Modified factor VIII

FIELD OF THE INVENTION

[0003] This invention relates generally to a modified mammalian factor VIII having amino acid substitutions which reduce its immunogenicity and/or antigenicity as compared to the proteins from which they were derived or other factor VIII preparations such as human factor VIII.

BACKGROUND OF THE INVENTION

[0004] Blood clotting begins when platelets adhere to the cut wall of an injured blood vessel at a lesion site. Subsequently, in a cascade of enzymatically regulated reactions, soluble fibrinogen molecules are converted by the enzyme thrombin to insoluble strands of fibrin that hold the platelets together in a thrombus. At each step in the cascade, a protein precursor is converted to a protease that cleaves the next protein precursor in the series. Co-factors are required at most of the steps.

[0005] Factor VIII circulates as an inactive precursor in blood, bound tightly and non-covalently to von Willebrand factor. Factor VIII is proteolytically activated by thrombin or factor Xa, which dissociates it from von Willebrand factor and activates its procoagulant function in the cascade. In its active form, the protein factor VIIIa is a cofactor that increases the catalytic efficiency of factor IXa toward factor X activation by several orders of magnitude.

[0006] People with deficiencies in factor VIII or antibodies against factor VIII who are not treated with factor VIII suffer uncontrolled internal bleeding that may cause a range of serious symptoms, from inflammatory reactions in joints to early death. Severe hemophiliacs, who number about 10,000 in the United States, can be treated with infusion of human factor VIII, which will restore the blood's normal clotting ability if administered with sufficient frequency and concentration. The classical definition of factor VIII is that substance present in normal blood plasma that corrects the clotting defect in plasma derived from individuals with hemophilia A.

[0007] The development of antibodies (“inhibitors” or “inhibitory antibodies”) that inhibit the activity of factor VIII is a serious complication in the management of patients with hemophilia. Autoantibodies develop in approximately 20% of patients with hemophilia A in response to therapeutic infusions of factor VIII. In previously untreated patients with hemophilia A who develop inhibitors, the inhibitors usually develops within one year of treatment. Additionally, autoantibodies that inactivate factor VIII occasionally develop in individuals with previously normal factor VIII levels. Inhibitory antibodies (inhibitors) to factor VIII (fVIII) either develop as alloantibodies in hemophilia A patients following fVIII infusions or as autoantibodies in nonhemophiliacs (Hoyer, L. W. and D. Scandella, 1994, “Factor VIII inhibitors: structure and function in autoantibody and hemophilia A patients,” Semin.Hematol. 31:1-5). Antibodies to epitopes in the A2, ap-A3, and C2 domains within the A1-A2-B-ap-A3-C1-C2 fVIII molecule are responsible for all anticoagulant activity in most inhibitor plasmas (Prescott, R. et al., 1997, “The inhibitory antibody response is more complex in hemophilia A patients than in most nonhemophiliacs with fVIII autoantibodies,” Blood 89:3663-3671; Barrow, R. T. et al., 2000, “Reduction of the antigenicity of factor VIII toward complex inhibitory plasmas using multiply-substituted hybrid human/porcine factor VIII molecules,” Blood 95:557-561). The 18-kDa C2 domain, defined as residues Ser2173-Tyr2332 in single chain human fVIII, contains a phospholipid membrane-binding site that is necessary for the normal procoagulant function of fVIII. Human C2-specific anti-fVIII antibodies inhibit this interaction (Arai, M. et al., 1989, “Molecular basis offactor-VIII inhibition by human antibodies—antibodies that bind to the factor-VIII light chain prevent the interaction of factor-VIII with phospholipid,” J. Clin. Invest. 83:1978-1984). Consistent with this, phospholipid protects fVIII from inactivation by fVIII inhibitors (Arai et al., supra; Barrowcliffe, T. W. et al., 1983, “Binding to phospholipid protects factor VIII from inactivation by human antibodies,” J. Lab. Clin. Med. 101:34-43). The C2 domain also contains part of the von Willebrand factor (vWf) binding site (Saenko, E. L. et al., 1994, “A role for the C2 domain of factor binding to von Willebrand factor. J. Biol. Chem. 269:11601-11605; Saenko, E. L. and Scandella, D., 1997, “The acidic region of the factor VIII light chain and the C2 domain together form the high affinity binding site for von Willebrand factor,” J. Biol. Chem. 272:18007-18014). Some inhibitors may act by interfering with this interaction (Shima, M. et al., 1995, “Common inhibitory effects of human anti-C2 domain inhibitor alloantibodies on factor VIII binding to von Willebrand factor,” Br. J. Haematol. 91:714-721; Saenko, E. L. et al., 1996, “Slowed release of thrombin-cleaved factor VIII from von Willebrand factor by a monoclonal and human antibody is a novel mechanism for factor VIII inhibition,” J. Biol. Chem. 271:27424-27431; Gilles, J G. et al., 1999, “Some factor VIII (FVIII) inhibitors recognize a FVIII epitope(s) that is present only on FVIII-vWf complexes,” Thromb. Haemost. 82:40-45).

[0008] Patients can be managed by increasing the dose of factor VIII provided the inhibitor titer is low enough. However, often the inhibitor titer is so high that it cannot be overwhelmed by factor VIII. An alternative strategy is to bypass the need for factor VIII during normal hemostasis using factor IX complex preparations (for example, KONYNE®, Proplex®) or recombinant human factor VIIIa. Additionally, since porcine factor VIII usually has substantially less reactivity with inhibitors than human factor VIII, a partially purified porcine factor VIII preparation (HYATE:C®) is used. Many patients who have developed inhibitory antibodies to human factor VIII have been successfully treated with porcine factor VIII and have tolerated such treatment for long periods of time. However, administration of porcine factor VIII is not a complete solution because inhibitors may develop to porcine factor VIII after one or more infusions.

[0009] Several preparations of human plasma-derived factor VIII of varying degrees of purity are available commercially for the treatment of hemophilia A. These include a partially-purified factor VIII derived from the pooled blood of many donors that is heat- and detergent-treated for viruses but contain a significant level of antigenic proteins; a monoclonal antibody-purified factor VIII that has lower levels of antigenic impurities and viral contamination; and recombinant human factor VIII, clinical trials for which are underway. Unfortunately, human factor VIII is unstable at physiologic concentrations and pH, is present in blood at an extremely low concentration (0.2 μg/ml plasma), and has low specific clotting activity.

[0010] Hemophiliacs require daily replacement offactor VIII to prevent bleeding and the resulting deforming hemophilic arthropathy. However, supplies have been inadequate and problems in therapeutic use occur due to difficulty in isolation and purification, immunogenicity, and the necessity of removing the AIDS and hepatitis infectivity risk. The use of recombinant human factor VIII or partially-purified porcine factor VIII will not resolve all the problems.

[0011] The problems associated with the commonly used, commercially available, plasma-derived factor VIII have stimulated significant interest in the development of a better factor VIII product. There is a need for a more potent factor VIII molecule so that more units of clotting activity can be delivered per molecule; a factor VIII molecule that is stable at a selected pH and physiologic concentration; a factor VIII molecule that is less apt to cause production of inhibitory antibodies; and a factor VIII molecule that evades immune detection in patients who have already acquired antibodies to human factor VIII.

[0012] U.S. Pat. No. 6,180,371 to Lollar describes amino acid substitutions in the A2 domain of human factor VIII which alter the antigenicity of the resulting factor VIII molecules. The '371 patent does not disclose or suggest specific amino acid substitutions in the C2 domain which reduces antigenicity or immunogenicity as compared to wild-type factor VIII or the corresponding recombinant factor VIII.

[0013] U.S. Pat. No. 5,859,204 to Lollar discloses the site specific replacement of amino acids in the 484-509 region of human factor VIII. More specifically, the '204 patent teaches modified factor VIII with amino acid substitutions at positions 485, 487, 488, 489, 492, 495, 501 or 508 relative to the human protein. The '204 patent does not disclose or suggest specific amino acid substitutions in the C2 domain which reduce antigenicity or immunogenicity as compared to wild-type factor VIII or the corresponding recombinant factor VIII.

[0014] U.S. Pat. No. 5,888,974 to Lollar et al. discloses hybrid procoagulant factor VIII produced by the isolation and recombination of human and other non-human factor VIII subunits or domains.

[0015] U.S. Pat. No. 5,744,446 to Lollar et al. describes hybrid factor VIII having amino acid substitutions in the A2 domain.

[0016] U.S. Pat. No. 5,663,060 to Lollar et al. describes hybrid factor VIII comprised of combinations of non-human and human heavy and light chain subunits. U.S. Pat. No. 5,583,209 describes nucleic acids encoding the hybrid factor VIII molecules in the '060 patent.

[0017] U.S. Pat. No. 5,364,771 describes purified hybrid factor VIII comprised of human and porcine combinations of the heavy and light subunits. Also disclosed is human factor VIII with porcine A2 domain swapped for the human A2 domain.

[0018] U.S. Pat. Nos. 6,180,371; 5,888,974; 5,859,204; 5,744,446; 5,663,060; 5,583,209; and 5,364,771 (all of which are incorporated herein by reference) do not disclose substitutions or suggest specific amino acid substitutions in the C2 domain of factor VIII which reduce antigenicity or immunogenicity as compared to wild-type factor VIII or the corresponding recombinant factor VIII. Despite years of intensive research from laboratories around the world, it appears that the invention regarding the C2 domain of factor VIII described in detail herein has not been described or suggested elsewhere.

