Factor VIII binds to von Willebrand factor via its Mr-80,000 light chain

Pharmacokinetic evaluation of efanesoctocog alfa: breakthrough factor VIII therapy for hemophilia A

INTRODUCTION

Blood coagulation factor (F)VIII functions as a cofactor in the tenase complex responsible for phospholipid-dependent FIXa-mediated activation of FX in plasma. Congenital defect of FVIII causes severe bleeding disorder, hemophilia (H) A. Intravenous FVIII replacement therapy is the gold standard therapy in patients with HA (PwHA) but requirement for frequent dosing of FVIII owing to pharmacokinetics burdens PwHA a lot. Efanesoctocog alfa is a new class of recombinant FVIII and has the ability to overcome conceivable unmet needs in treatment for PwHA.

AREAS COVERED

Efanesoctocog alfa is a B domain-deleted single-chain fusion FVIII connected to the Fc-region of human immunoglobulin G1, D'D3-fragment of von Willebrand factor (VWF), and unstructured hydrophilic recombinant polypeptides (XTEN). Owing to its novel design, it can function independently of endogenous VWF and elicits 2 to 4 times longer half-life compared to other existing FVIII products. The prolonged half-life contributes to maintain high level of FVIII activity for most of the week and has led to excellent hemostatic effect by once-weekly administration in phase 3 clinical trials.

EXPERT OPINION

Efanesoctocog alfa with outstanding pharmacological properties, well tolerated in the clinical trials, is a promising FVIII therapy for PwHA. Future studies should include long-term safety, especially in previously untreated patients.

Molecular determinants of the factor VIII/von Willebrand factor complex revealed by BIVV001 cryo-electron microscopy

Interaction of factor VIII (FVIII) with von Willebrand factor (VWF) is mediated by the VWF D'D3 domains and thrombin-mediated release is essential for hemostasis after vascular injury. VWF-D'D3 mutations resulting in loss of FVIII binding are the underlying cause of von Willebrand disease (VWD) type 2N. Furthermore, the FVIII-VWF interaction has significant implications for the development of therapeutics for bleeding disorders, particularly hemophilia A, in which endogenous VWF clearance imposes a half-life ceiling on replacement FVIII therapy. To understand the structural basis of FVIII engagement by VWF, we solved the structure of BIVV001 by cryo-electron microscopy to 2.9 Å resolution. BIVV001 is a bioengineered clinical-stage FVIII molecule for the treatment of hemophilia A. In BIVV001, VWF-D'D3 is covalently linked to an Fc domain of a B domain-deleted recombinant FVIII (rFVIII) Fc fusion protein, resulting in a stabilized rFVIII/VWF-D'D3 complex. Our rFVIII/VWF structure resolves BIVV001 architecture and provides a detailed spatial understanding of previous biochemical and clinical observations related to FVIII-VWF engagement. Notably, the FVIII acidic a3 peptide region (FVIII-a3), established as a critical determinant of FVIII/VWF complex formation, inserts into a basic groove formed at the VWF-D'/rFVIII interface. Our structure shows direct interaction of sulfated Y1680 in FVIII-a3 and VWF-R816 that, when mutated, leads to severe hemophilia A or VWD type 2N, respectively. These results provide insight on this key coagulation complex, explain the structural basis of many hemophilia A and VWD type 2N mutations, and inform studies to further elucidate how VWF dissociates rapidly from FVIII upon activation.

D' domain region Arg782-Cys799 of von Willebrand factor contributes to factor VIII binding

In the complex with von Willebrand factor (VWF) factor VIII (FVIII) is protected from rapid clearance from circulation. Although it has been established that the FVIII binding site resides in the N-terminal D'-D3 domains of VWF, detailed information about the amino acid regions that contribute to FVIII binding is still lacking. In the present study, hydrogen-deuterium exchange mass spectrometry was employed to gain insight into the FVIII binding region on VWF. To this end, time-dependent deuterium incorporation was assessed in D'-D3 and the FVIII-D'-D3 complex. Data showed reduced deuterium incorporation in the D' region Arg782-Cys799 in the FVIII-D'-D3 complex compared to D'-D3. This implies that this region interacts with FVIII. Site-directed mutagenesis of the six charged amino acids in Arg782-Cys799 into alanine residues followed by surface plasmon resonance analysis and solid phase binding studies revealed that replacement of Asp796 affected FVIII binding. A marked decrease in FVIII binding was observed for the D'-D3 Glu787Ala variant. The same was observed for D'-D3 variants in which Asp796 and Glu787 were replaced by Asn796 and Gln787. Site-directed mutagenesis of Leu786, which together with Glu787 and Cys789 forms a short helical region in the crystal structure of D'-D3, also had a marked impact on FVIII binding. The combined results show that the amino acid region Arg782-Cys799 is part of a FVIII binding surface. Our study provides new insight into FVIII-VWF complex formation and defects therein that may be associated with bleeding caused by markedly reduced levels of FVIII.

Interaction Between the a3 Region of Factor VIII and the TIL'E' Domains of the von Willebrand Factor

The von Willebrand factor (VWF) and coagulation factor VIII (FVIII) are intricately involved in hemostasis. A tight, noncovalent complex between VWF and FVIII prolongs the half-life of FVIII in plasma, and failure to form this complex leads to rapid clearance of FVIII and bleeding diatheses such as hemophilia A and von Willebrand disease (VWD) type 2N. High-resolution insight into the complex between VWF and FVIII has so far been strikingly lacking. This is particularly the case for the flexible a3 region of FVIII, which is imperative for high-affinity binding. Here, a structural and biophysical characterization of the interaction between VWF and FVIII is presented with focus on two of the domains that have been proven pivotal for mediating the interaction, namely the a3 region of FVIII and the TIL'E' domains of VWF. Binding between the FVIII a3 region and VWF TIL'E' was here observed using NMR spectroscopy, where chemical shift changes were localized to two β-sheet regions on the edge of TIL'E' upon FVIII a3 region binding. Isothermal titration calorimetry and NMR spectroscopy were used to characterize the interaction between FVIII and TIL'E' as well as mutants of TIL'E', which further highlights the importance of the β-sheet region of TIL'E' for high-affinity binding. Overall, the results presented provide new insight into the role the FVIII a3 region plays for complex formation between VWF and FVIII and the β-sheet region of TIL'E' is shown to be important for FVIII binding. Thus, the results pave the way for further high-resolution insights into this imperative complex.

Binding of Factor VIII to Lipid Nanodiscs Increases its Clotting Function in a Mouse Model of Hemophilia A

Background

Hemophilia A is a congenital bleeding disorder caused by defective or deficient factor VIII (FVIII). The active form of FVIII is the co-factor for the serine protease factor IXa (FIXa) in the membrane-bound intrinsic tenase (FVIIIa-FIXa) complex. The assembly of the FVIIIa-FIXa complex on the activated platelet surface is critical for successful blood clotting.

Objectives

To characterize the role of lipid nanodiscs (ND) for on FVIII function in vivo and test the lipid ND as a delivery system for FVIII. To evaluate the potential of binding recombinant FVIII to ND as improved treatment for Hemophilia A.

Methods

Recombinant porcine FVIII (rpFVIII) was expressed and characterized in solution, and when bound to ND. The rpFVIII, ND and rpFVIII-ND complexes were characterized via transmission electron microscopy. Functional studies were carried out using aPTT tests and time resolved tail snip studies of hemophilic mice.

Results

Functional rpFVIII was successfully assembled on lipid ND. When injected in hemophilic mice, the rpFVIII-ND complexes showed a pronounced pro-coagulant effect, which was stronger than that of rpFVIII alone. While injection of the ND alone showed a pro-coagulant effect this effect was not additive, implying that the rpFVIII-ND complexes have a synergistic effect on the clotting process in hemophilic mice.

