Marianne Kjalke

Recombinant activated factor VII (rFVIIa): characterization, manufacturing, and clinical development.

Recombinant activated coagulation factor VII (rFVIIa) (NovoSeven) was developed for treatment of bleeding in hemophilia patients with inhibitors (antibodies) against factors VIII or IX. rFVIIa initiates the coagulation cascade by binding to tissue factor at the site of injury and causes the formation of sufficient amounts of thrombin to trigger coagulation. Patients with a variety of other coagulation deficiencies than hemophilia characterized by an impaired thrombin generation and life-threatening bleeding have been reported as successfully treated with rFVIIa. Data are now entered into clinical registries established to further monitor this experimental treatment with NovoSeven. rFVIIa is produced free of any added human protein. The amino acid sequence of rFVIIa is identical to plasma-derived FVIIa (pdFVIIa). Posttranslational modifications (i.e., gamma-carboxylations, N- and O-glycosylations) are qualitatively identical in pdFVIIa and rFVIIa although some quantitative differences exist. The activities of rFVIIa and pdFVIIa are indistinguishable. Manufacturing of rFVIIa involves expression in baby hamster kidney (BHK) cells followed by purification, including three ion-exchange and one immunoaffinity chromatography steps. The last anion-exchange chromatography step ensures completion of the autoactivation of recombinant factor VII (rFVII) to rFVIIa. This review describes the mechanism of action, characterization, manufacturing, and preclinical and current clinical evidence for the efficacy and safety of rFVIIa.

High-dose factor VIIa increases initial thrombin generation and mediates faster platelet activation in thrombocytopenia-like conditions in a cell-based model system

Clinical experience has shown that high doses of recombinant factor VIIa (rFVIIa) may ensure haemostasis in thrombocytopenic patients. We have used a cell‐based model system to mimic thrombocytopenia and analyse the effect of rFVIIa. Lowering the platelet density from 200 × 109/l (reflecting normal conditions) to 100, 50, 20 and 10 × 109/l revealed a platelet density‐dependent decrease in the maximal rate of thrombin generation, a prolongation in the time to maximal thrombin activity and a lower maximal level of thrombin formed. The platelet activation, measured as the time to half‐maximal P‐selectin (CD62) exposure, was not significantly dependent on the platelet density in the range of 200 × 109/l to 10 × 109/l, although there was a tendency for slower platelet activation at 20 × 109 and 10 × 109 platelets/l than at the higher platelet densities. Addition of 50–500 nmol/l rFVIIa to samples with 20 × 109 or 10 × 109 platelets/l shortened the lag phase of thrombin generation as well as the time to half‐maximal platelet activation. Our data indicate that high doses of rFVIIa may help to provide haemostasis in thrombocytopenic patients by increasing the initial thrombin generation, resulting in faster platelet activation and thereby compensating for the lower number of platelets present.

Rational design of coagulation factor VIIa variants with substantially increased intrinsic activity

A trace amount of coagulation factor VII (FVII) circulates in the blood in the activated form, FVIIa (EC 3.4.21.21), formed by internal proteolysis. To avoid disseminated thrombus formation, FVIIa remains in a conformation with zymogen-like properties. Association with tissue factor (TF), locally exposed upon vascular injury, is necessary to render FVIIa biologically active and initiate blood clotting. We have designed potent mutants of FVIIa by replacing residues believed to function as determinants for the inherent zymogenicity. The TF-independent rate of factor X activation was dramatically improved, up to about 100-fold faster than that obtained with the wild-type enzyme and close to that of the FVIIa-soluble TF complex. The mutants appear to retain the substrate specificity of the parent enzyme and can be further stimulated by TF. Insights into the mechanism behind the increased activity of the mutants, presumably also pertinent to the TF-induced, allosteric stimulation of FVIIa activity, were obtained by studying their calcium dependence and the accessibility of the N terminus of the protease domain to chemical modification. The FVIIa analogues promise to offer a more efficacious treatment of bleeding episodes especially in hemophiliacs with inhibitory antibodies precluding conventional replacement therapy.

A novel B-domain O-glycoPEGylated FVIII (N8-GP) demonstrates full efficacy and prolonged effect in hemophilic mice models

Frequent infusions of intravenous factor VIII (FVIII) are required to prevent bleeding associated with hemophilia A. To reduce the treatment burden, recombinant FVIII with a longer half-life was developed without changing the protein structure. FVIII-polyethylene glycol (PEG) conjugates were prepared using an enzymatic process coupling PEG (ranging from 10 to 80 kDa) selectively to a unique O-linked glycan in the FVIII B-domain. Binding to von Willebrand factor (VWF) was maintained for all conjugates. Upon cleavage by thrombin, the B-domain and the associated PEG were released, generating activated FVIII (FVIIIa) with the same primary structure and specific activity as native FVIIIa. In both FVIII- and VWF-deficient mice, the half-life was found to increase with the size of PEG. In vivo potency and efficacy of FVIII conjugated with a 40-kDa PEG (N8-GP) and unmodified FVIII were not different. N8-GP had a longer duration of effect in FVIII-deficient mouse models, approximately a twofold prolonged half-life in mice, rabbits, and cynomolgus monkeys; however, the prolongation was less pronounced in rats. Binding capacity of N8-GP on human monocyte-derived dendritic cells was reduced compared with unmodified FVIII, resulting in several-fold reduced cellular uptake. In conclusion, N8-GP has the potential to offer efficacious prevention and treatment of bleeds in hemophilia A at reduced dosing frequency.

