Use of the Thrombin Generation Test to Evaluate Response to Treatment With Recombinant Activated Factor VII

Concizumab improves clot formation in hemophilia A under flow

BACKGROUND

Inhibition of tissue factor pathway inhibitor (TFPI) is an emerging therapeutic strategy for treatment of hemophilia. Concizumab is a monoclonal antibody that binds TFPI and blocks its inhibition of factor (F)Xa thereby extending the initiation of coagulation and compensating for lack of FVIII or FIX.

OBJECTIVES

The objective of this in vitro study was to evaluate how concizumab affects clot formation in hemophilia A under flow.

METHODS

Blood was collected from normal controls or people with hemophilia A. An anti-FVIII antibody was added to normal controls to simulate hemophilia A with inhibitory antibodies to FVIII. Whole blood and recombinant activated FVII (rFVIIa, 25 nM) or concizumab (200, 1000, and 4000 ng/mL) were perfused at 100 s-1 over a surface micropatterned with tissue factor (TF) and collagen-related peptide. Platelet and fibrin(ogen) accumulation were measured by confocal microscopy. Static thrombin generation in plasma was measured in response to rFVIIa and concizumab.

RESULTS

Concizumab (1000 and 4000 ng/mL) and rFVIIa both rescued (93%-101%) total platelet accumulation, but only partially rescued (53%-63%) fibrin(ogen) incorporation to normal control levels in simulated hemophilia A. Results using congenital hemophilia A blood confirmed effects of rFVIIa and concizumab. While these 2 agents had similar effect on clot formation under flow, concizumab enhanced thrombin generation in plasma under static conditions to a greater extent than rFVIIa.

CONCLUSION

TFPI inhibition by concizumab enhanced activation and aggregation of platelets and fibrin clot formation in hemophilia A to levels comparable with that of rFVIIa.

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.

Thrombin generation for monitoring hemostatic therapy in hemophilia A: A narrative review

Patients with severe hemophilia A (HA) have an increased risk of spontaneous and trauma-related bleeding because of a congenital absence of factor VIII (FVIII). Most severe HA patients use prophylactic FVIII concentrate, the effect of which can be monitored with FVIII activity level measurement. However, FVIII activity level is less valuable in predicting the potential clinical bleeding risk. Some patients still experience breakthrough bleeds despite adequate FVIII trough levels, whereas others do not bleed with trough levels below threshold. This difference may be caused by inter-individual differences in pro- and anticoagulant factors, the so-called hemostatic balance. Thrombin generation assays (TGAs) measure the hemostatic balance as a whole. Thereby, the TGAs may be a better tool in the guidance and monitoring of treatment in HA patients. In addition, TGAs offer the opportunity to determine the response to bypassing agents and treatment with non-factor replacement therapy, in which FVIII activity assays are not suitable for monitoring. This review summarizes the current knowledge about monitoring different HA treatment modalities by TGA, as a single treatment option and when used in a concomitant fashion.

Thrombin generation in plasma of patients with haemophilia A and B with inhibitors: Effects of bypassing agents and antithrombin reduction

Antithrombin (AT) reduction has been shown to improve thrombin generation (TG) in haemophilia with or without inhibitors. As treatment with bypassing agents (BPAs) may be required in patients with breakthrough bleeding while receiving AT-lowering therapy, we assessed TG in platelet-poor plasma samples from haemophilia patients in the presence of BPA (recombinant activated factor VII [rFVIIa; 1.25 or 2.5 μg mL^-1] or activated prothrombin complex concentrate [aPCC; 0.5 or 1 U mL^-1]) and AT reduction (anti-AT antibody). Mean ± SEM baseline peak thrombin height was 19.9 ± 4.3 nM in plasma from haemophilia patients (n = 12) and 230.5 ± 9.8 nM in healthy males (n = 24). Reduced AT improved mean peak thrombin height in haemophilia patient plasma to 75.4 ± 17.4 nM. Spiking of 90% AT-reduced haemophilia patient plasma with 2.5 μg mL^-1 rFVIIa or 1 U mL^-1 aPCC increased the mean peak thrombin height to 82.5 ± 12 nM and 134.8 ± 18.7 nM, respectively. Peak thrombin levels did not exceed the range for healthy volunteers when plasma samples from haemophilia patients with in vitro AT reduction were treated with BPAs, suggesting the potential use of BPAs in conjunction with AT reduction. Further clinical investigations are needed to confirm the safety of this approach.

