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

Mitigation of T-cell dependent immunogenicity by reengineering factor VIIa analogue

Vatreptacog alfa (VA), a recombinant activated human factor VII (rFVIIa) variant with 3 amino acid substitutions, was developed to provide increased procoagulant activity in hemophilia patients with inhibitors to factor VIII or factor IX. In phase 3 clinical trials, changes introduced during the bioengineering of VA resulted in the development of undesired anti-drug antibodies in some patients, leading to the termination of a potentially promising therapeutic protein product. Here, we use preclinical biomarkers associated with clinical immunogenicity to validate our deimmunization strategy applied to this bioengineered rFVIIa analog. The reengineered rFVIIa analog variants retained increased intrinsic thrombin generation activity but did not elicit T-cell responses in peripheral blood mononuclear cells isolated from 50 HLA typed subjects representing the human population. Our algorithm, rational immunogenicity determination, offers a broadly applicable deimmunizing strategy for bioengineered proteins.

Enhanced potency of recombinant factor VIIa with increased affinity to activated platelets

BACKGROUND

Recombinant factor VIIa (rFVIIa) enhances thrombin generation in a platelet-dependent manner; however, rFVIIa binds activated platelets with relatively low affinity. Triggering receptor expressed on myeloid cells (TREM)-like transcript (TLT)-1 is expressed exclusively on activated platelets.

OBJECTIVE

To enhance the potency of rFVIIa via binding TLT-1.

METHODS

Recombinant FVIIa was conjugated to a TLT-1 binding Fab. In vitro potency of this platelet-targeted rFVIIa (PT-rFVIIa) was evaluated using factor X activation assays and by measuring viscoelastic changes in whole blood. In vivo potency was evaluated using a tail vein transection model in F8-/- mice expressing human TLT-1.

RESULTS

PT-rFVIIa and rFVIIa had similar dissociation constant values for tissue factor binding and similar tissue factor-dependent factor X activation. However, PT-rFVIIa had increased catalytic efficiency on TLT-1-loaded vesicles and activated platelets. The in vitro potency in normal human blood with antibody-induced hemophilia A was dependent on assay conditions used; with maximally activated platelets, the half maximal effective concentration for clot time for PT-rFVIIa was 49-fold lower compared with rFVIIa. In the murine bleeding model, a 53-fold lower half maximal effective concentration was observed for blood loss for PT-rFVIIa, supporting the relevance of the assay conditions with maximally activated platelets. In vitro analysis of blood from subjects with hemophilia A confirmed the data obtained with normal blood.

CONCLUSIONS

Increasing the affinity of rFVIIa to activated platelets resulted in approximately 50-fold increased potency both in vitro and in the mouse model. The correlation of in vivo with in vitro data using maximally activated platelets supports that these assay conditions are relevant when evaluating platelet-targeted hemostatic concepts.

Eptacog beta: a novel recombinant human factor VIIa for the treatment of hemophilia A and B with inhibitors

INTRODUCTION

Hemophilia A and B are X-linked recessive disorders caused by the deficiency of factor VIII or factor IX, respectively. Bleeding episodes are treated with factor replacement therapy. The most serious complication of this treatment is the development of inhibitors. In such patients, bypassing agents, such as activated recombinant human factor VII (rhFVIIa) or plasma-derived activated prothrombin complex concentrates, are administered to prevent or treat bleeding episodes. The high cost of the current bypassing agents limits their availability in emerging countries. Areas covered: Authors reviewed the published data on the development and clinical testing of eptacog beta, a new second-generation rhFVIIa produced in the milk of transgenic rabbits. The available data indicate that activated eptacog beta exhibits structural (N- and O- glycosylation), pharmacodynamic and pharmacokinetic characteristics similar to activated eptacog alfa, its main competitor, but binds slightly better to platelets and HUVEC, and it is safe and effective. Expert commentary: This critical review of available data on activated eptacog beta shows that it represents an alternative source of rhFVIIa at potentially lower cost with easily expandable manufacturing capacity that could contribute to cover the future patient needs.

Human platelets express endothelial protein C receptor, which can be utilized to enhance localization of factor VIIa activity

Essentials Factor VIIa binds activated platelets to promote hemostasis in hemophilia patients with inhibitors. The interactions and sites responsible for platelet-FVIIa binding are not fully understood. Endothelial cell protein C receptor (EPCR) is expressed on activated human platelets. EPCR binding enhances the efficacy of a FVIIa variant and could impact design of new therapeutics.

