Use of albumin fusion technology to prolong the half-life of recombinant factor VIIa

A significant proportion of patients with haemophilia A develop inhibitors to administered factor VIII (FVIII) and require therapy with bypassing agents such as activated factor VII (FVIIa) or activated prothrombin complex concentrates. NovoSeven is a commercially available recombinant FVIIa (rFVIIa) with a very short half-life of approximately 2.4 hours. As a result, patients generally require multiple, frequent infusions for the management of bleeding episodes. Thus, there is growing interest in extending the circulating half-life of coagulation factors through the use of innovative drug delivery and formulation technologies. One such approach uses albumin fusion technology in which human albumin is genetically fused to the C-terminus of rFVIIa via a flexible glycine serine linker. The properties of this rFVIIa fusion protein (rVIIa-FP) have recently been examined in pre-clinical studies. Results from these investigations demonstrate the feasibility of this approach, which successfully extended the half-life and biological activity of rFVIIa without compromising haemostatic efficacy. These data suggest that rVIIa-FP may be a promising therapy for the treatment of haemophilia patients with inhibitors and warrants further investigation in clinical trials.     

Recombinant fusion protein linking factor VIIa with albumin (rVIIa­FP): Tissue distribution in rats

Introduction

A novel fusion protein linking coagulation factor VIIa with albumin (rVIIa-FP) is currently undergoing clinical investigations.

Objective

This study was conducted to examine the biodistribution of rVIIa-FP in comparison to recombinant factor VIIa (rFVIIa).

Materials and Methods

[³H]-rVIIa-FP (10 mg kg^− 1) or [³H]-rFVIIa (1.6 mg kg^− 1) were administered intravenously to rats, followed by quantitative whole-body and knee joint autoradiography for 24 ([³H]-rFVIIa) or 240 ([³H]-rVIIa-FP) hours post-dose. Pharmacokinetic and excretion balance analyses were performed.

Results

In contrast to [³H]-albumin, the tissue distributions of [³H]-rVIIa-FP and [³H]-rFVIIa were similar. Within the knee, both were rapidly present within synovial and mineralized regions. Importantly, rVIIa-FP- and albumin-derived radioactivity were detectable up to 72–120 hours, whereas [³H]-rFVIIa signals were already close to detection limits at 24 hours. The longest rVIIa-FP retention times were observed in bone marrow and endosteum, in which the retention times were up to 5 times longer for rVIIa-FP compared with rFVIIa. Up to 8 hours post-dose, 100% of radioactivity was assigned to unchanged [³H]-rVIIa-FP. Elimination of both proteins occurred primarily via the urine.

Conclusions

The data suggest that the FVIIa moiety is directing rVIIa-FP’s tissue distribution while the albumin moiety is responsible for the prolonged tissue retention. Importantly, rVIIa-FP is highly concentrated and retained over a long period in the growth plate of the knee joint − a vulnerable site in haemophilia patients. Overall, these improved tissue distribution characteristics of rVIIa-FP may enhance compliance and allow a more convenient dosing frequency.     

Prolonged half-life of glycoPEGylated rFVIIa variants compared to native rFVIIa

Introduction

Bleeding episodes in haemophilia patients with inhibitors are primarily treated with by-passing agents such as recombinant activated FVII (rFVIIa). Prophylactic treatment with rFVIIa has been shown to significantly reduce the number of bleeding episodes as compared to conventional on-demand haemostatic therapy, and a reduced dosing frequency could present an improved treatment option in inhibitor patients.

Materials and methods

A series of glycoPEGylated rFVIIa derivatives (5-40 K PEG) has been produced and their effect and pharmocokinetics have been investigated in several animal species.