Plasma Lipoproteins Enhance Tissue Factor–Independent Factor VII Activation

The effect of plasma lipoprotein fractions (large very-low-density lipoprotein, small very-low-density lipoprotein, intermediate-density lipoprotein, and low-density lipoprotein) on initiation of blood coagulation by supporting factor VII activation or by stimulating monocytes to express tissue factor was investigated in vitro. Endotoxin-free preparations of lipoprotein fractions did not induce functional tissue factor in monocytes, whereas all lipoprotein fractions enhanced tissue factor-independent activation of factor VII by factor Xa and by factors Xa/Va. In contrast, no or only slight enhancement of factor IXa-, factor IXa/VIIIa-, factor XIa-, or thrombin-mediated factor VII activation was observed. The effect of small very-low-density lipoprotein was less than that of large very-low-density lipoprotein, and intermediate-density and low-density lipoproteins caused an even lower but still significant increase of factor Xa- and factor Xa/Va-mediated factor VII activation. When the data were normalized for apolipoprotein B-100 content, differences remained between lipoprotein fractions. In contrast, when phospholipid content was used for normalization, differences between lipoprotein fractions in factor Xa- and factor Xa/Va-mediated factor VII activation disappeared, indicating that phospholipids were involved in factor VII activation. This was supported by enhancement of factor Xa-mediated factor VII activation by synthetic phospholipid vesicles containing negatively charged phospholipids.

High-dose erythropoietin alters platelet reactivity and bleeding time in rodents in contrast to the neuroprotective variant carbamyl-erythropoietin (CEPO)

The haematopoietic hormone erythropoietin (EPO) has neuroprotective properties and is currently being explored for treatment of stroke and other neurological disorders. Short-term, high-dose treatment with EPO seems to improve neurological function of stroke patients but may be associated with increased thrombotic risk, whereas alternative non-erythropoietic neuroprotective derivatives of EPO, such as carbamylated EPO (CEPO), may be devoid of such side-effects. We investigated the effects of short-term, high-dose treatment with EPO and CEPO on platelet function and haemostasis in healthy mice and rats. Animals received three daily doses of EPO or CEPO (50 microg/kg), and blood was compared with respect to alterations in haematology and platelet reactivity. In rats, treatment with EPO increased the haematocrit to >50% and the mean platelet volume by 37%, while CEPO had no effect on these parameters. Platelets from EPO-treated rats showed an increased sensitivity to thrombin receptor agonist peptides and elevated plasma levels of soluble P-selectin (sP-selectin) were found in treated mice. Further indicators of platelet hyperreactivity in EPO, but not CEPO-treated animals, were significantly increased aggregatory responses to collagen in whole blood and platelet-rich plasma (PRP). The increased platelet reactivity was paralleled by a decreased bleeding time after tail transection in rats. Samples from EPO-treated rats showed an attenuated response to ADP in whole blood aggregometry and thrombelastography (TEG) platelet mapping but not in apyrase-treated PRP, suggesting involvement of ADP receptor desensitization. These findings suggest that while EPO affects various aspects of platelet function, CEPO is devoid of such effects.

Functional characteristics of N8, a new recombinant FVIII

Summary.  The aim of this study was to evaluate the in vitro function of the new recombinant factor VIII (FVIII) compound, N8. The specific activity of N8 as measured in a FVIII:C one‐stage clot assay was 9300 ± 400 IU mg−1 based on the analysis of seven individual batches. The ratio between the FVIII:C activity measured in clot and chromogenic assays was 1.00 (95% confidence interval 0.97–1.03). N8 bound to von Willebrand factor with Kd values of 0.2 nm when measured by ELISA and by surface plasmon resonance. FVIIIa cofactor activity was determined from the kinetic parameters of factor IXa‐catalysed factor X (FX) activation. The rate of activation of N8 by thrombin as well as Km and kcat for FX activation was in the same range as those observed for Advate®. The rate of activated protein C (APC)‐catalysed inactivation was similar for activated N8 and Advate®. N8 improved thrombin generation in a dose‐dependent manner and induced similar rates of thrombin generation as Advate® and the plasma‐derived FVIII product Haemate®. Using thromboelastography (TEG®), N8 was shown to improve the clot formation and clot stability in whole blood from haemophilia A patients. Comparable potency and efficacy of N8 and Advate® was found based on TEG® parameters. Finally, similar binding profiles to immobilized lipoprotein receptor‐related protein (LRP) of N8 and Advate® were observed. The study demonstrated that N8 is fully functional in a variety of assays measuring FVIII activity. No functional differences were found between N8 and comparator compounds.