Conclusions

Binding of rpFVIII to ND prior to its injection in hemophilic mice significantly improves the therapeutic function of the protein. This represents a meaningful step towards a new approach to modulate blood coagulation at the membrane-bound FVIII level and the assembly of the intrinsic tenase complex.

Mapping the interaction between factor VIII and von Willebrand factor by electron microscopy and mass spectrometry

Association with the D'D3 domain of von Willebrand factor (VWF) stabilizes factor VIII (FVIII) in the circulation and maintains it at a level sufficient to prevent spontaneous bleeding. We used negative-stain electron microscopy (EM) to visualize complexes of FVIII with dimeric and monomeric forms of the D'D3 domain. The EM averages show that FVIII interacts with the D'D3 domain primarily through its C1 domain, with the C2 domain providing a secondary attachment site. Hydrogen-deuterium exchange mass spectrometry corroborated the importance of the C1 domain in D'D3 binding and implicates additional surface regions on FVIII in the interaction. Together, our results establish that the C1 domain is the major binding site on FVIII for VWF, reiterate the importance of the a3 acidic peptide in VWF binding, and suggest that the A3 and C2 domains play ancillary roles in this interaction.

Visualization of a protein-protein interaction at a single-molecule level by atomic force microscopy

Atomic force microscopy is unmatched in terms of high-resolution imaging under ambient conditions. Over the years, substantial progress has been made using this technique to improve our understanding of biological systems on the nanometer scale, such as visualization of single biomolecules. For monitoring also the interaction between biomolecules, in situ high-speed imaging is making enormous progress. Here, we describe an alternative ex situ imaging method where identical molecules are recorded before and after reaction with a binding partner. Relocation of the identical molecules on the mica surface was thereby achieved by using a nanoscale scratch as marker. The method was successfully applied to study the complex formation between von Willebrand factor (VWF) and factor VIII (FVIII), two essential haemostatic components of human blood. FVIII binding was discernible by an appearance of globular domains appended to the N-terminal large globular domains of VWF. The specificity of the approach could be demonstrated by incubating VWF with FVIII in the presence of a high salt buffer which inhibits the interaction between these two proteins. The results obtained indicate that proteins can maintain their reactivity for subsequent interactions with other molecules when gently immobilized on a solid substrate and subjected to intermittent drying steps. The technique described opens up a new analytical perspective for studying protein-protein interactions as it circumvents some of the obstacles encountered by in situ imaging and other ex situ techniques.

Factor VIII inhibitors: von Willebrand factor makes a difference in vitro and in vivo

BACKGROUND

The important association between von Willebrand factor (VWF) and factor VIII (FVIII) has been investigated for decades, but the effect of VWF on the reactivity of FVIII inhibitory antibodies, referred to as inhibitors, is still controversial.

OBJECTIVE

To investigate the interaction among VWF, FVIII and FVIII inhibitory antibodies.

METHODS

Three sources of inhibitors were used for in vitro studies, including the plasma from immunized VWF(null) FVIII(null) mice, purified plasma IgG from human inhibitor patients, or human monoclonal antibody from inhibitor patients' B-cell clones. Inhibitors were incubated with recombinant human FVIII (rhFVIII) either with or without VWF. The remaining FVIII activity was determined by chromogenic assay and inhibitor titers were determined. For in vivo studies, inhibitors and rhFVIII were infused into FVIII(null) or VWF(null) FVIII(null) mice followed by a tail clip survival test.

RESULTS

VWF has a dose-dependent protective effect on FVIII, limiting inhibitor inactivation of FVIII in both mouse and human samples. A preformed complex of VWF with FVIII provides more effective protection from inhibitors than competitive binding of antibodies and VWF to FVIII. The protective effect of VWF against FVIII inactivation by inhibitors was further confirmed in vivo by infusing inhibitors and FVIII into FVIII(null) or VWF(null) FVIII(null) mice followed by a tail clip survival test.

CONCLUSION

Our results demonstrate that VWF exerts a protective effect, reducing inhibitor inactivation of FVIII, both in vitro and in vivo.

Influence of single nucleotide polymorphisms in factor VIII and von Willebrand factor genes on plasma factor VIII activity: the ARIC Study

Factor VIII (FVIII) functions as a cofactor for factor IXa in the contact coagulation pathway and circulates in a protective complex with von Willebrand factor (VWF). Plasma FVIII activity is strongly influenced by environmental and genetic factors through VWF-dependent and -independent mechanisms. Single nucleotide polymorphisms (SNPs) of the coding and promoter sequence in the FVIII gene have been extensively studied for effects on FVIII synthesis, secretion, and activity, but impacts of non-disease-causing intronic SNPs remain largely unknown. We analyzed FVIII SNPs and FVIII activity in 10,434 healthy Americans of European (EA) or African (AA) descent in the Atherosclerosis Risk in Communities (ARIC) study. Among covariates, age, race, diabetes, and ABO contributed 2.2%, 3.5%, 4%, and 10.7% to FVIII intersubject variation, respectively. Four intronic FVIII SNPs associated with FVIII activity and 8 with FVIII-VWF ratio in a sex- and race-dependent manner. The FVIII haplotypes AT and GCTTTT also associated with FVIII activity. Seven VWF SNPs were associated with FVIII activity in EA subjects, but no FVIII SNPs were associated with VWF Ag. These data demonstrate that intronic SNPs could directly or indirectly influence intersubject variation of FVIII activity. Further investigation may reveal novel mechanisms of regulating FVIII expression and activity.

Unraveling the scissile bond: how ADAMTS13 recognizes and cleaves von Willebrand factor

von Willebrand factor (VWF) is a large adhesive glycoprotein with established functions in hemostasis. It serves as a carrier for factor VIII and acts as a vascular damage sensor by attracting platelets to sites of vessel injury. VWF size is important for this latter function, with larger multimers being more hemostatically active. Functional imbalance in multimer size can variously cause microvascular thrombosis or bleeding. The regulation of VWF multimeric size and platelet-tethering function is carried out by ADAMTS13, a plasma metalloprotease that is constitutively active. Unusually, protease activity of ADAMTS13 is controlled not by natural inhibitors but by conformational changes in its substrate, which are induced when VWF is subject to elevated rheologic shear forces. This transforms VWF from a globular to an elongated protein. This conformational transformation unfolds the VWF A2 domain and reveals cryptic exosites as well as the scissile bond. To enable VWF proteolysis, ADAMTS13 makes multiple interactions that bring the protease to the substrate and position it to engage with the cleavage site as this becomes exposed by shear. This article reviews recent literature on the interaction between these 2 multidomain proteins and provides a summary model to explain proteolytic regulation of VWF by ADAMTS13.

Efficacy and safety of long-term prophylaxis in severe hemophilia A dogs following liver gene therapy using AAV vectors

Developing adeno-associated viral (AAV)-mediated gene therapy for hemophilia A (HA) has been challenging due to the large size of the factor VIII (FVIII) complementary DNA and the concern for the development of inhibitory antibodies to FVIII in HA patients. Here, we perform a systematic study in HA dogs by delivering a canine FVIII (cFVIII) transgene either as a single chain or two chains in an AAV vector. An optimized cFVIII single chain delivered using AAV serotype 8 (AAV8) by peripheral vein injection resulted in a dose-response with sustained expression of FVIII up to 7% (n = 4). Five HA dogs administered two-chain delivery using either AAV8 or AAV9 via the portal vein expressed long-term, vector dose-dependent levels of FVIII activity (up to 10%). In the two-chain approach, circulating cFVIII antigen levels were more than fivefold higher than activity. Notably, no long-term immune response to FVIII was observed in any of the dogs (1/9 dogs had a transient inhibitor). Long-term follow-up of the dogs showed a remarkable reduction (>90%) of bleeding episodes in a combined total of 24 years of observation. These data demonstrate that both approaches are safe and achieve dose-dependent therapeutic levels of FVIII expression, which supports translational studies of AAV-mediated delivery for HA.

