Accurate evaluation of factor VIII activity of efanesoctocog alfa in the presence of emicizumab.

Abstract

Background: Efanesoctocog is a B-domain-deleted, Fc-fusion FVIII linked to the D'D3 domain of VWF and two XTEN polypeptides, designed for an ultra-extended half-life for prophylaxis in hemophilia A, but also aiding in managing acute bleeding or surgery in patients on long-term emicizumab. However, no current laboratory method accurately measures FVIII levels in the presence of emicizumab. We hypothesized that the chromogenic (CSA) FVIII assay, specifically calibrated for efanesoctocog using bovine coagulation factors, could provide an accurate assessment of efanesoctocog activity.

Materials&methods: Seven centers across five countries received 12 plasma samples to measure in triplicate using two calibration methods across three independent days. Samples (n=6) contained either only efanesoctocog (FVIII :C=150 to 5 IU/dL), or efanesoctocog (FVIII:C=150 to 5 IU/dL) in combination with emicizumab (50 μg/mL)(n=5). One sample contained efanesoctocog (FVIII :C=50 IU/dL) and a high dose of emicizumab (80 μg/mL), another sample contained efanesoctocog (FVIII :C=50 IU/dL) with a low dose of emicizumab (20 μg/mL). Each center used its own analyzers, along with their usual reagents.

Results: CSA calibrated with standard calibrators highly overestimate FVIII activity. However, specific calibration with efanesoctocog enabled accurate measurement of FVIII activity, with low inter- and intra-laboratory variability, and no interference from emicizumab. All CSA reagents used in the study demonstrated low variability across different laboratories (Inter-laboratory coefficient of variation ranges between 9% and 20%).

Conclusion: Specific calibration of the CSA FVIII assay using efanesoctocog and bovine reagents allows for accurate measurement of FVIII activity in patients receiving efanesoctocog, even in the presence of emicizumab.