Enhanced Global Hemostatic Potentials with a Bispecific Antibody to Factors IXa and X (ACE910) in the Whole Blood By Rotation Thromboelastometry (ROTEM)

Background: ACE910, a humanized bispecific antibody to factor (F) IXa and FX mimicking the functions of FVIII, exerts tenase activities without FVIII (a) (Kitazawa et al. Nature Medicine. 2012;18, 1570). In primate hemophilia A (HA) models, the hemostatic enhancing effect of ACE910 has been reported and the clinical study investigating the effect and safety of ACE910 for human HA patients is on-going. However, the hemostatic effect of ACE910 remains unquantified and difficult to be evaluated.

Objectives: In this study, we evaluated the viscoelastmetric parameters in the whole blood obtained from HA patients under long-term treatment of ACE910 in phase 1 and its extension studies, utilizing rotation thromboelastometry (ROTEM), in order to investigate global hemostatic function of HA patients under treatment with ACE910.

Methods: Ca²⁺-triggered hemostatic functions were assessed by ROTEM in the citrated whole blood samples obtained and stabilized at the indicated points from five severe HA cases with inhibitors (Case 1 : 39 BU/ml, 2 : 41 BU/ml, 3 : 17 BU/ml, 4 : 11 BU/ml and 5 : 111 BU/ml ) and two severe cases without inhibitors (Case 6 and 7) under subcutaneous administration of ACE910, with the different types of dosing regimen (group A with 0.3 mg/kg/week subcutaneous injection (loading dose of 1 mg/kg) for Case 1, 2, group B with 1 mg/kg/week (loading dose of 3 mg/kg) for Case 3, 4, group C with 3 mg/kg/week for Case 6 and group D changing from group A to group C for Case 5, 7) in a course of clinical study. The samples from the subjects prior to administration of ACE910 were spiked ex vivo with ACE910 and the hemostatic functions of them were also evaluated by ROTEM. The parameters of clot time (CT), clot formation time (CFT), maximum clot formation (MCF) and alpha angle were evaluated. The hemostatic potentials in the samples from twenty healthy volunteers and other ten HA patients with the various FVIII:C were evaluated by ROTEM as controls and the correlation between FVIII:C and CT was obtained. Annualized bleeding rates (ABR) of each case were calculated. The studies were approved by local ethics committee and the informed consent was obtained from each patient.

Results: Addition of ACE910 (f.c. 10 or 30 μg/ml) into the sample from each case prior to administration of ACE910 shortened CT from 5,562 ± 374 sec (median 5,924 sec) to 1,475 ± 138 or 1,131 ± 82 sec, equivalent to FVIII:C 3.3 or 12.2 IU/dL, respectively, in an ACE910 concentration dependent manner. In group A, the plasma concentration of ACE910 got to 12 ± 5 μg/ml at 12 week. CT was shortened to 1,443 ± 24 sec, equivalent to FVIII:C 3.7 IU/dL maximally at 47 week consistent with the result of the spiked result. ABR decreased from 38.6 ± 25.8 to 1.1 ± 0.8, showing 97% decrease. As for Case 3 in group B (ACE910: 31 μg/ml at 12 week), CT was shortened to 1,164 sec, equivalent to FVIII:C 11.9 IU/dL maximally at 47 week consistent with the spiked data. ABR apparently decreased from 38.6 to 3.6. In Case 4 who dropped out the clinical study at 4 week, CT was shortened to 977 sec, equivalent to FVIII:C 22 IU/dL, maximally at 3 week, and continued to be shortened till 29 week (1,758 sec, equivalent to FVIII:C 1.1 IU/dL). In group C, the shortening of CT from 1,594 ± 103 sec, equivalent to FVIII:C 2.0 ± 1.0 (median 3.6) IU/dL (before administration of ACE910), to 1,226 ± 71 sec (8.5 ± 2.5 (median 10.4) IU/dL) (at steady state) was observed in consistent with the decrease of ABR from 8.1 to 1.1. In group D, CT was shortened from 3,405 ± 386 sec to 1,409 ± 85 sec, equivalent to FVIII:C 4.2 IU/dL after middle to high dose escalation in consistent with decrease of ABR from 22.3 ± 9.6 to 3.8 ± 2.6. Among all cases except Case 4, CT and ABR were of clinically relevant difference between before and after administration of ACE910.

Conclusions: The comprehensive hemostatic function evaluated by ROTEM in hemophilia A patients irrespective of presence of inhibitors was improved after administration of ACE910 in a dose dependent fashion, resulting in the reduction of ABR, which suggested the hemostatic effectiveness of ACE910 for HA patients.