Factor VIIa analogue (V158D/E296V/M298Q-FVIIa) normalises clot formation in whole blood from patients with severe haemophilia A

Crystal structure, epitope, and functional impact of an antibody against a superactive FVIIa provide insights into allosteric mechanism

BACKGROUND

Blood coagulation factor VIIa (FVIIa) plays its critical physiological role in the initiation of hemostasis. Even so, recombinant FVIIa is successfully used as a bypassing agent for factor VIII or IX in the treatment of bleeds in patients with severe hemophilia with inhibitors. To investigate the utility of more potent FVIIa variants with enhanced intrinsic activity, molecules such as V21D/E154V/M156Q-FVIIa (FVIIaDVQ) were designed.

METHODS

Surface plasmon resonance was used to characterize the binding of mAb4F5 to FVIIaDVQ and related variants. X-ray crystallography was used to determine the structure of the Fab fragment of mAb4F5 (Fab4F5). Molecular docking and small angle X-ray scattering led to a model of FVIIaDVQ:Fab4F5 complex.

RESULTS

The binding experiments, functional effects on FVIIaDVQ and structure of mAb4F5 (originally intended for quantification of FVIIaDVQ in samples containing FVII(a)) pinpointed the epitope (crucial role for residue Asp21) and shed light on the role of the N-terminus of the protease domain in FVIIa allostery. The potential antigen-combining sites are composed of 1 hydrophobic and 1 negatively charged pocket formed by 6 complementarity-determining region (CDR) loops. Structural analysis of Fab4F5 shows that the epitope interacts with the periphery of the hydrophobic pocket and provides insights into the molecular basis of mAb4F5 recognition and tight binding of FVIIaDVQ.

CONCLUSION

The binary complex explains and supports the selectivity and functional consequences of Fab4F5 association with FVIIaDVQ and illustrates the potentially unique antigenicity of this FVIIa variant. This will be useful in the design of less immunogenic variants.

Factor VIII activation by factor VIIa analog (V158D/E296V/M298Q) in tissue factor-independent mechanisms

Factor (F)VIIa with tissue factor (TF) is a primary trigger of blood coagulation. The recombinant (r)FVIIa analog, NN1731 (V158D/E296V/ M298Q) containing a thrombin/FIXa-mimicking catalytic domain, is ~30-fold more effective on activated platelets without TF, but ~1.2-fold with TF, than rFVIIa for FX activation. We have recently demonstrated the FVIIa/TF-dependent FVIII activation in the early coagulation phase. We assessed the action of NN1731 on FVIII activation. NN1731/TF increased FVIII activity ~2.9-fold within 30 seconds, followed by rapid inactivation, and was slightly more active than rFVIIa/TF. NN1731-catalysed activation, however, was enhanced ~6-fold at 5 minutes (min), and its peak level persisted for ~30 min. NN1731/TF proteolysed FVIII at Arg740, Arg372, and Arg336, similar to rFVIIa/TF, but cleavage by NN1731 alone was much slower at Arg336 than at Arg740 and Arg372. The Km and Vmax for NN1731/TF-catalysed activation were ~1.8-fold lower and ~2.3-fold greater than rFVIIa/TF. The Km for NN1731 alone was ~1.3-fold lower than rFVIIa, whilst the Vmax was ~7.9-fold greater, indicating that the efficiency of FVIII activation by NN1731 and NN1731/TF was ~11- and ~4-fold greater, respectively, than equivalent reactions with rFVIIa. In SPR-based assays, NN1731 bound to FVIII and the heavy chain (Kd; 0.62 and 1.9 nM) with ~1.4- and ~3.1-fold higher affinity than rFVIIa, and the A2 domain contributed to this increase. Von Willebrand factor moderated NN1731-catalysed activation more significantly than NN1731/TF. In conclusion, NN1731 was a greater potential than rFVIIa in up-regulating FVIII activity, and the TF-independent FVIII activation might represent a potential extra mode of its enhanced haemostatic effect.

