Functional Characterization of BAX 855, a PEGylated Recombinant FVIII

Abstract 4359

Baxter and Nektar have developed BAX 855, a longer-acting PEGylated form of Baxter’s recombinant FVIII (ADVATE process) using stable PEG technology from Nektar. BAX 855 was functionally characterized in vitro and its features were compared with those of the unmodified parent rFVIII. The overall hemostatic potency of BAX 855 was assessed using a thrombin generation assay. Human FVIII-deficient plasma containing less than 1% of FVIII was supplemented with different concentrations of BAX 855 and unmodified rFVIII and coagulation was triggered by adding a small amount of recombinant human tissue factor complexed with phospholipid (PL) micelles to the plasma. Similar to unmodified rFVIII, BAX 855 corrected the impaired thrombin generation of the FVIII deficient plasma in a concentration-dependent manner. The role of FVIII within the tenase complex was determined by measuring the kinetics of FXa generation with a FIXa-cofactor activity assay, using either untreated or thrombin activated BAX 855. Comparison of the kinetic parameters and the maximum FXa generated revealed similar characteristics between BAX 855 and unmodified rFVIII. A similar approach revealed that BAX 855 fully retained its ability to be activated and inactivated by thrombin. The susceptibility of BAX 855 to activated protein C (APC) inactivation was also similar for BAX 855 and unmodified rFVIII. The binding affinities for VWF were similar for unmodified rFVIII (K_D 0.6 nM) and BAX 855 (K_D 0.8 nM) and the binding capacity of BAX 855 was also only slightly reduced. In contrast, the binding capacity of BAX 855 to the low-density lipoprotein-receptor-related protein (LRP) clearance receptor was 55% less than that of the unmodified rFVIII. In summary, the functional properties of BAX 855 were fully retained, indicating that PEGylation did not have an impact on the functional properties of rFVIII.