A bispecific antibody that mimics the natural activity of the essential clotting protein cofactor Factor VIII in the blood could solve the problems associated with current treatments for hemophilia A, researchers claim. The humanized IgG antibody, designated hBS23, targets both factor IXa (FIXa) and factor X (FX), and effectively does the same job as FVIII in the blood-clotting cascade by bringing the two target molecules into correct contact with each other, to allow the FXIa-catalyzed conversion of FX to its activated form FXa.

Moreover, claim the antibody’s developers at Nara Medical University in Japan, and Chugai Pharmaceuticals’ Fuji-Gotemba Research Laboratories, when compared with native FVIII, hBS23 has a much longer half life, could potentially be administered subcutaneously rather than intravenously, and shouldn’t trigger the formation of inhibitory antibodies that represent a major problem with FVIII therapy.

Hemophilia A is currently treated using FVIII replacement therapy, but short half life and low subcutaneous bioavailability means that patients need to receive frequent intravenous injections. And while modified FVIII agents such as FVIII Fc fusion protein in clinical trials may demonstrate longer half lives than native FVIII, they are just as likely to be affected by one of the major issues associated with FVIII therapy, which is the development of inhibitory antibodies, state Fuji-Gotemba Research Laboratories’ Kunihiro Hattori, Ph.D., Takehisa Kitazawa, Ph.D., and colleagues. In fact, about 30% of patients with severe hemophilia A and some patients with less severe forms of the disease will develop antibodies to the administered FVIII molecule, rendering the treatment ineffective. These individuals need alternative treatments such as recombinant activated factor VII and activated prothrombin complex concentrates, which aren’t always effective, are expensive, and have even shorter half lives than FVIII.

As an alternative approach the investigators have developed a humanized bispecific IgG antibody, hBS23, which recognizes FIXa with one arm and FX with the other, and effectively mimics the activity of native FVIII. Initial tests using purified coagulation factors showed that hBS23 enhances FX activation specifically in the presence of FIXa. Subsequent experiments in human FVIII-deficient plasma also confirmed that addition of the hBS23 antibody markedly reduced clotting time, and led to more efficient thrombin formation than native FVIII, even in the presence of inhibitors that would render human FVIII completely ineffective.

The investigators subsequently evaluated hSB23 in a cynomolgus monkey model of acquired hemophilia A, in which disease was generated using an FVIII neutralizing antibody, and bleeding artificially induced. While monkeys in the control group showed progressive anemia and expanded bruising, treatment using a bolus intravenous injection of hSB23 markedly prevented the drop in hemoglobin levels, and led to reduced bruising.

Interestingly the kinetics of hSB23 and native FVIII activity appear quite different, the authors note in their published paper in Nature Medicine. “Bispecific binding of FIXa and FX by hBS23 improves the interaction between these factors more efficiently than does binding by FVIIIa, but hBS23 has less potential to increase the turnover of the enzyme complex than does FVIIIa. hBS23 was calculated to have 1/14th the catalytic efficiency (kcat/Km) of FVIIIa.”

Pharmacokinetic studies in the treated monkeys showed that the hBS23 antibody exhibits a half life of about 14 days (slightly longer than other humanized or fully human IgG antibodies), and subcutaneous bioavailability of 84%, making the prospect of subcutaneous treatment at least feasible. Encouragingly, a simulation study of multiple-dose injections of hBS23 in cynomolgus monkeys indicated that once weekly subcutaneous administration of the antibody would be enough to maintain hemostasis.

Humanized antibodies generally display low immunogenicity, and the team’s sequence comparisons of hBS23 and FVIII indicated that therapy with the antibody would be unlikely to result in the development of hBS23-sepcific antibodies that are cross-reactive to FVIII. Thus, one of the most important potential benefits associated with the use of an antibody such as hBS23 is that treatment would not only be unlikely to trigger production of antibody-specific inhibitors, but also that it would be suitable for treating patients who have already developed anti-FVIII inhibitors, the investigators stress.

They admit that their prototype antibody will need optimization before it could be considered for human use. Modifications will be needed to enable large-scale clinical manufacturing, increase FVIII-mimetic activity, reduce any chance of immunogenicity, and increase half life and physicochemical properties to enable a subcutaneous formulation with an even longer dosing interval. Once this has been achieved, the team suggests, “this type of bispecific antibody may be able to overcome current limitations and reduce the burden of care for the treatment of hemophilia A.”

Drs. Hattori and Kitazawa describe their work in a paper titled “A bispecific antibody to factors IXa and X restores factor VIII hemostatic activity in a hemophilia A model.”

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