Activated Protein C-Catalyzed Factor Va Inactivation Predominantly Contributes to the Downregulation of Coagulation Rather Than Factor VIIIa Inactivation

Activated protein C (APC) directly inactivates factor (F)Va and FVIIIa dependently of protein S (PS), and furthermore inactivates FVIIIa through cofactor function of FV. Any impairment in these inactivation pathways may increase thrombotic risk. A previous report described that the APC resistance (APCR) associated with FV-Leiden was mainly due to the impaired cofactor activity of FV (Castoldi et al. Blood 2004;103, 4173). We have recently reported that FV-Nara carrying W1920R mutation exhibited APCR caused by a loss of FVa susceptibility to APC and APC cofactor activity resulting in more serious thrombotic potential than FV-Leiden (Nogami et al. Blood 2014;123, 2420). The contribution of each inactivation pathway associated with FVIII(a) and/or FV(a) for the hemostatic regulation remains unclear, however. An ACE910, bispecific antibody (Ab) to FIXa and FX mimicking the functions of FVIII, exerts tenase activities without FVIII(a) (Kitazawa et al. Nature Medicine. 2012;18, 1570). In this study, to elucidate the contribution of each pathway, we investigated the regulatory mechanism(s) by APC utilizing ACE910. FXa generation with ACE910 (10μg/ml) or with FVIIIa (0.3 nM) in FVIII-deficient plasma (ΔFVIII) was evaluated by the addition of APC (0-0.5 nM), PS (5 nM), and FV (1 nM). FXa generation with FVIIIa was reduced by ~60% maximally in an APC dose-dependent manner, whilst that with ACE910 was unaffected, indicating that ACE910 itself was not susceptible to inactivation by APC through the cofactor activity of FV. Thrombin generation in ΔFVIII with ACE910 (0-30μg/ml) was enhanced in an ACE910 dose-dependent fashion and ~10-fold of peak thrombin (PeakTh) (442 ± 7.0 nM), equivalent to 77.5% of PeakTh in pool normal plasma (PNP), was observed with ACE910 (30μg/ml) compared to that without ACE910 (44.6 ± 5.0 nM). To investigate the effect of APC upon the hemostatic enhancement by ACE910 in ΔFVIII, thrombin generation in ΔFVIII mixed with ACE910 (30μg/ml) and APC (0-16 nM) was examined. PeakTh showed the APC-dependent reduction to 156 ± 22nM and the %inhibition was 64.7%. Subsequently, we evaluated the effects of APC on thrombin generation in PNP or in PNP with an anti-FVIII Ab (FVIIIAb) (16BU/ml) supplemented by ACE910. Of note, the %inhibition of PeakTh by APC in latter with ACE910 and FVIIIAb (64%) was greater than that in PNP alone (35%) by ~1.9-fold. To elucidate the contribution of APC-catalyzed inactivation of FV(a) to the hemostatic regulation, we evaluated the thrombin generation in FV-diluted plasma (FV-dil) consisting of FV-deficient plasma and FV (8nM) corresponded to ~25% of its physiological concentration mixed with or without ACE910 and FVIIIAb in the presence or absence of APC. The %inhibition of PeakTh by APC in FV-dil with ACE910 and FVIIIAb (46.5%), being reduced in comparison with that of PNP with both Abs, was greater than that in FV-dil alone (18.3%) by ~2.5-fold, indicating that the inhibition of thrombin generation by APC-catalyzed FV(a) inactivation was FV(a) dose-dependent but enhanced in the absence of FVIII(a). Furthermore, the effects on thrombin generation with APC (0-16 nM) in plasmas carrying FV-Leiden without or with FVIIIAb and ACE910 (30μg/ml) were examined. It was of surprise that PeakTh in latter with FVIIIAb and ACE910 reduced exponentially by ~40% in an APC dose-dependent manner, whilst APC showed any little effects upon thrombin generation in FV-Leiden plasma alone indicating APCR. The susceptibility index to APC calculated by the rate of inhibition with FVIIIAb and ACE910 was 0.174, greater than that with FV-Leiden plasma alone (0.085) by ~2-fold. These results strongly suggested that APC could regulate the hemostatic enhancement by inactivation of FV(a) even FV-Leiden in the absence of another substrate, FVIII(a). We conclude that APC potentiates its catalytic activity to FVa more in the absence of FVIII(a) than in the presence of FVIII(a) and this mechanism would play significant roles in the downregulation of coagulation as one of the breaking systems.