Durability of ITI Utilizing Rfviiifc

Introduction: The eradication of inhibitors using immune tolerance induction (ITI) remains the mainstay of therapy in patients with severe hemophilia A who develop inhibitors. The long-acting recombinant factor VIII Fc fusion protein, rFVIIIFc (Eloctate™), which is safe and effective in the prevention and treatment of bleeding events, may promote tolerance to FVIII as shown in preclinical animal models and an inhibitor prone child, as Fc suppresses immunoregulatory Tcells to proteins to which Fc is attached. We therefore previously hypothesized rFVIIIFc would provide effective ITI, specifically shortening and simplifying ITI, and have previously described successful inhibitor eradication in three patients. Long-term follow-up data after successful ITI in patients with severe hemophilia remains limited. In the International Immune Tolerance Induction study, at 1-year follow-up, 6 of 66 subjects who had achieved tolerance demonstrated evidence of relapse at a median of 9.5 months. Of these 6 subjects, 1 had a measurable inhibitor titer and 5 had reduced FVIII recovery. We aim to provide follow-up data on our cohort of patients who had successful inhibitor eradication utilizing rFVIIIFc for ITI.

Methods: Immune tolerance induction was initiated in three patients with severe hemophilia A and anti-VIII >5 B.U., in two as initial ITI (Pt. 1, 3), and one as salvage (Pt. 2) after failing to achieve ITI with standard rFVIII due to poor compliance. Follow-up was scheduled every 6-8 weeks, with planned determination of FVIII half-life once the anti-FVIII fell to <0.6 B.U. Tolerance was a priori defined as achieving anti-FVIII <0.6 B.U., FVIII recovery of at least 60%, and half-life (t½) >6 hours. Once a t½ >6 hours was documented, incremental reduction to rFVIIIFc occurred. Patients continued to be followed by their local HTC as per standard of care.

Results: ITI was initiated with rFVIIIFc at a dose of 100-200 IU/kg rFVIIIFc every other day or three times weekly per MD discretion. The time to initial anti-FVIII <0.6 B.U. was 4-12 weeks. Patient 1 and 2 were able to achieve tolerance, with a FVIII recovery of at least 60%, and half-life (t½) >6 hours, at weeks 18 and 17, respectively, after initiation of ITI. Patient 3 has improved but is not yet fully tolerized, as evidenced by 57% recovery and a t½ of approximately 7 hours. Anti-VIII inhibitor titers remain negative at 15, 16 and 15 months, from the initiation of ITI in patients 1, 2, and 3 respectively. Patients 1 and 2 have been able to decrease their post ITI prophylaxis dosing regimen to 80 IU/kg and 65 IU/kg three times a week while maintaining a FVIII trough of >1%. No patients were maintained on bypassing prophylaxis during ITI and no patients have experienced hemarthroses or other major bleeding event since the initiation of ITI.

Discussion: Immune tolerance induction was successful in three children with inhibitors using rFVIIIFc, including a child previously failing rFVIII ITI. The time to anti-FVIII=0 was 4-12 weeks, significantly shorter than with current rFVIII ITI. At a mean duration of follow up of 15.3 months, all patients achieved an anti-VIII inhibitor titer of 0 B.U. Repeat pharmacokinetics studies will be available at planned subsequent follow-up visit. To date, these data indicate that rFVIIIFc safely and effectively induced immune tolerance to FVIII in three children with inhibitors, and has provided durable and continuing immune tolerance to FVIII. Whether rFVIIIFc ITI will be successful and durable in a larger cohort of children with severe hemophilia A will require prospective studies. A prospective observational study of rFVIIIFc ITI pre- and post-ITI T cell responses in children with hemophilia and inhibitors, the Hemophilia Inhibitor Response to Eloctate (HIRE) Study, has begun recruitment.