Tissue factor-dependent coagulation activation by heme: A thromboelastometry study

Heme has been characterized as potent trigger of inflammation. In hemostasis, although heme has been shown to both induce and inhibit different compartments of hemostasis, its net effect on the hemostatic balance, and the biological relevance of these effects remain to be determined. Herein we evaluated the effect of heme on hemostasis using a global assay able to generate clinically relevant data in several other complex hemostatic diseases. Citrated whole blood samples from healthy participants were stimulated by heme or vehicle and incubated for 4h at 37°C. Rotational thromboelastometry was immediately performed. The participation of tissue factor in coagulation activation was evaluated using inhibitory antibody. Heme was able of inducing ex vivo coagulation activation in whole blood, affecting predominantly parameters associated with the initial phases of clot formation. This activation effect was at least partially dependent on hematopoietic tissue factor, since the effects of heme were partially abrogated by the inhibition of human tissue factor. In conclusion, using a global hemostasis assay, our study confirmed that heme is able to activate coagulation in whole blood, in a tissue factor-dependent way. These findings could explain the disturbance in hemostatic balance observed in conditions associated with the release of heme such as sickle cell disease.

New treatments in hemophilia: insights for the clinician

Hemophilia has evolved from an often fatal hereditary bleeding disorder to a disorder for which safe and effective treatment is available. However, there are several challenges remaining in the treatment of hemophilia. Prophylaxis to prevent bleeding is costly and requires frequent intravenous injections, which are cumbersome for patients. Venous access is often difficult to achieve, especially in small children where central venous lines may need to be implanted. Development of inhibitory antibodies makes treatment of acute bleeds difficult and prophylaxis in patients with inhibitors must also be better addressed. In order to improve treatment, new products are being developed, some of which are already in clinical trials. There are several approaches to prolonging half-lives such as PEGylation, Fc fusion and albumin fusion. Increased activity has been demonstrated in preclinical trials for factor IX and in a human trial with factor VII where the activity of the molecules has been increased by manipulation of the molecular composition. Additional approaches, including blockage of inhibitors of clotting, are also under investigation. Factor VIII and factor IX gene therapy have become a tangible possibility since phase I data recently have been published. Results are promising and there is hope that in the near future substantial progress will be made, perhaps making hemophilia the first genetic condition to be cured.

Thrombin generation and whole blood viscoelastic assays in the management of hemophilia: current state of art and future perspectives

Hemophilia is a bleeding disorder that afflicts about 1 in 5000 males. Treatment relies upon replacement of the deficient factor, and response to treatment both in clinical research and practice is based upon subjective parameters such as pain and joint mobility. Existing laboratory assays quantify the amount of factor in plasma, which is useful diagnostically and prognostically. However, these assays are limited in their ability to fully evaluate the patient's clot-forming capability. Newer assays, known as global assays, provide a far more detailed view of thrombin generation and clot formation and have been studied in hemophilia for about 10 years. They have the potential to offer a more objective measure of both the hemophilic phenotype as well as the response to treatment. In particular, in patients who develop inhibitors to deficient clotting factors and in whom bypassing agents are required for hemostasis, these assays offer the opportunity to determine the laboratory response to these interventions where traditional coagulation assays cannot. In this article we review the existing literature and discuss several controversial issues surrounding the assays. Last, a vision of future clinical uses of these assays is briefly described.

Global haemostasis and point of care testing

The evaluation of the coagulation profile has used so far either clotting-based or chromogenic assays with different endpoints. Clotting-based techniques are the most used worldwide, and they certainly are useful for diagnosis of clotting factor deficiencies. However, the information provided is relatively limited, and therefore the individual profile of coagulation is poorly assessed. This is reflected by the weak correlation between the results of these assays and the clinical phenotype. Among the assays that benefited from technological advances, thrombin generation and thromboelastography are probably the most actively investigated, but they require specific instruments and are not fully automated. Their standardisation level is rapidly progressing, and they are progressively entering the clinical scene, with the attempt to provide additional information on the coagulation process and a meaningful clinical correlation. These inherited bleeding disorders frequently require replacement therapy using clotting factor concentrates that increase the plasma level of the missing clotting factor. The classical adjustment of the therapy is mainly based on the measurement of the plasma clotting activity of the protein administered. If one considers that a certain level of thrombin generated would predict clinical efficacy, monitoring of thrombin formation might offer new possibilities to individually predict the bleeding phenotype, select the most adapted therapeutic product and tailor the dose. The same holds true for thromboelastography/thromboelastometry which evaluate fibrin formation as well as clot resistance to fibrinolytic challenge, one step further down in the coagulation process. In this regard, these 2 assays could be seen as complementary in terms of information provided on the coagulation profile at the individual level.

Recombinant factor VIIa analog (vatreptacog alfa [activated]) for treatment of joint bleeds in hemophilia patients with inhibitors: a randomized controlled trial

BACKGROUND

A recombinant factor VIIa analog (NN1731; vatreptacog alfa [activated]) was developed to provide safe, rapid and sustained resolution of bleeds in patients with hemophilia and inhibitors.

