Prospective Efficacy and Safety of a Novel Bypassing Agent, FVIIa/FX Mixture (MC710) for Hemophilia Patients with Inhibitors

. These results suggest that MC710 has a relatively high specificity for TF compared to APCC .


Introduction
Hemophilia A and B are hereditary bleeding disorders caused by a deficiency of coagulation factors VIII (FVIII) and IX (FIX), respectively. In substitution therapies using FVIII and FIX concentrates for the management of bleeding, the development of inhibitory antibodies is a serious complication in ~28% and ~7% of hemophilia A and B patients, respectively [1, 2]. Currently, two bypassing agents, plasma-derived activated prothrombin complex concentrates (APCC, FEIBA ® ) and recombinant activated factor VII (rFVIIa, NovoSeven ® ), are available for the management of bleeding in hemophilia patients with inhibitors including acquired hemophilia patients. Retrospective studies showed the efficacy of rFVIIa and APCC in 12~36 h in a standard administration regime is assessed to be 66~95% and 39~76%, respectively [3]. A recent comparative study seemed to indicate equivalence between rFVIIa and APCC [4]; however, a considerable number of patients experience treatment failure or insufficient efficacy. In 2002, an anecdotal report suggested that the sequential administration of rFVIIa and prothrombin complex concentrate (PCC) to hemophilia patients with inhibitors in order to obtain a stronger hemostatic effect than with rFVIIa alone [5]. Further, it has been reported that the combination of rFVIIa and APCC appeared to confer beneficial hemostatic synergy in patients refractory to each individual therapy [6][7][8]. However, repeated infusion of APCC may cause an accumulation of prothrombin and factor X (FX), thereby increasing the risk of thrombosis. Moreover, a lack of suitable laboratory tests for monitoring hemostatic effects is a major concern with current bypassing therapies. As a consequence and in appreciation of the incomplete efficacy and safety of currently available bypassing agents, new drugs are in development, such as rFVIIa analogues featuring higher hemostatic potential [9], glycoPEGylated rFVIIa with a longer half-life than rFVIIa [10], and non-anticoagulant sulphated polysaccharides (AV513) [11]. In order to solve these problems, we searched for alternative factors for enhancing and promoting FVIIa's hemostatic activity, and found that a combination of plasma-derived FVIIa and FX (FVIIa/FX) may overcome the disadvantages of rFVIIa therapy for hemophilia patients with inhibitors. That combination allows improving APTT remarkably in the www.intechopen.com patient plasma which is useful laboratory test for monitoring the hemostatic effect of hemophilia patients. An FVIIa/FX mixture, MC710, was designed as a dry-heated product prepared by mixing plasma-derived FVIIa and FX at a weight ratio of 1:10 under acidic conditions to suppress the generation of FXa. A Phase I clinical study encompassing pharmacokinetics (PK), pharmacodynamics (PD), and safety has been completed. In this article, we outline the rationale for the combined use of FVIIa and FX, the manufacturing method of MC710, the treatment's prospective efficacy and safety, and in the final section, the results of Phase I clinical study in non-bleeding hemophilia patients with inhibitors.

