Development of BAX 826, a Polysialylated Full-Length rFVIII with Significantly Improved PK Properties

Post-translational glycosylation determines the pharmacodynamic and pharmacokinetic properties of therapeutic proteins. While desialylation of FVIII/VWF (asialo-FVIII/VWF) has a significantly reduced half-life compared to normal FVIII/VWF complex (Sodetz et al 1977), we introduced additional sialic acids on recombinant (r) FVIII to obtain the opposite: prolonged survival of rFVIII in the circulation. Additional sialic acid residues were introduced to rFVIII by covalently binding polysialic acid (PSA) to full length (FL) rFVIII. The resulting drug candidate BAX 826, polysialylated human rFVIII, is manufactured from octocog alfa which is expressed in Chinese Hamster Ovary Cells by a plasma/albumin free cell culture method and is the active substance in Baxalta´s licensed product ADVATE.

The manufacturing process for BAX 826 comprises several steps, starting with the bulk drug substance (BDS) of the ADVATE process which is subjected to polysialylation, i.e. covalent attachment of PSA of an average molecular weight of 20 kDa to rFVIII. Polysialylation is followed by a sequence of chromatographic purification steps and concentration of the conjugate by an ultra-/diafiltration step leading to the pre-formulated BDS. Final formulation of the BDS includes a filling and lyophilization step to obtain the final drug product. The process described is suited to manufacture BAX 826 in large scale and showed a good batch to batch consistency, ensuring an equivalent product quality for each batch.

BAX 826 was extensively structurally and functionally characterized. The methods used included reducing and non-reducing peptide mapping to determine amino acid sequence and post translational modifications, qualitative analysis of modification sites, SDS-PAGE and Western blot analysis, assessment of three dimensional structure similarities of BAX 826 and rFVIII, ADVATE, by Fourier-transformed infrared spectroscopy (FTIR), dynamic light scattering (DLS) and circular dichroism (CD). Functional characterization was performed by assessment of the kinetics of the assembly and activity of FIXa-FVIII (tenase) complex, determination of the rate of activation and inactivation of BAX 826 by thrombin, determination of overall hemostatic potency by a thrombin generation assay, measurement of the rate of inactivation of untreated or thrombin-activated BAX 826 by activated protein C, measurement of kinetics of the binding of BAX 826 to von Willebrand factor (VWF), to phospholipids, and to low-density lipoprotein (LDL)-receptor-related protein 1 (LRP1) by surface plasmon resonance spectroscopy.

While BAX 826 retained full hemostatic functionality of FVIII as a co-factor of the tenase complex, and can thus be considered a fully active FVIII molecule, it was found to have a reduced binding to VWF and to LRP1. Together with the conception that VWF as FVIII's chaperone dictates the maximally achievable terminal half-life extension for FVIII reduced binding to VWF and FVIII's major clearance receptor LRP1likely explains the prolonged pharmacokinetic (PK) properties when comparing BAX 826 with unmodified FL rFVIII in animal models. In summary, BAX 826, a polysialylated rFVIII derivative, can be manufactured reproducibly without relevant changes to the protein structure characteristic for a functional FVIII molecule with high specific activity.