Disulfide bond-stabilized factor VIII has prolonged factor VIIIa activity and improved potency in whole blood clotting assays

Background: Genetically engineered disulfide bonds in B-domain-deleted factor (F) VIII variants (C662-C1828 FVIII and C664-C1826 FVIII) improve FVIIIa stability by blocking A2 domain dissociation because the new disulfide covalently links the A2 and A3 domains in FVIIIa. Aim: The aim of this study was to assess the hypothesis that these variants have physiologically relevant properties because of prolonged thrombin generation and improved clot formation in whole blood. Methods: Clot-formation properties in whole blood were measured in thromboelastogram assays. The thrombin generation capabilities of the wild-type (WT) FVIII and FVIII variants were determined, and half-lives of FVIIIa variants were determined in fresh whole blood serum. Results: Thromboelastogram assays were performed with fresh, severe hemophilia whole blood reconstituted with variant and WT FVIII. The two disulfide bond-stabilized FVIII variants and WT FVIII had comparable clotting times at all studied concentrations. However, when compared with WT FVIII at low concentrations, the two FVIII variants required only 10% as much FVIII to achieve comparable clot-formation rates, clot-formation times and clot firmness values. The differences between WT and FVIII variants were quite pronounced at low FVIII concentrations. Measurement of the endogenous thrombin potential in FVIII-deficient plasma supplemented with these FVIII variants confirmed that the disulfide bond-stabilized variants supported high levels of thrombin generation at lower concentrations than did WT FVIII. During the course of clot generation in whole blood, the disulfide bond-stabilized FVIIIa variants had approximately 5-fold increased half-lives relative to WT FVIIIa. Conclusion: C662-C1828 FVIII and C664-C1826 FVIII have physiologically relevant superior clot-forming properties in a whole blood environment, most likely due to the increased half-life of these FVIIIa variants in whole blood.