Results

The glycoPEGylated rFVIIa derivatives exhibit significant prolongation of half-life in mice, dogs and pigs as measured by rFVIIa clot activity. The clearance of rFVIIa, rFVIIa-5 K PEG, rFVIIa-10 K PEG, rFVIIa-20 K PEG and rFVIIa-40 K PEG in minipigs were estimated to 59, 27, 22, 8.7 and 3.1 ml/h/kg, respectively. Across species a reduction in clearance as a function of the size of the attached PEG was observed. By allometric scaling, the compiled pharmacokinetics predicts a human half-life for rFVIIa-10 K PEG and rFVIIa-40 K PEG of approximately 7 and 12 h, respectively. The rFVIIa-10 K PEG and rFVIIa-40 K PEG are efficacious in stopping a bleed in the haemophilia A mouse tail-bleeding model after intravenous administration.

Conclusions

GlycoPEGylation of rFVIIa significantly increases the rFVIIa exposure in three animal models, glycoPEGylated rFVIIa compounds are effective in vivo and thus, represents a potential prophylactic treatment option for patients with inhibitors.     

Blocking the initiation of coagulation by RNA aptamers to factor VIIa

The tissue factor/factor VIIa complex is thought to be the primary initiator of most physiologic blood coagulation events. Because of its proximal role in this process, we sought to generate new inhibitors of tissue factor/factor VIIa activity by targeting factor VIIa. We employed a combinatorial RNA library and in vitro selection methods to isolate a high affinity, nuclease-resistant RNA ligand that binds specifically to coagulation factor VII/VIIa. This RNA inhibits the tissue factor-dependent activation of factor X by factor VIIa. Kinetic analyses of the mechanism of action of this RNA suggest that it antagonizes factor VIIa activity by preventing formation of a functional factor VII/tissue factor complex. Furthermore, this RNA significantly prolongs the prothrombin time of human plasma in a dose dependent manner, and has an in vitro half-life of approximately 15 h in human plasma. Thus, this RNA ligand represents a novel class of anticoagulant agents directed against factor VIIa.     

Heparin affinity of factor VIIa: implications on the physiological inhibition by antithrombin and clearance of recombinant factor VIIa

Factor VIIa (FVIIa), a trypsin-like serine protease, plays an essential role in haemostasis by initiating the coagulation in complex with its cofactor, tissue factor (TF). The TF pathway inhibitor is the main physiological inhibitor of FVIIa-TF complex, but FVIIa can also be inhibited by antithrombin, although little is known about this process.Functional analyses by second order kinetic determination and identification of FVIIa-antithrombin complex by electrophoresis, evaluating the effect of different cofactors: pentasaccharide, low molecular weight heparin (LMWH) and unfractionated heparin (UFH), confirmed that any activation of antithrombin significantly enhanced the inhibition of FVIIa. The analysis of the binding of FVIIa to heparin by surface plasmon resonance identified a high affinity interaction under physiologic conditions (K_D = 3.38 μM, with 0.15 M of ionic strength) strongly dependent on Ca²⁺ and ionic strength. This interaction was verified in cell models, indicating that FVIIa also binds to the surface of endothelial cells with similar requirements. Structural modeling suggests the presence of a potential exosite II in FVIIa. However, the binding of heparin did not display significant changes on both the intrinsic fluorescence and the associated functional consequences of FVIIa.These results indicate that FVIIa binds to exposed glycosaminglycans of the endothelium through an exosite II, structurally similar to that reported for thrombin and suggested for FIXa. This binding may favor its inhibition by antithrombin in the absence of TF, contributing to the physiological control of this protease. This process may also play an important role in the clearance of recombinant FVIIa administered to patients.     

Over-expression of factor VIIa in vivo

Although recombinant factor VIIa has been shown to bypass the deficiency in factor VIII or factor IX, efforts to reduce factor consumption and subsequently treatment cost have been focused on continuous infusion regimens with variable success. Using an engineered factor VII that can be secreted as factor VIIa, viral-mediated delivery of this transgene in the mouse liver resulted in phenotypic correction of murine hemophilia B, at expression levels of approximately 1 microg/ml. This model of factor VIIa continuous infusion can be further used to address the potential risk of thrombosis, in a transgenic model approach. The relative contribution of the extrinsic and/or intrinsic pathways in such risk can be dissected by crossing over-expressing FVIIa mice to recently described models of low expression of tissue factor or factor X. Our current data support the potential of factor VIIa gene transfer as a therapeutic alternative to bolus dosing but effective monitoring and modulation of factor VIIa expression will most likely be required.     

