Activated prothrombin complex concentrate (APCC)-mediated activation of factor (F)VIII in mixtures of FVIII and APCC enhances hemostatic effectiveness

In vitro effects of combining Mim8 with factor VIII, FVIIa, and activated prothrombin complex concentrates in thrombin generation assays

BACKGROUND

Mim8 is a novel antifactor IXa/antifactor X bispecific antibody in clinical development for prophylactic treatment of hemophilia A with and without inhibitors. Patients treated with Mim8 may need supplementary bleed treatment under certain conditions such as surgery or major trauma.

OBJECTIVES

This study aimed to better understand the response of Mim8 in thrombin generation assays (TGAs) alone or in combination with other hemostatic proteins.

METHODS

We used TGAs with different activators (tissue factor (TF) and activated factor XI) to better understand the similarities and differences between the mode of action of Mim8 and factor VIII (FVIII). Following this, we investigated the effects of mixing Mim8 with the main bleed treatment options for persons with hemophilia A with or without inhibitors: FVIII, activated factor VII (FVIIa), and activated prothrombin complex concentrates (aPCC).

RESULTS

The results indicated that for patients without inhibitors, Mim8 does not interfere with FVIII's mode of action. For patients with inhibitors, Mim8 mixed with aPCC results in a strong synergistic effect causing thrombin generation far exceeding the normal levels. Contrary to this, mixing Mim8 with FVIIa results in a more controlled additive effect, visible only when using TF as a trigger, which does not exceed the normal level of thrombin generation.

CONCLUSION

These findings support the use of approved clinical doses of FVIIa for bleed treatment of patients with FVIII inhibitors treated with Mim8. Additionally, the findings suggest that concomitant use of FVIII and Mim8 is safe for managing breakthrough bleeds.

Potential role of activated factor VIII (FVIIIa) in FVIIa/tissue factor-dependent FXa generation in initiation phase of blood coagulation

Factor VIIa/tissue factor (FVIIa/TF) initiates blood coagulation by promoting FXa generation (extrinsic-Xa). Subsequent generation of intrinsic FXa (intrinsic-Xa) amplifies thrombin formation. Previous studies suggested that FVIIa/TF activates FVIII rapidly in immediate coagulation reactions, and FVIIa/TF/FXa activates FVIII prior to thrombin-dependent feedback. We investigated FVIII/FVIIa/TF/FXa relationships in early coagulation mechanisms. Total FXa generated by FVIIa/TF and FVIIa/TF-activated FVIII (FVIIIa_VIIa/TF) was 22.6 ± 1.7 nM (1 min); total FXa with FVIIa-inhibitor was 3.4 ± 0.7 nM, whereas FXa generated by FVIIa/TF or FVIII/TF was 10.4 ± 1.1 or 0.74 ± 0.14 nM, respectively. Little Xa was generated by FVIII alone, suggesting that intrinsic-Xa mechanisms were mediated by FVIIIa_VIIa/TF and FVIII/TF in the initiation phase. Intrinsic-Xa was delayed somewhat by von Willebrand factor (VWF). FVIII activation by FXa with FVIIa/TF was comparable to activation with Glu-Gly-Arg-inactivated-FVIIa/TF. TF counteracted the inhibitory effects of VWF on FXa-induced FVIII activation mediated by Arg³⁷² cleavage. The FVIII-C2 domain bound to cytoplasmic domain-deleted TF (TF^1-243), and VWF blocked this binding by > 80%, indicating an overlap between VWF- and TF^1-243-binding site(s) on C2. Overall, these data suggest that FVIII-associated intrinsic-Xa, governed by both FVIIa/TF-induced and FXa-induced FVIII activation mediated by FVIII-TF interactions, together with FVIIa-dependent extrinsic-Xa mechanisms, may be central to the initiation phase of coagulation.

A Practical Review of the Emerging Direct Anticoagulants, Laboratory Monitoring, and Reversal Agents

Millions of patients in the United States use anticoagulation for a variety of indications, such as the prevention of stroke in those with atrial fibrillation (AF) and the treatment and prevention of venous thrombosis. For over six decades warfarin was the only available oral anticoagulant, but now several DOACs are available and their use has become more prevalent in recent years. In spite of this increased use, many physicians remain reluctant to prescribe DOACs due to concerns about bleeding and reversibility.

Combination of hemostatic therapies for treatment of patients with hemophilia A and inhibitors

BACKGROUND

Therapy application and monitoring of patients with hemophilia A (HA) and inhibitors are challenging. In the current study, combined FVIII - bypass therapy was implemented for a cohort of severe HA patients with inhibitors.

METHODS

Plasma of 15 HA patients with inhibitors was spiked ex vivo with FVIII, rFVIIa, FEIBA and their combinations and thrombin generation (TG) was studied. Some patients who experienced hemarthroses or required minor surgeries were treated by a combined concomitant administration of FVIII+FEIBA as IV bolus doses.

RESULTS

TG spiking studies showed individual responses not correlated to inhibitor titer. Combinations of agents augmented TG as compared to any single agent, while combined FVIII+FEIBA yielded the highest TG, supporting it as a potential treatment. Following emergent successful surgery of child treated by concomitant FVIII+FEIBA, a total of 396 episodes in 7/15 patients were treated with concomitant FVIII+FEIBA. Five patients were treated for bleeding episodes only, whereas 2 were children undergoing immune tolerance induction (ITI) with FEIBA prophylaxis. Four minor surgeries were performed on FVIII+FEIBA repeated infusions. Neither thrombosis nor any other adverse events were documented.

CONCLUSION

A combination of FVIII+FEIBA may be effective and safe as an alternative treatment option for some high-responding inhibitor patients.

Estimating the potential cost of a high dose immune tolerance induction (ITI) therapy relative to the cost of a combined therapy of a low dose ITI therapy with bypassing agent prophylaxis

INTRODUCTION

The International Immune Tolerance Study (I-ITI) demonstrated comparable success rates between low (FVIII 50 IU/kg/TIW) and high dose (FVIII 200 IU/kg/day) regimens. While costlier, the high dose ITI regimen achieved shorter time-to-treatment success with fewer bleeding episodes compared to the low dose ITI regimen. Adding bypassing agent prophylaxis (BAP) to a low dose ITI regimen may reduce bleeding while still being less costly than high dose ITI.

AIM AND METHODS

An economic model was developed to compare high dose ITI to low dose ITI with BAP. All model inputs were derived from clinical trials. The I-ITI study indicated a median time to negative inhibitor titre of 4.6 and 9.2 months and average number of bleeds/patient of 4.2 and 9.9 for the high and low dose regimens respectively. Based on the BAP trials, aPCC (85 U/kg/TIW) and rFVIIa (90 μg/kg/day) achieved a 62% and 45% reduction in bleeding frequency respectively. Cost analysis was from a US third party payer perspective and limited to drug costs. One-way, two-way and probabilistic sensitivity analyses were performed.

RESULTS

Costs of low dose ITI with aPCC prophylaxis until negative inhibitor titre is achieved was 24.0% less compared to high dose ITI. Low dose ITI with rFVIIa prophylaxis cost 46.5% more compared to high dose ITI. Model results were robust in the majority of the sensitivity analyses.

CONCLUSION

A low dose ITI regimen with aPCC prophylaxis may be cost saving compared to a high dose ITI regimen with the potential to reduce morbidity by lowering the risk for breakthrough bleeds.