Phosphatidylinositol containing lipidic particles reduces immunogenicity and catabolism of factor VIII in hemophilia a mice

Factor VIII (FVIII) is an important cofactor in blood coagulation cascade. It is a multidomain protein that consists of six domains, NH2-A1-A2-B-A3-C1-C2-COOH. The deficiency or dysfunction of FVIII causes hemophilia A, a life-threatening bleeding disorder. Replacement therapy using recombinant FVIII (rFVIII) is the first line of therapy, but a major clinical complication is the development of inhibitory antibodies that abrogate the pharmacological activity of the administered protein. FVIII binds to anionic phospholipids (PL), such as phosphatidylinositol (PI), via lipid binding region within the C2 domain of FVIII. This lipid binding site not only consists of immunodominant epitopes but is also involved in von Willebrand factor binding that protects FVIII from degradation in vivo. Thus, we hypothesize that FVIII-PL complex will influence immunogenicity and catabolism of FVIII. The biophysical studies showed that PI binding did not alter conformation of the protein but improved intrinsic stability as measured by thermal denaturation studies. ELISA studies confirmed the involvement of the C2 domain in binding to PI containing lipid particles. PI binding prolonged the in vivo circulation time and reduced catabolism of FVIII in hemophilia A mice. FVIII-PI complex reduced inhibitor development in hemophilia A mice following intravenous and subcutaneous administration. The data suggest that PI binding reduces catabolism and immunogenicity of FVIII and has potential to be a useful therapeutic approach for hemophilia A.

The "normal" factor VIII concentration in plasma

INTRODUCTION

The quantitation of factor (F)VIII by activity-based assays is influenced by the method, procedure, the quality and properties of reagents used and concentrations of other plasma proteins, including von Willebrand factor (VWF).

OBJECTIVE

To compare FVIII concentrations measured by activity-based assays with those obtained by an immunoassay and to establish the influence of plasma dilution on the FVIII clotting activity (FVIIIc).

METHODS

The APTT, a chromogenic assay (Coatest) and two in-house immunoassays were used. Albumin-free recombinant FVIII was used as the calibrator in all assays.

RESULTS

For a group of 44 healthy individuals (HI), the mean value observed for FVIII antigen (FVIIIag; 1.22+/-0.56 nM; S.D.) is substantially higher than that for FVIIIc (0.65+/-0.29 nM) and the chromogenic assay (FVIIIch; 0.50+/-0.23 nM). A positive correlation between FVIIIag and VWFag with R(2)=0.20 was observed. Since plasma VWF has an inhibitory effect on FVIIIc, we evaluated the influence of plasma dilutions on FVIIIc in HI (n=105). At a 4-fold dilution, estimates of FVIIIc by clotting assay were much lower than FVIIIag (0.77+/-0.31 vs. 1.14+/-0.48 nM). At 10- and 25-fold dilutions, the estimated FVIIIc increased to 0.87+/-0.36 and 0.94+/-0.44 nM, respectively.

CONCLUSIONS

1) In plasma, FVIIIag is higher than FVIIIc and FVIIIch; and 2) Real FVIII concentrations in plasma can be estimated by measuring FVIIIag.

The molecular basis of factor V and VIII procofactor activation

Activation of precursor proteins by specific and limited proteolysis is a hallmark of the hemostatic process. The homologous coagulation factors (F)V and FVIII circulate in an inactive, quiescent state in blood. In this so-called procofactor state, these proteins have little, if any procoagulant activity and do not participate to any significant degree in their respective macromolecular enzymatic complexes. Thrombin is considered a key physiological activator, cleaving select peptide bonds in FV and FVIII which ultimately leads to appropriate structural changes that impart cofactor function. As the active cofactors (FVa and FVIIIa) have an enormous impact on thrombin and FXa generation, maintaining FV and FVIII as inactive procofactors undoubtedly plays an important regulatory role that has likely evolved to maintain normal hemostasis. Over the past three decades there has been widespread interest in studying the proteolytic events that lead to the activation of these proteins. While a great deal has been learned, mechanistic explanations as to how bond cleavage facilitates conversion to the active cofactor species remain incompletely understood. However, recent advances have been made detailing how thrombin recognizes FV and FVIII and also how the FV B-domain plays a dominant role in maintaining the procofactor state. Here we review our current understanding of the molecular process of procofactor activation with a particular emphasis on FV.

Factor VIII and von Willebrand factor interaction: biological, clinical and therapeutic importance

The interaction of factor VIII (FVIII) with von Willebrand Factor (VWF) is of direct clinical significance in the diagnosis and treatment of patients with haemophilia A and von Willebrand disease (VWD). A normal haemostatic response to vascular injury requires both FVIII and VWF. It is well-established that in addition to its role in mediating platelet to platelet and platelet to matrix binding, VWF has a direct role in thrombin and fibrin generation by acting as a carrier molecule for the cofactor FVIII. Recent studies show that the interaction affects not only the biology of both FVIII and VWF, and the pathology of haemophilia and VWD, but also presents opportunities in the treatment of haemophilia. This review details the mechanisms and the molecular determinants of FVIII interaction with VWF, and the role of FVIII-VWF interaction in modulating FVIII interactions with other proteases, cell types and cellular receptors. The effect of defective interaction of FVIII with VWF as a result of mutations in either protein is discussed.

The influence of von Willebrand factor on factor VIII activity measurements

BACKGROUND

It has been reported by multiple laboratories that the quantitation of factor (F)VIII by activity-based assays is influenced by the method, procedure and the quality of reagents used in the assays.

OBJECTIVE

To evaluate the influence of von Willebrand factor (VWF) on FVIII activity in vitro.

METHODS

The activated partial thromboplastin time (APTT) and synthetic coagulation proteome assays were used. Citrated FVIII/VWF-depleted substrate plasma (SP) (both antigens < 0.5%) was used in all APTT assays.

RESULTS

The concentration of FVIII antigen in pooled plasma from healthy donors [normal plasma (NP)] was 1.5 nm. The SP reconstituted with 1.5 nm recombinant (r)FVIII clotted in 23.8 +/- 0.2 s (standard deviation). The addition of 10 microg mL(-1) VWF to the SP increased the clotting time to 28.7 +/- 0.1 s; that is, the activity of rFVIII (FVIIIc) decreased to 50%. This inhibitory effect of VWF decreased with decreasing rFVIII concentration in SP, and became negligible at rFVIII

CONCLUSIONS

VWF has an inhibitory effect on the measurement of FVIII clotting activity. This effect depends upon the structure and formulation of the FVIII product.

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Key Words

• factor viii
• von willebrand factor
• clotting activity
• factor viii products
• tissue factor
• synthetic coagulation proteome

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MESH Headings

• Blood Coagulation/drug effects
• Blood Coagulation Tests
• Dose-Response Relationship, Drug
• Factor VIII/antagonists & inhibitors
• Factor VIII/pharmacology
• Humans
• Partial Thromboplastin Time
• von Willebrand Factor/pharmacology

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Grants

• P01 HL046703 NHLBI NIH HHS
• P01 HL046703-16A1 NHLBI NIH HHS
• P01 HL046703-17 NHLBI NIH HHS
• P01 HL46703 NHLBI NIH HHS

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Affiliation(s)

S Butenas Department of Biochemistry, College of Medicine, University of Vermont, Burlington, VT 05446, USA.
B Parhami-Seren
K G Mann

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A systematic overview of the first pasteurised VWF/FVIII medicinal product, Haemate P/ Humate -P: history and clinical performance