Background
The blood coagulation system involves a so-called "cascade reaction" of enzymes and substrates which finally achieves the formation of a fibrin clot. In this reaction, Ca 2+ and phosolipids (PL) assume the role of cofactors which remarkably enhance the affinity between enzymes and substrates, resulting in the promotion of a coagulation reaction at the site of injury. FVIIa circulates in blood at a concentration of around 0.1 nM (3~5 ng mL -1 ) in plasma [12, 13] with a much longer half life, 2~3 h, than other activated coagulation factors because of its zymogen-like conformation [14]. Tissue factor (TF) is a membrane glycoprotein expressed in various tissues and plays the role of a cofactor of FVIIa. Vascular TF is present in the adventitia, hidden from the circulating blood. When a vessel is injured, TF is exposed at the site. FVIIa binds to TF forming a stoichiometric complex (the initiator of the extrinsic pathway) and remarkably enhances catalytic activity to activate FIX and FX. FIXa forms a FIXa/FVIIIa/PL complex (FXase) and converts FX to FXa. Also, FXa forms a FXa/FVa/PL complex (prothrombinase) and converts prothrombin to thrombin. Thrombin activates FXI to form FXIa and promotes the activation of FIX (feedback activation of the intrinsic pathway), and converts the fibrinogen to a fibrin clot via limited proteolysis [15]. Platelets are essential for hemostasis. In the artery, circulating platelets in blood attach and adhere to the extracellular matrix (collagen) under the layer of endothelial cells at the site of damage using GPVI and integrin  2  1 (GPIa/IIa), and adhere to the collagen via the interaction of VWF (von Willebrand factor) and GPIbIX, and finally aggregate through the interaction of  IIb  3 (GPIIb/IIIa)and VWF/fibrinogen [16][17][18][19]. Through these processes, localized platelets are activated by thrombin or chemical mediators (ADP, et al) released from activated platelet granules. The PL membrane essential for thrombin generation is mainly supplied by the activated platelets. In the hemostasis of hemophilia patients with inhibitors against FIX or FVIII, the intrinsic pathway is completely blocked. Therefore, it is necessary to enhance the potential for coagulation based on the extrinsic pathway, the activation of FX by FVIIa, followed by the activation of prothrombin to form thrombin. In 1996, rFVIIa therapy was launched to give a super-physiological concentration to the patient plasma to enhance the extrinsic pathway of hemophilia patients with inhibitors. However, the mechanism of the bypassing activity of rFVIIa is controversial due to the requirement for a high dose (60~120 µg kg -1 ), which gives a high FVIIa concentration in plasma (0.5~1.0 µg mL -1 ) [20].
There are two hypotheses to elucidate the requirement for the high dose ( Fig. 1-A). The first is "TF-dependent FX activation". It was reported that rFVIIa-induced thrombin generation in the presence of a low concentration of TF and in the absence of FVIII is inhibited by physiological concentrations of FVII, and at least 10 nM (0.5 µg mL -1 ) of rFVIIa is required to overcome the inhibition by FVII in order to induce the bypassing activity [21,22]. The second is "TF-independent FX activation". It was reported that 5 nM (0.25 µg mL -1 ) of rFVIIa could convert FX to FXa on the surface of activated platelets, leading to thrombin generation independent of TF [23]. Clinical experience has suggested that the dose or blood level of rFVIIa required for hemostasis in any given hemophilia patient with inhibitors is not always predictable. Differences in platelet procoagulant properties could influence the response to a high-dose of rFVIIa [24,25]. Also, it has been reported that rFVIIa binds to GPIbIX on activated platelets and localizes at the site of injury [26]. Recently, mega-dose therapy with an injection of rFVIIa (270 µg kg -1) has been applied to the control of bleeding to diminish repeated administrations in 2~3 h and raise the hemostatic potential of rFVIIa by increasing the C max in plasma above the standard dose [27,28]. The exact mechanism of rFVIIa is not clear at present, but its potential for FXa and thrombin generation and subsequent fibrin clot formation will be essential to obtain a hemostatic effect with rFVIIa, in either a TF-dependent or TF-independent manner.

Rationale for the combined use of FVIIa and FX
It was reported that TF binds to FX or FIX as well as to FVIIa and this ternary complex of enzyme/cofactor/substrate (FVIIa/TF/ FIX or FX) is the real trigger of the extrinsic pathway to generating FIXa or FXa and the subsequent generation of thrombin [29][30][31]. However, it was demonstrated that FIX is a much better substrate for FVIIa-TF than FX [32]. We investigated the kinetic parameters of FVIIa-mediated FX activation under several conditions. The results of the kinetic analysis are shown in Table 1. K m values in the presence of PL and relipidated TF were 180 ± 70 nM and 160 ± 60 nM, while k cat values were 0.38 ± 0.09 s -1 and 11.5 ± 4.7 s -1 , respectively, similar to values published previously [32]. The FX level in normal plasma is 8~12 μg mL -1 (140~210 nM) [34], and the K m values were within this range; therefore, it was suggested that the FX concentration in plasma might not be sufficient to achieve the generation of FXa by FVIIa required for hemostasis ( Fig. 1-B). These results indicate that a higher concentration of FX is required to enhance the catalytic efficacy of FVIIa and to complete coagulation in the plasma of hemophilia patients.  Table 1, Km values in the presence of PL and relipidated TF were in the range of FX levels in normal plasma (140~210 nM), and the Km value in activated platelets is far above that range. In the extrinsic pathway, the increase of FX concentration in plasma two or three times (2~3 x Km) might facilitate the increase of the FX actvation rate, and promote the formation of FVIIa/TF/FX complex.