Current status on tissue factor activation of factor VIIa

Free factor VIIa displays a zymogen-like behavior with low intrinsic activity. Formation of a complex between factor VIIa and tissue factor is necessary to enhance the procoagulant activity of factor VIIa, not only by providing membrane localization, substrate exosites and positioning the active site at an appropriate distance above the surface but also by allosteric enhancement of the enzymatic activity, and this event signals initiation of blood coagulation. The interaction is of high affinity and all the domains are engaged at the interface. The crosstalk between the protease domain of factor VIIa, in particular residue Met-306, and the N-terminal domain of tissue factor provides the starting point for the allosteric activation of factor VIIa. The pathway(s) of conformational transitions in factor VIIa ensuing tissue factor binding has not been entirely mapped. The present paper is a brief compilation of our current knowledge of the allosteric mechanism by which tissue factor induces and stabilizes the active conformation of factor VIIa.     

Differential gene expression in activated monocyte-derived macrophages following binding of factor VIIa to tissue factor

Factor VIIa/tissue factor (FVIIa/TF) interaction has been reported to induce intracellular signalling in cells constitutively expressing TF, independently of downstream activation of the coagulation cascade. It is unknown, however, whether binding of FVII to its cofactor TF alters the gene expression profile in cells which inducible express TF under inflammatory conditions. To address this issue, gene expression patterns in cultured LPS-stimulated monocyte-derived macrophages with or without exposure to FVIIa were compared by cDNA macro-array analysis. Of the 1176 genes examined on the array, a small set of six genes (IL-6, IL-8,TNF-a, GRO-beta alpha-thymosin, cathepsin H) were consistently up-regulated and one gene suppressed (alpha-antitrypsin) in response to FVIIa in activated monocyte-derived macrophages. Among the seven genes identified by array analysis, five genes were finally confirmed by real-time RT-PCR. Interestingly, all of these genes differentially regulated in response to FVIIa (GRO-beta, IL-6, IL-8, TNF-alpha and alpha-antitrypsin) are critical in inflammation. The changes in gene expression were reflected by corresponding changes in the protein concentrations of IL-6 and IL-8 as demonstrated by ELISA. Active site-inhibited FVIIa had no effect on gene expression indicating that FVIIa-induced gene alteration is dependent on the proteolytic activity of FVIIa. The FVIIa-induced alterations in gene expression were found to be TF-dependent but independent of downstream coagulation proteins like thrombin and FXa. In summary, this study demonstrates that binding of FVIIa to its cofactor TF enhances restricted pro-inflammatory genes in activated monocyte-derived macrophages. By up-regulation of chemokines critical for leukocyte recruitment, FVIIa/TF interaction on activated monocyte-derived macrophages could be relevant to prepare monocytes/macrophages for extravasation and may represent a novel amplification loop of leukocyte recruitment.     

Thioester chromogenic substrates for human factor VIIa: substituted isocoumarins are inhibitors of factor VIIa and in vitro anticoagulants

Arginine thiobenzyl esters are convenient chromogenic substrates of factor VIIa (Z-Arg-SBzl, Kcat/KM = 1,600 M-1 s-1) and were used to study the kinetics of inhibition of factor VIIa by several mechanism-based isocoumarin inhibitors of trypsin-like enzymes. Isocoumarin derivatives substituted with a 7-guanidino or 3-isothiureidopropoxy group were good inhibitors of factor VIIa and acted as anticoagulants in human and rabbit plasma. With normal citrated human plasma, 4-chloro-3-ethoxy-7-guanidinoisocoumarin (3) and 7-amino-4-chloro-3-(3-isothiureidopropoxy) isocoumarin (ACITIC, 6) prolonged the prothrombin time (PT) ca. two-fold and prolonged the activated partial thromboplastin time (APTT) more than 4.5-fold at 20-30 microM. Both compounds had smaller effects in rabbit plasma. The short half-life of ACITIC and related isocoumarins in plasma should make these compounds uniquely useful as anticoagulants in therapeutic situations where it is desirable to have anticoagulant effects for a short defined time period.     