Patients with von Willebrand disease (VWD) and haemophilia A (HA) lack, to varying degrees, the von Willebrand factor (VWF) and coagulation factor VIII (FVIII) that are critical for normal haemostasis. These conditions in turn make patients prone to uncontrolled bleeding. Historically, patients with severe forms of VWD or HA were crippled before adulthood and their life expectancy was significantly reduced. Over the past decades, specific coagulation factor replacement therapies including Haemate P, have been developed to help patients achieve and maintain normal haemostasis. Haemate P is a human, plasma-derived VWF/FVIII medicinal product, which was first licensed in Germany in 1981 for the treatment of HA-associated bleeding. It has since then come to be accepted as the gold standard for both the treatment and prophylaxis of bleeding in VWD, especially in cases where desmopressin [1-deamino-8-D-arginine vasopressin (DDAVP)] has been ineffective. Haemate P was the first effectively virus-inactivated (pasteurisation: 60 degrees C for 10 h in aqueous solution) FVIII product, whereby the risk of potentially threatening infective complications of plasma-derived products was reduced. Haemate P was also shown to have a VWF multimer profile remarkably close to that of normal plasma. This bibliographic review presents previously unpublished clinical data of Haemate P, based upon internal clinical study reports of the proprietor, CSL Behring, in addition to data already presented in other publications. The data demonstrate a predictable and well-characterised pharmacokinetic profile, and a proven record of short- and long-term safety, while effectively correcting the haemostatic defects in VWD and HA. Recently available data have also shown Haemate P to be of haemostatic value in exceptional clinical circumstances including surgical interventions. By virtue of its plasma-derived combination of VWF and FVIII, in addition to its high VWF:FVIII content ratio (2.4:1), Haemate P is also associated with successful immune tolerance induction in those patients developing inhibitor antibodies. Although the theoretical risk of thromboembolic complications does exist while receiving Haemate P, as it does with any FVIII replacement therapy, the incidence of such complications has remained notably low. Given the robust data that have accumulated for the use of Haemate P, dosing recommendations are also described in this review; the recommendations are tailored to patient-specific contexts including baseline VWF and FVIII levels in plasma and the type of surgical intervention being undertaken. A wide variety of studies have also provided data on paediatric and geriatric populations, all of which have suggested that Haemate P can be safely and effectively used in a wide variety of clinical circumstances.

Activation of factor VIII and mechanisms of cofactor action

The factor VIII procofactor circulates as a metal ion-dependent heterodimer of a heavy chain and light chain. Activation of factor VIII results from limited proteolysis catalyzed by thrombin or factor Xa, which binds the factor VIII substrate over extended interactive surfaces. The proteases efficiently cleave factor VIII at three sites, two within the heavy and one within the light chain resulting in alteration of its covalent structure and conformation and yielding the active cofactor, factor VIIIa. The role of factor VIIIa is to markedly increase the catalytic efficiency of factor IXa in the activation of factor X. This effect is manifested in a dramatic increase in the catalytic rate constant, k(cat), by mechanisms that remain poorly understood.

Successful treatment for patients with von Willebrand disease undergoing urgent surgery using factor VIII/VWF concentrate (Humate-PR)

von Willebrand disease (VWD) is characterized by insufficient von Willebrand factor (VWF) activity. It has been proposed that VWF:ristocetin cofactor (VWF:RCo) activity may be useful in evaluating the response to VWD treatment in patients who require replacement therapy. This prospective, open-label, non-randomized study evaluated the safety and efficacy of a factor VIII (FVIII)/VWF concentrate (Humate-P) used in treatment regimens based on VWF:RCo activity in subjects with VWD in situations requiring urgent and necessary surgery. This article summarizes the results for 39 subjects with 42 evaluable surgical treatment events, 100% of which were rated as excellent/good for overall efficacy (achievement of haemostasis). The median loading dose based upon VWF:RCo activity was 82.3 international units/kilogram (IU kg(-1); range 32.5-216.8 IU kg(-1)), and the median maintenance dose per infusion was 52.8 IU kg(-1) (range 24.2-196.5 IU kg(-1)) for a median of 3 days (range 1-50 days). The median number of infusions per event was 6 (range 1-67 infusions). Three unanticipated adverse events (peripheral oedema, extremity pain and pseudo-thrombocytopenia) from two surgical treatment events were reported that were potentially treatment-related. No serious drug-related adverse events (AEs) were observed, and no thrombotic events were reported in this study. This study supports the safety and efficacy of the FVIII/VWF concentrate Humate-P for the prevention of surgical haemorrhage in patients with VWD when administered in doses calculated in VWF:RCo units.

Successful treatment of urgent bleeding in von Willebrand disease with factor VIII/VWF concentrate (Humate-P®): use of the ristocetin cofactor assay (VWF:RCo) to measure potency and to guide therapy

This prospective, open-label, non-randomized study evaluated the safety and efficacy of factor VIII (FVIII)/von Willebrand Factor (VWF) concentrate (Humate-P) using treatment regimens based on VWF:ristocetin cofactor (VWF:RCo) activity in patients with von Willebrand Disease (VWD) in (i) urgent bleeding episodes, or (ii) in patients undergoing urgent and necessary surgery. This article summarizes the results of treatment for the 33 patients with 53 urgent bleeding events. The median loading dose of FVIII/VWF concentrate was 67.0 international units per kilogram (IU kg(-1)) VWF:RCo (range 25.7-143.2 IU kg(-1)), and the median daily maintenance dose per infusion was 74.0 IU kg(-1) (range 16.4-182.9 IU kg(-1)) for a median duration of 2 days (range 1-34 days). The overall efficacy (achievement of haemostasis) of FVIII/VWF concentrate was rated as excellent/good for 98% of the urgent bleeding events. No unexpected treatment-related adverse events or serious drug-related adverse events (AEs) were observed. This study supports the safety and efficacy of Humate-P administered in doses calculated in VWF:RCo units for the treatment of urgent bleeding episodes in patients with VWD.

Molecular mechanisms of mild and moderate hemophilia A

Mutations responsible for mild/moderate hemophilia A were extensively characterized over the last 15 years and more than 200 mutations have been identified. However, most of the molecular mechanisms responsible for the reduced factor (F)VIII levels in patients' plasma were determined only recently. Recent progresses in the study of the FVIII molecule three-dimensional structure provided a major insight for understanding molecular events leading to mild/moderate hemophilia A. This allowed prediction of mutations impairing FVIII folding and intracellular processing, which result in reduced FVIII secretion. Mutations potentially slowing down FVIII activation by thrombin were also identified. A number of mutations were also predicted to result in altered stability of activated FVIII. Biochemical analyses allowed identification of mutations reducing FVIII production. Mutations impairing FVIII stability in plasma, by reducing FVIII binding to von Willebrand factor (VWF) were also characterized. Defects in FVIII activity, notably slow activation by thrombin, or abnormal interaction with FIXa, were also recently demonstrated. Biochemical analysis of FVIII variants provided information regarding the structure/function relationship of the FVIII molecule and validated predictions of the three-dimensional structure of the molecule. These observations also contributed to explain the discrepant activities recorded for some FVIII variants using different types of FVIII assays. Altogether, the study of the biochemical properties of FVIII variants and the evaluation of the effects of mutations in three-dimensional models of FVIII identified molecular mechanisms potentially explaining reduced FVIII levels for a majority of patients with mild/moderate hemophilia A. It is expected that these studies will improve diagnosis and treatment of this disease.

Haemophilia A and haemophilia B: molecular insights

This review focuses on selected areas that should interest both the scientist and the clinician alike: polymorphisms within the factor VIII and factor IX genes, their linkage, and their ethnic variation; a general assessment of mutations within both genes and a detailed inspection of the molecular pathology of certain mutations to illustrate the diverse cause-effect relations that exist; a summary of current knowledge on molecular aspects of inhibitor production; and an introduction to the new areas of factor VIII and factor IX catabolism. An appendix defining various terms encountered in the molecular genetics of the haemophilias is included, together with an appendix providing accession numbers and locus identification links for accessing gene and sequence information in the international nucleic acid databases.

