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.