Factor IX mutants with enhanced catalytic activity

Generation of hyperfunctional recombinant human factor IX variants expressed in human cell line SK-Hep-1

OBJECTIVE

To develop recombinant factor IX (FIX) variants with augmented clotting activity.

RESULTS

We generated three new variants, FIX-YKALW, FIX-ALL and FIX-LLW, expressed in SK-Hep-1 cells and characterized in vitro and in vivo. FIX-YKALW showed the highest antigen expression level among the variants (2.17 µg^-mL), followed by FIX-LLW (1.5 µg^-mL) and FIX-ALL (0.9 µg^-mL). The expression level of FIX variants was two-five fold lower than FIX-wild-type (FIX-WT) (4.37 µg^-mL). However, the biological activities of FIX variants were 15-31 times greater than FIX-WT in the chromogenic assay. Moreover, the new variants FIX-YKALW, FIX-LLW and FIX-ALL also presented higher specific activity than FIX-WT (17, 20 and 29-fold higher, respectively). FIX variants demonstrated a better clotting time than FIX-WT. In hemophilia B mice, we observed that FIX-YKALW promoted hemostatic protection.

CONCLUSION

We have developed three improved FIX proteins with potential for use in protein replacement therapy for hemophilia B.

Expression and characterization of a novel human recombinant factor IX molecule with enhanced in vitro and in vivo clotting activity

INTRODUCTION

Hemophilia B is an inherited X-linked recessive bleeding disorder, due to a defect in human factor IX (FIX). The main treatment for hemophilia B is replacement therapy using FIX concentrates. Prophylactic treatment in severe hemophilia B is very effective but is limited by cost issues. Production of a recombinant FIX (rFIX) with enhanced clotting activity, offering the possibility of fewer infusions and fewer costs with similar efficacy, is one of the current challenges for hemophilia B treatment. The present study focused on an important amino acid sequence known to be involved in the interaction of activated FIX (FIXa) with its cofactor, activated factor VIII (FVIIIa).

MATERIALS AND METHODS

Using site-directed mutagenesis of glutamate E410 (c240, chymotrypsin numbering), four recombinant FIX-E410 (E410H, A, L and N) mutants were developed and produced by the human hepatoma cell line Huh-7.

RESULTS

The in-vitro clotting activity of mutant FIX molecules was 3 to 5-fold higher than wild-type recombinant FIX (FIX-WT). FIX-E410H compound showed the highest in-vitro procoagulant activity. Enhanced specific activity was confirmed using thrombin generation assay. FIX-E410H induced 5.2-fold higher thrombin generation than FIX-WT. In hemophilia B mice, we observed significantly higher in-vivo clotting activity and thrombin generating capacity with FIX-E410H compared to FIX-WT. We demonstrated that increased procoagulant activity of FIX-E410H was mainly explained by 2.5- fold enhanced affinity of the mutant for human FVIIIa.

CONCLUSION

We have engineered and characterized four improved FIX proteins with enhanced in-vitro and in-vivo activity. Future studies are required to evaluate the immunogenicity of FIX-E410.

Next generation FIX muteins with FVIII-independent activity for alternative treatment of hemophilia A

BACKGROUND

FVIII neutralizing antibodies are the main complication of substitution therapy in hemophilia A (HA); auto-antibodies against FVIII causing acquired HA can also occur. Treatment of inhibitor patients remains challenging because prophylactic treatment with existing FVIII bypassing agents, all based on constitutively active coagulation factors, is difficult due to their short half-life.

OBJECTIVES

To generate zymogenic FIX variants with FVIII-independent activity for gene- and protein-based therapy for HA.

METHODS

Modifications were introduced into FIX based on current knowledge of FIX structure and FVIII-independent function followed by random screening. Activity, thrombin generation and FX activation by FIX mutants were characterized in the presence and absence of FVIII. Phenotype correction of promising candidates was assessed by the tail-clip assay in FVIII-knockout mice.

RESULTS

About 1600 clones were screened and three mutations (L6F, S102N and E185D) identified, which improved FVIII-independent activity in combination with our previously described variant FIX-ITV. By systematic combination of all mutations, six FIX mutants with the desired bypassing activity were designed. Candidate mutants FIX-IDAV and FIX-FIAV demonstrated the most efficient thrombin generation in FVIII-deficient plasma and had considerably increased activities towards FX in the absence of FVIII, in that they showed an up to 5-fold increase in catalytic efficiency. Expression of FIX-IDAV in FVIII knockout mice reduced blood loss after the tail-clip assay, even in the presence of neutralizing FVIII antibodies.

CONCLUSION

Activatable bioengineered FIX molecules (as opposed to pre-activated coagulation factors) with FVIII-independent activity might be a promising tool for improving HA treatment, especially for patients with inhibitors.