Haemophilia A and haemophilia B: molecular insights

This review focuses on selected areas that should interest both the scientist and the clinician alike: polymorphisms within the factor VIII and factor IX genes, their linkage, and their ethnic variation; a general assessment of mutations within both genes and a detailed inspection of the molecular pathology of certain mutations to illustrate the diverse cause-effect relations that exist; a summary of current knowledge on molecular aspects of inhibitor production; and an introduction to the new areas of factor VIII and factor IX catabolism. An appendix defining various terms encountered in the molecular genetics of the haemophilias is included, together with an appendix providing accession numbers and locus identification links for accessing gene and sequence information in the international nucleic acid databases.

A novel cause of mild/moderate hemophilia A: mutations scattered in the factor VIII C1 domain reduce factor VIII binding to von Willebrand factor

The mechanisms responsible for the low factor VIII (fVIII) activity in the plasma of patients with mild/moderate hemophilia A are poorly understood. In such patients, we have identified a series of fVIII mutations (Ile2098Ser, Ser2119Tyr, Asn2129Ser, Arg2150His, and Pro2153Gln) clustered in the C1 domain and associated with reduced binding of fVIII to von Willebrand factor (vWf). For each patient plasma, the specific activity of mutated fVIII was close to that of normal fVIII. Scatchard analysis showed that the affinity for vWf of recombinant Ile2098Ser, Ser2119Tyr, and Arg2150His fVIII mutants was reduced 8-fold, 80-fold, and 3-fold, respectively, when compared with normal fVIII. Given the importance of vWf for the stability of fVIII in plasma, these findings suggested that the reduction of fVIII binding to vWf resulting from the above-mentioned mutations could contribute to patients' low fVIII plasma levels. We, therefore, analyzed the effect of vWf on fVIII production by Chinese hamster ovary (CHO) cells transfected with expression vectors for recombinant B domain-deleted normal, Ile2098Ser, Ser2119Tyr, and Arg2150His fVIII. These 3 mutations impaired the vWf-dependent accumulation of functional fVIII in culture medium. Analysis of fVIII production by transiently transfected CHO cells indicated that, in addition to the impaired stabilization by vWf, the secretion of functional Ile2098Ser and Arg2150His fVIII was reduced about 2-fold and 6-fold, respectively, by comparison to Ser2119Tyr and normal fVIII. These findings indicate that C1-domain mutations resulting in reduced fVIII binding to vWf are an important cause of mild/moderate hemophilia A.

A novel cause of mild/moderate hemophilia A: mutations scattered in the factor VIII C1 domain reduce factor VIII binding to von Willebrand factor

The mechanisms responsible for the low factor VIII (fVIII) activity in the plasma of patients with mild/moderate hemophilia A are poorly understood. In such patients, we have identified a series of fVIII mutations (Ile2098Ser, Ser2119Tyr, Asn2129Ser, Arg2150His, and Pro2153Gln) clustered in the C1 domain and associated with reduced binding of fVIII to von Willebrand factor (vWf). For each patient plasma, the specific activity of mutated fVIII was close to that of normal fVIII. Scatchard analysis showed that the affinity for vWf of recombinant Ile2098Ser, Ser2119Tyr, and Arg2150His fVIII mutants was reduced 8-fold, 80-fold, and 3-fold, respectively, when compared with normal fVIII. Given the importance of vWf for the stability of fVIII in plasma, these findings suggested that the reduction of fVIII binding to vWf resulting from the above-mentioned mutations could contribute to patients' low fVIII plasma levels. We, therefore, analyzed the effect of vWf on fVIII production by Chinese hamster ovary (CHO) cells transfected with expression vectors for recombinant B domain-deleted normal, Ile2098Ser, Ser2119Tyr, and Arg2150His fVIII. These 3 mutations impaired the vWf-dependent accumulation of functional fVIII in culture medium. Analysis of fVIII production by transiently transfected CHO cells indicated that, in addition to the impaired stabilization by vWf, the secretion of functional Ile2098Ser and Arg2150His fVIII was reduced about 2-fold and 6-fold, respectively, by comparison to Ser2119Tyr and normal fVIII. These findings indicate that C1-domain mutations resulting in reduced fVIII binding to vWf are an important cause of mild/moderate hemophilia A.

A human antibody directed to the factor VIII C1 domain inhibits factor VIII cofactor activity and binding to von Willebrand factor

The occurrence of factor VIII (fVIII) inhibitory antibodies is a rare complication of fVIII substitution therapy in mild/moderate hemophilia A patients. fVIII mutations in certain regions such as the C1 domain are, however, more frequently associated with inhibitor, for reasons which remain unclear. To determine whether inhibitors could map to the mutation site, we analyzed at the clonal level the immune response of such a patient with an inhibitor to wild-type but not self-fVIII and an Arg2150His substitution in the C1 domain. Immortalization of the patient B lymphocytes provided a cell line producing an anti-fVIII IgG4κ antibody, LE2E9, that inhibited fVIII cofactor activity, following type 2 kinetics and prevented fVIII binding to von Willebrand factor. Epitope mapping with recombinant fVIII fragments indicated that LE2E9 recognized the fVIII C1 domain, but not the Arg2150His-substituted C1 domain. Accordingly, LE2E9 did not inhibit Arg2150His fVIII activity. These observations identify C1 as a novel target for fVIII inhibitors and demonstrate that Arg2150His substitution alters a B-cell epitope in the C1 domain, which may contribute to the higher inhibitor incidence in patients carrying such substitution. (Blood. 2000; 95:156-163)

A human antibody directed to the factor VIII C1 domain inhibits factor VIII cofactor activity and binding to von Willebrand factor

The occurrence of factor VIII (fVIII) inhibitory antibodies is a rare complication of fVIII substitution therapy in mild/moderate hemophilia A patients. fVIII mutations in certain regions such as the C1 domain are, however, more frequently associated with inhibitor, for reasons which remain unclear. To determine whether inhibitors could map to the mutation site, we analyzed at the clonal level the immune response of such a patient with an inhibitor to wild-type but not self-fVIII and an Arg2150His substitution in the C1 domain. Immortalization of the patient B lymphocytes provided a cell line producing an anti-fVIII IgG4κ antibody, LE2E9, that inhibited fVIII cofactor activity, following type 2 kinetics and prevented fVIII binding to von Willebrand factor. Epitope mapping with recombinant fVIII fragments indicated that LE2E9 recognized the fVIII C1 domain, but not the Arg2150His-substituted C1 domain. Accordingly, LE2E9 did not inhibit Arg2150His fVIII activity. These observations identify C1 as a novel target for fVIII inhibitors and demonstrate that Arg2150His substitution alters a B-cell epitope in the C1 domain, which may contribute to the higher inhibitor incidence in patients carrying such substitution. (Blood. 2000; 95:156-163)

In vivo evaluation of a novel epitope-tagged human factor VIII-encoding adenoviral vector

Haemophilia A is caused by a deficiency in coagulation factor VIII (FVIII) and is an attractive target for gene therapy. Adenoviral vectors encoding a human B-domain deleted (BDD) FVIII cDNA have been shown previously to mediate expression of high levels of human FVIII and correct the bleeding defect in haemophiliac mice and dogs. While vector assessment in a non-human primate model would have a significant preclinical benefit, a haemophiliac non-human primate model is not available, and assays that distinguish human FVIII from monkey FVIII have not been developed successfully. As a first step to enable vector evaluation in non-human primates, we have constructed an epitope-tagged FVIII molecule by the addition of 16 amino-acids to the carboxy terminus of the BDD protein (BDD-E). Following vector administration to normal mice, therapeutic levels of BDD-E FVIII were expressed for at least 20 weeks. Treatment of haemophiliac mice revealed that the BDD-E protein was biologically active in vivo. To distinguish the BDD-E protein from non-human primate FVIII, a sensitive immunoprecipitation/Western assay was developed that reproducibly detected 1 ng mL-1 of the epitope-tagged human FVIII in the presence of monkey plasma. These data demonstrate that the addition of an epitope tag had no effect on FVIII function or immunogenicity, and suggest that the BDD-E vector will be an effective reagent for non-human primate studies.