A highly sensitive thrombin generation assay for assessment of recombinant activated factor VII therapy in haemophilia patients with an inhibitor

Bypass agents are the common treatment for haemophilia patients who develop inhibitory antibodies. Laboratory assessment of the efficacy of bypassing agent therapy is a challenge. In the present work we modified the conditions triggering thrombin generation (TG) assay in order to find the most sensitive assay for detection of rFVIIa and its analogue NN1731 in haemophilic plasma. TG was measured in samples of normal plasma, plasma of haemophilia patient with inhibitors, as well as haemophilia induced plasma. Recalcification-induced TG was compared to tissue factor (TF) -induced TG in the presence and absence of rFVIIa and NN1731. Recalcification-induced TG (without TF) in haemophilic plasma yielded baseline flat curves, with increased TG as a consequence of spiking the plasma rFVIIa. Using our system, we observed both dose-dependence and time-dependence of rFVIIa effect on TG. Elevated concentrations of TF mask the difference between rFVIIa-treated and non-treated haemophilic plasma. NN1731 yielded normal-isation of recalcification-induced TG curves (without TF) which may reflect high potency. In conclusion, we suggest that triggering TG by recalcification-only may be the most sensitive assay for determining the impact of bypassing agents in haemophilic plasma, and may serve as a caution surrogate safety marker in future studies.

Factor VIIa analog has marked effects on platelet function and clot kinetics in blood from patients with hemophilia A

To evaluate the hemostatic effects of NN1731 and rFVIIa, an ex-vivo study in hemophilia patients used the Hemodyne Hemostasis Analysis System (HAS) to measure platelet contractile force (PCF), clot elastic modulus (CEM), and force onset time (FOT), and the Haemoscope Thrombelastograph (TEG) to measure reaction time (R), kinetics time (K), and maximum amplitude (MA). Blood samples from 10 healthy volunteers and 10 Factor VIII-deficient patients of varying severity (mild, moderate, severe), were spiked with rFVIIa and NN1731 (both 0.64 and 1.28 microg/ml, respectively) and analyzed to characterize platelet function and clot kinetics. There was wide variability in the rFVIIa response. NN1731 had greater and more consistent effects on PCF, CEM, FOT, R, and K relative to rFVIIa, in all hemophilia groups. The lowest NN1731 concentration (0.64 microg/ml) shortened R and FOT, and increased CEM and PCF more than rFVIIa 1.28 microg/ml. NN1731 normalized clotting parameters equivalent to values obtained in healthy volunteers. FOT and R were highly correlated (r = 0.96). No correlation was observed between CEM and MA. NN1731 produced less variable, more pronounced and predictable ex-vivo hemostatic effects on PCF, CEM, FOT, R and K than rFVIIa in all hemophilia groups. HAS and TEG assays provided similar estimates of FOT and R, however CEM appeared to be more sensitive than MA to changes in clot firmness.

Monitoring rFVIIa 90 μg kg⁻¹ dosing in haemophiliacs: comparing laboratory response using various whole blood assays over 6 h

Recombinant FVIIa is a haemostatic agent administered to patients with severe FVIII or FIX deficiency with inhibitors. Although rFVIIa is effective at stopping bleeding, a reliable assay to monitor its effect is lacking. To characterize the pharmacokinetics and global coagulation effects of rFVIIa for 6 h following a IV dose of 90 μg kg⁻¹. Ten non-bleeding subjects with severe FVIII or FIX deficiency were infused with a single-dose of rFVIIa 90 μg k⁻¹ body weight and blood was collected before and at 0.5, 1, 2, 4 and 6 h postdose. Global haemostasis was characterized throughout the study utilizing whole blood analyses (Hemodyne HAS, TEG, ROTEM). The clearance and half-life of factor FVII:C was estimated as 39.0 ± 8.8 mL h⁻¹ kg⁻¹ and 2.1 ± 0.2 h respectively. There was good inter-assay agreement with respect to clot initiation parameters (R, CT and FOT) and these parameters all fell to a mean of approximately 9 min following rFVIIa dosing. The platelet contractile force (PCF) and clot elastic modulus (CEM) were positively correlated to FVII:C (P < 0.0001), and these parameters were dynamic throughout the 6-h period. The MA and MCF did not correlate to FVII:C nor did they significantly change during the study. Prothrombin F1 + 2 significantly increased following rFVIIa dosing (P < 0.001), but remained steady throughout the study. There was no change in D-dimer concentrations over time. The FOT, R and CT characterized clot initiation following rFVIIa dosing. The PCF and CEM were correlated to FVII:C and characterized the dynamics of platelet function and clot strength over the rFVIIa dosing interval. The clinical significance of these findings needs additional study.

