Variability of thromboelastographic responses following the administration of rFVIIa to haemophilia A dogs supports the individualization of therapy with a global test of haemostasis

Clot formation and fibrinolysis assays reveal functional differences among hemostatic agents in hemophilia A plasma

Background

Measuring the activity of hemostatic agents used to treat hemophilia A often requires drug-specific assays. In vitro assays show hemophilic clots have abnormal characteristics, including prolonged clotting time and decreased resistance to fibrinolysis. The ability of certain agents to correct these parameters in vitro is associated with hemostatic efficacy in vivo.

Objectives

To compare effects of established and emerging hemostatic agents on clot formation and fibrinolysis in hemophilia A plasma.

Methods

Pooled and individual hemophilia A platelet-poor plasmas were spiked with replacement (recombinant factor VIII [rFVIII], PEGylated rFVIII, polysialylated rFVIII, and porcine rFVIII) or bypassing (emicizumab, rFVIIa, and activated prothrombin complex concentrate) products. Effects on tissue factor-initiated clot formation and fibrinolysis were measured by turbidity.

Results

Compared to normal pooled plasma, hemophilia-pooled plasma showed reduced clot formation and increased fibrinolysis, and all replacement agents improved these characteristics. rFVIII and PEGylated rFVIII produced similar effects at similar concentrations, whereas polysialylated rFVIII produced slightly higher and porcine rFVIII slightly lower effects at these concentrations. Bypassing agents enhanced clot formation and stability, but patterns differed from replacement agents. The clotting rate showed a concentration-response relationship for all agents. High concentrations of all products produced effects that exceeded the normal range in at least some parameters. Responses of individual donors varied, but all agents improved clot formation and stability in all donors tested.

Conclusion

Clotting and fibrinolysis assays reveal hemostatic effects of replacement and bypassing therapies at clinically relevant concentrations. These assays may help characterize hemostatic agents and optimize dosing.

Monitoring low dose recombinant factor VIIa therapy in patients with severe factor XI deficiency undergoing surgery

Although factor XI (FXI) concentrate is an effective replacement therapy in severe FXI deficiency without inhibitors, some patients are unwilling to receive it because it is plasma-derived. We report on the use and monitoring of low dose, recombinant factor VIIa (rFVIIa, NovoSeven®), to cover surgery (caesarean section, cholecystectomy and abdominoplasty) in four female patients (FXI:C 2–4 IU/dl, aged 32–51 years) who wished to avoid exposure to plasma. None of our patients had inhibitors to FXI. Our aim was to find the optimal dose of rFVIIa by in vitro spiking of patient samples and to correlate this with the response to rFVIIa in vivo. Prior to surgery, venous blood was collected into sodium citrate with corn trypsin inhibitor and spiked with 0.25–1.0 μg/ml rFVIIa in vitro, equivalent to a 15–70 μg/kg dose of rFVIIa in vivo. Analysis using thromboelastometry and thrombin generation assays, triggered with tissue factor, showed that the thrombin generation assay was insufficiently sensitive to the haemostatic defect in these patients. A concentration of 0.5 μg/ml was as effective as 1.0 μg/ml FVIIa in normalising thromboelastometry in vitro in all four patients. Therefore, patients received 15–30 μg/kg rFVIIa at 2–4 hourly intervals with tranexamic acid 1g every six hours. Post treatment samples were taken at 10–240 minutes and showed initial normalisation of thromboelastometry with gradual return to baseline after 2–4 hours. In conclusion, low-dose rFVIIa therapy was successfully used in four patients with severe FXI deficiency undergoing surgery to prevent bleeding and can be monitored using thromboelastometry.

Rationale for individualizing haemophilia care

Owing to the heterogeneity in the clinical phenotype of haemophilia A and B, it is now recognized that disease severity (based on factor VIII/IX activity) may no longer be the most appropriate guide for treatment and that a 'one-size-fits-all' approach is unlikely to achieve optimal therapy. Based on the present literature and consensus views of a group of experts in the field, this article highlights key gaps in the understanding of the diverse relationships between bleeding phenotype and factors such as joint health, genetic susceptibility, laboratory parameters, quality of life and management of pain. Early prophylaxis is a potential 'gold standard' therapy and issues surrounding inhibitor development, variations in its clinical use and long-term outcomes are discussed. Comprehensive treatment should be individualized for all patients (including those with mild or moderate haemophilia and carriers). Wherever possible all patients should be given prophylaxis. However, adult patients with a milder haemophilia phenotype may be candidates for ceasing prophylaxis and switching to on-demand treatment. Regardless, all treatment (on-demand and prophylaxis) should be tailored towards both the patient's personal needs and their clinical profile. In addition, as the associations between risk factors (psychosocial, condition-related and treatment-related) and clinical features are unique to each patient, an individualized approach is required to enable patients to alter their behaviour in response to them. The practical methodologies needed to reach this goal of individualized haemophilia care, and the health economic implications of this strategy, are ongoing topics for discussion.