The application of bypassing-agent prophylaxis in haemophilia A patients with inhibitors: a meeting report

Spanish Consensus Guidelines on prophylaxis with bypassing agents in patients with haemophilia and inhibitors

Prophylaxis with the blood clotting factor, factor VIII (FVIII) is ineffective for individuals with haemophilia A and high-titre inhibitors to FVIII. Prophylaxis with the FVIII bypassing agents activated prothrombin complex concentrates (aPCC; FEIBA® Baxalta) or recombinant activated factor VII (rFVIIa; Novo-Seven®, Novo Nordisk) may be an effective alternative. It was our aim to develop evidence -and expert opinion- based guidelines for prophylactic therapy for patients with high-titre inhibitors to FVIII. A panel of nine Spanish haematologists undertook a systematic review of the literature to develop consensusbased guidance. Particular consideration was given to prophylaxis in patients prior to undergoing immune tolerance induction (ITI) (a process of continued exposure to FVIII that can restore sensitivity for some patients), during the ITI period and for those not undergoing ITI or for whom ITI had failed. These guidelines offer guidance for clinicians in deciding which patients might benefit from prophylaxis with FVIII bypassing agents, the most appropriate agents in various clinical settings related to ITI, doses and dosing regimens and how best to monitor the efficacy of prophylaxis. The paper includes recommendations on when to interrupt or stop prophylaxis and special safety concerns during prophylaxis. These consensus guidelines offer the most comprehensive evaluation of the clinical evidence base to date and should be of considerable benefit to clinicians facing the challenge of managing patients with severe haemophilia A with high-titre FVIII inhibitors.

Hemophilia and inhibitors: current treatment options and potential new therapeutic approaches

The immune response to infused factor concentrates remains a major source of morbidity and mortality in the treatment of patients with hemophilia A and B. This review focuses on current treatment options and novel therapies currently in clinical trials. After a brief review of immune tolerance regimens, the focus of the discussion is on preventing bleeding in patients with hemophilia and inhibitors. Recombinant factor VIIa and activated prothrombin complex concentrates are the mainstays in treating bleeds in patients with inhibitors. Both agents have been shown to reduce bleeding episodes to a similar degree when infused prophylactically; however, individual patients may respond better to one agent over the other at any given time. The international immune tolerance trial revealed that a high-dose factor VIII regimen provided significantly better bleeding protection than the low-dose regimen. Given the high cost of treatment and the potential for a high-dose immune tolerance regimen to prevent bleeding in some patients, we discuss how we treat patients to maximize the prevention of bleeds while minimizing cost. Novel approaches to treatment of these patients are in development. These include agents that mimic factor VIII or augment thrombin generation by bypassing the inhibitor, as well as agents that inhibit the natural anticoagulants.

Mechanisms and monitoring of bypassing agent therapy

Understanding the mechanism of action of normal hemostasis and how the bypassing agents recombinant activated factor VII (rFVIIa; NovoSeven) and plasma-derived activated prothrombin complex concentrate (Factor Eight Inhibitor Bypassing Agent [FEIBA]) control abnormal bleeding is imperative for healthcare professionals who treat patients with hemophilia and other bleeding disorders. A cell-based model has improved our understanding of in vivo mechanisms of hemostasis and the basis of the bleeding tendency in hemophilia. Bypassing agents do not restore the normal pathways of hemostasis in hemophilia, but rather boost thrombin generation in spite of a lack of platelet surface FVIIIa-FIXa ('tenase') activity. Thus, the common clinical laboratory coagulation assays do not reflect the clinically relevant hemostatic activity of bypassing agents, and no validated assay is available with which to measure the in vivo efficacy of these agents or predict individual patient responses to treatment. Global hemostasis assays measuring overall coagulation capacity have potential for assessment of the effects of bypassing agents. This review will focus on the mechanisms of clotting and their relationship to understanding the mechanisms of action of the bypassing agents in vivo and the methodologies that are emerging to monitor the clinical efficacy of bypassing agent therapy.

When should prophylaxis therapy in inhibitor patients be considered?