Prospective assessment of thrombin generation test for dose monitoring of bypassing therapy in hemophilia patients with inhibitors undergoing elective surgery

Clinical response to bypassing agents (BPAs) may vary between patients. Surgery is a particular situation, requiring effective hemostasis during the procedure and for several days postoperatively to obtain satisfactory wound healing. However, the optimal dose of BPA in different surgical situations has not been clearly established. We report here a prospective assessment of thrombin generation test (TGT) in monitoring the effectiveness of BPA during 10 elective invasive procedures performed in 6 patients with severe hemophilia and high-titer inhibitors. A standardized 3-step protocol was used in all cases to individually tailor BPA. Thrombin-generating capacity of patients increased after in vitro and ex vivo addition of BPA in a dose-dependent manner. Our results also showed a correlation between in vivo clinical response to BPA and thrombin-generating capacity. These data suggest that TGT may represent a surrogate marker for monitoring bypassing therapies in surgical situations.

Thrombin generation testing in haemophilia comprehensive care centres

SUMMARY

Haemophilia comprehensive care centres (HCCC) were first created more than 50 years ago. Their first objective was educating the patient and healthcare professionals in the management of bleeding. Today HCCCs are centres of excellence with multidisciplinary specialists, which continue to provide essential services that are continually reassessed in light of new scientific information. In addition, HCCCs make significant research contributions by studying new methods to improve the well-being of patients with haemophilia. Laboratory expertise is one of the central pillars of HCCCs with a direct impact on diagnosis and management of the haemophilia disease. Vast efforts have been made for the standardization of factor VIII (FVIII) and FIX measurements and inhibitor detection. Molecular biology has improved diagnostics and made it possible to develop new, more secure FVIII and FIX concentrates for replacement therapy. However, phenotyping of each haemophilia patient with an accurate prediction of the individual bleeding risk and also the individual response of patients to antihaemophilic treatment still remains a challenge. In the last 5 years, an expanding interest of haematologists for thrombin generation testing (TGT) reflects the need for new laboratory tools able to evaluate the overall coagulating capacity of patients. This study will review unmet laboratory needs in haemophilia and the potential applications of TGT in the management of haemophiliacs. Furthermore, technical and standardization issues of the method will be discussed.

Thrombin generation in haemophilia A patients with factor VIII inhibitors after infusion of recombinant factor VIIa

BACKGROUND

Development of factor VIII inhibitors is a serious complication in haemophilia A patients. Recombinant factor VIIa (rVIIa) is clinically effective, but its effects on haemostatic system need still to be fully elucidated.

MATERIAL AND METHODS

In an open controlled study, we measured thrombin generation (peak thrombin) in venous blood and prothrombin fragment F1 + 2 (F1 + 2) and D-dimer in venous and in shed blood in five haemophilia A patients with inhibitors before and after rVIIa infusion. A total of five healthy individuals who did not receive rVIIa served as controls.

RESULTS

At baseline, patients had lower mean (min-max) peak thrombin levels than controls [0.12 (0.0-0.6) vs. 186.9 (116.0-254.4) nM, P = 0.001]. After infusion, peak thrombin levels increased in average to 40.7 (28.3-51.6) nM, which translates into 80.2% (95% CI 65.4-88.6%) lower levels compared to that of controls. Mean (min-max) F1 + 2 levels in venous blood did not differ significantly between patients and controls [160.7 (89.8-331.3) vs. 160.8 (104.4-242.3) pmol L(-1)], but increased in average (min-max) by 39.4% (14.1-58.5%) after infusion. In blood emerging from incisions made to determine the bleeding (shed blood), F1 + 2 levels were lower in patients than controls [1383.3 (906.4-2044.6) vs. 2981.7 (1610.0-4539.6) pmol L(-1); P = 0.04], but were not affected by rVIIa; D-dimer levels were significantly higher in haemophiliacs than in controls and remained unchanged after infusion.

CONCLUSIONS

Haemophilia A patients with factor VIII inhibitors have low thrombin generation. After rVIIa, the extent of coagulation activation as measured by levels of F1 + 2 is increased, but thrombin generation is restored to only 20%. Peak thrombin levels could reflect the effects of rVIIa on coagulation mechanisms, and their relevance with regard to the clinical coagulation defect of haemophilia A patients with factor VIII inhibitors might be evaluated.