PATIENTS/METHODS

This global, prospective, randomized, double-blinded, active-controlled, dose-escalation trial evaluated and compared one to three doses of vatreptacog alfa at 5, 10, 20, 40, and 80 lg kg(-1) with one to three doses of recombinant FVIIa (rFVIIa) at 90 lg kg(-1) in the treatment of acute joint bleeds in hemophilia patients with inhibitors. The primary endpoint comprised adverse events; secondary endpoints were evaluations of immunogenicity, pharmacokinetics, and efficacy.

RESULTS AND CONCLUSIONS

Overall, 96 joint bleeds in 51 patients (> 12 years of age) were dosed. Vatreptacog alfa was well tolerated, with a low frequency of adverse events. No immunogenic or thrombotic events related to vatreptacog alfa were reported. A high efficacy rate of vatreptacog alfa in controlling acute joint bleeds was observed; 98% of bleeds were controlled within 9 h of the initial dose in a combined evaluation of 20–80 lg kg(-1) vatreptacog alfa. The efficacy rate observed for rFVIIa (90%) is consistent with data from published clinical trials. The trial was not powered to compare efficacy, and further trials are needed to investigate the efficacy of vatreptacog alfa as compared with that of rFVIIa. The trial was registered at ClinicalTrials.gov (

REGISTRATION NUMBER

NCT00486278).

Recombinant factor VIIa analog NN1731 (V158D/E296V/M298Q-FVIIa) enhances fibrin formation, structure and stability in lipidated hemophilic plasma

INTRODUCTION

The bypassing agent recombinant factor VIIa (rFVIIa) is efficacious in treating bleeding in hemophilia patients with inhibitors. Efforts have focused on the rational engineering of rFVIIa variants with increased hemostatic potential. One rFVIIa analog (V158D/E296V/M298Q-FVIIa, NN1731) improves thrombin generation and clotting in purified systems, whole blood from hemophilic patients and factor VIII-deficient mice.

METHODS

We used calibrated automated thrombography and plasma clotting assays to compare effects of bypassing agents (rFVIIa, NN1731) on hemophilic clot formation, structure, and ability to resist fibrinolysis.

RESULTS

Both rFVIIa and NN1731 shortened the clotting onset and increased the maximum rate of fibrin formation and fibrin network density in hemophilic plasma clots. In the presence of tissue plasminogen activator, both rFVIIa and NN1731 shortened the time to peak turbidity (TTPeak(tPA)) and increased the area under the clot formation curve (AUC(tPA)). Phospholipids increased both rFVIIa and NN1731 activity in a lipid concentration-dependent manner. Estimated geometric mean concentrations of rFVIIa and NN1731 producing similar onset, rate, TTPeak(tPA), and AUC(tPA) as seen with 100% factors VIII and IX were: 24.5, 74.3, 29.7, and 37.1 nM rFVIIa, and 8.6, 31.2, 9.0, and 11.3 nM NN1731, respectively. In each case, the NN1731 concentration was significantly lower than rFVIIa.

CONCLUSIONS

These findings suggest that like rFVIIa, NN1731 improves the formation, structure, and stability of hemophilic clots. Higher lipid concentrations may facilitate assessment of both rFVIIa and NN1731 activity. NN1731 appears likely to support rapid clot formation in tissues with high endogenous fibrinolytic activity.

Procoagulant activity in hemostasis and thrombosis: Virchow's triad revisited

Virchow's triad is traditionally invoked to explain pathophysiologic mechanisms leading to thrombosis, alleging concerted roles for abnormalities in blood composition, vessel wall components, and blood flow in the development of arterial and venous thrombosis. Given the tissue-specific bleeding observed in hemophilia patients, it may be instructive to consider the principles of Virchow's triad when investigating mechanisms operant in hemostatic disorders as well. Blood composition (the function of circulating blood cells and plasma proteins) is the most well studied component of the triad. For example, increased levels of plasma procoagulant proteins such as prothrombin and fibrinogen are established risk factors for thrombosis, whereas deficiencies in plasma factors VIII and IX result in bleeding (hemophilia A and B, respectively). Vessel wall (cellular) components contribute adhesion molecules that recruit circulating leukocytes and platelets to sites of vascular damage, tissue factor, which provides a procoagulant signal of vascular breach, and a surface upon which coagulation complexes are assembled. Blood flow is often characterized by 2 key variables: shear rate and shear stress. Shear rate affects several aspects of coagulation, including transport rates of platelets and plasma proteins to and from the injury site, platelet activation, and the kinetics of fibrin monomer formation and polymerization. Shear stress modulates adhesion rates of platelets and expression of adhesion molecules and procoagulant activity on endothelial cells lining the blood vessels. That no one abnormality in any component of Virchow's triad fully predicts coagulopathy a priori suggests coagulopathies are complex, multifactorial, and interactive. In this review, we focus on contributions of blood composition, vascular cells, and blood flow to hemostasis and thrombosis, and suggest that cross-talk among the 3 components of Virchow's triad is necessary for hemostasis and determines propensity for thrombosis or bleeding. Investigative models that permit interplay among these components are necessary to understand the operant pathophysiology, and effectively treat and prevent thrombotic and bleeding disorders.