[0019] Pratt et al. (1999, “Structure of the C2 domain of human factor VIII at 1.5 Å resolution,” Nature 402:43 9-442) have reported the crystal structure of the C2 domain of human factor VIII at 1.5 Å resolution. Pratt et al. reported that the structure partly explains why mutations in the C2 region of factor VIII lead to bleeding disorders. In fact, 21 residues in the C2 region were reported to be sites of deleterious point mutations in patients with hemophilia A. For example, V2223 is known to be a position where a point mutation occurs and is associated with bleeding disorders. Thus, one of ordinary skill in the art would not expect V2223 to be a reasonable candidate for substitution to provide modified factor VIII for therapeutic activity. Indeed, Shima et al. report C2 binding antibody inhibitors interfere with factor VIII with respect to phospholipid and Von Willebrand factor binding. Thus, it is taught by Pratt et al. that C2 inhibitors, i.e., those related to some bleeding disorders in individuals with hemophilia A, interfere with the binding of the C2 domain to phospholipid and Von Willebrand factor. This conclusion, combined with their determination that M2199, F2200, L2251, L2252, V2223, and R2220 appear at the protein-phospholipid interface, suggests that mutation of these residues would lead to altered phospholipid and/or Von Willebrand binding along with an associated increase in bleeding disorders. It is not clear from these studies which amino acid residues and corresponding substitutions would lead to improved factor VIII molecules.

[0020] Unexpectedly it was discovered by the inventor of the instant invention that mutations in the 3 hydrophobic feet identified in the recently available x-ray structure for the C2 domain of factor VIII have reduced binding to inhibitory antibodies, improved properties, and/or reduced immunogenicity.

[0021] It is therefore an object of the present invention to provide a factor VIII that corrects hemophilia in a patient deficient in factor VIII or having inhibitors to factor VIII.

[0022] It is a further object of the present invention to provide methods for treatment of hemophiliacs.

[0023] It is still another object of the present invention to provide a factor VIII that is stable at a selected pH and physiologic concentration.

[0024] It is yet another object of the present invention to provide a factor VIII that has greater coagulant activity than human factor VIII.

SUMMARY OF THE INVENTION

[0025] The present invention generally relates to compositions comprising recombinant mammalian factor VIII. The composition of the invention comprise isolated, purified recombinant mammalian factor VIII molecules with coagulant activity wherein the recombinant factor VIII has amino acid substitutions in the C2 domain which reduce antigenicity as compared to the proteins from which they were derived or other factor VIII preparations. DNA sequences encoding the novel compositions of the invention as well as methods of producing the novel compositions comprising factor VIII are also provided. Methods of treating patients in need of treatment with factor VIII are also within the scope of this invention.

[0026] A first embodiment of the invention provides novel compositions comprising recombinant mammalian factor VIII with amino acid substitution(s) in the C2 domain. The amino acid substitution(s) in the C2 domain of the modified recombinant factor VIII reduce the anticoagulant activity of inhibitory antibodies as compared to the proteins from which they were derived or other available factor VIII preparations. The novel composition of this embodiment have coagulant activity and reduced binding to inhibitory antibodies. Substitutions at residues that participate in binding of fVIII to phospholipid membranes and/or to inhibitory antibodies are preferred. Preferred substitutions at positions homologous to human factor VIII include, but are not limited to, Met2199, Phe2200, Val2223, Leu2251, and Leu2252. The novel compositions of this embodiment can be a single mutant, a double mutant, a triple mutant, or a quadruple mutant. Examples of amino acid substitutions of the invention include, but are not limited to, Met2199Ile, Phe2200Leu, Leu2252Phe, Met2199Ile/Phe2200Leu,Val2223Ala/Lys2227Glu, Met2199Ile/Phe2200Leu/Va2223 Ala/Lys2227Glu, all of which are referenced to the human factor VIII numbering system wherein amino acid number 1 is the amino terminal alanine of mature factor VIII. Substitutions in either recombinant porcine or human factor VIII are preferred.

[0027] A second embodiment of the invention provides novel hybrid factor VIII compositions comprising recombinant factor VIII with amino acid substitution(s) in the C2 domain. The novel compositions of this embodiment are constructed by preparing hybrid factor VIII with amino acid substitutions in the C2 domain. The other domains of factor VIII may be derived from a variety of mammals such as human, mouse, pig, rat, and canine and so on. The novel compositions of this embodiment have coagulant activity and reduced binding to inhibitory antibodies. Examples of amino acid positions that can be mutated to provided the novel compositions of this embodiment include, but are not limited to, Met2199, Phe2200, Val2223, Leu2251, and Leu2252, all of which are referenced to human factor VIII. Examples of amino acid substitutions in the C2 domain encompassed within this embodiment include, but are not limited to, Met2199Ile, Phe2220Leu, Leu2252Phe, Met2199Ile/Phe2200Leu,Val2223Ala/Lys2227Glu, Met2199Ile/Phe2200Leu/-Val2223Ala/Lys2227Glu, all of which are referenced to the human factor VIII.

[0028] Another embodiment of the invention provides DNA sequences comprising coding sequences for the novel compositions of the invention. Yet another embodiment of the invention provides methods of producing the novel compositions of the invention.

[0029] The invention also provides a method for reducing the immunogenicity of factor VIII molecules as well as recombinant factor VIII with reduced immunogenicity produced by the method. In particular, modified recombinant factor VIII molecule and methods of making such molecules with reduced immunogenicity that have substitutions in the C2 domain are described.

[0030] Also provided are pharmaceutical compositions and methods for treating patients having factor VIII deficiency comprising administering recombinant factor VIII and hybrid version thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]
FIG. 1. Putative fVIII residues involved in phospholipid binding. Shown are aligned sequences of the C2 domains of human (Vehar, G. A. et al., supra, 1984; Toole, J. J. et al., 1984, “Molecular cloning of a cDNA encoding human antihaemophilic factor” Nature 312:342-347), porcine (Healey, J. F. et al., 1996, “The cDNA and derived amino acid sequence of porcine factor VIII,” Blood 88:4209-4214), murine (Elder, B. et al., 1993, “Sequence of the murine factor VIII cDNA,” Genomics 16:374-379) and canine (Cameron, C. et al., 1998, “The canine factor VIII cDNA and 5′ flanking sequence,” Thromb.Haemostas. 79:317-322) fVIII. Proposed phospholipid-binding residues in human fVIII (Pratt, K. P. et al., 1999, “Structure of the C2 domain of human factor VIII at 1.5 Å resolution,” Nature 402:439-442) and homologous residues are underlined and shown in bold.

[0032]
FIG. 2. Mutated sites in the human fVIII C2 domain. A. Ribbon diagram showing hydrophobic residues proposed to be involved in phospholipid membrane binding and Lys2227, one of the four putative positively-charged binding residues (Pratt, K. P. et al., 1999, “Structure of the C2 domain of human factor VIII at 1.5 Å resolution,” Nature 402:439-442). Met2199, Phe2200, Val2223, Lys2227, and Leu2252 were mutated in this study. Leu2251, which is conserved in human, porcine, murine, and canine fVIII, was not mutated. B. Space filling model rotated, as if looking up from the membrane.

[0033]
FIG. 3. Bethesda titers of patient polyclonal anti-fVIII antibodies. Recombinant fVIII was diluted into hemophilia A plasma and Bethesda titers of antibodies AA, AJ, HR, LK, and RvR were determined as described in “Materials and Methods”. Shown are means and standard deviations determined by nonlinear least-squares regression analysis. C2 D1 is the Met2199Ile/Phe2200Leu double mutant. C2 D2 is the Val2223Ala/Lys2227Glu double mutant. C2 Q is the Met2199Ile/Phe2200Leu/Val2223Ala/Lys2227Glu quadruple mutant. HP20 is a B-domainless hybrid human/porcine fVIII molecule containing human A1, A2, ap-A3, and C1 domains and the porcine C2 domain. Confidence levels for differences between mutants and HB- are indicated as “**” at the 99.9% level and “*” at the 99% level. NS, not significant.

[0034]
FIG. 4. Bethesda titers of patient monoclonal antibody BO2C11. Abbreviations and notations are as described in FIG. 3 legend.

[0035]
FIG. 5. Bethesda titers of murine monoclonal antibody NMC VIII-5. Abbreviations and notations are as described in FIG. 3 legend

DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention generally relates to compositions comprising recombinant mammalian factor VIII. The composition of the invention comprise isolated, purified recombinant mammalian factor VIII molecules with coagulant activity. It was surprisingly discovered that mutations in the C2 domain of factor VIII, in three hydrophobic feet identified in a recently available x-ray structure, reduced the binding of inhibitory antibodies of the mutants as compared to the proteins from which they were derived and/or other factor VIII preparations. Thus, the novel compositions of the invention comprise recombinant factor VIII with amino acid substitutions in the C2 domain which reduce antigenicity as compared to the proteins from which they were derived. Furthermore, the invention also provides recombinant factor VIII with amino acid substitutions in the C2 domain which reduce antigenicity as compared to other available factor VIII preparations. Related embodiments of the invention provide for methods of treating patients in need offactor VIII treatment, methods of producing the novel recombinant factor VIII compositions of the invention, DNA sequences comprising coding sequences of the novel recombinant factor VIII proteins, and pharmaceutical compositions comprising the novel factor VIII proteins.

[0037] The present invention further provides active recombinant hybrid factor VIII molecules or fragments thereof, the nucleic acid sequences encoding these hybrids, methods of preparing and isolating them, and methods for characterizing them. These hybrids comprise human/animal, animal/animal, or other such hybrid factor VIII molecules, and further comprise at least one specific amino acid sequence in the C2 domain including one or more unique amino acids of the factor VIII of one species substituted for the corresponding amino acid sequence (or amino acid) of the factor VIII of the other species; or comprises at least one sequence in the C2 domain including one or more amino acids having no known sequence identity to factor VIII substituted for specific amino acid sequence in human, animal, or hybrid factor VIII. The resulting recombinant hybrid factor VIII has reduced or no immunoreactivity to factor VIII inhibitory antibodies, compared to human or porcine factor VIII.

