Multicenter evaluation of the hemostatic activity of emicizumab in patients with severe hemophilia A

Background: Emicizumab has been approved for the prophylaxis of patients with hemophilia A with or without inhibitors. However, spontaneous and trauma-induced breakthrough bleeds have been reported in patients on emicizumab prophylaxis, and no laboratory assay has been validated to evaluate the hemostatic activity of emicizumab. Objectives: The thrombin generation assay (TGA) could be a surrogate marker of the hemostatic ef ficacy of emicizumab. The correlation between TGA and the methods used to measure emicizumab blood concentration was evaluated in this study. Methods: TGA was modi fied by the use of a trigger reagent combining a very low concentration of tissue factor and activated factor (F)XI. Emicizumab quanti fication was performed by 3 methods: the modi fied 1-step FVIII assay and 2 methods based on liquid chromatography and mass spectrometry (LC-MS). Results: Using tissue factor/activated FXI -triggered TGA and platelet-poor plasma, a relationship was observed between the area under the thrombin generation curve (endogenous thrombin potential [ETP]) and the clinical response of patients to emicizumab. The ultrastructure of fibrin clots was consistent with ETP results and showed that emicizumab had a hemostatic activity equivalent to 20 to 30 IU/dL of FVIII. Finally, pharmacokinetic/pharmacodynamic analyses showed no correlation between ETP and LC-MS nor with modi fied 1 -stage FVIII assay, but a statistically signi ficant correlation between the LC-MS methods and the time-to-peak results of the TGA. Conclusion: Using a modi fied TGA, this study showed that patients who experienced breakthrough bleeds while on emicizumab had a lower thrombin-generating capacity compared with others with good clinical response to emicizumab.