SUMMARY

Background High-dose factor VIIa (FVIIa) is routinely used as an effective bypassing agent to treat hemophilia patients with inhibitory antibodies that compromise factor replacement. However, the mechanism by which FVIIa binds activated platelets to promote hemostasis is not fully understood. FVIIa-DVQ is an analog of FVIIa with enhanced tissue factor (TF)-independent activity and hemostatic efficacy relative to FVIIa. Our previous studies have shown that FVIIa-DVQ exhibits greater platelet binding, thereby suggesting that features in addition to lipid composition contribute to platelet-FVIIa interactions. Objectives Endothelial cell protein C receptor (EPCR) also functions as a receptor for FVIIa on endothelial cells. We therefore hypothesized that an interaction with EPCR might play a role in platelet-FVIIa binding. Methods/results In the present study, we used flow cytometric analyses to show that platelet binding of both FVIIa and FVIIa-DVQ is partially inhibited in the presence of excess protein C or an anti-EPCR antibody. This decreased binding results in a corresponding decrease in the activity of both molecules in FXa and thrombin generation assays. Enhanced binding to EPCR was sufficient to account for the increased platelet binding of FVIIa-DVQ compared with wild-type FVIIa. As EPCR protein expression has not previously been shown in platelets, we confirmed the presence of EPCR in platelets using immunofluorescence, flow cytometry, immunoprecipitation, and mass spectrometry. Conclusions This work represents the first demonstration that human platelets express EPCR and suggests that modulation of EPCR binding could be utilized to enhance the hemostatic efficacy of rationally designed FVIIa analogs.

Factor VIIa interaction with EPCR modulates the hemostatic effect of rFVIIa in hemophilia therapy: Mode of its action

Recent studies established that clotting factor VIIa (FVIIa) binds endothelial cell protein C receptor (EPCR). It has been speculated that FVIIa interaction with EPCR might augment the hemostatic effect of rFVIIa in therapeutic conditions. The present study is carried out to investigate the mechanism by which FVIIa interaction with EPCR contributes to the hemostatic effect of rFVIIa in hemophilia therapy. Active-site inhibited FVIIa, which is capable of binding to EPCR but has no ability to activate factor X, reduced the concentration of rFVIIa required to correct the bleeding following the saphenous vein injury in mouse hemophilia model systems. Higher doses of rFVIIa were required to restore hemostasis in EPCR overexpressing hemophilia mice compared to hemophilia mice expressing normal levels of EPCR. Administration of FVIII antibody induced only mild hemophilic bleeding in EPCR-deficient mice, which was corrected completely with a low dose of rFVIIa. Administration of therapeutic concentrations of rFVIIa increased plasma protein C levels in EPCR overexpressing mice, indicating the displacement of protein C from EPCR by rFVIIa. EPCR levels did not significantly alter the bioavailability of rFVIIa in plasma. Overall, our data indicate that EPCR levels influence the hemostatic effect of rFVIIa in treating hemophilia. Our present findings suggest that FVIIa displacement of anticoagulant protein C from EPCR that results in down-regulation of activated protein C generation and not the direct effect of EPCR-FVIIa on FX activation is the mechanism by which FVIIa interaction with EPCR contributes to the hemostatic effect of rFVIIa in hemophilia therapy.

Differences in N-glycosylation of recombinant human coagulation factor VII derived from BHK, CHO, and HEK293 cells

BACKGROUND &

METHODS

Recombinant factor VII (rFVII), the precursor molecule for recombinant activated FVII (rFVIIa), is, due to its need for complex post translational modifications, produced in mammalian cells. To evaluate the suitability of a human cell line in order to produce rFVII with post-translational modifications as close as possible to pdFVII, we compared the biochemical properties of rFVII synthesized in human embryonic kidney-derived (HEK)293 cells (HEK293rFVII) with those of rFVII expressed in Chinese hamster ovary (CHO, CHOrFVII) and baby hamster kidney (BHK, BHKrFVII) cells, and also with those of plasma derived FVII (pdFVII), using various analytical methods. rFVII was purified from selected production clones derived from BHK, CHO, and HEK293 cells after stable transfection, and rFVII isolates were analyzed for protein activity, impurities and post-translational modifications. RESULTS &

DISCUSSION

The analytical results showed no apparent gross differences between the various FVII proteins, except in their N-linked glycosylation pattern. Most N-glycans found on rFVII produced in HEK293 cells were not detected on rFVII from CHO and BHK cells, or, somewhat unexpectedly, on pdFVII; all other protein features were similar. HEK293rFVII glycans were mainly characterized by a higher structural variety and a lower degree of terminal sialylation, and a high amount of terminal N-acetyl galactosamines (GalNAc). All HEK293rFVII oligosaccharides contained one or more fucoses (Fuc), as well as hybrid and high mannose (Man) structures.