[0038] A “corresponding” nucleic acid or amino acid or sequence of either, as used herein, is one present at a site in a factor VIII molecule or fragment thereof that has the same structure and/or function as a site in the factor VIII molecule of another species, although the nucleic acid or amino acid number may not be identical. A DNA sequence “corresponding to” another factor VIII sequence substantially corresponds to such sequence, and hybridizes to the sequence of the designated SEQ ID NO. under stringent conditions. A DNA sequence “corresponding to” another factor VIII sequence also includes a sequence that results in the expression of a factor VIII or fragment thereof and would hybridize to the designated SEQ ID NO. but for the redundancy of the genetic code.

[0039] A “unique” amino acid residue or sequence, as used herein, refers to an amino acid sequence or residue in the factor VIII molecule of one species that is different from the homologous residue or sequence in the factor VIII molecule of another species.

[0040] “Specific activity,” as used herein, refers to the activity that will correct the coagulation defect of human factor VIII deficient plasma. Specific activity is measured in units of clotting activity per milligram total factor VIII protein in a standard assay in which the clotting time of human factor VIII deficient plasma is compared to that of normal human plasma. One unit of factor VIII activity is the activity present in one milliliter of normal human plasma. In the assay, the shorter the time for clot formation, the greater the activity of the factor VIII being assayed. Porcine factor VIII has coagulation activity in a human factor VIII assay.

[0041] “Expression” refers to the set of processes that occur whereby genetic information is utilized to yield a product. A DNA encoding the amino acid sequence of porcine factor VIII can be “expressed” within a mammalian host cell to yield modified factor VIII protein. The materials, genetic structures, host cells and conditions which permit expression of a given DNA sequence to occur are well-known in the art and can be manipulated to affect the time and amount of expression, as well as the intra- or extra-cellular location of the expressed protein. For example, by including DNA encoding a signal peptide at the 5′ end of the DNA encoding porcine factor VIII (the 5′ end being, by convention, that end encoding the NH2 terminus of the protein) the expressed protein becomes exported from the interior of the host cell into the culture medium. Providing a signal peptide coding DNA in combination with the porcine factor VIII coding DNA is advantageous because the expressed factor VIII is exported into the culture medium which simplifies the process of purification. A preferred signal peptide is a mammalian factor VIII signal peptide.

[0042] The human factor VIII cDNA nucleotide and predicted amino acid sequences are shown in SEQ ID NOs: 1 and 2, respectively. Factor VIII is synthesized as an approximately 300 kDa single chain protein with internal sequence homology that defines the “domain” sequence NH2-A1-A2-B-A3-C1-C2-COOH. In a factor VIII molecule, a “domain”, as used herein, is a continuous sequence of amino acids that is defined by internal amino acid sequence identity and sites of proteolytic cleavage by thrombin. Unless otherwise specified, factor VIII domains include the following amino acid residues, when the sequences are aligned with the human amino acid sequence (SEQ ID NO: 2): A1, residues Ala1-Arg372; A2, residues Ser373-Arg740; B, residues Ser741-Arg1648; A3, residues Ser1690-Ile2032; C1, residues Arg2033-Asn2172; C2, residues Ser2173-Tyr2332. The A3-C1-C2 sequence includes residues Ser1690-Tyr2332. The remaining segment, residues Glu1649-Arg1689, is usually referred to as the factor VIII light chain activation peptide. Factor VIII is proteolytically activated by thrombin or factor Xa, which dissociates it from von Willebrand factor, forming factor VIIIa, which has procoagulant function. The biological function of factor VIIIa is to increase the catalytic efficiency of factor IXa toward factor X activation by several orders of magnitude. Thrombin-activated factor VIIIa is a 160 kDa A1/A2/A3-C1-C2 heterotrimer that forms a complex with factor IXa and factor X on the surface of platelets or monocytes. A “partial domain” as used herein is a continuous sequence of amino acids forming part of a domain.

[0043] “Subunits” of human or animal factor VIII, as used herein, are the heavy and light chains of the protein. The heavy chain of factor VIII contains three domains, A1, A2, and B. The light chain of factor VIII also contains three domains, A3, C1, and C2.

[0044] The terms “epitope,” “antigenic site,” and “antigenic determinant,” as used herein, are used synonymously and are defined as a portion of the human, or animal factor VIII or fragment thereof that is specifically recognized by an antibody. It can consist of any number of amino acid residues, and it can be dependent upon the primary, secondary, or tertiary structure of the protein.

[0045] The term “immunogenic site, ” as used herein, is defined as a region of the human or animal factor VIII, or fragment thereof, that specifically elicits the production of antibody to the factor VIII, or fragment, in a human or animal, as measured by routine protocols, such as immunoassay, e.g. ELISA, or the Bethesda assay, described herein. It can consist of any number of amino acid residues, and it can be dependent upon the primary, secondary, or tertiary structure of the protein. In some embodiments, the hybrid or hybrid equivalent factor VIII or fragment thereof is nonimmunogenic or less immunogenic in an animal or human than human or porcine factor VIII.

[0046] “Factor VIII deficiency,” as used herein, includes deficiency in clotting activity caused by production of defective factor VIII, by inadequate or no production of factor VIII, or by partial or total inhibition of factor VIII by inhibitors. Hemophilia A is a type of factor VIII deficiency resulting from a defect in an X-linked gene and the absence or deficiency of the factor VIII protein it encodes.

[0047] As used herein, “diagnostic assays” include assays that in some manner utilize the antigen-antibody interaction to detect and/or quantify the amount of a particular antibody that is present in a test sample to assist in the selection of medical therapies. There are many such assays known to those of skill in the art. As used herein, human, porcine or modified porcine factor VIII DNA or fragment thereof and protein expressed therefrom, in whole or in part, can be substituted for the corresponding reagents in the otherwise known assays, whereby the modified assays may be used to detect and/or quantify antibodies to factor VIII. It is the use of these reagents, the factor VIII DNA or fragment thereof or protein expressed therefrom, that permits modification of known assays for detection of antibodies to human or animal factor VIII. Such assays include, but are not limited to ELISAs, immunodiffusion assays, and immunoblots. Suitable methods for practicing any of these assays are known to those of skill in the art. As used herein, the factor VIII or fragment thereof that includes at least one epitope of the protein can be used as the diagnostic reagent. Examples of other assays in which human, porcine or modified porcine factor VIII or fragment thereof can be used include the Bethesda assay and anticoagulation assays.

[0048] The term “DNA encoding a protein, such as porcine factor VIII” means a polydeoxynucleic acid whose nucleotide sequence embodies coding information to a host cell for the amino acid sequence of the protein, e.g. porcine factor VIII, according to the known relationships of the genetic code.

[0049] The “expression product” of a DNA encoding a human or animal factor VIII or a modified factor VIII is the product obtained from expression of the referenced DNA in a suitable host cell, including such features of pre- or post-translational modification of protein encoded by the referenced DNA, including but not limited to glycosylation, proteolytic cleavage and the like. It is known in the art that such modifications can occur and can differ somewhat depending upon host cell type and other factors, and can result in molecular isoforms of the product, with retention of procoagulant activity. See, e.g. Lind, P. et al., Eur. J. Biochem. 232:1927 (1995), incorporated herein by reference.

[0050] An “expression vector” is a DNA element, often of circular structure, having the ability to replicate autonomously in a desired host cell, or to integrate into a host cell genome and also possessing certain well-known features which permit expression of a coding DNA inserted into the vector sequence at the proper site and in proper orientation. Such features can include, but are not limited to, one or more promoter sequences to direct transcription initiation of the coding DNA and other DNA elements such as enhancers, polyadenylation sites and the like, all as well known in the art. The term “expression vector” is used to denote both a vector having a DNA coding sequence to be expressed inserted within its sequence, and a vector having the requisite expression control elements so arranged with respect to an insertion site that it can serve to express any coding DNA inserted into the site, all as well-known in the art. Thus, for example, a vector lacking a promoter can become an expression vector by the insertion of a promoter combined with a coding DNA.

[0051] Discovery of Mutations in Factor VIII Which Reduce Binding of Inhibitory Antibodies

[0052] Recently, a 1.5 Å X-ray structure of the human fVIII C2 domain was reported (Pratt, K. P. et al., 1999, “Structure of the C2 domain of human factor VIII at 1.5 A resolution,” Nature 402:439-442). Examination of this structure revealed three solvent-exposed hydrophobic “feet” consisting of Met2199/Phe2200, Val2223, and Leu2251/Leu2252. A ring of positively charged residues, including Arg 2213, Arg 2220, Lys 2227, and Lys 2249, surrounds these residues. This motif suggests that membrane binding consists of the insertion of the hydrophobic feet into the membrane bilayer and is stabilized by electrostatic interaction with negatively charged phospholipid.

[0053] Most fVIII inhibitors cross-react poorly with porcine fVIII. This observation led to the mapping of a major determinant of the C2 epitope to residues Glu2181-Val2243 using a series of constructs that contained porcine substitutions in the human fVIII C2 domain (Healey, J. F. et al., 1998, “Residues Glu2181-Val2243 contain a major determinant of the inhibitory epitope in the C2 domain of human factor VIII,” Blood 92:3701-3709). In the present invention, residues in porcine, murine, or canine fVIII that are homologous to residues Met2199, Phe2200, Val2223, Lys2227, and/or Leu2252 in human fVIII were used as the basis for creating a series of recombinant fVIII molecules. A significant reduction in antigenicity was observed associated with mutations at Met2199, Phe2200, and Leu2252, indicating that these residues participate in binding of fVIII to phospholipid membranes and often to inhibitory antibodies.

[0054]
FIG. 1 shows the alignment of the human, porcine, murine and canine fVIII C2 domains. At four of the five proposed hydrophobic phospholipid binding residues there is one species that differs from human fVIII: Met2199→Ile (porcine), Phe2200→Leu (canine), Val2223→Ala (canine), and Leu2252→Phe (murine). There is a species difference in only one of the four proposed basic residues, Lys2227→Glu (porcine). Accordingly, Met2199Ile, Phe2200Leu, Val2223Ala, Leu2252Phe, and Lys2227Glu single mutants in human B-domainless fVIII were made. Additionally, two double mutants, Met2199Ile/Phe2200Leu (designated C2 D1) and is Val2223Ala/Leu2252Phe (C2 D2), and a quadruple mutant, Met2199Ile/Phe2200Leu/Val2223Ala/Leu2252Phe (C2 Q) were made. The locations of the mutated residues in the X-ray structure of fVIII are shown in FIG. 2. Met2199/Phe2200 and Leu2251/Leu2252 project from two β-hairpin loops. Val2223 projects from an adjacent loop and is near Lys2227.

