Pharmacokinetics Of Recombinant Factor VIII Fc Fusion Protein (rFVIIIFc) in Pediatric Subjects With Hemophilia A: An Interim Analysis Of The Kids A-LONG Study

Prophylactic factor VIII (FVIII) administration is the standard of care for patients with severe hemophilia A; however, frequent injections are required to maintain protective factor levels. To reduce injection frequency, we developed a long-lasting recombinant FVIII Fc fusion protein (rFVIIIFc) consisting of one rFVIII molecule covalently linked to the Fc domain of immunoglobulin G₁ (IgG₁). rFVIIIFc had a 1.53-fold higher half-life and a 36% reduction in clearance (CL) versus FVIII (Advate®) in a phase 3 study of adults and adolescents (J Thromb Haemost. 2013;11[2]:169). The Kids A-LONG study (NCT01458106) was designed to investigate the pharmacokinetics (PK), safety, and efficacy of rFVIIIFc in pediatric subjects with hemophilia A who were previously treated with FVIII products. The objective of this planned interim analysis was to determine the PK parameters of rFVIIIFc in pediatric subjects and compare these parameters to those of the subjects' prescribed FVIII products.

This multi-center, open-label, phase 3 study is currently enrolling previously-treated subjects aged<12 years with severe hemophilia A (≤1 IU/dL endogenous FVIII), at least 50 exposure days (EDs) to FVIII products, and no history of or current inhibitors to FVIII. Subjects are stratified into two age cohorts (<6 years and 6 to<12 years). All subjects are started on a twice-weekly rFVIIIFc prophylactic regimen 25 IU/kg on day 1 and 50 IU/kg on day 4 with subsequent dosing adjustment based on PK data and bleeding frequency. The primary endpoint is the incidence of inhibitor development. A sequential PK analysis is performed to compare the PK parameters of rFVIIIFc with that of the prescribed FVIII product. Subjects undergo a washout period of at least 72 hours before receiving the first dose of either FVIII or rFVIIIFc. For FVIII PK analysis, subjects receive 50 IU/kg of their currently prescribed FVIII product with sampling at baseline and at 4 additional time points after for up to 48 hours. For rFVIIIFc PK assessment, subjects receive 50 IU/kg rFVIIIFc, with sampling prior to rFVIIIFc administration and at 5 additional time points after for up to 72 hours. PK parameters were derived from FVIII activity-over-time profiles estimated by the non-compartmental analysis using the PK data analysis software Phoenix™ WinNonlin 6.2.1.51. FVIII activity was measured by the one-stage clotting assay calibrated against a commercially available FVIII plasma standard. A data cut-off date of 8 February 2013 was used to report PK data in this interim analysis.

At the time of this analysis, 52 subjects were enrolled and received at least one dose of FVIII and/or rFVIIIFc. Of 37 subjects with evaluable PK profiles, 30 received both FVIII and rFVIIIFc. For PK assessment of FVIII, 7 different FVIII products were used, of which Advate ®, Haemosolvate®, and Kogenate FS® were the most common. A comparison of PK parameters for rFVIIIFc versus FVIII for both age cohorts demonstrated that rFVIIIFc had a longer half-life (∼1.5 fold increase) and a lower CL (30% to 50% reduction) than FVIII (Table 1).

In comparison to currently available FVIII products, rFVIIIFc had an extended half-life and reduced CL in children. These results are in agreement with those previously observed in adults and adolescents. The final analysis of the Kids A-LONG study will provide additional PK information and evaluate the safety and efficacy of rFVIIIFc in children.

Young:Novo Nordisk: Consultancy, Honoraria; Biogen Idec, Baxter, Kedrion: Consultancy. Mahlangu:Bayer, Novo Nordisk: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity’s Board of Directors or advisory committees; Biogen Idec: Research Funding. Brown:Novo Nordisk, Biogen Idec, Baxter: Membership on an entity’s Board of Directors or advisory committees. Valentino:Baxter, Bayer, Biogen Idec, GTC Biotherapeutics, Inspiration Biopharmaceuticals, Novo Nordisk: Consultancy, Membership on an entity’s Board of Directors or advisory committees. Liesner:Bayer, Baxter, Novo Nordisk, Pfizer: Consultancy, Sponsorship Other; Octapharma: Consultancy, Research Funding, Sponsorship, Sponsorship Other; Inspiration Biopharmaceuticals: Research Funding. Dong:Biogen Idec: Employment, Equity Ownership. Diao:Biogen Idec: Employment, Equity Ownership. Jiang:Biogen Idec: Employment; Biogen Idec: Equity Ownership. Nugent:Biogen Idec: Employment, Equity Ownership. Pierce:Biogen Idec: Employment, Equity Ownership. Allen:Biogen Idec: Employment, Equity Ownership.