In vitro effects of emicizumab on activated clotting time in blood samples from cardiac surgical patients

Abstract

Background Heparin management in hemophilia A (HA) patients with a factor VIII (FVIII) inhibitor can be challenging due to severe activated clotting time (ACT) prolongations. It is important to better understand the impact of emicizumab, a FVIII mimetic on ACT, and tissue factor (TF)‐based coagulation assays. Methods Whole blood from 18 patients undergoing cardiopulmonary bypass (CPB) were mixed in vitro with pooled normal plasma, FVIII‐deficient or FVIII‐inhibitor plasma to affect functional FVIII levels. ACTs and heparin concentration by protamine titration were measured in whole blood mixture with/without emicizumab (50‐100 μg/ml). Thrombin generation and plasmin generation were measured in the patient's plasma mixed with normal plasma or FVIII‐inhibitor plasma to assess the impact of emicizumab under low TF activation. Results FVIII inhibitors prolonged ACTs by 2.2‐fold compared to those in normal plasma mixture at baseline. During CPB, ACTs in normal plasma mixture, and FVIII‐deficient mixture were in 400s, but ACTs reached 900s in FVIII‐inhibitor mixture. Emicizumab shortened ACTs by up to 100s in normal plasma mixture, and FVIII‐deficient mixtures. ACTs remained over 600s in FVIII‐inhibitor mixture, despite adding emicizumab at 100 μg/ml. Heparin concentration measured by TF‐based protamine titration was unaffected. Emicizumab enhanced thrombin peak in the presence of FVIII inhibitors, whereas plasmin generation was mainly affected by thrombin generation, and systemic use of ɛ‐aminocaproic acid. Conclusions FVIII inhibitors extensively prolong ACTs in heparinized whole blood, and clinical levels of emicizumab partially reverse ACT values. Protamine titration should be considered for optimal heparin monitoring in emicizumab‐treated patients with FVIII inhibitors.

Patient blood management and patient safety

“…As seen in Figure 1, emicizumab is a bispecific antibody that can bind factors X and IXa, an action that in intact hemostasis, factor VIII would perform. Therefore, it can significantly reduce bleeding in patients with either one, inherited factor VIII deficiency with inhibitors, and acquired factor VIII deficiency [6]. In acute settings of acquired hemophilia, bleeding is terminated and provides time to perform further diagnosis.…”