[0055] The mutants were stably expressed in serum-free medium from a baby hamster kidney-derived cell line and then were partially purified. The specific coagulant activities of the hybrids based on an ELISA assay were equal or greater than that of HB- as described in “Materials and Methods”, indicating that they were suitable for antigenicity studies. The interaction of the mutants with C2-specific fVIII inhibitors was measured using the Bethesda assay as described in “Materials and Methods”. The results were compared to human B-domainless fVIII (HB-) and a hybrid human/porcine FVIII molecule, HP20, which is human except for substitution of the entire porcine C2 domain (Healey, J. F. et al., 1998, “Residues Glu2181-Val2243 contain a major determinant of the inhibitory epitope in the C2 domain of human factor VIII,” Blood 92:3701-3709).

[0056] Most FVIII inhibitors are polyclonal IgG populations directed against epitopes both within and outside the C2 domain (Prescott, R. et al., supra, 1997; Fulcher, C. A. et al. 1985, “Localization of human factor FVIII inhibitor epitopes to two polypeptide fragments,” Proc. Natl. Acad. Sci. USA 82:7728-7732). However, some inhibitors are C2-specific and are useful for evaluating the effects of substitution of non-human sequence into the C2 domain (Healey, J. F. et al., 1998, “Residues Glu2181-Val2243 contain a major determinant of the inhibitory epitope in the C2 domain of human factor VIII,” Blood 92:3701-3709). C2-specific polyclonal inhibitors from five patients, AA, AJ, HR, LK, RvR (FIG. 3) were used in these studies. A reduction in antigenicity due to mutations at Met2199, Phe2200, and/or Leu2252 always was observed, although individual inhibitors varied in the residues they recognized. Surprisingly, frequently there was a significant increase in Bethesda titer, most notably with the Val2223Ala mutant. The double mutant Met2199Ile/Phe2200Leu exhibited low antigenicity toward all five antibodies, consistent with the fact that the antigenicity of Met2199Ile and/or Phe2200Leu always was reduced. Paradoxically, the double mutant Val2223Ala/Lys2227Glu displayed a reduction in antigenicity toward all five polyclonal antibodies even though in three cases (AA, AJ, and HR) the corresponding individual mutants displayed unchanged or increased antigenicity. The antigenicity of the quadruple mutant Met2199Ile/Phe2200Leu/Val2223Ala/Lys2227Glu was equal or lower than the single or double mutants. The antigenicity of HP20 was the lowest of all the mutants. This is consistent with the existence of antigenic residues in addition to Met2199, Phe2200, and Leu2252 that were not mutated in this study.

[0057] The Bethesda titers of antibodies BO2C11 (Jacquemin, M. G. et al., 1998, “Mechanism and kinetics of factor VIII inactivation: study with an IgG4 monoclonal antibody derived from a hemophilia A patient with inhibitor,” Blood 92:496-506) and NMC VIII-5 (Shima, M., D. et al., 1993, “A factor VIII neutralizing monoclonal antibody and a human inhibitor alloantibody recognizing epitopes in the C2 domain inhibit factor VIII binding to von Willebrand factor and to phosphatidylserine,” Thromb. Haemost. 69:240-246) toward HB- and the mutant fVIII molecules are shown in FIGS. 4 and 5, respectively. BO2C11 is a C2-specific human IgG4κ monoclonal antibody derived from transformed B cells of a hemophilia A inhibitor patient. It is the only C2-specific human antibody that has been cloned to date. BO2C11 and NMC VIII-5 both recognize the C2 domain of fVIII and inhibit the binding of fVIII to vWf and phospholipid. NMC VIII-5 can compete for the binding of human polyclonal inhibitors to fVIII. The results with BO2C11 and NMC VIII-5 were similar to those obtained using polyclonal antibody HR (FIG. 3). In all three antibodies, Phe2200 is antigenic, whereas Val2223 and Lys2227 appear to reduce antigenicity.

[0058] Mutations at Met2199, Phe2200, and/or Leu2252 were associated with a decrease in antigenicity in most of the seven antibodies tested (Table 1), which was frequently pronounced (FIGS. 3-5). This is consistent with the hypothesis that the Met2199/Phe2200 and Leu2251/Leu2252 loops participate in membrane binding. Even though all seven inhibitors recognized the M2199/Phe2200 loop, the effects of mutations at Met2199 and Phe2200 often differed. For example, Met2199Ile displayed decreased antigenicity and Phe2200Leu displayed increased antigenicity toward antibody AJ, whereas the opposite was true for BO2C11. Thus, the amino acid specificity of AJ and BO2C11 varies, although both recognize the Met2199/Phe2200 loop.

[0059] Previously, a series of recombinant hybrid human/porcine FVIII molecules was used to map a major determinant of the C2 epitope(s) to a segment bounded by residues Glu2181-Val2243 (Healey, J. F. et al., 1998, “Residues Glu2181-Val2243 contain a major determinant of the inhibitory epitope in the C2 domain of human factor VIII,” Blood 92:3701-3709). The Met2199/Phe2200 loop is contained within this region. The Leu2251/Leu2252 loop was neither included nor excluded by this analysis because porcine FVIII also contains leucines at residues 2251 and 2252. Substitution of the entire porcine C2 domain into human fVIII, which produces a molecule designated HP20, is associated with lower antigenicity than the more limited substitutions made in the present study (FIGS. 3-5). This indicates that there are residues outside the Met2199/Phe2200 and Leu2251/Leu2252 loops that contribute to binding by C2 inhibitors.

[0060] Recently, X-ray structures of two conformations of the factor V C2 domain in the absence of phospholipid were reported (Macedo-Ribeiro, S. et al., 1999, “Crystal structures of the membrane-binding C2 domain of human coagulation factor V,” Nature 402:434-439). The authors proposed a model for phospholipid membrane binding that involves a loop containing tryptophans at positions 26 and 27 (human factor V C2 domain numbering), which are homologous to Met2199 and Phe2200 in fVIII. A considerable amount of evidence exists to support the involvement of this loop in phospholipid membrane binding. An inhibitory monoclonal antibody, HV-1 that blocks the binding of factor V to PS maps to this loop (Kim, S. W. et al., 2000, “Identification of functionally important amino acid residues within the C2-domain of human factor V using alanine-scanning mutagenesis” Biochemistry 39:1951-1958; Ortel, T. L. et al., 1994 “Localization of functionally important epitopes within the second C-type domain of coagulation factor V using recombinant chimeras,” J. Biol. Chem. 269:15898-15905; Ortel, T. L. et al., 1998, “Inhibitory anti-factor V antibodies bind to the factor V C2 domain and are associated with hemorrhagic manifestations,” Blood 91:4188-4196). Substitution of alanine for residues equivalent to Trp26 and Trp27 in factor Va is associated with decreased binding to PS and loss of coagulant activity (Kim, S. W. et al., supra, 2000, “Identification of functionally important amino acid residues within the C2-domain of human factor V using alanine-scanning mutagenesis,” Biochemistry 39:1951-1958).

[0061] Additionally, a loop containing Leu79, which is homologous to Leu2251 in fVIII, and a loop containing residues Asn41-Asn51, were also proposed to participate in phospholipid membrane binding based on proximity to the Trp26/Trp27 loop (Macedo-Ribeiro, S. et al., 1999, “Crystal structures of the membrane-binding C2 domain of human coagulation factor V,” Nature 402:434-439). The fVIII segment that is homologous to the Asn41-Asn51loop, His2076-Asn2082, has not been proposed as a phospholipid membrane-binding site (Pratt, K. P., 1999, “Structure of the C2 domain of human factor VIII at 1.5 Å resolution,” Nature 402:439-442). Conversely, the loop in factor V that is homologous to the Val2223 loop in fVIII was not proposed to participate in phospholipid membrane binding. In the present study, Val2223Ala and Lys2227Glu mutations were usually associated with an increase in antigenicity (Table 1). Thus, these results do not support the hypothesis that these residues participate in phospholipid membrane binding. However, it is possible that they do bind phospholipid but are not frequently targeted by inhibitory antibodies.

[0062] The two factor V C2 structures have different conformations, designated “open” and “closed”, which are associated with major movements of Trp26 and Trp27 at the phospholipid membrane binding site. These states are proposed to switch the phospholipid membrane binding state to on and off, respectively (Macedo-Ribeiro, S., supra, 1999). The reduction in antigenicity associated with Val2223 and Lys2227 may result because these residues stabilize a similar “closed” conformational state in FVIII that is associated with low affinity membrane and antibody binding. Relaxation of this state by the Val2223Ala and Lys2227Glu mutations would then lead to high affinity antibody binding. Alternatively, Val2223 and Lys2227 may simply interfere with an antigen-antibody lock-and-key interaction that involves high affinity contacts with other fVIII residues (e.g., Met2199, Phe2200, etc.).

[0063] The human C2-specific monoclonal antibody, BO2C11, is important to compare to polyclonal inhibitors because of the heterogeneity that may confound the analysis of the latter. The functional properties of BO2C11 are similar to the murine monoclonal antibody NMC VIII-5. Both antibodies inhibit the binding of fVIII to PS and to vWf and promote dissociation of the fVIII-vWf complex (Jacquemin, M. G. et al., 1998, “Mechanism and kinetics of factor VIII inactivation: study with an IgG4 monoclonal antibody derived from a hemophilia A patient with inhibitor,” Blood 92:496-506; Shima, M. et al., 1993, “A factor VIII neutralizing monoclonal antibody and a human inhibitor alloantibody recognizing epitopes in the C2 domain inhibit factor VIII binding to von Willebrand factor and to phosphatidylserine,” Thromb.Haemost. 69:240-246). These results indicate that Phe2200 but not Met2199 is an important part of the epitope recognized by both antibodies (FIGS. 4 and 5). However, NMC VIII-5 recognizes Leu2252 but BO2C11 does not. Val2223 and Lys2227 reduce antigenicity with respect to both antibodies. Thus, BO2C11 and NMC VIII-5 appear to recognize overlapping but not identical epitopes.

