Haemostatic effect of adding tranexamic acid to emicizumab prophylaxis in severe haemophilia A: A preclinical study

Background Patients with severe haemophilia have impaired haemostatic response, delayed clot formation and fibrin clots that are vulnerable to fibrinolysis. Emicizumab is a bispecific antibody that mimics activity of activated factor VIII (FVIII) and increases haemostatic capacity to the level of moderate‐to‐mild haemophilia, thereby used for prophylaxis. Regardless of the impressive clinical performance of emicizumab, breakthrough bleeds may still occur. We aimed to study, in FVIII knockout mice (FVIII‐KO), whether haemostasis is improved with the addition of tranexamic acid (TxAc) to emicizumab. Methods FVIII‐KO mice received prophylaxis with emicizumab or emicizumab+TxAc before trauma. FVIII‐KO mice were given emicizumab 1.5 mg/kg via IV injection. A second retro‐orbital IV injection containing human FIX and FX (both 100U/kg) was given 24 h later and 5 min before the tail amputation or knee trauma. After trauma‐induced knee joint bleeding, magnetic resonance imaging (MRI), and histological analysis were used to compare haemostatic efficacy of the two prophylactic strategies. Thrombin generation (TG) was measured and clots obtained with TG experiment were analysed by scanning electron microscopy. Results In FVIII‐KO mice, blood loss after tail clip was lower after prophylaxis with emicizumab+TxAc compared to emicizumab. MRI results and histological analysis of knee joints showed that the addition of TxAc significantly decreased joint bleeding. Fibrin fibre diameters of mice treated with emicizumab only was thicker than those who received combined prophylaxis with emicizumab+TxAc. Conclusion Our results suggest a potential benefit of TxAc when used in combination with emicizumab in prophylactic settings, especially in patients presenting breakthrough bleeds.