoroughly characterized ion-exchange fractions of a 2+1 TCB. In addition, the new method facilitated the criticality assessment of post-translational modifications in stress studies and structural variants in early stage clone selection. CD3 AC-MS has high impact for streamlining the integration of functional and structural characterizations of the large landscape of therapeutic CD3 targeting TCBs from early stage research to late stage characterization.