A Novel Hemostatic Monitoring System Convertible to FVIII Activity Based on Non-Activated ROTEM (NATEM) for Hemophilia a Patients during Emicizumab Prophylaxis

Emicizumab is a factor (F) VIII(a) mimicking bispecific antibody to FIX(a) and FX, which is a new therapeutic agent for hemophilia A patients (PWHA) with inhibitor. Although the clinical effect and safety were confirmed in the clinical studies (NEJM 2016, NEJM 2017, Blood Adv. 2017), there exists difficulty in monitoring of emicizumab by APTT-based assay. In the present study, we challenged to establish a hemostatic monitoring system convertible to FVIII activity (FVIII:C) for PWHA under the emicizumab prophylaxis by utilizing non-activated rotational thromboelastometry (ROTEM). The studies were approved by local ethics committee and the informed consent was obtained from each patient.

We firstly prepared the FVIII-deficient whole blood samples treated with a neutralizing anti-FVIII polyclonal IgG (30BU/mL), and compared the in vitro hemostatic response in the presence of emicizumab (0, 5 and 50µg/mL) spiked into these samples among three modes of ROTEM such as NATEM, EXTEM and INTEM. According to CaCl₂ triggered-NATEM without any other activator, the sum of clot time and clot formation time (CT+CFT) was 1269 ± 197 sec (mean ± SD) for normal control and 6713 ± 855 sec for the samples without emicizumab. The value of CT+CFT was significantly shortened in the presence of emicizumab at 5 µg/ml and 50 µg/ml to 2157 ± 192 sec and 1521 ± 313 sec, respectively (p = 0.025). By contrast, measured by EXTEM utilizing tissue factor (0.5 pM) and CaCl₂ as triggers, the values of CT+CFT for control, 0, 5 and 50 µg/mL of emicizumab were 552 ± 97 sec, 771 ± 186 sec, 658 ± 135 sec and 778 ± 90 sec, respectively. There was no significant difference (p = 0.44) among them. By INTEM, triggered by ellagic acid with CaCl₂, the values of CT+CFT for control, 0, 5, and 50 µg/mL of emicizumab were 662 ± 202 sec, 5854 ± 705 sec, 951 ± 170 sec and 814 ± 216 sec, respectively. The value of CT+CFT was shortened in the presence of emicizumab, however, little difference was observed between two doses. The other parameters, maximum clot firmness and alpha angle, were not suitable for quantitative evaluation irrespective of the mode of measurement. These suggested that CT+CFT based on NATEM is a suitable parameter for the monitoring of emicizumab.

In order to develop a NATEM-based hemostatic scale convertible to FVIII:C, we secondly measured CT+CFT in the samples (n = 81) from PWHA with various FVIII:C levels in our hospital. The samples were classified based on FVIII:C into three groups, such as Tertile (T)1 with undetectable FVIII:C (<0.2 IU/dL,n = 22), T2 (0.2≤ FVIII:C <12 IU/dL, median FVIII:C 1.7 IU/dL, interquartile range 1.1-5.6 IU/dL, n = 35) and T3 group (12≤ FVIII:C <60 IU/dL, 19 IU/dL, 13-39 IU/dL, n = 24). T2 and T3 were delimited by FVIII:C 12 IU/dL in which zero joint-bleeding can be expected (den Uijil, Haemophilia 2011). The value of CT+CFT in T1, T2 and T3 group was 6702 ± 1517 sec, 2747 ± 817 sec and 2024 ± 630 sec, respectively, with the significant difference (p < 0.01).

By utilizing this NATEM scale, we investigated the hemostatic function of four PWHA with inhibitor (patient 1, 2, 3 and 5) and two without inhibitor (patient 4, 6) enrolled in the phase 1 study (ACE001JP) and its extension study (ACE002JP) with the different types of dosing-regimen (0.3, 1.0 and 3.0 mg/kg/week after loading dose) (Table). The baseline value of CT+CFT among all of the patients was 6502 ± 1297 sec in the range of T1-level classified as above. The value was shortened and maintained to T2-level at the concentration of ~10 μg/ml after emicizumab infusion at all three dosing. The value reached up to T3-level of ~20 μg/ml in 1.0 and 3.0 dosing groups. The hemostatic function reached up to T2- and T3-level during loading phase before achievement to the steady state. No joint bleeding was observed among the patients who achieved T3-level except for patient 5 and 6 who developed breakthrough bleedings in target joints.

In conclusion, the global hemostatic function during emicizumab prophylaxis can be quantitatively evaluated by NATEM-based scaling before steady state among PWHA.

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