Oral gene therapy for hemophilia B using chitosan-formulated FIX mutants

BACKGROUND

Oral gene delivery of non-viral vectors is an attractive strategy to achieve transgene expression. Although expected efficacy from non-viral delivery systems is relatively low, repeated vector administration is possible and may help to obtain durable transgene expression in a therapeutic range.

OBJECTIVES

To test the principle feasibility of using factor (F) IX variants with improved function combined with an optimized oral delivery system in hemophilia B (HB) mice.

METHODS

FIX modifications were introduced by site-directed mutagenesis into plasmid- or minicircle-based expression cassettes. Vectors were formulated as chitosan nanoparticles for oral delivery to HB mice. Protection of vector DNA in nanoparticle constructs and transfection efficiency were characterized. HB mice received eGFP-formulated chitosan nanoparticles to confirm gene transfer in vivo. FIX expression, phenotype correction and the potential of nanoparticles to induce immunotolerance (ITI) against exogenous FIX were evaluated after repeated oral administration.

RESULTS

Transfection of HEK 293T cells or livers of FIX-knockout mice with nanoparticles resulted in GFP or functional FIX expression. Oral administration of FIX mutants resulted in exclusive FIX expression in the small intestine, as confirmed by RT-PCR and fluorescence staining. HB mice demonstrated transient FIX expression reaching > 14% of normal activity and partial phenotype correction after oral delivery of FIX mutants with high specific activity and improved tissue release.

CONCLUSION

The feasibility of oral, non-viral delivery of FIX was established and improved by bioengineered FIX proteins and optimized vectors. Thus, these data might point the way for development of a clinically applicable oral gene transfer strategy for hemophilia B.

FIXing factor VIII inhibitors

In this issue of Blood, Milanov and colleagues demonstrate that a Factor IX (FIX) variant that does not require activated Factor VIII (FVIIIa) for activity induces coagulation in hemophilia A mice with FVIII inhibitors.1 This protein might be developed as a bypass agent.

Engineered factor IX variants bypass FVIII and correct hemophilia A phenotype in mice

The complex of the serine protease factor IX (FIX) and its cofactor, factor VIII (FVIII), is crucial for propagation of the intrinsic coagulation cascade. Absence of either factor leads to hemophilia, a disabling disorder marked by excessive hemorrhage after minor trauma. FVIII is the more commonly affected protein, either by X-chromosomal gene mutations or in autoimmune-mediated acquired hemophilia. Whereas substitution of FVIII is the mainstay of hemophilia A therapy, treatment of patients with inhibitory Abs remains challenging. In the present study, we report the development of FIX variants that can propagate the intrinsic coagulation cascade in the absence of FVIII. FIX variants were expressed in FVIII-knockout (FVIII-KO) mice using a nonviral gene-transfer system. Expression of the variants shortened clotting times, reduced blood loss after tail-clip assay, and reinstalled clot formation, as tested by in vivo imaging of laser-induced vessel injury. In addition, we confirmed the therapeutic efficacy of FIX variants in mice with inhibitory Abs against FVIII. Further, mice tolerant to wild-type human FIX did not develop immune responses against the protein variants. Our results therefore indicate the feasibility of using variants of FIX to bypass FVIII as a novel treatment approach in hemophilia with and without neutralizing FVIII Abs.

Recent advances in the development of coagulation factors and procoagulants for the treatment of hemophilia

Hemophilia is a family of rare bleeding disorders. The two primary types, hemophilia A and hemophilia B, are caused by recessive X-chromosome linked mutations that result in deficiency of coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Clinically, hemophilia is manifested by spontaneous bleeding, particularly into the joints (haemarthrosis) and soft tissue, and excessive bleeding following trauma or surgery. The total overall number of hemophilia patients worldwide is approximately 400,000, however only about 100,000 of these individuals are treated. The first treatment of hemophilia was initiated when it was determined that the clotting deficiency could be corrected by a plasma fraction taken from normal blood. The discovery of factor VIII enrichment by cryoprecipitation of plasma opened a new era of therapy which eventually led to the production of factor concentrates and the subsequent development of highly purified forms of plasma factors. The most significant improvements have been the availability of recombinant forms of factors VIII and IX. Unfortunately, recombinant factors still retain some of the limitations of plasma concentrates. These limitations include development of antibody responses in patients and the relatively short half-life of the molecules requiring frequent injection to maintain effective concentration. Treatment beyond replacement of native factors has been tried. They include the development of modified factor VIII and IX molecules with improved potency, stability and circulating half-life and enhancement of a prothrombotic responses and/or stabilization of coagulation factors via inhibition of key negative regulatory pathways. These approaches will be reviewed in this commentary.