Overcoming delayed in-vitro response to rFVIIa: effects of rFVIIa and rFVIIa analogue (vatreptacog alfa) concentration escalation in whole blood assays

In a previous pharmacokinetic/pharmacodynamic study in nonbleeding hemophilia patients, variability in laboratory response to recombinant factor VIIa (rFVIIa) 90 μg/kg was noted, and the patients were described as delayed or rapid laboratory responders based on time to clot formation. The current study determined whether in-vitro experiments could reproduce previous in-vivo findings; whether the delayed laboratory response to rFVIIa 90 μg/kg is improved by spiking with high-dose rFVIIa or rFVIIa analogue (vatreptacog alfa); whether a dose-response is observed with our method. In-vitro experiments were conducted in our previous patient cohort using rFVIIa 1.28 and 3.84 μg/ml and vatreptacog alfa 0.28 and 0.56 μg/ml. Whole blood studies were conducted using the Hemodyne Hemostasis Analysis System (platelet contractile force, clot elastic modulus, force onset time) and rotational thromboelastometry (clotting time, maximum clot firmness). Spiking with rFVIIa 1.28 μg/ml showed the same distribution of delayed and rapid laboratory response as observed previously. Increasing in-vitro rFVIIa concentrations improved the coagulation parameters; however, there remained delayed and rapid responders. Vatreptacog alfa improved the coagulation parameters at all concentrations tested, and the 0.56 μg/ml concentration normalized the force onset time, platelet contractile force, clot elastic modulus and clotting time parameters. A dose-response was observed with both assays. There was good agreement between the laboratory responses obtained after intravenous administration of rFVIIa 90 μg/kg and in-vitro spiking studies. Escalating rFVIIa and vatreptacog alfa concentrations improved coagulation parameters in all patients compared to rFVIIa 1.28 μg/ml. Vatreptacog alfa produced more pronounced coagulation effects at lower concentrations than rFVIIa; and the 0.56 μg/ml concentration completely normalized responses in all patients.

Platelet binding and activity of a factor VIIa variant with enhanced tissue factor independent activity

BACKGROUND AND OBJECTIVES

Platelet binding and activity play important roles in the efficacy of factor VIIa (FVIIa) as a bypassing agent for hemophilia treatment. An analog of FVIIa with increased tissue factor (TF)-independent activity, NN1731, has been produced by introducing three amino acid changes in the protease domain. NN1731 has a conformation similar to TF-bound FVIIa, even in the absence of TF. This results in much greater intrinsic proteolytic activity, but similar activity in the presence of TF.

OBJECTIVES

We hypothesized that these changes would not alter binding to platelets or phospholipid, a characteristic thought to be localized to the Gla domain. The goal of the current work was to compare platelet binding and activity of NN1731 and wild-type FVIIa.

METHODS/RESULTS

FVIIa and NN1731 bound identically to phospholipid vesicles as assessed by both activity assays and electrophoretic quasielastic light scattering techniques. However, NN1731 bound to a greater number of sites on activated platelets than FVIIa, as assessed by flow cytometry. Removal of the Gla domain abolished binding of both FVIIa and NN1731. Inhibition of the active site did not reduce NN1731 binding to the level of FVIIa. When corrected for the amount of protein bound, NN1731 had greater activity than FVIIa on platelet surfaces.

CONCLUSIONS

While the Gla domain is essential for FVIIa binding to platelets, changes in the protease domain in NN1731 enhanced platelet binding as well as proteolytic activity. Features in addition to lipid composition appear to contribute to binding of rFVIIa and, especially, NN1731 to platelets.

