Phospholipid binding of factor VIII in different therapeutic concentrates

Kinetics and regulation of coagulation factor X activation by intrinsic tenase on phospholipid membranes

BACKGROUND

Factor X activation by the phospholipid-bound intrinsic tenase complex is a critical membrane-dependent reaction of blood coagulation. Its regulation mechanisms are unclear, and a number of questions regarding diffusional limitation, pathways of assembly and substrate delivery remain open.

METHODS

We develop and analyze here a detailed mechanism-driven computer model of intrinsic tenase on phospholipid surfaces. Three-dimensional reaction-diffusion-advection and stochastic simulations were used where appropriate.

RESULTS

Dynamics of the system was predominantly non-stationary under physiological conditions. In order to describe experimental data, we had to assume both membrane-dependent and solution-dependent delivery of the substrate. The former pathway dominated at low cofactor concentration, while the latter became important at low phospholipid concentration. Factor VIIIa-factor X complex formation was the major pathway of the complex assembly, and the model predicted high affinity for their lipid-dependent interaction. Although the model predicted formation of the diffusion-limited layer of substrate for some conditions, the effects of this limitation on the fXa production were small. Flow accelerated fXa production in a flow reactor model by bringing in fIXa and fVIIIa rather than fX.

CONCLUSIONS

This analysis suggests a concept of intrinsic tenase that is non-stationary, employs several pathways of substrate delivery depending on the conditions, and is not particularly limited by diffusion of the substrate.

Alloantibodies to therapeutic factor VIII in hemophilia A: the role of von Willebrand factor in regulating factor VIII immunogenicity

The rising incidence of neutralizing antibodies (inhibitors) against therapeutic factor VIII prompted the conduct of studies to answer the question as to whether this rise is related to the introduction of recombinant factor VIII products. The present article summarizes current opinions and results of non-clinical and clinical studies on the immunogenic potential of recombinant compared to plasma-derived factor VIII concentrates. Numerous studies provided circumstantial evidence that von Willebrand factor, the natural chaperone protein present in plasma-derived factor VIII products, plays an important role in protecting exogenous factor VIII from uptake by antigen presenting cells and from recognition by immune effectors. However, the definite contribution of von Willebrand factor in reducing the inhibitor risk and in the achievement of immune tolerance is still under debate.

A solvent/detergent-treated and 15-nm filtered factor VIII: a new safety standard for plasma-derived coagulation factor concentrates

BACKGROUND

Since the early 1990 s the Committee for Proprietary Medicinal Products has set the mandatory requirement that all manufacturing processes for blood products include two virus removal/inactivation steps that are complementary in their action.

OBJECTIVES

The objective was to develop a manufacturing process for factor VIII (FVIII) including two complementary steps of viral inactivation/elimination.

METHODS

A 35-15 nm nanofiltration step was added to a former FVIII manufacturing process that included solvent/detergent (S/D) treatment to generate a new FVIII concentrate called Factane. The impact of nanofiltration on the structural and functional characteristics of FVIII, as well as virus/transmissible spongiform encephalopathy reduction factors were assessed.

RESULTS

Using an innovative approach, FVIII was successfully nanofiltered at 35-15 nm, while the biological properties of the active substance were unmodified. FVIII coagulant and antigen content for Factane and previous S/D-treated FVIII (FVIII-LFB, commercialized as Facteur VIII-LFB) were comparable. The FVIII one-stage chromogenic and coagulant/antigen ratios confirmed that nanofiltered FVIII was not activated. After nanofiltration, the copurified von Willebrand factor (vWF) was reduced but vWF/FVIII binding properties were unaffected. Phospholipid binding and thrombin proteolysis studies displayed no differences between Factane and FVIII-LFB. The rate of factor Xa generation was slightly lower for Factane when compared to FVIII-LFB. Viral validation studies with different viruses showed no detectable virus in the filtrate.

CONCLUSIONS

Nanofiltration of FVIII at 15 nm is feasible despite the large molecular weight of FVIII and vWF. Nanofiltration has been proven to be highly effective at removing infectious agents while preserving the structural and functional integrity of FVIII.

Influence of the type of factor VIII concentrate on the incidence of factor VIII inhibitors in previously untreated patients with severe hemophilia A

Inhibitor development is the major treatment complication in children with severe hemophilia A. It is not clear whether the risk of inhibitors is higher with recombinant factor VIII or with plasma-derived factor VIII. We used multivariate analysis to compare 2 cohorts of previously untreated patients (PUPs) with severe hemophilia A: 62 patients treated with the same brand of high-purity plasma-derived FVIII (pFVIII) containing von Willebrand factor (VWF) and 86 patients treated with full-length recombinant FVIII (rFVIII). In addition to the usual end points (all inhibitors, high inhibitors), we also examined a third end point (high inhibitors and/or immune tolerance induction). The risk of inhibitor development was higher in patients treated with rFVIII than in patients treated with pFVIII, regardless of other risk factors (F8 genotype; nonwhite origin; history of inhibitors in patients with a family history of hemophilia; age at first FVIII infusion). The adjusted relative risk (RRa) for inhibitor development with rFVIII versus pFVIII was 2.4 (all inhibitors), 2.6 (high inhibitors), and 3.2 (high inhibitors and/or immune tolerance induction), respectively, depending on the end point (above). The pathophysiology of this large effect must be understood in order to improve the characteristics of recombinant products and to reduce the incidence of inhibitors to FVIII.