Fig. 1. Advantage of co-administration of FVIIa and FX
The results of the kinetic analysis were consistent with those of the thrombin generation (TG) assay (Fig. 2). TG assay using a fluorosubstrate (Z-G-G-R-MCA) specific for thrombin was developed by Hemker et al [35]. In this assay system, thrombin generation is analyzed in the following three steps [36, 37]: 1. Initiation: initiation of the cascade reaction to start thrombin generation. 2. Propagation: explosive thrombin production. 3. Termination: attenuation of thrombin generation. The TG assay is used to analyze the clinical efficacy of rFVIIa and APCC [38]. The TG parameters are lag time (time to initial thrombin formation (min)), peak thrombin level (nM), time to peak (ttPeak) (min), and endogenous thrombin potential (ETP) (nM min) [39]. We used this assay to examine the hemostatic potential of the combination of FVIIa and FX. The thrombin-generating potential of hemophilic plasma was raised by increasing the concentration of FX added to the plasma in the range of 2.5~20 μg mL -1 without the addition of FVIIa (Figs. 2A-a and 2B-a). Further, the combination of FVIIa (0.25 and 1.0 μg mL -1 ) and FX (2.5~20 μg mL -1 ) gave more thrombin-generating potential to the plasma than did FVIIa alone, resulting in a shortening of the ttPeak and an increase in the peak thrombin level ( Figs. 2A-b and -c, and 2B-b and -c)  decrease faster than that of FX, but a high FX level might help to generate bypassing activity which results in longer-lasting hemostatic potential than FVIIa alone. This idea was proven in our previous experiment using a monkey acquired hemophilia B model in which the hemostatic efficacy of FVIIa/FX (co-administration of FVIIa (80 μg kg -1 ) and FX (800 μg kg -1 )) and FVIIa alone 80 μg kg -1 was compared by measuring thromboelastography (TEG). As shown in Fig. 3, administration of FVIIa/FX remarkably normalized TEG parameters. Even 6 h after its administration, FVIIa/FX had hemostatic potential above that immediately after the administration of FVIIa alone [44].   (40) APTT is used for monitoring the management of bleeding or determining the supplemental level of FVIII or FIX concentrate in hemophilia patients, but not in therapy using bypassing agents because of the poor improvement. We reported that more than 5 µg mL -1 of FVIIa alone is required to reduce APTT in hemophilic plasma to levels equivalent to those after replacement-therapy (10 % of FVIII or FIX activity in hemophilic plasma); on the other hand, the mixture of FX and FVIIa caused a significant improvement of APTT in a concentrationdependent manner. In plasma containing 0.5~1.5 μg mL -1 of FVIIa (obtained after intravenous rFVIIa administration at standard doses) when FX is added at 5~15 μg mL -1 , the poor coagulant activity in hemophilic plasma is remarkably improved to levels achieved www.intechopen.com with replacement therapy [44]. Therefore, a mixture ratio 1: 10 of FVIIa to FX in MC710 was designed to optimize the bypassing effect of FVIIa (0.5~1.5 μg mL -1 ) in plasma.
A hemophilia B model using a cynomolgus monkey was produced by the administration of goat anti-FIX antibodies to be in < 5% of FIX activity. FVIIa (80 µg/kg) was administered to the monkey and Thromboelastography (TEG) was measured pre-administration, and 6 h and 12 h post-administration. Also, FVIIa (80 µg/kg) and FX (800 µg/kg) were continuously injected into the monkey. The TEG patterns are shown in the figure and r + k values are described on the right side of each pattern. ND means "not detected". In the production of plasma-derived FVIIa, FVII is converted to FVIIa with the following two steps to achieve high recovery and quality: (1) partial activation on anion exchange resin and, (2) further activation in the solution after eluting from the resin [46]. A flow diagram of the preparation of MC710 is shown in Fig. 4. In the first purification step, a crude vitamin K-dependent protein fraction is extracted from cryoprecipitate-poor plasma using anion exchange chromatography. Next, this fraction is applied to an immunoaffinity chromatography column containing gels bound with Ca 2+ -dependent anti-FVII or anti-FX monoclonal antibody as a ligand. The FVII or FX fraction eluted with a buffer containing EDTA is treated in solvent and detergent (0.3% TNBP and 1% polysorbate 80) for virus inactivation. After the treatment, the FVII fraction is applied to DEAE Sepharose-FF to obtain partially activated FVIIa and the eluted FVII/FVIIa mixture is completely converted to FVIIa in the solution. The FX fraction eluted from the immunoaffinity chromatography