Use of surface plasmon resonance for studies of protein-protein and protein-phospholipid membrane interactions. Application to the binding of factor VIII to von Willebrand factor and to phosphatidylserine-containing membranes

The surface plasmon resonance phenomenon is used for real time measurements of protein-protein and protein-membrane interactions. In the present study two surface plasmon resonance-based binding assays permitting study of the interaction of coagulation factor VIII (fVIII) with von Willebrand factor (vWf) and phospholipid have been developed. These interactions of fVIII are required for maintenance of fVIII concentration in circulation and for the assembly of the functional factor Xase complex, respectively. With these binding assays, the role of the light chain (LCh) in fVIII binding to vWf and to immobilized phospholipid monolayers and intact vesicles containing 25% phosphatidylserine (PS) and 4% PS was examined. The finding that Kd of LCh binding to vWf (3.8 nM) is 9.5 times higher than that of fVIII (0.4 nM), indicates that the heavy chain (HCh) is required for the maximal affinity of fVIII for vWf. In contrast, affinities of LCh for 25/75 PS/phosphatidylcholine (PC) monolayers and 4/76/20 PSPC-phosphatidylethanolamine (PE) vesicles are similar to that of fVIII, indicating that LCh is solely responsible for these interactions. It was also examined how removal of the acidic region affects the binding affinity of the remaining part of LCh for vWf and phospholipid. It was demonstrated that the loss of the LCh acidic region upon thrombin cleavage leads to an 11 and 160-fold increase in the dissociation rate constant (k(off) value) and a 165 and 1500-fold increase in the Kd value of the binding of fVIII fragment A3-C1-C2 to vWf compared to that of LCh and fVIII, respectively. In contrast, the binding affinity of A3-C1-C2 for PS-containing membranes was 8-11-fold higher than that of LCh. Possible conformational change(s) in C2 domain upon removal of the acidic region were studied using anti-fVIII monoclonal antibody NMC-VIII/5 with an epitope within the C2 domain of LCh as a probe. The determined lower binding affinity of A3-C1-C2 for NMC-VIII/5 immobilized to a sensor chip than that of LCh, indicates that these conformational changes do occur.

Proteolysis of blood coagulation factor VIII by the factor VIIa-tissue factor complex: generation of an inactive factor VIII cofactor

Activation of factor VIII by thrombin occurs via limited proteolysis at R372, R740, and R1689. The resultant active factor VIIIa molecule consists of three noncovalently associated subunits: A1-a1, A2-a2, and A3-C1-C2 (50, 45, and 73 kDa respectively). Further proteolysis of factor VIIIa at R336 and R562 by activated protein C subsequently inactivates this cofactor. We now find that the factor VIIa-tissue factor complex (VIIa-TF/PL), the trigger of blood coagulation with restricted substrate specificity, can also catalyze limited proteolysis of factor VIII. Proteolysis of factor VIII was observed at 10 sites, producing 2 major fragments (47 and 45 kDa) recognized by an anti-factor VIII A2 domain antibody. Time courses indicated the slow conversion of the large fragment to 45 kDa, followed by further degradation into at least two smaller fragments. N-Terminal sequencing along with time courses of proteolysis indicated that VIIa-TF/PL cleaved factor VIII first at R740, followed by concomitant cleavage at R336 and R372. Although cleavage of the light chain at R1689 was observed, the majority remained uncleaved after 17 h. Consistent with this, only a transient 2-fold increase in factor VIII clotting activity was observed. Thus, heavy chain cleavage of factor VIII by VIIa-TF/PL produces an inactive factor VIII cofactor no longer capable of activation by thrombin. In addition, VIIa-TF/PL was found to inactivate thrombin-activated factor VIII. We hypothesize that these proteolyses may constitute an alternative pathway to regulate coagulation under certain conditions. In addition, the ability of VIIa-TF/PL to cleave factor VIII at 10 sites greatly expands the known protein substrate sequences recognized by this enzyme-cofactor complex.

Pharmacokinetics, efficacy and safety of Humate-P in von Willebrand disease

In a pharmacokinetic study with Humate-P including six patients with various types of von Willebrand disease, a median half-life of 11.3 h for vWF:RCoF and of 15.2 h for vWF:Ag was found. The median value of in vivo recovery (IVR) was estimated for vWF:RCoF as 2.10 IU dL-1 plasma per 1 substituted IU kg-1 b.w. (or 73%), for vWF:Ag as 1.88 IU dL-1 plasma per 1 substituted IU kg-1 b.w. (or 69%); and for FVIII:C as 2.69 IU dL-1 plasma per 1 IU kg-1 b.w. (or 99%). Transient postinfusion shortening or normalization of previously prolonged bleeding time was observed in all patients. In a retrospective study involving 97 patients with various von Willebrand disease types, clinical efficacy and safety of treatment with Haemate-P in 73 surgical interventions, 344 separate bleeding events, 93 other events and 20 cycles of prophylactic treatment were evaluated. The clinical efficacy was rated good to excellent in 99% of the surgeries, in 97% of the bleeding episodes, in 86% of the other events, and in all prophylactic treatments. The overall tolerability was good. Adverse events possibly or probably associated with use of Humate-P/Haemate-P were rare, of non-serious nature and mild to moderate in their intensity.

von Willebrand Factor Elevates Plasma Factor VIII Without Induction of Factor VIII Messenger RNA in the Liver

Factor VIII and von Willebrand factor (vWF) circulate in the plasma as a noncovalent protein complex. Circulating levels of factor VIII are coordinately regulated with circulating levels of vWF in which the ratio is maintained at 1 molecule of factor VIII for 50 to 100 vWF subunits. Infusion of vWF into vWF-deficient animal models and human patients yields a secondary increase in circulating levels of factor VIII. We have studied the mechanism of the secondary rise in factor VIII in a porcine model of vWF deficiency. On infusion of vWF into a vWF-deficient pig there was an approximately fivefold increase in circulating factor VIII activity. Liver biopsies were taken pre- and post-vWF infusion for isolation of total messenger RNA (mRNA). Factor VIII–specific mRNA was measured by an RNAse protection assay. The results showed no difference in the liver-specific factor VIII mRNA on vWF infusion. These results indicate that the secondary rise in factor VIII levels in response to exogenous vWF infusion is not dependent on increased steady-state levels of factor VIII mRNA in the liver.

von Willebrand Factor Elevates Plasma Factor VIII Without Induction of Factor VIII Messenger RNA in the Liver

Factor VIII and von Willebrand factor (vWF) circulate in the plasma as a noncovalent protein complex. Circulating levels of factor VIII are coordinately regulated with circulating levels of vWF in which the ratio is maintained at 1 molecule of factor VIII for 50 to 100 vWF subunits. Infusion of vWF into vWF-deficient animal models and human patients yields a secondary increase in circulating levels of factor VIII. We have studied the mechanism of the secondary rise in factor VIII in a porcine model of vWF deficiency. On infusion of vWF into a vWF-deficient pig there was an approximately fivefold increase in circulating factor VIII activity. Liver biopsies were taken pre- and post-vWF infusion for isolation of total messenger RNA (mRNA). Factor VIII–specific mRNA was measured by an RNAse protection assay. The results showed no difference in the liver-specific factor VIII mRNA on vWF infusion. These results indicate that the secondary rise in factor VIII levels in response to exogenous vWF infusion is not dependent on increased steady-state levels of factor VIII mRNA in the liver.

