Estimating the Factor VIII-Equivalent Activity of Emicizumab Using Global Assays of Haemostasis

Background

Emicizumab is a bispecific monoclonal antibody mimicking factor (F) VIII activity and an effective therapy for prophylaxis in people with haemophilia A (PWHA). The FVIII-equivalent activity (FVIIIeq) of emicizumab remains unknown.

Objectives

To estimate FVIIIeq of emicizumab using global assays of haemostasis.

Patients/Methods

We sampled plasma from (1) PWHA of all severities and therapeutic regimens and (2) persons with severe haemophilia A on emicizumab. Thrombin generation assay (TGA) and clot formation and lysis assay (CLA) were measured by commercially available (Technothrombin, Technoclone) and turbidimetric assays, respectively. FVIII was measured by a chromogenic (CSA) and emicizumab by a modified one-stage assay (OSA). Using principal components (PC) or individual parameters as dependent and independent variables in linear regression with (1) FVIII and (2) emicizumab levels, respectively, we estimated FVIIIeq (95% CI) of emicizumab.

Results

We collected 253 samples from 110 PWHA and 41 samples from nine individuals on emicizumab prophylaxis. TGA parameters of emicizumab-treated subjects clustered distinctly, with no correlation amongst samples with only endo-/exogenous FVIII. Consequently, FVIIIeq varied substantially between parameters: Using individual TGA parameters, endogenous thrombin potential (ETP) and thrombin peak resulted in conversion factors of 0.85 (0.60–1.24) and 0.25 (0.15–0.37) IU/dL FVIIIeq per µg/mL emicizumab, respectively. CLA parameters showed broader overlap, with an FVIIIeq estimate of 0.82 (0.44–1.38).

Conclusion

Distinct clustering of emicizumab and FVIII samples in TGA leads to conflicting FVIIIeq estimates between parameters, highlighting systematic limitations and questioning their clinical usefulness. A, possibly conservative, FVIIIeq conversion factor estimate as determined by the thrombin peak could range from 0.15 to 0.37.