Gene Therapy For Hemophilia B Using CB 2679d-GT: A Novel Factor IX Variant With Higher Potency Than Factor IX Padua

Abstract

Sustained expression of therapeutic factor IX (FIX) levels has been achieved after adeno-associated viral (AAV) vector-based gene therapy in patients with hemophilia B. Nevertheless, patients are still at risk of vector dose-limiting toxicity, particularly liver inflammation justifying the need for more efficient vectors and a lower dosing regimen. A novel increased potency FIX (designated as CB 2679d-GT), containing three amino acid substitutions (R318Y, R338E, T343R), significantly outperformed the R338L-Padua variant after gene therapy. CB 2679d-GT demonstrated a statistically significant ~3-fold improvement in clotting activity when compared to R338L-Padua after AAV-based gene therapy in hemophilic mice. Moreover, CB 2679d-GT gene therapy showed a significantly reduced bleeding time (~5 to 8-fold) and total blood loss volume (~4-fold) compared with mice treated with the R338L-Padua, thus achieving a more rapid and robust hemostatic correction. FIX expression was sustained for at least 20 weeks with both CB 2679d-GT and R338L-Padua while immunogenicity was not significantly increased. This is a novel gene therapy study demonstrating the superiority of CB 2679d-GT highlighting its potential to obtain higher FIX activity levels and superior hemostatic efficacy following AAV directed gene therapy in hemophilia B patients than what is currently achievable with the R338L-Padua variant.