Selective human factor VIII activity measurement after analytical in‐line purification

BACKGROUND: It is essential to measure the activity of factor VIII (FVIII) throughout the life cycle of a coagulation FVIII concentrate. Such measurement in nonclinical pharmacokinetic studies is potentially biased by the presence of endogenous nonhuman FVIII, and certain manufacturing process–related additives can also impact the assay performance. Finally, the presence of FVIII activity–mimicking antibodies poses challenges when measuring FVIII in samples. Therefore, we developed an antibody‐based chromogenic FVIII assay, which facilitates the selective and sensitive activity measurement of human FVIII in the presence of animal plasma and interfering agents. METHODS: Plate‐bound monoclonal anti‐FVIII antibody specifically captured human FVIII, which was then measured with a chromogenic activity assay. A human reference plasma preparation was used to construct the calibration curve. Spike recovery was carried out in a citrated cynomolgus monkey plasma–solvent/detergent mixture and in the presence of the bispecific antibody emicizumab. RESULTS: The calibration curve ranged from 3.03 to 97.0 mIU FVIII/ml and was obtained repeatedly with good accuracy. B domain–deleted and full‐length FVIII did not differ in their responses. Recovery of spiked human FVIII in citrated cynomolgus monkey plasma was 102.7%, while neither native monkey plasma nor the other animal specimen tested showed any activity. Solvent/detergent solution and the bispecific antibody emicizumab had no influence on the assay. CONCLUSION: Combining antibody‐mediated specific capture of human FVIII and a chromogenic activity assay resulted in a selective and sensitive measurement of human FVIII with no interference by endogenous, nonhuman FVIII, manufacturing process additives, or an FVIII activity–mimicking antibody.