Abstract
Objectives
Emicizumab, a monoclonal antibody mimicking the function of factor (F) VIII in the activation of FX by FIXa, is widely used for prophylaxis in hemophilia patients with or without inhibitors to FVIII. Although it is administered at fixed dose, its measurement could be occasionally required. In principle, the emicizumab procoagulant effect could be assessed by the one-stage assay (OSA) currently used to measure FVIII. However, the OSA for FVIII presents with limitations. Furthermore, owing to its potent FVIII-like activity, emicizumab interferes with the measurement of the inhibitor to FVIII, which is often needed in patients on emicizumab.
Methods
We prepared test samples by spiking a FVIII-deficient plasma with graded amounts of emicizumab. We modified the OSA for FVIII and tested plasma samples for emicizumab concentrations. Furthermore the chromogenic assay (CA) for FVIII with bovine reagents was used to assess for the FVIII inhibitor in patients on emicizumab.
Results
Slight modification of the OSA for FVIII (i.e., higher test plasma dilution and longer coagulometer acquisition time) made the regular OSA as a reliable laboratory tool to measure emicizumab concentration as shown by the identity of the regression (observed vs. expected) lines. Furthermore, the inhibitors to FVIII in patients on emicizumab, which were negative when measured by the regular Bethesda assay, were reliably measured by the CA assay employing bovine reagents.
Conclusions
The methods currently used to measure FVIII can be easily modified to make the general clinical laboratory able to assist clinicians when dealing with patients on emicizumab.
Research funding: None declared.
Author contributions: AT conceived the work and wrote the manuscript. VC supervised the preparation of test plasmas, collected data and made statistical analysis. ES, MC, prepared test plasmas. CN, MBA and FR made laboratory testing. FP, revised the manuscript. All authors reviewed the data, revised the manuscript, accepted responsibility for the entire content of this manuscript and approved its submission.
Competing interests: AT reports speaker’s fees from Werfen and Stago. CN personal fees from werfen, Roche, Bayer, Novonordisk, Sobi, outside the submitted work. FP personal fees from Bioverativ, Grifols, Roche, Sanofi, Sobi, Spark, and Takeda, outside the submitted work. The other authors none to declare.
Informed consent: Informed consent was obtained from all individuals included in this study.
Ethical approval: The local Institutional Review Board deemed the study exempt from review as it was considered as an in vitro study.
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