Mechanism and Kinetics of Factor VIII Inactivation: Study With an IgG4 Monoclonal Antibody Derived From a Hemophilia A Patient With Inhibitor

The development of an immune response towards factor VIII (fVIII) remains a major complication for hemophilia A patients receiving fVIII infusions. The design of a specific therapy to restore unresponsiveness to fVIII has been hampered by the diversity of the anti-fVIII antibody. Molecular analysis of the specific immune response is therefore required. To this end, we have characterized an fVIII-specific human IgG4κ monoclonal antibody (BO2C11) produced by a cell line derived from the memory B-cell repertoire of a hemophilia A patient with inhibitor. BO2C11 recognizes the C2 domain of fVIII and inhibits its binding to both von Willebrand factor (vWF) and phospholipids. It completely inhibits the procoagulant activity of native and activated fVIII, with a specific activity of approximately 7,000 Bethesda units/mg. vWF reduces the rate of fVIII inactivation by BO2C11. The antibody-fVIII association rate constant (kass ∼7.4 × 105M−1 s−1) is eightfold lower than that for vWF-fVIII association, whereas its dissociation rate constant (kdiss ≤1 × 10−5s−1) is 100-fold lower than that for the vWF-fVIII complex, which suggests that BO2C11 almost irreversibly neutralizes fVIII after its dissociation from vWF. BO2C11 is the first human monoclonal anti-fVIII IgG antibody that has been isolated and allows the study of fVIII inactivation at the molecular level.

Binding of Factor VIII to von Willebrand Factor Is Enabled by Cleavage of the von Willebrand Factor Propeptide and Enhanced by Formation of Disulfide-Linked Multimers

von Willebrand factor (vWF) is a multimeric adhesive glycoprotein with one factor VIII binding site/subunit. Prior reports suggest that posttranslational modifications of vWF, including formation of N-terminal intersubunit disulfide bonds and subsequent cleavage of the propeptide, influence availability and/or affinity of factor VIII binding sites. We found that deletion of the vWF propeptide produced a dimeric vWF molecule lacking N-terminal intersubunit disulfide bonds. This molecule bound fluorescein-labeled factor VIII with sixfold lower affinity than multimeric vWF in an equilibrium flow cytometry assay (approximate KDs, 5 nmol/L v 0.9 nmol/L). Coexpression of propeptide-deleted vWF with the vWF propeptide in trans yielded multimeric vWF that displayed increased affinity for factor VIII. Insertion of an alanine residue at the N-terminus of the mature vWF subunit destroyed binding to factor VIII, indicating that the native mature N-terminus is required for factor VIII binding. The requirement for vWF propeptide cleavage was shown by (1) a point mutation of the vWF propeptide cleavage site yielding pro-vWF that was defective in factor VIII binding and (2) correlation between efficiency of intracellular propeptide cleavage and factor VIII binding. Furthermore, in a cell-free system, addition of the propeptide-cleaving enzyme PACE/furin enabled factor VIII binding in parallel with propeptide cleavage. Our results indicate that high-affinity factor VIII binding sites are located on N-terminal disulfide-linked vWF subunits from which the propeptide has been cleaved.

Mechanism and Kinetics of Factor VIII Inactivation: Study With an IgG4 Monoclonal Antibody Derived From a Hemophilia A Patient With Inhibitor

The development of an immune response towards factor VIII (fVIII) remains a major complication for hemophilia A patients receiving fVIII infusions. The design of a specific therapy to restore unresponsiveness to fVIII has been hampered by the diversity of the anti-fVIII antibody. Molecular analysis of the specific immune response is therefore required. To this end, we have characterized an fVIII-specific human IgG4κ monoclonal antibody (BO2C11) produced by a cell line derived from the memory B-cell repertoire of a hemophilia A patient with inhibitor. BO2C11 recognizes the C2 domain of fVIII and inhibits its binding to both von Willebrand factor (vWF) and phospholipids. It completely inhibits the procoagulant activity of native and activated fVIII, with a specific activity of approximately 7,000 Bethesda units/mg. vWF reduces the rate of fVIII inactivation by BO2C11. The antibody-fVIII association rate constant (kass ∼7.4 × 105M−1 s−1) is eightfold lower than that for vWF-fVIII association, whereas its dissociation rate constant (kdiss ≤1 × 10−5s−1) is 100-fold lower than that for the vWF-fVIII complex, which suggests that BO2C11 almost irreversibly neutralizes fVIII after its dissociation from vWF. BO2C11 is the first human monoclonal anti-fVIII IgG antibody that has been isolated and allows the study of fVIII inactivation at the molecular level.

Binding of Factor VIII to von Willebrand Factor Is Enabled by Cleavage of the von Willebrand Factor Propeptide and Enhanced by Formation of Disulfide-Linked Multimers

von Willebrand factor (vWF) is a multimeric adhesive glycoprotein with one factor VIII binding site/subunit. Prior reports suggest that posttranslational modifications of vWF, including formation of N-terminal intersubunit disulfide bonds and subsequent cleavage of the propeptide, influence availability and/or affinity of factor VIII binding sites. We found that deletion of the vWF propeptide produced a dimeric vWF molecule lacking N-terminal intersubunit disulfide bonds. This molecule bound fluorescein-labeled factor VIII with sixfold lower affinity than multimeric vWF in an equilibrium flow cytometry assay (approximate KDs, 5 nmol/L v 0.9 nmol/L). Coexpression of propeptide-deleted vWF with the vWF propeptide in trans yielded multimeric vWF that displayed increased affinity for factor VIII. Insertion of an alanine residue at the N-terminus of the mature vWF subunit destroyed binding to factor VIII, indicating that the native mature N-terminus is required for factor VIII binding. The requirement for vWF propeptide cleavage was shown by (1) a point mutation of the vWF propeptide cleavage site yielding pro-vWF that was defective in factor VIII binding and (2) correlation between efficiency of intracellular propeptide cleavage and factor VIII binding. Furthermore, in a cell-free system, addition of the propeptide-cleaving enzyme PACE/furin enabled factor VIII binding in parallel with propeptide cleavage. Our results indicate that high-affinity factor VIII binding sites are located on N-terminal disulfide-linked vWF subunits from which the propeptide has been cleaved.

Can we improve on nature? "Super molecules" of factor VIII

Treatment of haemophilia A requires frequent infusion of plasma- or recombinant-derived factor VIII. This regimen is limited due to the high cost and inconvenient access to peripheral veins. In addition, patients frequently develop inhibitory antibodies that limit available therapeutic regimens. Two major advances in factor VIII research over the past 15 years were the ability to isolate homogeneous preparations of factor VIII and the isolation of the factor VIII gene that provided for a detailed biochemical and structural characterization of the factor VIII molecule. With an increased understanding of the requirements for factor VIII function, studies have attempted to produce improved factor VIII molecules for replacement therapy. These findings have produced forms of factor VIII that are more efficiently produced, that are less immunogenic, and that have higher specific activity. The future will see the engineering of novel factor VIII molecules with increased therapeutic efficiency while minimizing inhibitor antibody development. In addition, there are now structural models of factor VIII available that should in the future direct development of novel peptidomimetics that may eventually overcome the requirement for replacement therapy with factor VIII protein.

The natural history of the immune response to exogenous factor VIII in severe haemophilia A

The development of inhibitory antibodies to factor VIII (fVIII) in severe haemophilia A patients is a serious therapeutic complication. Using a highly sensitive immunoprecipitation (IP) assay which measures all anti-fVIII antibodies, we have tested severe haemophilic plasmas from two clinical studies. Inhibitor titres in the range of 0.4 to 1 Bethesda units/ml (BU/ml) could not be verified by IP as being due to an immune response to fVIII in 35% of plasmas tested. Low fVIII recoveries were likewise correlated with the presence of antibodies in 29% of plasmas tested. However, 16% of plasmas without inhibitor titres had immune responses as measured by IP. The rapidity of antibody appearance did not allow their effective detection by IP before development of inhibitor titres. These results suggest that the IP assay can provide a valuable confirmation of anti-fVIII antibody production when the Bethesda assay is low or negative and where clinical observations suggest their presence, but they cannot be used reliably to detect early immune responses.