Platelet binding and activity of recombinant factor VIIa

Recombinant FVIIa was developed for the purpose of treating hemophiliacs with antibody inhibitors. It was initially assumed to act by enhancing factor X activation by a tissue factor-dependent mechanism. However, the very high levels of FVIIa required for hemostatic effect in vivo seemed inconsistent with this mechanism. After many years of debate, in now appears that platelet surface binding and activity play an important role in the efficacy of FVIIa as a bypassing agent in hemophilia. Platelet binding was initially suggested to be mediated by binding to anionic phospholipid exposed on platelet surfaces upon activation. It now appears that the glycoprotein Ib/IX/V complex also plays a role in FVIIa binding to platelets. However, the characteristics of FVIIa binding to GPIb/IX/V to not seem to fully explain platelet localization of FVIIa to platelets. Thus, there are still unanswered questions in fully understanding the mechanism of hemostatic action of recombinant FVIIa.     

Relationship between factor VII activity and clinical efficacy of recombinant factor VIIa given by continuous infusion to patients with factor VIII inhibitors.

A multicenter prospective study of recombinant activated factor VII (rFVIIa) given by continuous infusion (CI) to treat severe hemorrhages and to handle surgical procedures was carried out. Relations between clinical efficacy, dosages used and levels of FVII coagulant activity (FVII:C) achieved in plasma were also evaluated. Case material included 25 patients with hemophilia (9 children and 16 adults) with high-responding inhibitors and 3 patients with acquired factor VIII inhibitors. Overall, 35 CI courses were given for 10 spontaneous bleeding episodes, 11 major surgical procedures and 14 minor surgical procedures. Bolus doses of 90 to 150 microg/kg (median: 100) were followed by CI given at median rates of 20 microg/kg/h for major surgery and of 17 and 16 microg/kg/h for minor surgery and spontaneous hemorrhages. Satisfactory hemostasis was obtained in 30 of 35 courses (88%). rFVIIa CI was ineffective in 2 hemophiliacs undergoing surgical operations and in another hemophiliac with hemoperitoneum who had to be switched to other treatments (high doses of porcine or human factor VIII concentrates). rFVIIa CI was partially effective in 2 hemophiliacs who had mild local bleeding after minor surgery. The CI rates and the corresponding FVII:C levels in plasma were similar in effective, partially effective and ineffective courses (median rate: 17, 20 and 20 microg/kg/h, respectively; median FVII:C:14, 18 and 18 IU/ml, respectively). A single adverse event was observed, superficial thrombophlebitis. This study confirms that rFVIIa given by CI is effective in a high proportion of patients with factor VIII inhibitors. It also demonstrates that FVII:C levels attained in plasma do not always predict efficacy because similarly high levels were attained during successful treatments and in those that failed.     

Plasma factor VII-activating protease is increased by oral contraceptives and induces factor VII activation in-vivo

Oral contraceptive (OC) use influences the hemostatic system significantly and is a risk factor for development of cardiovascular disease. Factor VII-activating protease (FSAP) has potential effects on hemostasis. The 1601GA genotype of the 1601 G/A polymorphism in the FSAP gene expresses a FSAP alloenzyme with reduced pro-fibrinolytic activity. Presently, we address whether OC use and OC formulation affect FSAP measures in human blood.Healthy women (n = 588) were allocated to six cycles of OCs with estrogen contents of 20 μg (n = 158), 30 μg (n = 284), 35 μg (n = 79) or 50 μg (n = 67) combined with various progestins. FSAP genotypes, FSAP and factor VII (FVII) plasma measures were assessed at baseline and after 6 cycles of OC.The 1601GA genotype was present in 49 (8.3%) of the women and was associated with significantly reduced levels of FSAP (P ≤ 0.001). OC use increased FSAP antigen by 25% and FSAP activity by 59% (P < 0.001). The FSAP increase was comparable in the seven different OC treatment groups (P > 0.05). The relative increase in FSAP activity was significantly higher in women carrying the 1601GG genotype (63%) than in women carrying 1601GA genotype (50%) (P = 0.01) and was associated with an increased activation of FVII.In conclusion: OC use increases the plasma measures of FSAP. The increase in FSAP is comparable in the seven OC-groups studied but is more significant in women carrying the 1601GG genotype than in women with the 1601GA genotype and results in increased activation of FVII suggesting that FSAP-induced activation of FVII takes place in-vivo and not only in-vitro as hitherto described.     

