Additional Factor X Enhances Emicizumab-Driven Coagulation Function in Patients with
Hemophilia A and Hemophilia A Mice
Hemophilia A (HA) is a genetic bleeding disorder caused by a deficiency or defect
in the factor VIII (FVIII) procoagulant protein. Regular use of FVIII products results
in the development of anti-FVIII alloantibodies (inhibitors) in 20 to 30% of severe
HA patients, increasing the risk of morbidity and mortality and complicating the management
of these patients.[1] Until recently, the treatment options were limited to giving regular or as-needed
bypassing agents, like recombinant activated factor VII (factor VIIa) or activated
prothrombin complex concentrates, or inducing immune tolerance to eliminate FVIII
inhibitors.[2] Currently, prophylaxis with emicizumab, a humanized bispecific antibody, is increasingly
recognized as the standard of care for patients with HA, regardless of the presence
of inhibitors. Emicizumab mimics FVIII cofactor activity by binding to activated factor
IX and factor X (FX) to form a tenase complex that efficiently restores effective
hemostasis.[3] In the recommended therapeutic concentrations, emicizumab significantly decreases
bleeding events and corresponds to an FVIII activity level of 10 to 15 IU/dL assimilated
to mild-phenotype hemophilia.[4] However, additional use of bypassing agents or FVIII is still necessary in patients
receiving emicizumab in case of breakthrough bleeding or patients undergoing surgical
procedures with high bleeding risk. In HA with inhibitors on emicizumab prophylaxis,
the use of activated prothrombin complex concentrates was associated with thrombotic
events, and the use of factor VIIa is recommended as the first-line treatment of bleeds.[5] Even if such adverse events were not reported with factor VIIa, the number of patients
with HA with inhibitors treated for significant bleeding is limited until now; therefore,
venous or arterial thrombotic events cannot be excluded. Moreover, recombinant factor
VIIa should be administered frequently due to the short half-life of the product,
making this therapy inconvenient; in certain situations, the bleeding does not stop
after factor VIIa administration, and adding activated prothrombin complex concentrate
is necessary, leading to an even higher risk of thrombotic complications.
It is essential to understand how to increase the hemostatic activity of emicizumab
and to discover alternative methods for managing breakthrough bleeding in HA patients
with inhibitors who are using emicizumab. In the latest issue of Thrombosis and Haemostasis, Shimizu et al offer additional insight into the impact of elevating FX levels on
the coagulation potential in HA patients with inhibitors who are being treated with
emicizumab.[6] The authors used in vivo and in vitro experiments. They evaluated global coagulation
potential using the adjusted maximum coagulation velocity from clot waveform analysis,
peak thrombin, endogenous thrombin potential from thrombin generation assay (TGA),
and clotting time, clot formation time and α parameter from rotational thromboelastometry.[6]
In their in vitro experiments, the authors assessed the coagulation potential in FVIII-depleted
plasmas and in plasma samples from patients with HA without inhibitors in which they
added emicizumab (in a dose calculated to mimic the therapeutic concentration) and/or
FX in increasing concentrations. When emicizumab alone was added to the FVIII-depleted
plasma, the coagulation potential increased, and the supplemental addition of FX led
to a more significant increase compared with baseline values. Adding FX only without
emicizumab to FVIII-depleted plasma also led to an enhanced coagulation potential
but of a lower magnitude than that obtained with emicizumab alone ([Fig. 1A]). Interestingly, if the adjusted maximum coagulation velocity from clot waveform
analysis and the peak thrombin from TGA increased dose-dependently with the amount
of FX added to FVIII-depleted plasma with or without emicizumab, the other parameter
recorded from TGA, endogenous thrombin potential, did not show the same tendency to
increase in parallel with FX concentrations. In a second experiment, the authors demonstrated
similar effects on the coagulation potential when factor VIIa and FX (in adjusted
doses to obtain the concentrations attained in clinical use) were added to plasma
samples from patients with HA with inhibitors treated with emicizumab.
