Therapeutic doses of recombinant factor VIIa in hemophilia generates thrombin in platelet-dependent and -independent mechanisms

Reduced plasma factor X is associated with a lack of response to recombinant activated factor VII in patients with hemophilia A and inhibitor, but does not impair emicizumab-driven hemostasis in vitro

BACKGROUND

The hemostatic effect of recombinant (r) factor (F)VIIa after repetitive intermittent administration may be attenuated in patients with hemophilia A (PwHA) with inhibitors (PwHAwI) creating a clinically unresponsive status, although mechanism(s) remain to be clarified. In patients receiving prophylaxis treatment with emicizumab, concomitant rFVIIa is sometimes utilized in multiple doses for surgical procedures or breakthrough bleeding.

AIM AND METHODS

We identified 'unresponsiveness' to rFVIIa, based on global coagulation function monitored using rotational thromboelastometry (ROTEM) in 11 PwHAwI and 5 patients with acquired HA, and investigated possible mechanisms focusing on the association between plasma FX levels and rFVIIa-mediated interactions.

RESULTS

Our data demonstrated that FX antigen levels were lower in the rFVIIa-unresponsive group than in the rFVIIa-responsive group (0.46 ± 0.14 IU/mL vs. 0.87 ± 0.15 IU/mL, p < 0.01). This relationship was further examined by thrombin generation assays using a FX-deficient PwHAwI plasma model. The addition of FX with rFVIIa was associated with increased peak thrombin (PeakTh) generation. At low levels of FX (<0.5 IU/mL), rFVIIa failed to increase PeakTh to the normal range, consistent with clinical rFVIIa-unresponsiveness. In the presence of emicizumab (50 μg/mL), PeakTh was increased maximally to 80 % of normal, even at low levels of FX (0.28 IU/mL).

CONCLUSIONS

Unresponsiveness to rFVIIa was associated with reduced levels of FX in PwHAwI. Emicizumab exhibited in vitro coagulation potential in the presence of FX at concentrations that appeared to limit the clinical response to rFVIIa therapy.

Selective inhibition of activated protein C anticoagulant activity protects against hemophilic arthropathy in mice

Recurrent spontaneous or trauma-related bleeding into joints in hemophilia leads to hemophilic arthropathy (HA), a debilitating joint disease. Treatment of HA consists of preventing joint bleeding by clotting factor replacement, and in extreme cases, orthopedic surgery. We recently showed that administration of endothelial cell protein C receptor (EPCR) blocking monoclonal antibodies (mAb) markedly reduced the severity of HA in factor VIII (FVIII)-/- mice. EPCR blocking inhibits activated protein C (APC) generation and EPCR-dependent APC signaling. The present study was aimed to define the role of inhibition of APC anticoagulant activity, APC signaling, or both in suppressing HA. FVIII-/- mice were treated with a single dose of isotype control mAb, MPC1609 mAb, that inhibits anticoagulant, and signaling properties of APC, or MAPC1591 mAb that only blocks the anticoagulant activity of APC. Joint bleeding was induced by needle puncture injury. HA was evaluated by monitoring joint bleeding, change in joint diameter, and histopathological analysis of joint tissue sections for synovial hypertrophy, macrophage infiltration, neoangiogenesis, cartilage degeneration, and chondrocyte apoptosis. No significant differences were observed between MPC1609 and MAPC1591 in inhibiting APC anticoagulant activity in vitro and equally effective in correcting acute bleeding induced by the saphenous vein incision in FVIII-/- mice. Administration of MAPC1591, and not MPC1609, markedly reduced the severity of HA. MAPC1591 inhibited joint bleed-induced inflammatory cytokine interleukin-6 expression and vascular leakage in joints, whereas MPC1609 had no significant effect. Our data show that an mAb that selectively inhibits APC's anticoagulant activity without compromising its cytoprotective signaling offers a therapeutic potential alternative to treat HA.

The Reevaluation of Thrombin Time Using a Clot Waveform Analysis

Object: Although thrombin burst has attracted attention as a physiological coagulation mechanism, clinical evidence from a routine assay for it is scarce. This mechanism was therefore evaluated by a clot waveform analysis (CWA) to assess the thrombin time (TT). Material and Methods: The TT with a low concentration of thrombin was evaluated using a CWA. We evaluated the CWA-TT of plasma deficient in various clotting factors, calibration plasma, platelet-poor plasma (PPP), and platelet-rich plasma (PRP) obtained from healthy volunteers, patients with thrombocytopenia, and patients with malignant disease. Results: Although the TT-CWA of calibration plasma was able to be evaluated with 0.01 IU/mL of thrombin, that of FVIII-deficient plasma could not be evaluated. The peak time of CWA-TT was significantly longer, and the peak height significantly lower, in various deficient plasma, especially in FVIII-deficient plasma compared to calibration plasma. The second peak of the first derivative (1st DP-2) was detected in PPP from healthy volunteers, and was shorter and higher in PRP than in PPP. The 1st DP-2 was not detected in PPP from patients with thrombocytopenia, and the 1st DP-2 in PRP was significantly lower in patients with thrombocytopenia and significantly higher in patients with malignant disease than in healthy volunteers. Conclusion: The CWA-TT became abnormal in plasma deficient in various clotting factors, and was significantly affected by platelets, suggesting that the CWA-TT may be a useful test for hemostatic abnormalities.

Factor VIIa induces extracellular vesicles from the endothelium: a potential mechanism for its hemostatic effect

Recombinant factor FVIIa (rFVIIa) is used as a hemostatic agent to treat bleeding disorders in hemophilia patients with inhibitors and other groups of patients. Our recent studies showed that FVIIa binds endothelial cell protein C receptor (EPCR) and induces protease-activated receptor 1 (PAR1)-mediated biased signaling. The importance of FVIIa-EPCR-PAR1-mediated signaling in hemostasis is unknown. In the present study, we show that FVIIa induces the release of extracellular vesicles (EVs) from endothelial cells both in vitro and in vivo. Silencing of EPCR or PAR1 in endothelial cells blocked the FVIIa-induced generation of EVs. Consistent with these data, FVIIa treatment enhanced the release of EVs from murine brain endothelial cells isolated from wild-type (WT), EPCR-overexpressing, and PAR1-R46Q-mutant mice, but not EPCR-deficient or PAR1-R41Q-mutant mice. In vivo studies revealed that administration of FVIIa to WT, EPCR-overexpressing, and PAR1-R46Q-mutant mice, but not EPCR-deficient or PAR1-R41Q-mutant mice, increased the number of circulating EVs. EVs released in response to FVIIa treatment exhibit enhanced procoagulant activity. Infusion of FVIIa-generated EVs and not control EVs to platelet-depleted mice increased thrombin generation at the site of injury and reduced blood loss. Administration of FVIIa-generated EVs or generation of EVs endogenously by administering FVIIa augmented the hemostatic effect of FVIIa. Overall, our data reveal that FVIIa treatment, through FVIIa-EPCR-PAR1 signaling, releases EVs from the endothelium into the circulation, and these EVs contribute to the hemostatic effect of FVIIa.