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

Abstract

Background

Efanesoctocog is a B-domain-deleted, Fc-fusion factor (F)VIII linked to the D'D3 domain of von Willebrand factor and 2 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.

Objectives

To test whether the bovine chromogenic FVIII assay, specifically calibrated for efanesoctocog, could provide an accurate assessment of efanesoctocog activity.

Methods

Seven centers across 5 countries received 12 plasma samples to measure in triplicate using 2 calibration methods across 3 independent days. Samples (n = 6) contained either only efanesoctocog (FVIII activity [FVIII:C]= 5 to 150 IU/dL), or efanesoctocog (FVIII:C = 5 to 150 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

Chromogenic assay (CSA) calibrated with standard calibrators highly overestimates FVIII:C. However, specific calibration with efanesoctocog enabled accurate measurement of FVIII:C, 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 (interlaboratory coefficient of variation ranges between 9% and 20%).

Conclusion

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