[0064] The RvR antibody was obtained from a hemophilia A patient who was part of an inhibitor “epidemic” that resulted from exposure to a heat pasteurized FVIII product, CPS-A (Sawamoto, Y. et al., 1998, “C2 domain restricted epitope specificity of inhibitor antibodies elicited by a heat pasteurized product, factor VIII CPS-P, in previously treated hemophilia A patients without inhibitors,” Thromb.Haemostas. 79:62-68). In 1990 and 1991, several previously treated patients without inhibitors promptly developed C2-specific antibodies after exposure to this product in The Netherlands and Belgium. RvR antibodies block the binding of fVIII to both PS and vWf(Sawamoto, Y. et al., supra, 1998). The RvR epitope maps to the N-terminal, Glu2181-Val2243 region of the fVIII C2 domain recognized by most C2 inhibitors (Healey, J. F. et al., 1998, “Residues Glu2181-Val2243 contain a major determinant of the inhibitory epitope in the C2 domain of human factor VIII,” Blood 92:3701-3709). The high resolution mapping in the present study indicates that RvR is a typical C2 inhibitor that recognizes primarily the Met2199/Phe2200 and Leu2251/Leu2252 loops. Thus, the immunogenicity associated with the CPS-A appears to be due to enhanced immune recognition of a normal immunodominant epitope rather than to development of a neoepitope.

GENERAL DESCRIPTION OF METHODS

[0065] U.S. Pat. No. 5,364,771 described the discovery of hybrid human/porcine factor VIII molecules having coagulant activity, in which elements of the factor VIII molecule of human or pig are substituted for corresponding elements of the factor VIII molecule of the other species. U.S. Pat. No. 5,663,060 describes procoagulant hybrid human/animal and hybrid equivalent factor VIII molecules, in which elements of the factor VIII molecule of one species are substituted for corresponding elements of the factor VIII molecule of the other species.

[0066] Since current information indicates that the B domain has no inhibitory epitope and has no known effect on factor VIII function, in some embodiments the B domain is wholly or partially deleted in the active hybrid or hybrid equivalent factor VIII molecules or fragments thereof (“B(−) factor VIII”) prepared by any of the methods described herein.

[0067] The human factor VIII gene was isolated and expressed in mammalian cells, as reported by Toole, J. J. et al. (1984) Nature 312:342-347 (Genetics Institute); Gitschier, J. et al.(1984) Nature 312:326-330 (Genentech); Wood, W. I. et al. (1984) Nature 312:330-337 (Genentech); Vehar, G. A. et al. (1984) Nature 312:337-342 (Genentech); WO 87/04187; WO 88/08035; WO 88/03558; U.S. Pat. No. 4,757,006, and the amino acid sequence was deduced from cDNA. U.S. Pat. No. 4,965,199 to Capon et al. discloses a recombinant DNA method for producing factor VIII in mammalian host cells and purification of human factor VIII. Human factor VIII expression on CHO (Chinese hamster ovary) cells and BHKC (baby hamster kidney cells) has been reported. Human factor VIII has been modified to delete part or all of the B domain (U.S. Pat. No. 4,868,112), and replacement of the human factor VIII B domain with the human factor V B domain has been attempted (U.S. Pat. No. 5,004,803). The cDNA sequence encoding human factor VIII and predicted amino acid sequence are shown in SEQ ID NOs: 1 and 2, respectively. In SEQ ID NO: 1, the coding region begins at nucleotide position 208, the triplet GCC being the codon for amino acid number 1 (Ala) of the mature protein as given in SEQ ID NO: 2.

[0068] Porcine factor VIII has been isolated from plasma [Fass, D. N. et al. (1982) Blood 59:594]. Partial amino acid sequence of porcine factor VIII corresponding to portions of the N-terminal light chain sequence having homology to ceruloplasmin and coagulation factor V were described by Church et al. (1984) Proc. Natl. Acad. Sci. USA 81:6934. Toole, J. J. et al. (1984) Nature 312:342-347 described the partial sequencing of the N-terminal end of four amino acid fragments of porcine factor VIII but did not characterize the fragments as to their positions in the factor VIII molecule. The amino acid sequence of the B and part of the A2 domains of porcine factor VIII were reported by Toole, J. J. et al. (1986) Proc. Natl. Acad. Sci, USA 83:5939-5942. The CDNA sequence encoding the complete A2 domain of porcine factor VIII and predicted amino acid sequence and hybrid human/porcine factor VIII having substitutions of all domains, all subunits, and specific amino acid sequences were disclosed in U.S. Pat. No. 5,364,771 entitled “Hybrid Human/Porcine factor VIII” issued on Nov. 15, 1994, and in WO 93/20093 published Oct. 14, 1993. The cDNA sequence encoding the A2 domain of porcine factor VIII corresponding to residues 373-740 in mature human factor VIII, as shown in SEQ ID NO: 1, and the predicted amino acid sequence are shown in SEQ ID NOs: 3 and 4, respectively. More recently, the nucleotide and corresponding amino acid sequences of part of the Al domain lacking the first 198 amino acid and of the A2 domain of porcine factor VIII were reported in WO 94/11503, published May 26, 1994. The entire nucleotide sequence encoding porcine factor VIII, including the complete A1 domain, activation peptide, A3, C1 and C2 domains, as well as the encoded amino acid sequence, was finally obtained by Lollar, as disclosed in U.S. Pat. No. 5,859,204, issued Jan. 12, 1999, and in WO 97/49725, published Dec. 31, 1997, both incorporated herein by reference.

[0069] Both porcine and human factor VIII are isolated from plasma as a two subunit protein. The subunits, known as the heavy chain and light chain, are held together by a non-covalent bond that requires calcium or other divalent metal ions. The heavy chain of factor VIII contains three domains, A1, A2, and B, which are linked covalently. The light chain of factor VIII also contains three domains, designated A3, C1, and C2. The B domain has no known biological function and can be removed, or partially removed from the molecule proteolytically or by recombinant DNA technology methods without significant alteration in any measurable parameter of factor VIII. Human recombinant factor VIII has a similar structure and function to plasma-derived factor VIII, though it is not glycosylated unless expressed in mammalian cells.

[0070] Both human and porcine activated factor VIII (“factor VIIIa”) have three subunits due to cleavage of the heavy chain between the A1 and A2 domains. This structure is designated A1/A2/A3-C1-C2. Human factor VIIIa is not stable under the conditions that stabilize porcine factor Villa, presumably because of the weaker association of the A2 subunit of human factor VIIIa. Dissociation of the A2 subunit of human and porcine factor VIIIa is associated with loss of activity in the factor VIIIa molecule. Yakhyaev, A. et al. (1997) Blood 90:Suppl. 1, Abstract #126, reported binding of A2 domain by low density lipoprotein receptor-related protein, suggesting that cellular uptake of A2 mediated by such binding acts to down-regulate factor VIII activity.

[0071] Expression of “B-domainless factor VIII” is enhanced by including portions of the B-domain. The inclusion of those parts of the B domain designated “SQ” [Lind, P. et al. (1995) supra] was reported to result in favorable expression. “SQ” constructs lack all of the human B domain except for 5 amino acids of the B domain N-terminus and 9 amino acids of the B domain C-terminus.

[0072] The purified modified factor VIII or fragment thereof can be assayed for immunoreactivity and coagulation activity by standard assays including, for example, the plasma-free factor VIII assay, the one-stage clotting assay, and the enzyme-linked immunosorbent assay using purified recombinant human factor VIII as a standard.

[0073] Other vectors, including both plasmid and eukaryotic viral vectors, may be used to express a recombinant gene construct in eukaryotic cells depending on the preference and judgment of the skilled practitioner (see, for example, Sambrook et al., Chapter 16). Other vectors and expression systems, including bacterial, yeast, and insect cell systems, can be used but are not preferred due to differences in, or lack of, glycosylation.

[0074] Recombinant factor VIII protein can be expressed in a variety of cells commonly used for culture and recombinant mammalian protein expression. In particular, a number of rodent cell lines have been found to be especially useful hosts for expression of large proteins. Preferred cell lines, available from the American Type Culture Collection, Rockville, Md., include baby hamster kidney cells, and Chinese hamster ovary (CHO) cells which are cultured using routine procedures and media.

[0075] The basis for the greater coagulant activity of porcine factor VIII appears to be the more rapid spontaneous dissociation of the human A2 subunit from human factor VIIIa than the porcine A2 subunit from porcine factor VIIIa. Dissociation of the A2 subunit leads to loss of activity, [Lollar, P. et al. (1990) J. Biol. Chem. 265:1688-1692; Lollar, P. et al. (1992) J. Biol. Chem. 267:23652-23657; Fay, P. J. et al. (1992) J. Biol. Chem. 267:13246-13250].

[0076] Factor VIII Molecules with Reduced Immunoreactivity

[0077] Epitopes that are immunoreactive with antibodies that inhibit the coagulant activity of factor VIII (“inhibitors” or “inhibitory antibodies”) have been characterized based on known structure-function relationships in factor VIII. Presumably, inhibitors could act by disrupting any of the macromolecular interactions associated with the domain structure of factor VIII or its associations with von Willebrand factor, thrombin, factor Xa, factor IXa, or factor X. However, most inhibitory antibodies to human factor VIII act by binding to epitopes located in the 40 kDa A2 domain or 20 kDa C2 domain of factor VIII, disrupting specific functions associated with these domains, as described by Fulcher et al. (1985) Proc. Natl. Acad. Sci. USA 82:7728-7732; and Scandella et al. (1988) Proc. Natl. Acad. Sci. USA 85:6152-6156. In addition to the A2 and C2 epitopes, there may be a third epitope in the A3 or C1 domain of the light chain of factor VIII, according to Scandella et al. (1993) Blood 82:1767-1775. The significance of this putative third epitope is unknown, but it appears to account for a minor fraction of the epitope reactivity in factor VIII.

