Emicizumab promotes factor Xa generation on endothelial cells

Area of Science:

• Hemostasis and Thrombosis Research
• Pharmacology and Drug Development

Background:

• Hemophilia A treatment traditionally relies on Factor VIII (FVIII) replacement, but neutralizing alloantibodies limit efficacy in up to one-third of severe cases.
• Emicizumab, a bispecific antibody mimicking FVIIIa, has transformed hemophilia A treatment.
• Thrombotic complications, including microangiopathy, have been reported with emicizumab combined with activated prothrombin complex concentrate (FEIBA).

Purpose of the Study:

• To investigate the hypothesis that thrombotic complications from emicizumab and FEIBA result from excessive procoagulant activity on endothelial cells.
• To examine the mechanism underlying thrombotic events in patients treated with emicizumab and FEIBA.

Main Methods:

• Utilized two distinct cell culture models to assess emicizumab's capacity to facilitate Factor X (FX) activation on endothelial cells.
• Quantified FXa generation under varying concentrations of FIXa and in the presence of FEIBA or FVIII(a).

Main Results:

• Endothelial cells effectively support emicizumab-mediated FX activation by FIXa, with FXa generation levels dependent on FIXa concentration.
• FEIBA significantly and dose-dependently increased FXa generation when combined with emicizumab on endothelial cells.
• FXa generation was further amplified by activated endothelial cells; FVIII(a) showed limited FXa generation with a lag phase, unlike emicizumab.

Conclusions:

• Emicizumab promotes FXa generation on endothelial cell surfaces, an effect markedly amplified by the co-administration of FEIBA.
• This study identifies a potential mechanism contributing to the thrombotic complications observed with the combined use of emicizumab and FEIBA.