Enhancing factor VIII and VIIIa stability by combining mutations at the A2 domain interface and A1-C2 domain interface

Protein-Engineered Coagulation Factors for Hemophilia Gene Therapy

Hemophilia A (HA) and hemophilia B (HB) are X-linked bleeding disorders due to inheritable deficiencies in either coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Recently, gene therapy clinical trials with adeno-associated virus (AAV) vectors and protein-engineered transgenes, B-domain deleted (BDD) FVIII and FIX-Padua, have reported near-phenotypic cures in subjects with HA and HB, respectively. Here, we review the biology and the clinical development of FVIII-BDD and FIX-Padua as transgenes. We also examine alternative bioengineering strategies for FVIII and FIX, as well as the immunological challenges of these approaches. Other engineered proteins and their potential use in gene therapy for hemophilia with inhibitors are also discussed. Continued advancement of gene therapy for HA and HB using protein-engineered transgenes has the potential to alleviate the substantial medical and psychosocial burdens of the disease.

Noncovalent stabilization of the factor VIII A2 domain enhances efficacy in hemophilia A mouse vascular injury models

An important negative regulator of factor VIIIa (FVIIIa) cofactor activity is A2 subunit dissociation. FVIII molecules with stabilized activity have been generated by elimination of charged residues at the A1-A2 and A2-A3 interfaces. These molecules exhibited reduced decay rates as part of the enzymatic factor Xa generation complex and retained their activities under thermal and chemical denaturing conditions. We describe here the potency and efficacy of 1 such stability variant, D519V/E665V, derived from B domain-deleted FVIII (BDD-FVIII). The major effect of A2 stabilization was on cofactor activity. D519V/E665V potency was increased twofold by the 2-stage chromogenic assay relative to BDD-FVIII. D519V/E665V demonstrated enhanced thrombin generation responses (fivefold by peak thrombin) relative to BDD-FVIII. In vivo consequences of enhanced cofactor activity of D519V/E665V included >fourfold increased maximal platelet-fibrin deposition after laser injury and twofold increased protection from bleeding in acute and prolonged vascular injury model in hemophilia A mice. These results demonstrate that noncovalent stabilization of the FVIII A2 subunit can prolong its cofactor activity, leading to differential enhancement in clot formation over protection from blood loss in hemophilia. The FVIII molecule described here is the first molecule with clear efficacy enhancement resulting from noncovalent stabilization of the A2 domain.

Effects of replacement of factor VIII amino acids Asp519 and Glu665 with Val on plasma survival and efficacy in vivo

Proteolytic cleavage of factor VIII (FVIII) to activated FVIIIa is required for participation in the coagulation cascade. The A2 domain is no longer covalently bound in the resulting activated heterotrimer and is highly unstable. Aspartic acid (D) 519 and glutamic acid (E) 665 at the A1-A2 and A2-A3 domain interfaces were identified as acidic residues in local hydrophobic pockets. Replacement with hydrophobic valine (V; D519V/E665V) improved the stability and activity of the mutant FVIII over the wild-type (WT) protein in several in vitro assays. In the current study, we examined the impact of mutations on secondary and tertiary structure as well as in vivo stability, pharmacokinetics (PK), efficacy, and immunogenicity in a murine model of Hemophilia A (HA). Biophysical characterization was performed with far-UV circular dichroism (CD) and fluorescence emission studies. PK and efficacy of FVIII was studied following i.v. bolus doses of 4, 10 and 40 IU/kg with chromogenic and tail clip assays. Immunogenicity was measured with the Bethesda assay and ELISA after a series of i.v. injections. Native secondary and tertiary structure was unaltered between variants. PK profiles were similar at higher doses, but at 4 IU/kg plasma survival of D519V/E665V was improved. Hemostasis at low concentrations was improved for the mutant. Immune response was similar between variants. Overall, these results demonstrate that stabilizing mutations in the A2 domain of FVIII can improve HA therapy in vivo.