CONCLUSIONS

From all rFVII isolates investigated, CHOrFVII contained the highest degree of sialylation and no terminal GalNAc, and CHO cells were therefore assumed to be the best option for the production of rFVII.

The endothelial protein C receptor enhances hemostasis of FVIIa administration in hemophilic mice in vivo

Recombinant activated human factor VII (rhFVIIa) is an established hemostatic agent in hemophilia, but its mechanism of action remains unclear. Although tissue factor (TF) is its natural receptor, rhFVIIa also interacts with the endothelial protein C receptor (EPCR) through its γ-carboxyglutamic acid (Gla) domain, with unknown hemostatic consequences in vivo. Here, we study whether EPCR facilitates rhFVIIa hemostasis in hemophilia using a mouse model system. Mouse activated FVII (mFVIIa) is functionally homologous to rhFVIIa, but binds poorly to mouse EPCR (mEPCR). We modified mFVIIa to gain mEPCR binding using 3 amino acid changes in its Gla domain (L4F/L8M/W9R). The resulting molecule mFVIIa-FMR specifically bound mEPCR in vitro and in vivo and was identical to mFVIIa with respect to TF affinity and procoagulant functions. In macrovascular injury models, hemophilic mice administered mFVIIa-FMR exhibited superior hemostatic activity compared with mFVIIa. This was abolished by blocking mEPCR and was absent in ex vivo whole blood coagulation assays, implicating a specific mFVIIa-FMR and endothelial mEPCR interaction. Because mFVIIa-FMR models the TF-dependent and EPCR binding properties of rhFVIIa, our data unmask a novel contribution of EPCR on the action of rhFVIIa administration in hemophilia, prompting the rational design of improved and safer rhFVIIa therapeutics.

Decreased generation of procoagulant platelets detected by flow cytometric analysis in patients with bleeding diathesis

BACKGROUND

A clinically relevant bleeding diathesis is a frequent diagnostic challenge, which sometimes remains unexplained despite extensive investigations. The aim of our work was to evaluate the diagnostic utility of functional platelet testing by flow cytometry in this context.

METHODS

In case of negative results after standard laboratory workup, flow cytometric analysis (FCA) of platelet function was done. We performed analysis of surface glycoproteins Ibα, IIb, IIIa; P-selectin expression and PAC-1 binding after graded doses of ADP, collagen, and thrombin; content/secretion of dense granules; and ability to generate procoagulant platelets.

RESULTS

Of 437 patients investigated with standard tests between January 2007 and December 2011, we identified 67 (15.3%) with high bleeding scores and nondiagnostic standard laboratory workup including platelet aggregation studies. Among these patients, FCA revealed some potentially causative platelet defects: decreased dense granule content/secretion (n = 13); decreased α-granule secretion induced by ADP (n = 10), convulxin (n = 4), or thrombin (n = 3); decreased fibrinogen receptor activation induced by ADP (n = 11), convulxin (n = 11), or thrombin (n = 8); and decreased generation of COAT platelets, that is, highly procoagulant platelets induced by simultaneous activation with collagen and thrombin (n = 16).

CONCLUSION

Our work confirms that storage pool defects are frequent in patients with a bleeding diathesis and normal coagulation and platelet aggregations studies. Additionally, FCA is able to identify discrete platelet activation defects. In particular, we show for the first time that a relevant proportion of these patients has an isolated impaired ability to generate COAT platelets--a conceptually new defect in platelet procoagulant activity, which is missed by conventional laboratory workup.

Superactivated Platelets

Platelets contribute to hemostasis by forming the platelet plug and then contributing to coagulation by providing a catalytic surface where thrombin generation occurs efficiently. This catalytic activity, known as the platelet procoagulant response, is being recognized as a nuanced response. This review examines platelets’ response to strong stimuli, which results in the formation of a platelet subpopulation (superactivated platelets) with several unique properties, including enhanced procoagulant activity. These platelets contribute uniquely to thrombus architecture and seem to have thrombus regulatory activity. Superactivated platelets’ role in diseases of thrombosis and hemostasis, as either potentiating or mitigating factors, is not currently known, but may be an important pharmacological target.

