Evaluation of One-Stage Assays for the Monitoring of Recombinant Human Factor IX Padua Activity After Etranacogene Dezaparvovec Gene Therapy.

INTRODUCTION

Accurate and reproducible measures of factor activity are required to guide clinical decision-making following gene therapy for haemophilia B (HB). Highly significant discrepancies have been observed in measurements of various factor IX (FIX) concentrates that carry molecular modifications to extend their half-life, arguing for the need for careful analysis of new HB treatment modalities with respect to FIX assay performance.

AIM

To further characterise variability in FIX activity measured using different one-stage assays (OSAs) and chromogenic assays (CAs) in patients with HB receiving gene therapy utilising the FIX Padua variant and to assess whether assay differences were due to the FIX-Padua variant.

METHODS

FIX activity was assessed centrally (OSA and CA) and locally (OSA only) using plasma samples collected from a phase 2b and phase 3 study of etranacogene dezaparvovec and in an in vitro study of wild-type (wt) recombinant human FIX (rhFIX) and rhFIX-Padua.

RESULTS

Lower CA than OSA FIX activity for plasma samples from the phase 3 trial was observed (CA:OSA ratio: 0.408 [±0.049]-0.547 [±0.062]). Local OSA:central OSA FIX activity ratios were 0.789 (±0.314)-1.021 (±0.159). Local OSA:central OSA FIX activity ratios across methods and/or reagents were 0.81 (±0.02)-1.28 (±0.04) for rhFIX-wt-spiked samples and 0.67 (±0.02)-1.13 (±0.09) for rhFIX-Padua-spiked samples.

CONCLUSION

FIX activity differences between central and local OSAs were modest; similar differences were observed in vitro with rhFIX-wt versus rhFIX-Padua. Commonly available OSAs can be used to monitor patients post-etranacogene dezaparvovec administration; we recommend using the same assay platform throughout the post-treatment period.