APTT and PT waveform analysis
APTT waveforms are used to analyze the overall process of fibrin clotting by measuring the turbidity and calculating the coagulation rate (dT/dt) and coagulation acceleration (second derivative of transmittance and time; d 2 T/dt 2 ). It was reported that these parameters are useful for the diagnosis of DIC [47,48]. Recently, Shima reported that APTT waveforms are applicable to the quantification of low levels of FVIII (<1 U dL -1 ) on the basis of the correlation of the FVIII activity with coagulation acceleration, and the waveform profile formed by rFVIIa was different from that for normal plasma [49]. In our analysis, MC710 with a FVIIa concentration of 1 µg mL -1 (the dose and concentration in plasma of MC710 are expressed as FVIIa amounts) exhibited greater clotting ability than 1 µg mL -1 FVIIa alone in hemophilia A patient plasma with inhibitors and FIX-deficient plasma samples (Figs. 5A-a  and 5B-a). Coagulation acceleration showed that MC710 at above 1-2 g mL -1 possessed a greater ability to shorten APTT and to induce accelerated clotting than did the same concentration of rFVIIa (Figs. 5A-b and 5B-b). Parameters for plasma from hemophilia A patients with inhibitors in the presence of 1 U mL -1 APCC were similar to those in the presence of 1 µg mL -1 MC710 (Figs. 5A-a and 5A-b). On the other hand, PT and its clot formation acceleration did not show any difference among the three agents (data not shown).
To evaluate the TF-specificity of the agents, thrombin generation in plasma of a hemophilia patient with inhibitors was measured in the presence or absence of relipidated TF or PL at various concentrations of MC710 and APCC, and 1.0 μg mL -1 of rFVIIa (Figs. 7A-a~c  and 7Ba~c). The 0.1 μg mL -1 of MC710 and 0.25 U mL -1 of APCC showed a greater thrombin generation potential than 1.0 μg mL -1 of rFVIIa (Figs. 7A-a and 7B-a). MC710 showed lower thrombin-generating potential than did APCC in the absence of TF or PL (Figs. 7A-b and -c,  and 7B-b and -c). These results suggest that MC710 has a relatively high specificity for TF compared to APCC. Normal plasma ("Normal") or FIX-DP supplemented with FIX at 1 U mL -1 was used as a control. The MC710 concentration is denoted by the FVIIa concentration in each panel.   Fig. 7. Changes in TG profiles induced by APCC and MC710 with or without TF (40) www.intechopen.com

Thrombogenic test using monkeys
It was reported that APCCs might induce thrombotic complications such as disseminated intravascular coagulation (DIC) and acute myocardial infarction [50 -52]. As the clearance of FX is much slower than that of FVIIa, repeated administrations of MC710 might induce the accumulation of FX in the blood raising concerns over safety regarding DIC or other thrombotic events. Therefore, it is important to confirm the safety of repeated administrations of MC710 alone and in combination with other bypassing agents. We performed multiple injections of MC710 (4 injections of 120 g kg -1 every 8 h (as FVIIa dose)), and rFVIIa (one injection of 90 g kg -1 and 2 injections of 120 g kg -1 every 2 h) or APCC (3 infusions of 100 U kg -1 every 12 h) at 8 h after the administration of MC710 (120 g kg -1 ) into the monkeys, and compared the DIC parameter changes with those of APCC (4 infusion of 100 U kg -1 every 12 h) (Fig. 8). No serious or severe event was observed in any monkey or in any group, and the fibrinogen level and platelet counts did not change. However, the FDP (fibrinogen degraded products) level increased in all groups and the rate of increase was lower in the group repeatedly administered MC710 than that repeatedly administered APCC (See the legend in Fig. 8). These results suggest the thrombogenic risk from the repeated administration of MC710 is equal to or lower than that of repeated administration of APCC. Thrombogenicity of MC710 was compared to APCC (FEIBA ® ) using cynomolgus monkeys. The experimental design is described in the figure. Schemes a)~c) show the time courses of injections or infusions of rFVIIa (NovoSeven ® ) and APCC after 120 µg kg -1 of MC710. MC710 dose is denoted as FVIa dose. Scheme d) shows the time course of repeated infusions of APCC. The experiment was performed using three monkeys in each group. At pre-administrations FDP level was 0.44 ± 0.24 ng mL -1 (n=12) and at 30 min after the final administration of the agents a), b), c), and d) were 3.37 ± 3.59, 5.87 ± 2.64, 2.93 ± 0.42, and 8.57 ± 2.17 ng mL -1 (n=3), respectively.

Outline of the trial
Phase I clinical study of MC710 has been completed. In this study, PK and PD parameters and the safety of single doses of MC710 were investigated in 11 hemophilia patients with inhibitors in a non-bleeding state. A total of 25 administrations of MC710 were made to the subjects (7 hemophilia A patients with inhibitors and 4 hemophilia B patients with inhibitors) at 5 doses of MC710 (20,40,80, 100 and 120 g kg -1 (as FVIIa dose)) in addition to the administrations of rFVIIa 120 μg kg -1 , and APCC 50 U kg -1 or 75 U kg -1 as active controls [53].