The acidic region of the factor VIII light chain and the C2 domain together form the high affinity binding site for von willebrand factor

A binding site for von Willebrand factor (vWf) was previously localized to the carboxyl terminus of the C2 domain of the light chain (LCh) of factor VIII (fVIII). The acidic region of the LCh, residues 1649-1689, also controls fVIII.vWf binding by an unknown mechanism. Although anti-acidic region monoclonal antibodies prevent formation of the fVIII.vWf complex, the direct involvement of the acidic region in this binding has not been demonstrated. By limited proteolysis of LCh with Staphylococcus aureus V8 protease, we prepared 14- and 63-kDa LCh fragments, which begin with fVIII residues 1672 and 1795, respectively. Using surface plasmon resonance to measure binding interactions, we demonstrated that the 14-kDa fragment binds to vWf, but its affinity for vWf (Kd 72 nM) was 19-fold lower than that of LCh. This was not due to an altered conformation of the acidic region within the 14-kDa fragment, since its affinity for an anti-acidic region monoclonal antibody was similar to that of LCh. All LCh derivatives lacking the acidic region (thrombin-cleaved LCh, recombinant C2, and 63-kDa fragment) had also greatly reduced affinities for vWf (Kd 564-660 nM) compared with LCh (Kd 3.8 nM). In addition, the similar affinities of these derivatives for vWf indicated that apart from its acidic region, the LCh contains no vWf binding site other than the one within C2. The reduced affinities of the LCh derivatives lacking the acidic region for monoclonal antibody NMC-VIII/5 (epitope, C2 residues 2170-2327) indicated that removal of the acidic region leads to a conformational change within C2. This change is likely to affect the conformation of the vWf binding site in C2, which overlaps the epitope of NMC-VIII/5; therefore, the acidic region also appears to be required to maintain the optimal conformation of this vWf binding site. Our results demonstrate that the acidic region and the C2 domain are both directly involved in forming a high affinity binding site for vWf.

Slowed release of thrombin-cleaved factor VIII from von Willebrand factor by a monoclonal and a human antibody is a novel mechanism for factor VIII inhibition

The anti-factor VIII (fVIII) C2 domain monoclonal antibody ESH8 inhibits fVIII activity only when fVIII is bound to von Willebrand factor (vWf). However, ESH8 binds with similar affinity to fVIII and fVIII.vWf complex, and it does not affect the kinetics of thrombin cleavage at positions 372 and 740 within the fVIII heavy chain and at 1689 within the light chain. The latter is required for fVIII release from vWf. We showed that ESH8 reduced the initial rate of thrombin-activated fVIII (fVIIIa) release from vWf by 4.3-fold compared to that in the absence of antibody. The complex of vWf. fVIII.ESH8 was activated, and the rate constant determined for fVIIIa dissociation from vWf was 4 x 10(-3) s-1. We constructed a mathematical model incorporating the measured rates for fVIIIa release from vWf and for inactivation of heterotrimeric fVIIIa due to the spontaneous loss of the A2 subunit and found that the decreased release rate is sufficient to explain our experimentally observed inhibition of fVIII activity by ESH8. We hypothesize that the slowed rate of fVIIIa release from vWf in the presence of ESH8 allows time for inactivation of unstable fVIIIa prior its participation in the formation of the factor Xase complex. The relevance of these findings is illustrated by our observation that reduction of fVIIIa release from vWf represents an additional mechanism of fVIII inhibition by an anti-C2 domain antibody (epitope 2218-2307) from a hemophilia A patient. This rare antibody binds to a more amino-terminal epitope than other human anti-C2 inhibitors, resulting in its lack of inhibition of fVIII binding to vWf but not to phospholipid. These two fVIII ligands therefore bind to C2 sites which do not overlap completely.

A mechanism for inhibition of factor VIII binding to phospholipid by von Willebrand factor

von Willebrand factor (vWf) acts as a carrier for blood coagulation factor VIII (fVIII) in the circulation. The amino-terminal 272 residues of mature vWf contain a high affinity fVIII binding site. Upon thrombin activation, fVIII is released from vWf, thereby allowing its binding to phospholipid which is required for its procoagulant activity. Although phospholipid and vWf compete for fVIII binding, it was previously suggested that their binding sites are not closely juxtaposed within the fVIII protein because only amino-terminal vWf proteolytic fragments larger than SPIII-T4 (1-272) were able to block the binding of fVIII to phospholipid. We have demonstrated, however, that SPIII-T4 is able to inhibit fVIII binding to phosphatidylserine (PS) in a dose-dependent fashion, but only at concentrations higher than those used in previous experiments. Our demonstration that the Kd values for vWf and SPIII-T4 for fVIII are 0.52 nM and 48 nM, respectively, explain this discrepancy. Inhibition (> 95%) of SPIII-T4 binding to fVIII by a purified recombinant fVIII C2 domain polypeptide demonstrated that SPIII-T4 binds directly to C2, as we had previously shown for vWf. The similarity of the C2 binding sites for vWf and SPIII-T4 was further confirmed by the identical inhibitory effects of synthetic peptides and monoclonal antibodies (mAbs) on vWf-fVIII or SPIII-T4 fVIII binding. In both cases, binding was inhibited by synthetic peptide 2303-2332, containing a PS binding site, and by mAb NMC-VIII/5 Fab' (epitope within C2 residues 2170-2327). We propose that vWf, via residues 1-272, and PS compete for fVIII binding because they recognize overlapping sites within fVIII C2 domain residues 2303-2332.

Cleavage of factor VIII light chain is required for maximal generation of factor VIIIa activity

Thrombin-catalyzed activation of heterodimeric factor VIII occurs by limited proteolysis, yielding subunits A1 and A2 derived from the heavy chain (HC) and A3-C1-C2 derived from the light chain (LC). The roles of these cleavages in the function of procoagulant activity are poorly understood. To determine whether LC cleavage contributes to the potentiation of factor VIII activity, factor VIII heterodimers were reconstituted from native HC and either thrombin-cleaved LC (A3-C1-C2) or intact LC and purified by Mono S chromatography. The reconstituted factor VIII form containing the A3-C1-C2 subunit had a specific activity (2 units/micrograms) that was approximately 3-fold greater than that of the reconstituted factor VIII form containing native LC (0.6 units/microgram). Factor Xa generation assays using the hybrid heterodimer showed an initial rate that was unaffected by the presence of von Willebrand factor and a reduced lag time when compared with the native heterodimer. The A1/A3-C1-C2 dimer was dissociated by chelation, and the purified A1 subunit was reacted with either the A3-C1-C2 subunit or the LC in the presence of Mn2+ to reconstitute the dimer. Factor VIIIa heterotrimers were reconstituted from either A1/A3-C1-C2 or A1/LC plus the A2 subunit. The authentic factor VIIIa heterotrimer (A1/A3-C1-C2/A2) had 3-fold greater activity than the form containing the LC. However, upon reaction with thrombin, the activity of the latter form was increased to that of the factor VIIIa form containing native subunits. The incremental increase in fluorescence anisotropy of fluorescein-Phe-Phe-Arg chloromethyl ketone-modified factor IXa was markedly greater in the presence of HC/A3-C1-C2 (delta r = 0.037) compared with HC/LC (delta r = 0.011) and approached the value obtained with factor VIIIa (delta r = 0.051). These results suggest that cleavage of factor VIII LC directly contributes to the potentiation of coagulant activity by modulating the conformation of the factor IXa active site.

Application of a new statistical approach to optimize the immunopurification of antihemophilia A factor

Our aim was to optimize the immunopurification process of human factor VIII. This purification was performed using a mouse monoclonal anti-factor VIII light-chain antibody. Previous dissociation of the factor VIII-von Willebrand factor complex with CaCl2 led to a 50% increase of the factor VIII adsorption on the immunosorbent. The optimization of the elution step required the analysis of the effects of two parameters, pH and ionic strength, on four different responses: elution yield, concentration, specific activity and stability of factor VIII. For this purpose, a multifunctional method using Doehlert matrices for statistically designed experiments was applied. This methodology allowed us to obtain, with only seven experiments, a 60% increase of the elution yield and a two-fold increase of the specific activity of factor VIII.