Recent advances in the development of coagulation factors and procoagulants for the treatment of hemophilia

Hemophilia is a family of rare bleeding disorders. The two primary types, hemophilia A and hemophilia B, are caused by recessive X-chromosome linked mutations that result in deficiency of coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Clinically, hemophilia is manifested by spontaneous bleeding, particularly into the joints (haemarthrosis) and soft tissue, and excessive bleeding following trauma or surgery. The total overall number of hemophilia patients worldwide is approximately 400,000, however only about 100,000 of these individuals are treated. The first treatment of hemophilia was initiated when it was determined that the clotting deficiency could be corrected by a plasma fraction taken from normal blood. The discovery of factor VIII enrichment by cryoprecipitation of plasma opened a new era of therapy which eventually led to the production of factor concentrates and the subsequent development of highly purified forms of plasma factors. The most significant improvements have been the availability of recombinant forms of factors VIII and IX. Unfortunately, recombinant factors still retain some of the limitations of plasma concentrates. These limitations include development of antibody responses in patients and the relatively short half-life of the molecules requiring frequent injection to maintain effective concentration. Treatment beyond replacement of native factors has been tried. They include the development of modified factor VIII and IX molecules with improved potency, stability and circulating half-life and enhancement of a prothrombotic responses and/or stabilization of coagulation factors via inhibition of key negative regulatory pathways. These approaches will be reviewed in this commentary.

Stabilization of fibrin clots by activated prothrombin complex concentrate and tranexamic acid in FVIII inhibitor plasma

Defective hemostasis in haemophilia patients with FVIII inhibitors results in a dramatic decrease in thrombin generation forming unstable fibrin clots that are susceptible to fibrinolyisis. In this study we tested whether the combination of plasma derived activated prothrombin complex concentrate (pd-aPCC) with tranexamic acid (TXA) may improve fibrin clot stability in FVIII inhibitor plasma. A microplate assay for clot lysis time was used to assess clot stability in FVIII inhibitor plasma. The effect of pd-aPCC on clot stability was first tested using the commercial FVIII inhibitor plasma. TXA (5 ~ 10 mg mL⁻¹) increased clot lysis time, but pd-aPCC (0.25 ~ 1.0 U mL⁻¹) had no effect on it. The combination of pd-aPCC and TXA significantly increased clot lysis time compared with TXA alone. The effect appeared to be limited to fibrin clot resistance to fibrinolysis, as TXA was found to have no effect on thrombin generation induced by pd-aPCC. The effect of pd-aPCC and TXA on clot stability was then tested and verified in plasma samples from ten patients with severe haemophilia A and inhibitors. The combination of TXA (10 mg mL⁻¹) and pd-aPCC (0.5 U mL⁻¹) significantly increased clot lysis time compared to TXA alone. Our results suggest that the combination of pd-aPCC with TXA improves clot stability in FVIII inhibitor plasma without additional increases in thrombin generation.

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.

Intravascular inhibition of factor VIIa and the analogue NN1731 by antithrombin

NN1731 is a recombinant activated factor VII (rFVIIa) analogue with increased intrinsic activity. This also applies to its reactivity towards antithrombin (AT), the role of which was investigated in a pharmacokinetic (PK) study. NN1731 or rFVIIa was administered to normal and haemophilia A dogs and elimination was measured by FVIIa clot activity, FVIIa- and FVIIa-AT antigen. In vitro AT complex formation was studied in canine plasma spiked with NN1731 or rFVIIa. Based on FVIIa antigen concentrations, PK profiles in normal and haemophilia A dogs were similar for NN1731 and rFVIIa with antigen half lives, t(½) ≈1·8 h. In contrast, PK profiles based on activity measurements were distinctly different. NN1731 induced a strong, short lasting (t(½) ≈0·5 h) pro-coagulant response, whereas rFVIIa induced a lower, longer lasting (t(½) ≈1·1 h) response. Western Blot and FVIIa-AT antigen analysis demonstrated in vivo AT complex formation that accounted for these divergences. AT complex formation with FVIIa or NN1731 in vitro in canine plasma was considerably slower than the in vivo reaction. The results suggest that in vivo inhibition by AT contributes significantly to define drug duration in haemophilia treatment with rFVIIa and in particular with the NN1731 analogue.