Impairment of the hemostatic potential of platelets during storage as evaluated by flow cytometry, thrombin generation, and thrombelastography under conditions promoting formation of coated platelets

BACKGROUND: The increasing demand for platelet (PLT) transfusions has focused attention on appropriate use. Coated PLTs are a subpopulation of highly procoagulant PLTs formed by simultaneous stimulation by the agonists collagen and thrombin hypothesized to drive clot formation at the site of vascular injury. Prolonged storage of PLTs may reduce their ability to support optimal hemostasis upon transfusion.

Factor VIII C1-domain spikes 2092-2093 and 2158-2159 comprise regions that modulate cofactor function and cellular uptake

Background: Antibody KM33 blocks factor VIII (FVIII) endocytosis and phospholipid binding. Results: Hydrogen-deuterium exchange mass spectrometry reveals that KM33 binds C1 domain spikes 2092–2093 and 2158–2159. Glycosylated FVIII-R2159N shows reduced endocytosis and decreased binding to phospholipid membranes with low phosphatidylserine content. Conclusion: Spikes 2092–2093 and 2158–2159 modulate FVIII endocytosis and phospholipid binding. Significance: Novel insight is obtained about the role of the C1 domain for FVIII biology. The C1 domain of factor VIII (FVIII) has been implicated in binding to multiple constituents, including phospholipids, von Willebrand factor, and low-density lipoprotein receptor-related protein (LRP). We have previously described a human monoclonal antibody called KM33 that blocks these interactions as well as cellular uptake by LRP-expressing cells. To unambiguously identify the apparent “hot spot” on FVIII to which this antibody binds, we have employed hydrogen-deuterium exchange mass spectrometry. The results showed that KM33 protects FVIII regions 2091–2104 and 2157–2162 from hydrogen-deuterium exchange. These comprise the two C1 domain spikes 2092–2093 and 2158–2159. Spike 2092–2093 has been demonstrated recently to contribute to assembly with lipid membranes with low phosphatidylserine (PS) content. Therefore, spike 2158–2159 might serve a similar role. This was assessed by replacement of Arg-2159 for Asn, which introduces a motif for N-linked glycosylation. Binding studies revealed that the purified, glycosylated R2159N variant had lost its interaction with antibody KM33 but retained substantial binding to von Willebrand factor and LRP. Cellular uptake of the R2159N variant was reduced both by LRP-expressing U87-MG cells and by human monocyte-derived dendritic cells. FVIII activity was virtually normal on membranes containing 15% PS but reduced at low PS content. These findings suggest that the C1 domain spikes 2092–2093 and 2158–2159 together modulate FVIII membrane assembly by a subtle, PS-dependent mechanism. These findings contribute evidence in favor of an increasingly important role of the C1 domain in FVIII biology.

Pharmacokinetics and pharmacodynamics of a new recombinant FVIII (N8) in haemophilia A mice

Summary.  N8 is a new recombinant factor VIII (rFVIII) compound produced and formulated without human‐ or animal‐derived protein. The aims of the present studies were to evaluate the pharmacokinetics and pharmacodynamics properties of N8 and to compare with a commercially available rFVIII product (Advate®) in haemophilia A mice. The pharmacokinetics were evaluated after single i.v. administration of 80, 120 and 280 IU kg−1 of N8 and Advate® and measurements of FVIII blood concentrations as a function of time. The efficacy and dose response curves of N8 and Advate® (1–200 IU kg−1) were evaluated in a tail bleeding model. Furthermore, the effects in a newly developed haemophilia knee joint haemarthrosis model were investigated. No significant differences were found in the pharmacokinetic parameters between N8 and Advate®. The clearances were 11 ± 1 vs. 10 ± 2 mL h−1 kg−1 (P = 0.14) and the half‐lives 7.2 ± 0.9 vs. 7.7 ± 1.4 h (P = 0.31) after administration of N8 and Advate® respectively. Dose‐independent pharmacokinetics was shown, and comparable efficacy and potency were shown between N8 and Advate® in the tail bleeding model. Both compounds normalized the bleeding at the dose of 200 IU kg−1, and for blood loss ED50 values of 27 IU kg−1 (N8) and 28 IU/kg (Advate®) were found (P = 0.97). In the haemarthrosis model, treatment with N8 and Advate® at 200 IU kg−1 reduced the mean increase in the joint diameter significantly from 1.23 ± 0.19 to 0.32 ± 0.08 mm (P < 0.01) and 0.25 ± 0.08 mm (P < 0.001) respectively. Pharmacokinetics and pharmacodynamics of N8 and Advate® were comparable after i.v. administration to haemophilia A mice.