Mechanism by which recombinant factor VIIa shortens the aPTT: activation of factor X in the absence of tissue factor

Clinical trials have recently begun using high concentrations of activated recombinant factor VII (rFVIIa) for the treatment of hemophilic patients with inhibitors. Unexpectedly, the activated partial thromboplastin time (aPTT) was observed to be significantly shortened during infusion of the rFVIIa. To determine the mechanism for this shortening, the effect of rFVIIa on both the prothrombin time (PT) and the aPTT of normal and various factor-deficient plasmas was examined. rFVIIa shortened the PT of all plasmas tested except FX and FV deficient plasmas. rFVIIa also shortened the aPTT of all plasmas tested except FX and FV deficient plasmas. Since there is no added tissue factor (TF) in aPTT reagents, rFVIIa appeared to shorten the aPTT in the absence of TF. To investigate this possibility, the activity of rFVIIa in a purified system containing only FX, phospholipid vesicles (1:1 PS:PC), and calcium was examined. In this system, rFVIIa activated factor X in the absence of TF. If any component of the purified system was omitted, there was no detectable activation of FX. Thus it appears that calcium and phospholipids are required for the activation of FX by rFVIIa in the absence of TF. Increasing the concentration of rFVIIa increased the rate of FX activation, but the rate of activation was always much lower than that observed with even trace amounts of tissue factor. We conclude that high concentrations of rFVIIa, in the presence of calcium and phospholipid, can directly activate FX in the absence of TF and hence account for the shortening of the aPTT in inhibitor patients treated with rFVIIa.     

Tissue factor/factor VIIa complex: role of the membrane surface

Blood clotting is triggered when the plasma serine protease factor VIIa binds to the cell-surface protein, tissue factor (TF); the resulting TF:FVIIa complex activates factors IX (FIX) and X (FX) by limited proteolysis. FVIIa, FIX and FX all bind reversibly to membranes via their gamma-carboxyglutamate-rich (GLA) domains, while TF is an integral membrane protein. Removing these proteases from the membrane surface is known to render them thousands of times less active, although the mechanisms by which blood clotting proteins bind to membranes-and the contributions of membranes to catalysis-remain very incompletely understood. Our recent and ongoing studies use a combination of nanoscale membrane bilayers (Nanodiscs), solid-state NMR and all-atom molecular dynamics (MD) simulations, enabling detailed insights into how GLA domains bind to phospholipid bilayers and how specific phospholipids enhance the catalytic activity of the TF:FVIIa complex.     

Modeling the action of factor VIIa in dilutional coagulopathy

Dilutional coagulopathy is observed in patients who have lost blood and had the blood volume replaced with components that do not have plasma procoagulants and anticoagulants. Since both procoagulant and anticoagulant mechanisms contribute to total thrombin generation, it is not clear whether the procoagulant or anticoagulant effects will dominate when both are decreased. Decreasing antithrombin levels leads to increasing thrombin generation in ex vivo models. However, in these same models decreasing prothrombin leads to decreasing thrombin generation. Previous studies can be extrapolated to suggest that the ratio of procoagulants and anticoagulants may be the critical component in determining thrombin generation. Limited clinical data suggest that factor VIIa has been useful in controlling intractable bleeding in some cases of dilutional coagulopathy. Preliminary data suggest that factor VIIa activation of factors X and IX on activated platelets enhances thrombin generation significantly in a model of dilutional coagulopathy. This may suggest a mechanism to account for the efficacy of factor VIIa in reducing blood loss in some settings of dilutional coagulopathy.