Key points

Introduction

The commonest severe inherited bleeding disorder in all ethnic groups worldwide is hemophilia A, followed by hemophilia B. These are X-linked recessive disorders that result from mutations in the genes for blood clotting factor VIII (FVIII) in hemophilia A or factor IX (FIX) in hemophilia B. The incidence of hemophilia A in live male births is approximately 1 in 5000, and of hemophilia B, 1 in 25,000. Bleeding tendency varies but correlates best with the residual circulating factor level, which in turn depends on the genotype of the mutation that prevents synthesis and/or interferes with function of the affected factor. If the residual factor level is 5% of normal or greater, subjects can be assigned to the mild hemophilia category, in which spontaneous bleeding is absent and only occurs after significant trauma. Wherein residual factor level is less than 5% but greater than 1%, patients are considered to have moderate hemophilia with a variable bleeding tendency; some in this group seldom have any bleeding, whereas others experience frequent bleeding after minor trauma. About half of patients with hemophilia A or B have factor levels less than 1% of normal. These individuals have a severe bleeding tendency with frequent spontaneous musculoskeletal and soft tissue bleeding. A recent careful study of the hemophilic patient population at a large Dutch clinic confirmed these correlations and the basic division into severe, moderate, and mild, but added the insight that those mildly affected patients whose residual factor level is 13% or greater never experienced joint bleeding. Thus, factor levels of greater than 13% could be considered as a target for gene therapy to attain. Among those patients who do bleed into their joints, the ankles are most commonly affected starting in early childhood, with knees and elbows affected later. Repeated episodes of intra-articular bleeding cause severe, progressive, destructive arthropathy with deformity leading to complete loss of joint function and attendant disability.

In the absence of replacement therapy, the life expectancy of a boy with severe hemophilia is only about 10 years. This severe shortening of life still applies in many less-developed countries. Even in developed countries until the 1960s, treatment of hemophilia was limited to infusion of fresh frozen plasma. In 1968, the first widely available concentrate for hemophilia A, cryoprecipitate, was introduced. During the 1970s and 1980s, many multidonor factor concentrates were developed to improve the purity, potency, stability, and convenience of administration of factor replacement therapy. However, these developments, depending as they did on large donor pools of often commercially sourced plasma, permitted transmission of human immunodeficiency virus (HIV) and hepatitis C virus. Almost a whole generation of hemophiliacs who were given the new products became HIV positive and died of AIDS before highly effective antiretroviral therapies were developed. During the period 1970 to 1986, every treated patient was also exposed to hepatitis C, and up to 25 years later, some are still succumbing to chronic liver failure resulting from continued infection. From 1986 onward, heat treatment and then the solvent detergent method inactivated both HIV and hepatitis C virus. Since then, there have been no new cases of transmission of those lipid enveloped viruses. Transmission by blood products of other pathogens resistant to inactivation, such as parvovirus, hepatitis A, and prions (variant Creutzfeldt-Jakob disease ), remain a major concern. Recombinant factor concentrates are of course free from blood-borne infections, but their availability has been limited to the most developed countries by very high cost and production constraints. With the expiry of patents on recombinant FVIII and FIX, biosimilars and other variants with enhanced pharmacokinetic or other properties are entering the market, with potential for wider availability than hitherto.

In developed countries, standard hemophilia care for severely affected patients now consists of home-administered prophylaxis with safe concentrates intended to maintain factor level greater than 1% of normal. This is a compromise based on cost and practical considerations, which reduces but does not eliminate bleeding. If started in early childhood after the first joint bleed, arthropathy can be largely prevented. When continued throughout life, prophylaxis leads to near normalization of life expectancy. However, the relatively short half-life of FVIII and FIX in the circulation necessitates frequent intravenous administration of factor concentrates (at least 2–3 times a week), which is demanding and extremely expensive; annualized costs of prophylaxis for an adult equal or exceed £120,000 for patients with hemophilia B. Even with prophylaxis, significant limitations remain because normal plasma clotting factor levels are not consistently restored; the short half-life of existing clotting factors results in peaks and troughs of circulating clotting factor associated with breakthrough bleeding. The saw-tooth pattern of factor level, high immediately after infusion, falling rapidly to near baseline, mandates careful planning of physical activities such as sports, which people living without hemophilia can hardly imagine. New modified synthetic formulations of FVIII and FIX that are pegylated or fused to proteins with long half-life such as albumin or Fcγ have greatly improved the activity profile for FIX but have been less impressive for FVIII because of the dominant role of von Willebrand factor in determining its half-life. In any case, these products do not remove the problems of lifelong intravenous administration, breakthrough bleeding, and ever-mounting cost. The cumulative effects of lifelong administration of pegylated proteins are unknown, as is the potential of fusion proteins to induce an immune response. Two other entirely novel approaches to normalizing thrombin generation in hemophilia are undergoing extensive trials at the time of this writing (January 2017). The first is a synthetic FVIII mimic consisting of linked antibodies, one of which binds factor IXa and the other factor X (Emicizumab). Although restoring thrombin generation to a degree comparable to FVIII level of about 15% in patients with or without inhibitory antibody, there is a major difference from wild-type FVIII. The mimic is under no control of its activity, being permanently active throughout the circulation, whereas native FVIII has very strictly controlled activity in both time and site of action. It circulates as a procofactor tightly bound to a carrier; it is activated only at sites of clot propagation, and it has a very short half-life after activation. The consequences of these differences have recently emerged in thrombotic events occurring in patients treated with Emicizumab and bypass clotting agents. The second alternative approach is to lower the natural antithrombin level with antisense RNA technology. Both approaches have shown efficacy in reducing the rate of bleeding, but their use may be limited by risk of thrombogenicity, and both still require lifelong injections without restoring normal hemostasis.

Even set against this scenario of widening therapeutic choice, gene therapy offers a strikingly attractive potential for cure by means of the endogenous production of FVIII or FIX following transfer of a normal copy of the respective gene. The hemophilias were recognized in the 1980s as good candidates for gene therapy because all their clinical manifestations are due to lack of a single protein that circulates in minute amounts in the bloodstream. Years of clinical experience and the natural experiment of moderate hemophilia prove that a small increase to 1% to 2% in circulating levels of the deficient clotting factor significantly modifies the bleeding diathesis; even a modest response to gene therapy can be effective. Tight regulation of transgene expression is unnecessary because a wide range of FIX or FVIII levels is without toxicity and effective at reducing bleeding. Animal models such as FVIII- and FIX-knockout mice, and dogs with hemophilia A or B, have facilitated extensive preclinical evaluation of gene therapy strategies. The efficiency of therapy can be assessed easily just by measuring plasma levels of FVIII or FIX. The complementary DNA (cDNA) for the gene encoding FIX is small and adaptable to gene transfer in many viral systems. In addition, its expression pathway is significantly less complex than that of FVIII, and it is natively expressed at higher levels. Consequently, more gene transfer studies have focused on hemophilia B than hemophilia A, but this is rapidly changing as the technology evolves.