Evaluation of Enhanced in Vitro Plasma Stability of a Novel Long Acting Recombinant FVIIIFc-VWF-XTEN Fusion Protein

INTRODUCTION

More than 95% of circulating clotting factor VIII (FVIII) exists in a non-covalent complex with von Willebrand Factor (VWF). While VWF stabilizes and protects FVIII from its clearance pathways, it also subjects FVIII to VWF-mediated clearance. Thus, interaction with VWF imposes a limitation on the extent of FVIII half-life extension achieved by current technologies (Fc fusion, PEGylation etc.). Recombinant FVIIIFc-VWF-XTEN (rFVIIIFc-VWF-XTEN) is a novel fusion protein, consisting of the FVIII binding D'D3 domains of VWF fused to a single chain rFVIIIFc (scFVIIIFc). Appending the domains of VWF to FVIII provides the protection and stability of endogenous VWF, while avoiding the limitation imposed by VWF clearance. Besides D'D3 domains, it also contains two XTEN linkers. XTEN is an unstructured polypeptide consisting of six amino acids repeats (Gly, Ala, Pro, Glu, Ser, Thr). Fusion of XTEN to a protein reduces the rate of clearance and degradation of the fusion protein. In rFVIIIFc-VWF-XTEN, one XTEN linker replaces the B-domain of FVIII and other is attached to the D'D3 domains. In preclinical studies, this protein has shown >4-fold prolonged half-life and similar in vivo acute efficacy compared to rFVIII. In the current study, we examined the impact of various modifications on the in vitro plasma stability of rFVIIIFc-VWF-XTEN protein.

MATERIALS AND METHODS

rFVIIIFc-VWF-XTEN is a fusion protein which is expressed as a dual chain molecule. One chain expresses the D'D3 domains linked to a Fc monomer through an XTEN linker. This polypeptide is co-expressed with a single chain rFVIIIFc monomer to generate a dimer, via the disulfide-bond between the Fc domains. To assess the in vitro plasma stability, fusion proteins were expressed in HEK293 cells, purified and incubated with plasma from FVIII KO (Hem A) or FVIII/VWF DKO mice, for various time periods at 37 degree centigrade. After the desired incubation time, plasma stability of the recombinant proteins was determined by FVIII chromogenic activity assay.

Results and Conclusions

rFVIIIFc-VWF-XTEN fusion protein showed significantly enhanced in vitro plasma stability compared to rFVIII. In FVIII KO plasma, rFVIII started losing activity by 4 hours, and by 24 hours it lost more than 80% of its activity. The decline in activity was more pronounced and rapid when rFVIII was incubated with FVIII/VWF DKO plasma, mainly due to the absence of protection provided by VWF. Conversely, in the case of rFVIIIFc-VWF-XTEN, there was no significant drop in activity even after 6 hours (in both FVIII KO and DKO plasma). By 24 hours, only 10-15% activity reduction was observed in FVIII KO plasma and about a 35% decrease in DKO plasma. Further studies were conducted to evaluate various parameters which contributed to the improved stability of this fusion protein. Our results suggest that there are multiple factors which contribute to the overall stability of rFVIII-VWF-XTEN protein. These include: presence of covalently attached D'D3 domains, enhanced stability of single chain FVIII isoform used in the fusion protein and presence of the XTEN linker in the B-domain of FVIII. These data suggest that superior plasma stability of this novel fusion protein might be a contributing factor to its prolonged in vivo half-life and efficacy.