[0078] Anti-A2 antibodies block factor X activation, as shown by Lollar et al. (1994) J. Clin. Invest. 93:2497-2504. Previous mapping studies by deletion mutagenesis described by Ware et al. (1992) Blood Coagul. Fibrinolysis 3:703-716, located the A2 epitope to within a 20 kDa region of the NH2-terminal end of the 40 kDa A2 domain. Competition immunoradiometric assays have indicated that A2 inhibitors recognize either a common epitope or narrowly clustered epitopes, as described by Scandella et al. (1992) Throm. Haemostas. 67:665-671, and as demonstrated in U.S. Pat. No. 5,859,204.

[0079] Modified factor VIII molecules can be tested in humans for their reduced antigenicity and/or immunogenicity in clinical trials. In one type of trial, designed to determine whether the factor VIII is immunoreactive with inhibitory antibodies, factor VIII is administered, preferably by intravenous infusion, to approximately 25 patients having factor VIII deficiency who have antibodies that inhibit the coagulant activity of therapeutic human factor VIII. The dosage of the animal or modified animal factor VIII is in a range between 5 and 50 Units/kg body weight, preferably 10-50 Units/kg, and most preferably 40 Units/kg body weight. Approximately 1 hour after each administration, the recovery of factor VIII from blood samples is measured in a one-stage coagulation assay. Samples are taken again approximately 5 hours after infusion, and recovery is measured. Total recovery and the rate of disappearance of factor VIII from the samples is predictive of the antibody titer and inhibitory activity. If the antibody titer is high, factor VIII recovery usually cannot be measured. The recovery results are compared to the recovery results in patients treated with plasma-derived human factor VIII, recombinant human factor VIII, plasma-derived porcine factor VIII, and other commonly used therapeutic forms of factor VIII or factor VIII substitutes.

[0080] After identification of clinically significant epitopes, recombinant factor VIII molecules can be expressed that have less than or equal cross-reactivity compared with plasma-derived porcine factor VIII when tested in vitro against a broad survey of inhibitor plasmas. Additional mutagenesis in epitopic regions can be done to reduce cross-reactivity. Reduced cross-reactivity, although desirable, is not necessary to produce a product that may have advantages over the existing plasma-derived porcine factor VIII concentrate, which can produce side effects due to contaminant porcine proteins or contaminant infectious agents such as viruses or prions. A recombinant porcine or modified porcine factor VIII molecule will not contain foreign porcine proteins.

[0081] Diagnostic Assays

[0082] The factor VIII cDNA and/or protein expressed therefrom, in whole or in part, can be used in assays as diagnostic reagents for the detection of inhibitory antibodies to human or animal factor VIII or modified animal VIII in substrates, including, for example, samples of serum and body fluids of human patients with factor VIII deficiency. These antibody assays include assays such as ELISA assays, immunoblots, radioimmunoassays, immunodiffusion assays, and assay of factor VIII biological activity (e.g., by coagulation assay). Techniques for preparing these reagents and methods for use thereof are known to those skilled in the art. For example, an immunoassay for detection of inhibitory antibodies in a patient serum sample can include reacting the test sample with a sufficient amount of the factor VIII to be tested that a detectable complex can be formed with the inhibitory antibodies in the sample of the test factor VIII is indeed antigenic.

[0083] Nucleic acid and amino acid probes can be prepared based on the sequence of the modified factor VIII cDNA or protein molecule or fragments thereof. In some embodiments, these can be labeled using dyes or enzymatic, fluorescent, chemiluminescent, or radioactive labels that are commercially available. The amino acid probes can be used, for example, to screen sera or other body fluids where the presence of inhibitors to human, animal, or hybrid human/animal factor VIII is suspected. Levels of inhibitors can be quantitated in patients and compared to healthy controls, and can be used, for example, to determine whether a patient with a factor VIII deficiency can be treated with an animal or modified animal factor VIII. The cDNA probes can be used, for example, for research purposes in screening DNA libraries.

[0084] Preparation of Recombinant Factor VIII

[0085] Recombinant factor VIII can be produced through the use of eukaryotic protein expression systems. In general, an eukaryotic cell line, which is deficient in a required gene, is transformed with a vector comprising the gene that it has a deficiency for, and the recombinant DNA which one wishes to express. Transformation can be accomplished by techniques such as electroporation or viral delivery. The cell line chosen to produce the protein is selected to be compatible with the protein of interest, capable of continuously expressing the protein of interests, capable of growing on a medium which facilitates purification of the protein of interest, along with other factors known to those skilled in the art. Examples of such techniques are disclosed in European Patent Application 0 302 968 A2 and U.S. Pat. No. 5,149,637 both of which are incorporated by reference in their entirety.

[0086] Testing of Recombinant Factor VIII Molecules

[0087] The recombinant factor VIII molecules can be tested in humans for their reduced antigenicity and/or immunogenicity in at least two types of clinical trials. In one type of trial, designed to determine whether the recombinant or recombinant hybrid factor VIII is immunoreactive with inhibitory antibodies, recombinant or recombinant hybrid factor VIII is administered, preferably by intravenous infusion, to approximately 25 patients having factor VIII deficiency who have antibodies to factor VIII that inhibit the coagulant activity of therapeutic human or porcine factor VIII. The dosage of the recombinant or recombinant hybrid factor VIII is in a range between 5 and 50 Units/kg body weight, preferably 10-50 Units/kg, and most preferably 40 Units/kg body weight. Approximately 1 hour after each administration, the recovery of factor VIII from blood samples is measured in a one-stage coagulation assay. Samples are taken again approximately 5 hours after infusion, and recovery is measured. Total recovery and the rate of disappearance of factor VIII from the samples is predictive of the antibody titer and inhibitory activity. If the antibody titer is high, factor VIII recovery usually cannot be measured. The recovery results are compared to the recovery of recovery results in patients treated with plasma-derived human factor VIII, recombinant human factor VIII, porcine factor VIII, and other commonly used therapeutic forms of factor VIII or factor VIII substitutes.

[0088] In a second type of clinical trial, designed to determine whether the recombinant or recombinant hybrid factor VIII is immunogenic, i.e., whether patients will develop inhibitory antibodies, recombinant or recombinant hybrid factor VIII is administered, as described in the preceding paragraph, to approximately 100 previously untreated hemophiliac patients who have not developed antibodies to factor VIII. Treatments are given approximately every 2 weeks over a period of 6 months to 1 year. At 1 to 3 month intervals during this period, blood samples are drawn and Bethesda assays or other antibody assays are performed to determine the presence of inhibitory antibodies. Recovery assays can also be done, as described above, after each infusion. Results are compared to hemophiliac patients who receive plasma-derived human factor VIII, recombinant human factor VIII, porcine factor VIII, or other commonly used therapeutic forms of factor VIII or factor VIII substitutes.

[0089] Pharmaceutical Compositions

[0090] Pharmaceutical compositions comprising recombinant or recombinant hybrid factor VIII, alone or in combination with appropriate pharmaceutical stabilization compounds, delivery vehicles, and/or carrier vehicles, are prepared according to known methods, as described in Remington's Pharmaceutical Sciences by E. W. Martin.

[0091] In one preferred embodiment, the preferred carriers or delivery vehicles for intravenous infusion are physiological saline or phosphate buffered saline.

[0092] In another preferred embodiment, suitable stabilization compounds, delivery vehicles, and carrier vehicles include but are not limited to other human or animal proteins such as albumin.

[0093] Phospholipid vesicles or liposomal suspensions are also preferred as pharmaceutically acceptable carriers or delivery vehicles. These can be prepared according to methods known to those skilled in the art and can contain, for example, phosphatidylserine/phosphatidylcholine or other compositions of phospholipids or detergents that together impart a negative charge to the surface, since factor VIII binds to negatively charged phospholipid membranes. Liposomes may be prepared by dissolving appropriate lipid(s) (such as stearoyl phosphatidyl ethanolamine, stearoyl phosphatidyl choline, arachadoyl phosphatidyl choline, and cholesterol) in an inorganic solvent that is then evaporated, leaving behind a thin film of dried lipid on the surface of the container. An aqueous solution of the hybrid factor VIII is then introduced into the container. The container is then swirled by hand to free lipid material from the sides of the container and to disperse lipid aggregates, thereby forming the liposomal suspension.

[0094] Recombinant or recombinant hybrid factor VIII can be combined with other suitable stabilization compounds, delivery vehicles, and/or carrier vehicles, including vitamin K dependent clotting factors, tissue factor, and von Willebrand factor (vWf) or a fragment of vWf that contains the factor VIII binding site, and polysaccharides such as sucrose.

[0095] Recombinant or recombinant hybrid factor VIII can also be delivered by gene therapy in the same way that human factor VIII can be delivered, using delivery means such as retroviral vectors. This method consists of incorporation of factor VIII cDNA into human cells that are transplanted directly into a factor VIII deficient patient or that are placed in an implantable device, permeable to the factor VIII molecules but impermeable to cells, that is then transplanted. The preferred method will be retroviral-mediated gene transfer. In this method, an exogenous gene (e.g., a factor VIII cDNA) is cloned into the genome of a modified retrovirus. The gene is inserted into the genome of the host cell by viral machinery where it will be expressed by the cell. The retroviral vector is modified so that it will not produce virus, preventing viral infection of the host. The general principles for this type of therapy are known to those skilled in the art and have been reviewed in the literature (e.g., Kohn, D. B. et al. [1989] Transfusion 29:812-820).