Thrombin generation and fibrinolysis in anti-factor IX treated blood and plasma spiked with factor VIII inhibitor bypassing activity or recombinant factor VIIa

Activated prothrombin complex concentrates (aPCC) and recombinant activated factor VIIa (rFVIIa) are two important therapies in haemophilia patients with inhibitors and improve clot stability. We hypothesized that potential differences in procoagulant and fibrinolytic actions of aPCC and rFVIIa may lie in the clot stability against fibrinolytic activation. We used thrombin generation, fluorescence detection and thromboelastometry in anti-factor IXa (FIXa) aptamer-treated whole blood (WB) and plasma to evaluate: (i) generation of thrombin and activated factor X (FXa) and (ii) viscoelastic properties of blood clots in the presence of tissue plasminogen activator (tPA) after addition of aPCC (0.4 U mL(-1)) or rFVIIa (60 nm). Peak thrombin generation increased from 85 +/- 19 nm in aptamer-treated plasma to 276 +/- 83 nm and 119 +/- 22 nm after addition of aPCC and rFVIIa respectively (P < 0.001). FXa activity increased within 20 min by 87 +/- 6% and by 660 +/- 97% after addition of aPCC and rFVIIa respectively (P < 0.001). TPA-induced lysis time increased from 458 +/- 378 s in aptamer-treated WB to 1597 +/- 366 s (P = 0.001) and 1132 +/- 214 s (P = 0.075), after addition of aPCC and rFVIIa respectively. In this haemophilia model using the anti-FIXa aptamer, the larger amount of thrombin was generated with aPCC compared with rFVIIa, while FXa generation was more rapidly increased in the presence of rFVIIa. Furthermore, clot formation in anti-FIXa aptamer-treated WB was less susceptible to tPA-induced fibrinolysis after adding aPCC compared with rFVIIa.

rFVIIa and NN1731 reduce bleeding in hydroxyethyl starch hemodiluted rabbits

BACKGROUND

Colloid plasma expanders are used to maintain blood pressure and ensure tissue perfusion during hypovolemia, e.g., caused by traumatic bleeding. Although colloids stabilize the cardiovascular system, they can also potentially cause coagulopathy. Consequently, bleeding tendency may increase, as well as the associated risk of morbidity and mortality. Thus, there is a need for hemostatic treatment options for these patients. rFVIIa (NovoSeven, Novo Nordisk A/S, Bagsvaerd, Denmark) is a hemostatic agent that effectively controls bleedings in patients with inhibitor-complicated hemophilia. rFVIIa works by enhancing thrombin generation on the activated platelet surface at the site of injury, leading to the formation of a stable fibrin clot. NN1731 is an rFVIIa analog with increased hemostatic potential and is currently under clinical development.

METHODS

In this study, the effect of rFVIIa and NN1731 on cuticle bleeding in rabbits 50% hemodiluted with hydroxyethyl starch (molecular weight ∼ 200,000) was tested. Cuticle bleeding was induced after a two-stage hemodilution procedure. After 5 minutes, the animals were treated with rFVIIa (2, 5, or 10 mg/kg), NN1731 (1 or 2 mg/kg), or vehicle, followed by 30 minutes of observation.

RESULTS

Hemodilution caused a significant increase in bleeding time and blood loss. rFVIIa dose-dependently reduced bleeding time and blood loss, reaching statistical significance at 10 mg/kg. However, 2 mg/kg NN1731 reduced bleeding time and blood loss significantly and to a similar extent as 10 mg/kg rFVIIa. This increased hemostatic potential of NN1731 compared with rFVIIa and was confirmed by findings using thromboelastography on ex vivo hemodiluted whole blood.

CONCLUSION

In summary, rFVIIa and NN1731 significantly and dose-dependently reduced bleeding in extensively hemodiluted rabbits.

Effects of a recombinant FVIIa analogue, NN1731, on blood loss and survival after liver trauma in the pig

BACKGROUND

We considered whether haemorrhage after a liver trauma would be reduced by early administration of a pro-haemostatic agent and evaluated the effect of i.v. vs i.m. administration of the coagulation factor VIIa analogue NN1731 on haemorrhage after a liver trauma in the pig.

METHODS

The pharmacokinetics of i.v. and i.m. NN1731 was evaluated in eight minipigs, and the effects of dose and administration route of NN1731 (i.v. 180 microg kg(-1), n=6; i.m. 540 microg kg(-1), n=4, or 2000 microg kg(-1), n=6) vs vehicle (n=16) were studied on a liver laceration injury in pigs. To simulate a pre-hospital setting, the administration of NN1731 was delayed by 1 min for i.m. administration and 7 min for i.v. administration, at which time fluid resuscitation also began.

RESULTS

In the minipigs, NN1731 exposure was similar after i.v. 180 microg kg(-1) and i.m. 540 microg kg(-1), with a bioavailability of approximately 35%. The injury and blood loss at 7 min was comparable between the four groups of pigs; however, after 60 min, the blood loss was lower in the i.v. treated animals: 1.3 (0.3) (i.v.) vs 2.2 (0.8) litres (i.m.(540), i.m.(2000), and vehicle) (P<0.001). Also, the survival time was increased: 117 (14) (i.v.) vs 84 (28) min (i.m.(540), i.m.(2000), and vehicle) (P<0.001).