Pharmacokinetic analysis
PK parameters were calculated based on FVII:C, FVII:Ag, FX:C and FX:Ag levels. As shown in Figs. 9A-D, those levels rapidly increased after administration of MC710. FVII:C and FVII:Ag levels returned to pre-administration values during 12 to 24 h after the administration, and increased levels of FX:C and FX:Ag persisted in the blood until 48 h after the administration of MC710 at 80 μg kg -1 or more.   Fig. 9. Changes in pharmacokinetic parameters after the administration of MC710 to hemophilia patients with inhibitors (53) www.intechopen.com

Pharmacodynamic analysis
APTT and PT were measured as PD parameters. APTT, prolonged 120 sec or more before administration, improved in a dose-dependent manner after administration of MC710, and the effect persisted for 12 h at all doses (Fig. 10A). At MC710 doses of more than 100 μg kg -1 ,

A. APTT B. PT
Time-dependent changes in APTT (Panel A) and PT (Panel B) are shown. The normal ranges for healthy individuals (---) for APTT were defined as 42.5 (upper limit) and 23.5 (lower limit) sec and for PT as 12.8 (upper limit) and 9.9 (lower limit) sec. The mark represents the mean ± SD. MC710, rFVIIa (NovoSeven ® ) and APCC (FEIBA ® ) doses are denoted by the following color symbols: MC710 (as FVIIa dose); 20 μg kg -1 , (-•-); 40 μg kg -1 , (-•-); 80 μg kg -1 , (-•-); 100 μg kg -1 , (-•-); 120 μg kg -1 , (-•-); rFVIIa; 120 μg kg -1 , (-▲-); APCC; 50 U kg -1 , (-■-); 75 U kg -1 , (-■-). Fig. 10. Changes in pharmacodynamic parameters after the administration of MC710 to hemophilia patients with inhibitors (53) the APTT was especially close to the normal range. Even 6 h after the administration of more than 100 μg kg -1 of MC710, the APTT was shorter than that immediately after the administration of 120 μg kg -1 of rFVIIa and 75 U kg -1 of APCC. It is expected that from the evaluation based on the level of improvement in APTT, the hemostatic effect immediately after the administration of MC710 at over 100 μg kg -1 might be equivalent to that of FVIII or FIX replacement therapy (replacement level 20 to 50% of these factors). The PT reached approximately 6 sec (the determination limit) after administration of all doses of MC710 except for 20 μg kg -1 and remained at that level for up to 2 h. At 6 h after the administration of 80, 100 and 120 μg kg -1 of MC710, the PT was shorter than that after the administration of 120 μg kg -1 of rFVIIa. The reduction in PT persisted for 12 h at all doses (Fig. 10B). The PT after the administration of 40, 80, 100 and 120 μg kg -1 of MC710 was shorter than that for 75 U kg -1 of APCC.

DIC and other safety concerns
TAT and F1+2 were increased after the administration of MC710 indicating the activation of prothrombin in blood flow; however, similar increases were also observed after the administration of rFVIIa and APCC [54,55]. No serious or severe adverse events were observed within 4 weeks after the administration of MC710 and no subject discontinued treatment due to an adverse event. Also, no clinical symptoms or changes in laboratory tests (platelet count, fibrinogen, D-dimer) indicating a hypercoagulable state such as DIC were detected (data not shown). In addition, the results of virologic and serologic tests confirmed that no subject developed a new viral antigen or produced a new antibody after the administration of MC710.

Conclusion and future perspectives
In this review, we described the rationale for the combined use of FVIIa and FX, the manufacturing process of FVIIa/FX mixture, MC710, and the treatment's prospective efficacy and safety. We also outlined the results of a Phase I clinical study. In the study, PK and PD parameters changed in a dose-dependent manner after the administration of MC710 and the changes in the PD parameters (APTT and PT) were equal to or greater than those in rFVIIa and APCC. Furthermore, MC710 was safely administered at doses of up to 120 μg kg -1 and no serious or severe adverse events, including DIC, were observed. It was recently reported that the combination of APCC and rFVIIa is safe and effective in the treatment of bleeding that is unresponsive to monotherapy [56]. This report supports our hypothesis that the FVIIa/FX mixture, MC710, would be a more potent bypassing agent than clinically available bypassing agents. Phase II clinical studies in hemophilia patients with inhibitors who are hemorrhaging have been completed in Japan and MC710 is expected to be used as an alternative to APCC and rFVIIa in the near future.