[0096] Recombinant or recombinant hybrid factor VIII can be stored bound to vWf to increase the half-life and shelf-life of the hybrid molecule. Additionally, lyophilization of factor VIII can improve the yields of active molecules in the presence of vWf. Current methods for storage of human and animal factor VIII used by commercial suppliers can be employed for storage of hybrid factor VIII. These methods include: (1) lyophilization of factor VIII in a partially-purified state (as a factor VIII “concentrate” that is infused without further purification); (2) immunoaffinity-purification of factor VIII by the Zimmerman method and lyophilization in the presence of albumin, which stabilizes the factor VIII; (3) lyophilization of recombinant factor VIII in the presence of albumin.

[0097] Additionally, hybrid factor VIII has been indefinitely stable at 4° C. in 0.6 M NaCl, 20 mM MES, and 5 mM CaCl2 at pH 6.0 and also can be stored frozen in these buffers and thawed with minimal loss of activity.

[0098] Methods of Treatment

[0099] Recombinant or recombinant hybrid factor VIII is used to treat uncontrolled bleeding due to factor VIII deficiency (e.g., intraarticular, intracranial, or gastrointestinal hemorrhage) in hemophiliacs with and without inhibitory antibodies and in patients with acquired factor VIII deficiency due to the development of inhibitory antibodies. The active materials are preferably administered intravenously.

[0100] Additionally, recombinant or recombinant hybrid factor VIII can be administered by transplant of cells genetically engineered to produce the hybrid or by implantation of a device containing such cells, as described above.

[0101] In a preferred embodiment, pharmaceutical compositions of recombinant or recombinant hybrid factor VIII alone or in combination with stabilizers, delivery vehicles, and/or carriers are infused into patients intravenously according to the same procedure that is used for infusion of human or animal factor VIII.

[0102] The treatment dosages of recombinant or recombinant hybrid factor VIII composition that must be administered to a patient in need of such treatment will vary depending on the severity of the factor VIII deficiency. Generally, dosage level is adjusted in frequency, duration, and units in keeping with the severity and duration of each patient's bleeding episode. Accordingly, the hybrid factor VIII is included in the pharmaceutically acceptable carrier, delivery vehicle, or stabilizer in an amount sufficient to deliver to a patient a therapeutically effective amount of the hybrid to stop bleeding, as measured by standard clotting assays.

[0103] Factor VIII is classically defined as that substance present in normal blood plasma that corrects the clotting defect in plasma derived from individuals with hemophilia A. The coagulant activity in vitro of purified and partially-purified forms offactor VIII is used to calculate the dose offactor VIII for infusions in human patients and is a reliable indicator of activity recovered from patient plasma and of correction of the in vivo bleeding defect. There are no reported discrepancies between standard assay of novel factor VIII molecules in vitro and their behavior in the dog infusion model or in human patients, according to: Lusher, J. M. et al. 328 New Engl. J. Med. 328:453-459; Pittman, D. D. et al., (1992)Blood 79:389-397; and Brinkhous et al. (1985) Proc. Natl. Acad. Sci. 82:8752-8755.

[0104] Usually, the desired plasma factor VIII level to be achieved in the patient through administration of the recombinant or recombinant hybrid factor VIII is in the range of 30-100% of normal. In a preferred mode of administration of the recombinant or recombinant hybrid factor VIII, the composition is given intravenously at a preferred dosage in the range from about 5 to 50 units/kg body weight, more preferably in a range of 10-50 units/kg body weight, and most preferably at a dosage of 20-40 units/kg body weight; the interval frequency is in the range from about 8 to 24 hours (in severely affected hemophiliacs); and the duration of treatment in days is in the range from 1 to 10 days or until the bleeding episode is resolved. See, e.g., Roberts, H. R., and M. R. Jones, “Hemophilia and Related Conditions—Congenital Deficiencies of Prothrombin (Factor II, Factor V, and Factors VII to XII),” Ch. 153, 1453-1474, 1460, in Hematology, Williams, W. J., et al., ed. (1990). Patients with inhibitors may require more recombinant or recombinant hybrid factor VIII, or patients may require less recombinant or recombinant hybrid factor VIII because of its higher specific activity than human factor VIII or decreased antibody reactivity or immunogenicity. As in treatment with human or porcine factor VIII, the amount of recombinant or recombinant hybrid factor VIII infused is defined by the one-stage factor VIII coagulation assay and, in selected instances, in vivo recovery is determined by measuring the factor VIII in the patient's plasma after infusion. It is to be understood that for any particular subject, specific dosage regimens should be adjusted over time according to the individual need and the professional judgment of the person administering or supervising the administration of the compositions, and that the concentration ranges set forth herein are exemplary only and are not intended to limit the scope or practice of the claimed composition.

[0105] Treatment can take the form of a single intravenous administration of the composition or periodic or continuous administration over an extended period of time, as required. Alternatively, recombinant or recombinant hybrid factor VIII can be administered subcutaneously or orally with liposomes in one or several doses at varying intervals of time.

[0106] Factor VIII can also be used to treat uncontrolled bleeding due to factor VIII deficiency in hemophiliacs who have developed antibodies to human factor VIII. In this case, coagulant activity that is superior to that of human or animal factor VIII alone is not necessary. Coagulant activity that is inferior to that of human factor VIII (i.e., less than 3,000 units/mg) will be useful if that activity is not neutralized by antibodies in the patient's plasma.

[0107] The recombinant or recombinant hybrid factor VIII molecule and the methods for isolation, characterization, making, and using it generally described above will be further understood with reference to the following non-limiting examples.

EXAMPLES

[0108] Materials—Citrated hemophilia A plasma and normal pooled human plasma (FACT) were purchased from George King Biomedical, Inc. (Overland Park, Kans.). Heparin-Sepharose was purchased from Sigma Chemical Co.(St. Louis, Mo.). Fetal bovine serum, geneticin, penicillin, streptomycin, DMEM/F12 medium and AIM-V medium were purchased from Life Technologies, Inc. (Gaithersburg, Md.). Pfu DNA polymerase and the phagemid pBlueScript II KS− were purchased from Stratagene (La Jolla, Calif.). Murine anti-human fVIII monoclonal antibodies ESH4 and ESH8 were purchased from American Diagnostica (Greenwich, Conn.). The murine fVIII C2-specific inhibitory monoclonal antibody NMC VIII-5 was obtained from Dr. Midori Shima, Nara Medical College, Japan. A human fVIII C2-specific IgG4κ monoclonal antibody, BO2C11, which was cloned from a transformed B cell line from patient with hemophilia, was prepared as described previously (Jacquemin, M. G. et al., 1998, “Mechanism and kinetics of factor VIII inactivation: study with an IgG4 monoclonal antibody derived from a hemophilia A patient with inhibitor,” Blood 92:496-506). Citrated human plasmas from five inhibitor patients, AA, AJ, HR LK, and RvR, were obtained from Dr. Dorothea Scandella. They were used either without further purification (HR, RvR, and AJ) or as IgG preparations (LK and AA). Inhibitor IgG was prepared as described previously (Scandella, D., L. et al., 1992, “A soluble recombinant factor VIII fragment containing the A2 domain binds to some human anti-factor VIII antibodies that are not detected by immunoblotting,” Thromb.Haemostas. 67:665-671). The inhibitors in HR, LK, AA, and RvR antibodies were specific for the C2 domain as judged by antibody neutralization assays (Prescott, R. et al., 1997, “The inhibitory antibody response is more complex in hemophilia A patients than in most nonhemophiliacs with FVIII autoantibodies,” Blood 89:3663-3671). AJ was identified as a C2-specific using a panel of recombinant hybrid human/porcine FVIII molecules (Barrow, R. T. et al., 2000, “Reduction of the antigenicity of factor VIII toward complex inhibitory plasmas using multiply-substituted hybrid human/porcine factor VIII molecules,” Blood 95:557-561). Albumin-free recombinant full-length fVIII was obtained from the Hyland-Immuno Division of Baxter Healthcare (Deerfield, Ill.). Synthetic oligonucleotides were purchased from Life Technologies, Inc. (Gaithersburg, Md.). Restriction enzymes were purchased from New England Biolabs (Beverly, Mass.) or Promega (Madison, Wis.). A cell line derived from baby hamster kidney cells was obtained from Dr. R. T. A. Macgillivray (Funk, W. D. et al., 1990, “Expression of the amino-terminal half-molecule of human serum transferrin in cultured cells and characterization of the recombinant protein,” Biochemistry 29:1654-1660). A B-domainless fVIII expression vector, designated HB-/ReNeo, containing a NotI site two bases 3′ to the stop codon and ampicillin and geneticin resistance genes was prepared as described previously (Healey, J. F. et al., 1998, “Residues Glu2181-Val2243 contain a major determinant of the inhibitory epitope in the C2 domain of human factor VIII,” Blood 92:3701-3709). HSQ/ReNeo, a human B-domainless fVIII molecule containing a fourteen amino acid segment, SerPheSerGlnAsnProPro ValLeuLysArgHisGlnArg, in place of the B domain in human fVIII (Lind, P. et al., 1995, “Novel forms of B-domain-deleted recombinant factor VIII molecules. Construction and biochemical characterization,” Eur. J. Biochem. 232:19-27) was constructed by splicing-by-overlap extension (SOE) mutagenesis (Horton, R. M. et al., 1993, “Gene splicing by overlap extension,” Methods Enzymol. 217:270-279) using HB-/ReNeo as template, essentially as described previously for the corresponding porcine molecule (Healey, J. F. et al., 1998, “Residues Glu2181-Val2243 contain a major determinant of the inhibitory epitope in the C2 domain of human factor VIII,” Blood 92:3701-3709). HP20, a B-domainless hybrid human/porcine fVIII molecule containing human A1, A2, ap-A3, and C1 domains and the porcine C2 domain was prepared as described previously (Healey, J. F., supra, 1998).