CONCLUSIONS

After a liver trauma in the pig, i.v. administration of NN1731 reduced the bleeding and increased the survival time. In contrast, i.m. administration had no effect, presumably because reduced muscle perfusion during haemorrhage reduced the uptake of NN1731.

Faster onset of effect and greater efficacy of NN1731 compared with rFVIIa, aPCC and FVIII in tail bleeding in hemophilic mice

BACKGROUND

Recombinant factor VIIa (rFVIIa, Novoseven) is currently used to control bleeding in hemophiliacs with inhibitors. A new rFVIIa variant, NN1731, with increased activity on the surface of activated platelets, has demonstrated a more potent and faster onset of reactivity than rFVIIa in various in vitro models. The present study aimed to investigate whether this translates into greater efficacy and faster promotion of hemostasis in vivo.

METHOD AND RESULTS

In a severe tail-bleeding model in hemophilia A mice, NN1731 demonstrated significantly greater efficacy than rFVIIa, plasma-derived activated prothrombin complex concentrate (pd-aPCC, FEIBA or FVIII (Refacto). Assessment of the blood loss over time showed that NN1731 significantly and dose-dependently reduced the blood loss in the first 5-min observation period, whereas the effect of rFVIIa, FVIII and pd-aPCC first became evident 5-10 min after injury.

CONCLUSION

This study shows that NN1731 has a greater efficacy and faster resolution of bleeding in a severe bleeding model in hemophilia A mice compared with any of the other agents tested.

Evaluation of the safety and pharmacokinetics of a fast-acting recombinant FVIIa analogue, NN1731, in healthy male subjects

BACKGROUND

NN1731 is a recombinant activated factor VII (rFVIIa) analog with enhanced activity.

OBJECTIVES

This clinical trial aimed to assess the safety and pharmacokinetics of single doses of NN1731 in healthy male subjects.

METHODS

This was a randomized, placebo-controlled dose-escalation trial with four dose tiers (NN1731 5-30 microg kg(-1)). Eight subjects were randomized to either NN1731 (n = 6) or placebo (n = 2) in each tier.

RESULTS

No thromboembolic or serious adverse events were reported and no antibody formation towards NN1731 was detected. NN1731 was demonstrated to be pharmacologically active based on coagulation-related parameters (prothrombin fragment 1+2, activated partial thromboplastin time and prothrombin time). There were five mild/moderate adverse events in three subjects. The FVIIa activity of NN1731 after ascending single-dose administration of NN1731 fits well with a two-compartment model, indicating a bi-exponential decline with a rapid initial distribution of approximately 73% FVIIa activity (half-life = 20 min), followed by a less rapid terminal elimination phase eliminating the remaining 27% (half-life = 3 h). Dose proportionality in healthy male subjects at the dose levels investigated (5-30 microg kg(-1)) was supported by the FVIIa activity data.

CONCLUSIONS

Based on the results of this trial, NN1731 appears safe and well tolerated in healthy subjects at doses up to 30 microg kg(-1). No immunogenic or thromboembolic events were reported. The pharmacokinetic profile of NN1731 as measured by FVIIa activity appears to follow two-compartment pharmacokinetics characterized by an initial rapid distribution phase followed by a less rapid elimination phase.

Further understanding of recombinant activated factor VII mode of action

Recombinant activated factor VII (rFVIIa) is being increasingly used to treat bleeding associated with a variety of non-hemophilic coagulopathic indications, and its mechanism of action in these areas is under active investigation. Numerous studies have shown that FVIIa binds with low affinity to activated platelets; rFVIIa can subsequently enhance platelet-surface thrombin generation by activating factor (F) X and by contributing additional FIXa to the hemostatic process. This FIXa can rapidly activate additional FX, which may explain why non-hemophilic coagulopathic bleeds respond to lower doses of rFVIIa than do hemophilic bleeds. However, the platelet surface may be able to process only a limited amount of FXa, accounting for the observation that some models of non-hemophilic coagulopathy show a plateau in the effect of rFVIIa.

Hemostatic effect of recombinant factor VIIa, NN1731 and recombinant factor VIII on needle-induced joint bleeding in hemophilia A mice

BACKGROUND

Hemophilia A is the most common serious bleeding disorder, and the hallmark of this disease is joint bleeding episodes. These result in hemophilic synovitis, an inflammatory and proliferative condition of the joint, which progresses into a chronic degenerative arthritis, hemophilic arthropathy.