Fig. 1 (A) Coagulation potential in FVIII-depleted plasmas and plasma samples from hemophilia
A (HA) patients after the addition of factor X and/or emicizumab. CWA, clot waveform
assay; TGA, thrombin generation assay; +Emi, added emicizumab; +FX, added factor X;
+Emi& FX, added emicizumab and factor X; the comparisons of coagulation potentials
are made with raw plasma samples. (B) Coagulation potential in HA mice administered emicizumab and/or factors IX and X
with factor X in increasing dosages. NATEM, nonactivated thromboelastometry; CT, clotting
time; CFT, clot formation time; α, α angle; the comparisons of coagulation potentials
are made with raw samples; +Emi, added emicizumab; +100/100, added human factor IX
(100 IU/kg), and human factor X (100 IU/kg); +emi& 100/100, added emicizumab and human
factor IX (100 IU/kg), and human factor X (100 IU/kg); +emi& 100/200, added emicizumab
and human factor IX (100 IU/kg), and human factor X (200 IU/kg); +emi& 100/400, added
emicizumab and human factor IX (100 IU/kg), and human factor X (100 IU/kg); the comparisons
of coagulation potentials are made with raw samples.
In vivo, animal experiments used nonactivated rotational thromboelastometry to assess
the hemostatic effects of increasing doses of FX in the presence of factor IX with
or without emicizumab in HA mice. When only emicizumab or only coagulation factors
were given, the coagulation parameters reflecting clotting initiation and propagation
were not changed compared with untreated animals and improved only in animals treated
simultaneously with emicizumab and coagulation factors ([Fig. 1B]). Clinically, the hemostatic changes induced by increasing doses of FX in the presence
of factor IX with or without added emicizumab were assessed by estimating the blood
loss after a standardized tail clip in HA mice. The findings showed that the hemostatic
effect of added coagulation factors was only visible in mice that received emicizumab
as a decreased blood loss compared with untreated mice. At the same time, the differences
in thrombotic markers measured in HA mice receiving emicizumab and mice receiving
emicizumab plus additional FX were not significant.
Using in vitro and animal experiments, the authors have elegantly demonstrated that
FX supplementation could enhance hemostatic mechanisms mediated by emicizumab in HA,
with or without inhibitors. The combined use of FX and emicizumab resulted in greater
hemostatic effects than either alone. Importantly, the resulting global coagulation
potential did not surpass the normal ranges, and the thrombotic risk was not enhanced
even with high concentrations of FX.[6]
The results from Shimizu et al are consistent with those of Nakajima et al, who previously
demonstrated improved hemostatic potential (but still lower than normal ranges) in
FVIII-deficient plasma after adding emicizumab.[7] Improving hemostasis by using factor concentrates different from FVIII and factor
IX, the missing factors responsible for hemophilias, is not new. Concentrates containing
FX, or variants of FX capable of activating thrombin, were shown to increase coagulation
potential in HA with or without inhibitors.[8]
[9]
[10] A recent article by Yada et al showed that the hemostatic response to factor VIIa
in patients with HA with inhibitors was dependent on FX levels, being decreased with
FX levels lower than 0.5 IU/mL.[11] However, in patients with HA with inhibitors treated with emicizumab, the coagulant
response to factor VIIa was enhanced even at low FX levels.[11] The importance of FX levels in HA patients receiving emicizumab was also demonstrated
by Kitazawa et al, who showed that more complexes are formed with emicizumab, factor
IX/IXa, and FX/factor Xa when plasmatic FX levels are higher, leading to improved
hemostatic potential.[12]
Currently, activated prothrombin complex concentrates and recombinant factor VIIa
are recommended for breakthrough bleeding therapy in patients with HA treated with
emicizumab. Considering the previous findings of increased thromboembolic risk with
enhanced factor IX activity in emicizumab-treated patients,[13] using FX to increase hemostatic potential in HA patients would be preferable to
using activated prothrombin complex concentrates. Another practical advantage is that,
compared with factor VIIa, the half-life of FX is longer, with less frequent infusions
needed. The study by Shimizu et al adds to the evidence in favor of exogenous FX therapy
in combination with emicizumab for treating breakthrough bleeding in HA patients with
inhibitors. Future research should focus on finding adequate therapy regimens using
this approach.