[0109] Plasmid DNA was purified using a Qiagen Plasmid Maxi Kit (Qiagen, Inc., Valencia, Calif.). PCR reactions were done using a Hybrid OmniGene thermocycler using Pfu DNA polymerase. PCR products were gel purified, precipitated with ethanol, and ligated into plasmid DNA using T4 DNA ligase (Rapid DNA Ligation Kit, Boehringer Mannheim, Indianapolis, Ind.). Insert-containing plasmids were used to transform E. coli Epicurean XL1-Blue cells. All novel fVIII DNA sequences generated by PCR were confirmed by dideoxy sequencing using an Applied Biosystems (Foster City, Calif.) 373a automated DNA sequencer and the PRISM dye terminator kit.

Example 1

Construction of fVIII Mutant cDNAs

[0110] Mutations were made in HSQ codons by SOE mutagenesis to produce the following proteins: Met2199Ile (human to porcine), ATG to ATC, Phe2200Leu (human to canine), TTT to TTG, Val2223Ala (human to canine), GTG to GCC, Lys2227Glu (human to porcine), AAA to GAG, Leu2252Phe (human to murine), CTT to TTC, Met2199Ile/Phe2200Leu, ATG to ATC and TTT to TTG, Val2223Ala/Lys2227Glu, GTG to GCC and AAA to GAG, Met2199Ile/Phe2200Leu/Val2223Ala/Lys2227Glu, ATG to ATC, TTT to TTT, GTG to GCC, and AAA to GAG.

[0111] HSQ/ReNeo was used as the template in the PCR reactions. The first PCR reaction used the human C1 primer, SEQ ID NO: 3, 5′-GTG GAT TCA TCT GGG ATA AAA CAC-3′, designated H3763+, corresponding to nucleotides 3763-3786 in the HSQ sequence, as the sense primer. The following primers were used as antisense primers:

1Met2199Ile,5′-AGG AGA CCA GGT GGC AAA GAT ATT GGT AAA GTA GGA TGA-3′,SEQ ID NO:4,Phe2200Leu,5′-TGA AGG AGA CCA GGT GGC CAA CAT ATT GGT AAA GTA GGA-3′,SEQ ID NO:5,Val2223Ala,5′-CCA CTC TTT TGG ATT ATT GGC CTG AGG TCT CCA GGC ATT-3′,SEQ ID NO:6,Lys2227Glu,5′-GTC CAC TTG CAG CCA CTC CTC TGG ATT ATT CAC CTG AGG-3′,SEQ ID NO:7,Leu2252Phe,5′-CTT CAC ATA CAT GCT GGT GAA CAG AGA TTT TAC TCC CTG-3′,SEQ ID NO:8,Met2199Ile/Phe2200Leu,5′-AGG AGA CCA GGT GGC CAA GAT ATT GGT AAA GTA GGA TGA-3′, andSEQ ID NO:9,Val2223Ala/Lys2227Glu,5′-CAC TTG CAG CCA CTC CTC TGG ATT ATT GGC CTG AGG TCT CCA GGC-3′.SEQ ID NO:10,

[0112] The second PCR reaction used the ReNeo primer, SEQ ID NO: 11, 5′-AGT TTT TCT ACA ACA GAG GAA GTG-3′, designated RE1110-, which is 3′ to the C2 domain, as antisense primer. The following primers were used as sense primers: Met2199Ile, SEQ ID NO: 12, 5′-TCA

2Met2199Ile,5′-TCA TCC TAC TTT ACC AAT ATC TTT GCC ACC TGG TCT CCT-3′,SEQ ID NO:12,Phe2200Leu,5′-TCC TAC TTT ACC AAT ATG TTG GCC ACC TGG TCT CCT TGA-3′,SEQ ID NO:13,Val2223Ala,5′-AAT GCC TGG AGA CCT CAG GCC AAT AAT CCA AAA GAG TGG-3′,SEQ ID NO:14,Lys2227Glu,5′-CCT CAG GTG AAT AAT CCA GAG GAG TGG CTG CAA GTG GAC-3′,SEQ ID NO:15,Leu2252Phe,5′-CAG GGA GTA AAA TCT CTG TTC ACC AGC ATG TAT GTG AAG-3′,SEQ ID NO:16,Met2199Ile/Phe2200Leu,5′-TCA TCC TAC TTT ACC AAT ATC TTG GCC ACC TGG TCT CCT-3′, andSEQ ID NO:17,Val2223Ala/Lys2227Glu,5′-GCC TGG AGA CCT CAG GCC AAT AAT CCA GAG GAG TGG CTG CAA GTG-3′.SEQ ID NO:18,

[0113] The SOE reaction used fragments from the PCR reactions as templates and H3763+ and RE1110 as primers. The SOE product and HSQ/ReNeo ligation fragments were generated using Swa I and Not I.

[0114] The Met2199Ile/Phe2200Leu/Val2223Ala/Lys2227Glu cDNA was constructed as follows. The Met2199Ile/Phe2200Leu cDNA was moved into pBluescript II KS- and digested with Bsu36 I. The Val2223Ala/Lys2227Glu cDNA also was digested with Bsu36 I and the appropriate fragments were ligated. The resulting Met2199Ile/Phe2200Leu/Val2223Ala/Lys2227Glu cDNA was moved into ReNeo by digestion with Swa I and Not I.

Example 2

Expression of Recombinant fVIII Molecules

[0115] Transfected cell lines were maintained in Dulbecco's modified Eagle's medium-F12 containing 10% fetal bovine serum, 50 U/ml penicillin, and 50 μg/ml streptomycin. Fetal bovine serum was heat inactivated for one hour at 56° C. before use. Mutant cDNAs in ReNeo were stably transfected into BHK cells, selected for geneticin resistance, switched to serum-free, AIM-V medium for expression, and partially purified by heparin-Sepharose chromatography as described previously (Healey, J. F. et al, supra, 1998).

Example 3

FVIII and fVIII Inhibitor Assays

[0116] The activity of recombinant fVIII proteins was measured by one-stage clotting assay (Bowie, E. J. W. and C. A. Owen, 1984, “The clinical and laboratory diagnosis of hemorrhagic disorders,” In Disorders of Hemostasis, O. D. Ratnoff and C. D. Forbes, editors. Grune & Stratton, Inc., Orlando, Fla. 43-72). One unit of fVIII is defined as the activity in one ml of normal citrated human plasma. FVIII inhibitor titers were measured by a modification of the Bethesda assay (Kasper, C. K. et al., 1975, “A more uniform measurement of factor VIII inhibitors,” Thromb. Diath. Haemorrh. 34:869-872) as follows. Recombinant FVIII was added to hemophilia A plasma to a final concentration of 0.8-1.2 units per ml and incubated with varying concentrations of inhibitor for 2 hours at 37° C. To determine the 50% inhibition point that defines the Bethesda unit, dilutions of inhibitor were made that produced residual activities that spanned at least the 35% to 65% range. In some cases, replicate dilutions were made, in which case the average was used. An average of 10 dilutions was made for the determination of each Bethesda titer. Semi-logarithmic plots of per cent residual activity versus the log of the reciprocal of the inhibitor dilution appeared linear in all cases. The data were fitted by nonlinear regression using the Marquardt algorithm (SigmaPlot 5.0, SPSS, Inc.) to the equation

% Residual activity=m(log x−log x50)+50

[0117] where the fitted parameter x50 is the reciprocal dilution that produces 50% inhibition, the fitted parameter m is the slope of the semi-log line and the independent variable x is the reciprocal dilution of the inhibitor sample. The standard error of the estimate (average deviation of data points from the regression line) for 62 Bethesda assays was 10.0±4.0 (mean±1 SD), indicative of the relatively low precision that is inherent in the assay.

[0118] The Bethesda titer equals x50−1. The estimate of the standard error (SD) of the Bethesda titer was calculated by multiplying the Bethesda titer by the coefficient of variation of x50. The Bethesda titers of fVIII mutants and HB- were compared by Student's t test. The mass concentration of fVIII in partially purified preparations was determined by a sandwich ELISA using ESH4 as capture antibody and biotinylated ESH8 as detection antibody as described previously (Lubin, I. M. et al., 1994, “Elimination of a major inhibitor epitope in factor VIII,” J. Biol. Chem. 269:8639-8641). Full-length recombinant FVIII was used as the standard and values were corrected for the difference in mass between full-length and B-domainless forms of fVIII. Samples were assayed in quadruplicate. The average coefficient of variation was 9.0%. The specific activity of FVIII molecules was calculated by dividing the coagulant activity by the concentration as determined by ELISA. The following values were obtained (units per mg): HB-, 7,800; Met2199Ile, 12,800; Phe2200Leu, 10,200; Val2223Ala, 19,600; Lys2227Glu, 36,200; Leu2252Phe, 10,100; Met2199Ile/Phe2200Leu, 10,000; Val2223Ala/Lys2227Glu, 33,200; Met2199Ile/Phe2200Leu/Val2223Ala/Lys2227Glu, 14,200. The apparent specific activity of some of the mutants is higher than HB-. This may be due to a relatively small decreased ability of the mutants to bind either the capture or detection antibody compared to HB-, leading to an underestimate of fVIII mass and an overestimate of the specific activity.

3TABLE 1Antigenicity of FVIII C2 mutants towards C2-SpecificInhibitory Antibodies Compared to Human FVIIIAntigenicityaMutantLessEqualMoreMet2199Ile4/70/73/7Phe2200Leu4/72/71/7Val2223Ala0/72/75/7Lys2227Glu2/71/74/7Leu2252Phe4/73/70/7Met2199Ile/Phe2200Leu6/71/70/7Val2223Ala/Lys2227Glu4/71/72/7Met2199Ile/Phe2200Leu/7/70/70/7Val2223Ala/Lys2227GluHP207/70/70/7aSignificant difference at the 99% confidence level

[0119]

	Number	Date	Country
	60236460	Sep 2000	US
	60234047	Sep 2000	US

Modified factor VIII

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