METHODS

In this paper, we describe the effect of recombinant factor VIIa (rFVIIa), and an analogue NN1731 as well as rFVIII on needle-induced bleeding in hemophilia A mice.

CONCLUSIONS

Here we show a reducing effect of rFVIIa and NN1731 on bleeding induced in hemophilic mice, and we show that preventive treatment with rFVIII normalizes bleeding.

The origins of enhanced activity in factor VIIa analogs and the interplay between key allosteric sites revealed by hydrogen exchange mass spectrometry

Factor VIIa (FVIIa) circulates in the blood in a zymogen-like state. Only upon association with membrane-bound tissue factor (TF) at the site of vascular injury does FVIIa become active and able to initiate blood coagulation. Here we used hydrogen exchange monitored by mass spectrometry to investigate the conformational effects of site-directed mutagenesis at key positions in FVIIa and the origins of enhanced intrinsic activity of FVIIa analogs. The differences in hydrogen exchange of two highly active variants, FVIIa(DVQ) and FVIIa(VEAY), imply that enhanced catalytic efficiency was attained by two different mechanisms. Regions protected from exchange in FVIIa(DVQ) include the N-terminal tail and the activation pocket, which is a subset of the regions of FVIIa protected from exchange upon TF binding. FVIIa(DVQ) appeared to adopt an intermediate conformation between the free (zymogen-like) and TF-bound (active) form of FVIIa and to attain enhanced activity by partial mimicry of TF-induced activation. In contrast, exchange-protected regions in FVIIa(VEAY) were confined to the vicinity of the active site of FVIIa. Thus, the changes in FVIIa(VEAY) appeared to optimize the active site region rather than imitate the TF-induced effect. Hydrogen exchange analysis of the FVIIa(M306D) variant, which was unresponsive to stimulation by TF, correlated widespread reductions in exchange to the single mutation in the TF-binding region. These results reveal the delicate interplay between key allosteric sites necessary to achieve the transition of FVIIa into the active form.

Thrombin generation, fibrin clot formation and hemostasis

Hemostatic clot formation entails thrombin-mediated cleavage of fibrinogen to fibrin. Previous in vitro studies have shown that the thrombin concentration present during clot formation dictates the ultimate fibrin structure. In most prior studies of fibrin structure, clotting was initiated by adding thrombin to a solution of fibrinogen; however, clot formation in vivo occurs in an environment in which the concentration of free thrombin changes over the reaction course. These changes depend on local cellular properties and available concentrations of pro- and anti-coagulants. Recent studies suggest that abnormal thrombin generation patterns produce abnormally structured clots that are associated with an increased risk of bleeding or thrombosis. Further studies of fibrin formation during in situ thrombin generation are needed to understand fibrin clot formation in vivo.

Effect of recombinant factor VIIa variant (NN1731) on platelet function, clot structure and force onset time in whole blood from healthy volunteers and haemophilia patients

NN1731 is a novel variant of recombinant factor VIIa (rFVIIa) that binds to activated platelets, but has greater enzymatic activity than rFVIIa in generating FXa and thrombin. The effect of NN1731 on clot structure and platelet function was characterized ex vivo in whole blood from healthy volunteers and haemophilic patients. Blood samples from six healthy volunteers, nine haemophilia A patients with and without inhibitors and one acquired haemophilia A patient, were spiked with increasing concentrations (0.32, 0.64 and 1.28 microg mL(-1)) of rFVIIa and NN1731. Platelet contractile force (PCF) or platelet function, clot elastic modulus (CEM) or clot structure, and force onset time (FOT) or the thrombin generation time (TGT) were determined using the Hemodyne Hemostasis Analysis System (HAS). Baseline PCF, CEM and FOT values in patients were abnormal compared to healthy volunteers' baseline values. Overall, haemophilia blood samples with or without inhibitors spiked with NN1731 had significantly greater PCF, CEM and shorter FOT values relative to samples spiked with corresponding doses of rFVIIa. The variability in response to treatment between patients was greater with rFVIIa compared to NN1731. At 1.28 microg mL(-1) (90 microg kg(-1)), NN1731 normalized PCF, CEM and FOT in nine of 10 patients, while rFVIIa normalized these parameters in four of 10 patients. Increasing in vitro concentrations of NN1731 normalized platelet function, clot structure and thrombin generation consistently in haemophilia blood with or without inhibitors. NN1731 may be a promising haemostatic agent for patients with bleeding disorders. These results should be confirmed in an in vivo study.