Currently, patients with severe haemophilia can expect to lead a relatively normal life including prevention of disabling arthropathy as a result of the development of factor replacement therapy and advances in the understanding of the use of such therapy given prophylactically. Unfortunately, a subset of patients develops neutralizing antibodies termed inhibitors rendering such therapy ineffective. These patients frequently develop recurrent joint bleeding resulting in arthropathy. Until recently, prophylactic therapy was not considered for patients with inhibitors because of the perceived lack of an effective therapeutic agent. However, an accumulation of case reports and a recent prospective study have suggested that prophylaxis with the currently available bypassing agents could be effective and appears to be safe in selected cases. This report will review the current data on prophylaxis with bypassing agents and suggest specific situations in which prophylaxis in inhibitor patients could be considered.

Clinical issues in inhibitors

SUMMARY

Anamestic inhibitors represent the major complication of haemophilia therapy now that clotting factor concentrates are virtually free of pathogen-transmission risk. Conventional clotting factor replacement is usually insufficient to prevent or treat bleeding in a haemophilia patient with a high responding inhibitor so that alternative treatment with bypassing agents is required. Despite their relative efficacy, their use does not achieve the same invariable haemostasis that patients without inhibitors do following treatment with factor concentrate replacement. This has led to the attempt to eradicate such inhibitors with immune tolerance induction. Success is not invariable, however, and many patients with long-term persistent high-titre inhibitors continue to experience great morbidity. Recently, this has given rise on a limited basis to attempts to use bypassing agents in prophylaxis regimens in an effort to alleviate this extreme morbidity. Each of these strategies is discussed in the context of their relative benefits and risks.

Biological rationale for new drugs in the bleeding disorders pipeline

Since the introduction of replacement coagulation factor infusions for the treatment of hemophilia in the 1970s and subsequent improvements in the safety profile of available factor VIII (FVIII) and factor IX (FIX) concentrates, mortality among patients with hemophilia has improved considerably and now parallels that of the noncoagulopathic population in developed countries. Substantial morbidity, however, continues from the development of inhibitory antibodies, a recognized complication of clotting factor replacement; from infections and thrombosis complicating placement of central venous catheters, which are required in children with hemophilia due to frequent prophylactic infusions of coagulation factors with defined half-lives; and from disabling joint disease in individuals without access to costly prophylaxis regimens. In response to the need for long-acting, more potent, less immunogenic, and more easily administered therapies, an impressive array of novel agents is nearly ready for use in the clinical setting. These therapeutics derive from rational bioengineering of recombinant coagulation factors or from the discovery of nonpeptide molecules that have the potential to support hemostasis through alternative pathways. The number of novel agents in clinical trials is increasing, and many of the initial results are promising. In addition to advancing treatment of bleeding episodes or enabling adherence to prophylactic infusions of clotting factor concentrate, newer therapeutics may also lead to improvements in joint health, quality of life, and tolerability of iatrogenic or comorbidity-associated bleeding challenges.

Meeting unmet needs in inhibitor patients

For patients with haemophilia, the development of inhibitors complicates treatment, and inhibitor patients may thus have a range of unmet needs. Although successful inhibitor eradication will render patients responsive to factor replacement therapy, with potentially beneficial effects on long-term outcomes, this may not always be possible. Physicians treating inhibitor patients should aim to achieve reliable control of bleeding episodes, and the prevention of joint disease should also be a priority. Patients with high-titre inhibitors require therapy with bypassing agents--recombinant activated factor VII (rFVIIa) or a plasma-derived activated prothrombin complex concentrate (pd-APCC)--for the treatment of bleeding. When treating joint haemorrhage in inhibitor patients, both aggressive treatment of intercurrent joint bleeds and prophylaxis should be considered, although evidence is needed as to whether prophylaxis with bypassing agents can significantly delay/prevent the development of osteochondral changes in patients with inhibitors. Despite physicians' best efforts, joint disease may ultimately occur in inhibitor patients, and in such instances optimizing treatment, of both early and late stages, is important. There is no single therapeutic modality for dealing with the various treatment challenges posed by inhibitor patients, but overall goals should be to improve quality of life, with the provision of cost-effective care that aims to maintain physical function.