Therapeutic Monoclonal Antibody─Antidrug Antibody Affinity Constant Determination Using Capillary Electrophoresis–Tandem Mass Spectrometry

Monoclonal antibody (mAbs) therapeutics cannot evade the occurrence of adverse effects. Thus, mAbs are commonly triggering immune responses corresponding to the expression of antidrug antibodies. Antidrug antibodies can neutralize mAbs, leading to their inhibition and hasten clearance, which dramatically hampers their therapeutic effects. Therefore, studies concerning the affinity between a mAbs and the corresponding antidrug antibody are demonstrating a growing interest to further understand the outcome of biotherapeutics after administration. We describe a novel analytical approach for the determination of the dissociation constant (<i>K</i>_d) between a mAb and an antidrug antibody using capillary electrophoresis coupled to mass spectrometry (CE-MS/MS). The CE-MS/MS method employs a competitive assay, followed by the quantification of the residual amount of the active mAbs. The methodology allowed for the measurement of <i>K</i>_d using serum samples without the implementation of immobilization to achieve protein-protein interaction. This characteristic enabled us to generate the interaction in conditions reflecting the physiological environment. A mathematical treatment was developed to calculate <i>K</i>_d from MS/MS data, taking into account the stoichiometry of the mAbs/antidrug antibody complex and the bivalent properties of the two immunoglobulins. Prior to CE-MS/MS analysis, the interaction of the two proteins was studied by using mass photometry (MP) to determine equilibrium conditions and complexation stoichiometry. CE-MS/MS was used to investigate the interaction between infliximab and a neutralizing anti-infliximab antibody. Results allowed to measure a <i>K</i>_d of 14.4 ± 2.9 nM. MS instrumentation sensitivity and specificity showed to be relevant to achieve accurate and robust <i>K</i>_d measurements for strong interactions in the nanomolar range.