Inhibitor incidence with recombinant vs. plasma-derived FVIII in previously untreated patients with severe hemophilia A: homogeneous results from four published observational studies

Impact of the Survey of Inhibitors in Plasma-Product Exposed Toddlers (SIPPET) study and its post hoc analyses on clinical practice in the United States: A survey of Haemophilia and Thrombosis Research Society members

INTRODUCTION

A recent randomized trial, the Survey of Inhibitors in Plasma-Product Exposed Toddlers (SIPPET), confirmed that exposure to recombinant FVIII (rFVIII) products doubled the risk of inhibitor development compared to plasma-derived FVIII (pdFVIII) in previously untreated (or minimally treated) patients (PUPs) with severe haemophilia A. SIPPET post hoc analyses showed that early exposure to rFVIII was more immunogenic and that rFVIII could harm low-risk PUPs with non-null mutations. Clinical implications of SIPPET findings for the haemophilia community were unclear.

AIM

Study the impact of the SIPPET study and its post hoc analyses on clinical practice for PUPs with severe haemophilia A in the United States.

METHODS

Members of the North American Hemophilia and Thrombosis Research Society (HTRS) completed two online questionnaires related to SIPPET publications and PUP management (study period: 12/2016-8/2018).

RESULTS

Over 50% participated the study. Sixty per cent expressed methodological concerns about the SIPPET study, yet 55% shared the study with new families. During the study period, rFVIII selection fell from 43/61 (70%) to 15/54 (28%) while use of pdFVIII and shared decision-making increased from 5/61 (8%) to 9/54 (17%) and from 4/61 (7%) to 10/54 (19%), respectively. Based on post hoc analyses, 44/54 (82%) would change their clinical practice with 31/44 (70%) using pdFVIII for PUPs. Barriers to translation of SIPPET analyses included study design concerns, non-inclusion of novel therapies, inability to perform genetic testing at diagnosis and risk of plasma-derived infections.

CONCLUSION

Despite the methodological concerns about the SIPPET study, this Grade I evidence appears to have influenced the clinical practice of haemophilia providers in the United States.

The C1 and C2 domains of blood coagulation factor VIII mediate its endocytosis by dendritic cells

The development of inhibitory antibodies to therapeutic factor VIII is the major complication of replacement therapy in patients with hemophilia A. The first step in the initiation of the anti-factor VIII immune response is factor VIII interaction with receptor(s) on antigen-presenting cells, followed by endocytosis and presentation to naïve CD4⁺ T cells. Recent studies indicate a role for the C1 domain in factor VIII uptake. We investigated whether charged residues in the C2 domain participate in immunogenic factor VIII uptake. Co-incubation of factor VIII with BO2C11, a monoclonal C2-specific immunoglobulin G, reduced factor VIII endocytosis by dendritic cells and presentation to CD4⁺ T cells, and diminished factor VIII immunogenicity in factor VIII-deficient mice. The mutation of basic residues within the BO2C11 epitope of C2 replicated reduced in vitro immunogenic uptake, but failed to prevent factor VIII immunogenicity in mice. BO2C11 prevents factor VIII binding to von Willebrand factor, thus potentially biasing factor VIII immunogenicity by perturbing its half-life. Interestingly, a factor VIII^Y1680C mutant, that does not bind von Willebrand factor, demonstrated unaltered endocytosis by dendritic cells as well as immunogenicity in factor VIII-deficient mice. Co-incubation of factor VIII^Y1680C with BO2C11, however, resulted in decreased factor VIII immunogenicity in vivo In addition, a previously described triple C1 mutant showed decreased uptake in vitro, and reduced immunogenicity in vivo, but only in the absence of endogenous von Willebrand factor. Taken together, the results indicate that residues in the C1 and/or C2 domains of factor VIII are implicated in immunogenic factor VIII uptake, at least in vitro Conversely, in vivo, the binding to endogenous von Willebrand factor masks the reducing effect of mutations in the C domains on factor VIII immunogenicity.

Factor VIII inhibitors: risk factors and methods for prevention and immune modulation

Patients with hemophilia A are deficient in coagulation Factor VIII. This bleeding disorder can be treated with Factor VIII replacement therapy, but close to a third of patients will be immunized to the treatment and begin to form inhibitory antibodies known as "inhibitors". These inhibitors will render the treatment ineffective and represent the most severe complication in the treatment of hemophilia A. In this review, we highlight factors involved in inhibitor development and emphasize research being done to modulate the immune response to this life-saving therapy.

A role for thrombin in the initiation of the immune response to therapeutic factor VIII

Administration of human factor VIII (FVIII) to FVIII knockout hemophilia mice is a useful small animal model to study the physiologic response in patients iatrogenically immunized to this therapeutic protein. These mice manifest a robust, T cell-dependent, antibody response to exogenous FVIII treatment, even when encountered through traditionally tolerogenic routes. Thus, FVIII given via these routes elicits both T- and B-cell responses, whereas a control, foreign protein, such as ovalbumin (OVA), is poorly immunogenic. When FVIII is heat inactivated, it loses function and much of its immunogenicity. This suggests that FVIII's immunogenicity is principally tied to its function and not its structure. If mice are treated with the anticoagulant warfarin, which depletes other coagulation factors including thrombin, there is a reduced immune response to FVIII. Furthermore, when mice are treated with the direct thrombin inhibitor, hirudin, the T-cell responses and the serum anti-FVIII antibody concentrations are again significantly reduced. Notably, when FVIII is mixed with OVA, it acts to increase the immune response to OVA. Finally, administration of thrombin with OVA is sufficient to induce immune responses to OVA. Overall, these data support the hypothesis that formation of thrombin through the procoagulant activity of FVIII is necessary to induce costimulation for the immune response to FVIII treatment.

Recombinant factor VIII in the management of hemophilia A: current use and future promise

Hemophilia A is a rare inherited bleeding disorder due to mutation of the gene that encodes the coagulation protein factor VIII. Historically, prior to the availability of treatment with factor VIII preparations, most boys died from uncontrolled bleeding, either spontaneous bleeding or after injury, before reaching 20 years of age. One of the most impressive triumphs of modern medicine is that with current recombinant factor VIII replacement therapy, a boy born in the 21st century with severe hemophilia A can anticipate a normal life expectancy with essentially no permanent complications from bleeding. For severe hemophilia A, current optimal treatment should have two goals: first, to provide sufficient factor VIII to prevent spontaneous bleeding, and second, to provide sufficient factor VIII to have normal coagulation function after any trauma. However, the replacement therapy requires tremendous resources for effective use, and remains extraordinarily expensive. Thus there are opportunities for further advances in therapy for hemophilia A. Two major concerns continue to trouble current optimal treatment approaches: some patients will develop neutralizing antibodies during the first 50 infusions of therapeutic factor VIII, and second, to administer therapeutic factor VIII every other day in young boys often requires placement of a central venous access device, and such use carries the life-threatening risks of infection and thrombosis. Because of the effectiveness of current therapy, any new developments in treatment will require significant concerns for safety, both immediate and in the long term. A number of research groups seek to prolong the biological efficacy of infused recombinant factor VIII. Currently, one such promising development is in the advanced stages of clinical trial. The goals will be to improve further the quality of life of an individual with severe hemophilia A, and to reduce the burden of current treatment strategies on families and medical resources. Hopefully, the hemophilia community will continue to participate actively in the clinical trials needed to address these new challenges.