Key Points

  • Emicizumab (≥1.5 mg/kg) partially corrects blood loss in a hemophilia A bleeding model, with a FVIII-equivalence of 9 U/dL

  • Emicizumab provides additive hemostatic activity when given in combination with low dose Factor VIII

The bispecific antibody Emicizumab is increasingly used for hemophilia A-treatment. However, its specificity for human factors IX and X (FIX, FX) has limited its in vivo functional analysis to primate models of acquired hemophilia. Here we describe a novel mouse model allowing to examine Emicizumab function. Briefly, FVIII-deficient mice receive Emicizumab intravenously 24h before performing a tail clip-bleeding model. A second infusion with human FIX and FX is administered 5 min before bleeding. This approach generates consistent levels of Emicizumab (0.7-19 mg/dL for 0.5-10 mg/kg dose) and of both FIX and FX (85 and 101 U/dL respectively, after dosing 100 U/kg). Plasmas from these mice display FVIII-like activity in a diluted aPTT and in thrombin generation assays, similar to human samples containing Emicizumab. Emicizumab doses of 1.5 mg/kg and higher significantly reduced blood loss in a tail clip-bleeding model using FVIII-deficient mice. However, reduction was incomplete compared to mice treated with human FVIII concentrate, and no difference in efficacy between doses was observed. From this model, we deducted a FVIII-like activity of Emicizumab that corresponds to a dose of 4.5 U FVIII/kg (i.e. 9.0 U/dL). Interestingly, combined with a low FVIII dose (5 U/kg), Emicizumab provided additive activity to allow a complete bleeding arrest. This model could be useful for further in vivo analysis of Emicizumab.

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