Factor VIII-Fc Fusion Protein Shows Extended Half-Life and Hemostatic Activity in Hemophilia A Dogs.

Abstract 545

A recombinant B-domain-deleted factor VIII-Fc (rFVIIIFc) fusion protein was created as an approach to extend the half-life of FVIII. The pharmacokinetics and pharmacodynamics of rFVIIIFc were evaluated in the Chapel Hill colony of hemophilia A dogs. These dogs have a severe hemophilic phenotype comparable to the severe form of human disease with F.VIII < 1%. A single intravenous dose (125 IU/kg) was administered to four dogs and immediately corrected the clotting to normal as measured by whole blood clotting time (WBCT) and aPTT. The WBCT remained below 20 min, the time consistent with FVIII:C > 1%, through approximately 96 h. The range of WBCT in our normal dogs is 8 to 12 min. The concentration of rFVIIIFc in the plasma was measured by ELISA and the terminal half-life was 15.7 ± 1.7 hr. Similar results were obtained when rFVIIIFc was measured using a FVIII-specific chromogenic activity assay (half-life was 15.4 ± 0.3 hr). The concentration vs. time curves were similar using both methods. The activity data were converted to ng/mL using the specific activity of the test article that was used to dose the animals, and these data correlated well with the ELISA data, thus demonstrating that the protein that was measured by ELISA was fully active.

Two of the dogs also received a single dose of recombinant B-domain deleted FVIII (rBDD-FVIII, ReFacto®), 114 IU/kg for one dog and 120 IU/kg rBDD-FVIII for the other, and then received rFVIIIFc (125 IU/kg) 72 hr later in a cross over design. Clotting was corrected to normal immediately after dosing with both rBDD-FVIII and rFVIIIFc (determined by WBCT and clotting activity measured using an aPTT assay). However, the WBCT normalization after rFVIIIFc lasted for approximately twice as long compared to rBDD-FVIII and the half-lives determined from the ELISA data for FVIIIFc (15.7 ± 1.7 hr) were twice those determined for rBDD-FVIII (7.0 hr and 6.7 hr). No adverse clinical signs were detected with any of the infusions. Therefore construction of an Fc fusion of FVIII produces a molecule with a defined mechanism of action that has an increased half life and the potential to provide prolonged protection from bleeding.