Mutation of the surface-exposed amino acid Trp to Ala in the FVIII C2 domain results in defective secretion of the otherwise functional protein

The C2 domain of factor VIII (FVIII) is important for FVIII-phospholipid (PL) and FVIII-von Willebrand factor (VWF) interactions. A FVIII structural model, derived by electron crystallography, suggests four hydrophobic loops at the FVIII C2 domain-PL interface. Within loop four, the solvent-exposed amino acid, Trp(2313), is believed to contribute to FVIII-PL binding. To analyse this interaction, the amino-acid exchange Trp(2313) to Ala (W2313A) was introduced into the C2 domain of B-domain-deleted FVIII (dBFVIII). Both proteins, dBFVIII and W2313A, were expressed in a mammalian expression system. Labelling experiments showed that the mutation W2313A resulted in reduced secretion but did not affect intracellular synthesis of the protein. Specific activity, kinetic parameters, binding to VWF and haemostatic potential in a murine model of haemophilia A were found to be similar for both proteins. Binding studies to synthetic 4% phosphatidyl-l-serine vesicles showed, however, a 28-fold higher K(D) for W2313A, indicating the important role of Trp(2313) in the FVIII-PL interaction. In conclusion, the C2-domain-surface-exposed residue Trp(2313), is critical for secretion of the protein. The W2313A mutation weakens binding to phosphatidyl-l-serine vesicles but the mutant protein has the same effector function as dBFVIII in vitro and in vivo.

Molecular defects in coagulation Factor VIII and their impact on Factor VIII function

Molecular defects in Factor VIII (FVIII), such as haemophilia A-related mutations or denaturative conformational changes, may affect the stability of FVIII as well as its interactions with physiological activators, von Willebrand Factor, phospholipid, or conformationally sensitive antibodies. We summarize the contemporary assays which allow identification of impaired functional interactions of FVIII that cause a reduction or loss of its cofactor activity and/or increased immunogenicity. These assays can potentially be used for detection of molecular defects in FVIII and elucidation of the function impaired by these defects.

Comparison of the properties of phospholipid surfaces formed on HPA and L1 biosensor chips for the binding of the coagulation factor VIII

Binding of a coagulation factor VIII to phosphatidylserine-containing membranes is critical for exerting its cofactor activity. The use of surface plasmon resonance allows studying factor VIII interaction with immobilized phospholipids. In the present study we compared factor VIII-binding properties of phospholipid surfaces immobilized on L1 and HPA Biacore chips in the form of a flexible bilayer and rigid monolayer, respectively. We demonstrated that immobilized phospholipid surfaces with physiological contents of PS and PE formed on L1 but not on HPA chip closely mimic intact phospholipid vesicles in their factor VIII and thrombin-activated factor VIII (factor VIIIa) binding properties.

Quantification of plasma-derived blood coagulation factor VIII by real-time biosensor measurements

Plasma-derived blood coagulation factor VIII was analyzed in real time using biosensor technology. Monoclonal antibodies directed against the heavy and against the light chain of factor VIII were immobilized on different carboxymethyl dextran surfaces. Different factor VIII concentrations were injected over the antibody surfaces in parallel and response levels were determined from the dissociation phase at a fixed time after sample injection. Serial dilutions of plasma-derived factor VIII with known concentrations determined by a commercial FVIIIC:Ag ELISA were used as standards. A quantification limit of 0.9 I.U./ml with antibody 530p and 1.5 I.U./ml with antibody 531p was calculated. Intra-assay precision expressed as percent coefficient of variation was below 10% for concentrations above 0.6 I.U./ml. Inter-assay precision for antibody 530p was below 20% for concentrations higher than 0.6 I.U./ml. For 531p, inter-assay precision was below 10% for concentrations higher than 2 I.U./ml. A sensor chip lifetime in respect to regeneration of at least 100 cycles for both antibodies was found. The small sample requirement of 35 microl allows fast analysis of different FVIII products and the use of two monoclonal antibodies directed against two different FVIII domains provides additional information about the integrity of the FVIII molecule.

Use of optical biosensors for the study of mechanistically concerted surface adsorption processes

The advent of commercial optical biosensors, such as the BIAcore from Pharmacia and IAsys from Affinity Sensors, has made available to the biochemist a powerful means to examine and characterize the interaction of biological macromolecules with a binding surface. By analysis of the kinetic and equilibrium aspects of the observed experimental adsorption isotherms, rate and affinity constants can be determined. This Review focuses on pertinent aspects of the technology and its use for the performance and quantitative characterization of some various types of mechanistically concerted adsorption behavior.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Variation in inhibitor reactivity in acquired haemophilia A with different concentrates

Acquired haemophilia A due to the development of auto-antibodies directed against factor VIII (FVIII) is a rare disorder that may result in serious haemorrhagic episodes. Although in many cases no associated underlying disorders are apparent, the condition has been reported in association with autoimmune disorders, haematological malignancies, treatment with certain drugs and pregnancy. The reaction kinetics of auto-antibodies to FVIII differ from those observed with allo-antibodies in congenital haemophilia. Previous studies in congenital haemophilia have raised the possibility that inhibitory antibodies vary in their reactivity with the factor VIII molecules in different concentrates used for treatment. However, the interaction of FVIII in concentrates and inhibitors in acquired haemophilia has never been previously studied. In this study, the effect of different FVIII concentrates was studied on neutralization in vitro by performing inhibitor titres using the New Oxford inhibitor assay method. The inhibitor titre in eight patients with acquired haemophilia A was assayed against five commercially available FVIII concentrates of varying purity. The intermediate purity concentrate 8Y and the high purity concentrate that contains normal amounts of von Willebrand's Factor (vWF) (Alphanate) gave lower titres than the high purity concentrates with low (Monoclate-P) or no (Kogenate) von Willebrand content. All but one antibody had very low reactivity with porcine FVIII. Further work will be required to establish whether concentrates manifesting a low level of in vitro reactivity with the inhibitor have a better haemostatic effect in vivo.