OvercomingElectrochemiluminescence Immunoassay (ECLIA)Limitations: Conformation-Independent Quantitation of a TherapeuticAntibody by LC–MS/MS

Monoclonal antibody therapies have transformed cancer treatment by enabling targeted intervention even in advanced disease stages. However, reliable monitoring of circulating therapeutic antibody levels is critical to ensuring efficacy and patient safety. This study investigates the limitations of an electrochemiluminescence immunoassay (ECLIA) in quantifying a therapeutic immunoglobulin G2 (IgG2) antibody, known to undergo conformational changes in vivo. We observed that the ECLIA failed to reflect increasing antibody concentrations in patient plasma, instead producing a plateau effect. This discrepancy was attributed to the ECLIA’s reliance on the intact structural conformation of the antibody, which may be altered during circulation. In contrast, a mass spectrometry (MS)-based approach successfully quantified the accumulating antibody levels over time, independent of conformational state. These findings highlight the need for alternative quantitation strategies in clinical settings, particularly when structural heterogeneity may compromise the ECLIA performance. Mass spectrometry provides a robust and reliable option for monitoring therapeutic antibodies under these conditions.