Intravenous Administration of Recombinant Factor VIII Induces Immune Tolerance to Subcutaneous Turoctocog Alfa Pegol (SC N8-GP) in Humanized Hemophilia A Mice

Introduction

Turoctocog alfa pegol (N8-GP) is a glycoPEGylated recombinant factor VIII (rFVIII) under development for treatment of hemophilia A (HA). Patients with severe HA receiving intravenous (IV) replacement therapy have 20-40% risk of developing inhibitory FVIII antibodies. It is unclear whether the absence of inhibitors in some patients is due to lack of response or due to peripheral tolerance. Induction of peripheral tolerance may be dependent on regulatory T-cells (Tregs) since they are suggested to be involved in regulation of the anti-FVIII antibody response.

In this study, tolerance induction following IV rFVIII administration was assessed in humanized HA mice, described as a suitable model for tolerance induction as only a fraction produce anti-FVIII antibodies after IV FVIII exposure (Steinitz KN, et al. Blood 2012;119(17):4073-4082).

Aims

To explore whether IV exposure to rFVIII induces peripheral tolerance to subcutaneous N8-GP (SC N8-GP) exposure in humanized HA mice, and whether Tregs play a role in tolerance induction.

Methods

HLA-DR transgenic FVIII-deficient mice lacking murine MHC class II were treated with rFVIII IV once per week for 8 weeks, then challenged with SC N8-GP once a week for 8 weeks. The role of CD25+ Tregs was studied by treating one group of mice with depleting anti-CD25 antibodies during the IV exposure period.

The presence of anti-FVIII antibodies was measured prior to IV and SC dosing, and at study end. Mice with anti-FVIII antibodies after IV rFVIII exposure were excluded from the analysis.

Results

No anti-FVIII antibodies were detected after challenge with SC N8-GP in mice pretreated with IV rFVIII. Depletion of CD25+ Tregs (confirmed by flow cytometry) during the IV exposure did not significantly affect the anti-FVIII antibody response.

Conclusion

It was demonstrated that peripheral tolerance to SC N8-GP was induced in humanized HA mice not responding to IV rFVIII exposure. Depletion of CD25+ Tregs during IV exposure was insufficient to abolish the induction of tolerance.