Evaluation of early prophylactic corticosteroid administration on early safety and efficacy outcomes of AAV gene therapy in the hemophilia dog model.

INTRODUCTION Prior to commercial withdrawal, adeno-associated virus (AAV) gene therapy was approved for the treatment of adults with severe hemophilia A. Mechanisms underlying variability, durability, and liver transaminitis are largely uncharacterized.

Objectives

To evaluate the effects of prophylactic corticosteroid administration on AAV gene therapy outcomes in severe hemophilia A dogs.

Methods

Seven hemophilia A dogs received 6e13 vector genomes/kg AAV5-canine factor VIII (AAV5-cFVIII). Four dogs received oral prednisolone (1 mg/kg/day) starting 3 hours pre-infusion with dose-tapering over 6 weeks; three control dogs received no corticosteroids. Percutaneous liver biopsies were performed on detection of alanine transaminase (ALT) >2-fold the upper limit of normal (ULN).

Results

All dogs expressed therapeutic FVIII:C (8.7-56.1%), improved whole blood clot time, and decreased bleeding rates (pre=6.11 vs. post=1.42 bleeds/year, p=0.016) over 2 years post-AAV5-cFVIII. Corticosteroid-treated dogs demonstrated a modest increase in mean FVIII:C at two years by chromogenic substrate assay (CSA) (39.1%) compared with controls (29.5%), driven by one female with markedly elevated FVIII:C from day 54 onward. When analyzed by sex, all female dogs exhibited increased mean FVIII:C CSA at 2 years (49.3%) compared with males (9.1%), regardless of prophylactic corticosteroid use. Prophylactic corticosteroids did not impact transient post-treatment elevations of pro-inflammatory cytokines, anti-AAV antibody formation, or ALT levels. One corticosteroid-treated dog experienced ALT >4.3-fold ULN at 18 weeks without impacting long-term FVIII:C expression. A liver biopsy showed diffuse, minimal periportal lymphocyte and neutrophil infiltration, consistent with non-specific minimal hepatitis.

Conclusions

Prophylactic corticosteroids were not clearly associated with increased transgene expression in hemophilia A dogs.