Low intensity laser therapy speeds wound healing in hemophilia by enhancing platelet procoagulant activity

Our group has previously shown that cutaneous wound healing is delayed and histologically abnormal in a mouse model of hemophilia. Hemostasis is not only required to stop bleeding at the time of wounding, but also produces bioactive substances that promote appropriate inflammatory and proliferative responses during healing. Low intensity laser therapy (LILT) has been reported to enhance impaired wound healing in a variety of animal and human studies. The current studies were conducted to test the hypothesis that LILT can improve healing in a hemophilia B mouse model. Three daily treatments with 12 J/sq cm of 650 nm laser illumination reduced the time to closure of a 3-mm cutaneous punch biopsy wound in the hemophilic mice. All wounds were closed at 13 days in the sham-treated hemophilic mice, compared with 10 days in the LILT-treated hemophilic mice, and 9 days in wild-type mice. While LILT can speed healing by enhancing proliferation of cutaneous cells, we found that an additional mechanism likely contributes to the efficacy of LILT in the hemophilic mice. LILT enhanced the mechanical rigidity and platelet activity of clots formed from human platelet-rich plasma. Illumination of isolated platelets increased the mitochondrial membrane potential and enhanced binding of coagulation factors to the surface of activated platelets. Thus, while LILT can directly promote proliferative responses during healing, it also appears to enhance hemostasis in an animal model with impaired coagulation. These data suggest that trials of LILT as an adjunct to the usual hemostatic therapies in hemophilia are warranted.

Mechanisms and monitoring of bypassing agent therapy

Understanding the mechanism of action of normal hemostasis and how the bypassing agents recombinant activated factor VII (rFVIIa; NovoSeven) and plasma-derived activated prothrombin complex concentrate (Factor Eight Inhibitor Bypassing Agent [FEIBA]) control abnormal bleeding is imperative for healthcare professionals who treat patients with hemophilia and other bleeding disorders. A cell-based model has improved our understanding of in vivo mechanisms of hemostasis and the basis of the bleeding tendency in hemophilia. Bypassing agents do not restore the normal pathways of hemostasis in hemophilia, but rather boost thrombin generation in spite of a lack of platelet surface FVIIIa-FIXa ('tenase') activity. Thus, the common clinical laboratory coagulation assays do not reflect the clinically relevant hemostatic activity of bypassing agents, and no validated assay is available with which to measure the in vivo efficacy of these agents or predict individual patient responses to treatment. Global hemostasis assays measuring overall coagulation capacity have potential for assessment of the effects of bypassing agents. This review will focus on the mechanisms of clotting and their relationship to understanding the mechanisms of action of the bypassing agents in vivo and the methodologies that are emerging to monitor the clinical efficacy of bypassing agent therapy.

Unifying the mechanism of recombinant FVIIa action: dose dependence is regulated differently by tissue factor and phospholipids

Recombinant factor VIIa (rFVIIa) is used for treatment of hemophilia patients with inhibitors, as well for off-label treatment of severe bleeding in trauma and surgery. Effective bleeding control requires supraphysiological doses of rFVIIa, posing both high expense and uncertain thrombotic risk. Two major competing theories offer different explanations for the supraphysiological rFVIIa dosing requirement: (1) the need to overcome competition between FVIIa and FVII zymogen for tissue factor (TF) binding, and (2) a high-dose-requiring phospholipid-related pathway of FVIIa action. In the present study, we found experimental conditions in which both mechanisms contribute simultaneously and independently to rFVIIa-driven thrombin generation in FVII-deficient human plasma. From mathematical simulations of our model of FX activation, which were confirmed by thrombin-generation experiments, we conclude that the action of rFVIIa at pharmacologic doses is dominated by the TF-dependent pathway with a minor contribution from a phospholipid-dependent mechanism. We established a dose-response curve for rFVIIa that is useful to explain dosing strategies. In the present study, we present a pathway to reconcile the 2 major mechanisms of rFVIIa action, a necessary step to understanding future dose optimization and evaluation of new rFVIIa analogs currently under development.

Hemostatic properties of the FVIIa analog NN1731

Recombinant activated coagulation factor VII (rFVIIa) has proven to be a useful prohemostatic agent in patients with hemophilia and antibody inhibitors. It has also been used off-label in other settings. A major mechanism of its hemostatic efficacy is its ability to activate factor X on the surface of activated platelets in a tissue factor (TF)-independent manner. Novel analogs of FVIIa have been designed to have greater platelet-surface activity in the hope that these will be more active and reliable hemostatic agents. The analog NN1731 (vatreptacog alfa) was designed to have greater activity in the absence of TF, with the same substrate specificity as FVIIa. Surprisingly, it also binds to a greater number of sites on activated platelets by an, as yet, unknown mechanism. This molecule exhibits greater potency than FVIIa in biochemical assays, in vitro and animal models, and in early phase clinical trials.

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.