FVIII activity following FVIII protein infusion or FVIII gene transfer predicts the bleeding risk in hemophilia A rats.

Background: Prophylactic replacement therapy in hemophilia A (HA) patients does not adequately prevent bleeds and arthropathic complications. A more refined understanding of the relationship between coagulation factor VIII (FVIII) levels and bleeding risk during protein prophylaxis, or with gene therapy, is needed to improve patient care.

Objectives: Investigate this relationship in the HA rat, a model exhibiting spontaneous bleeds and development of arthropathy similar to HA patients.

Methods: Human B domain-deleted FVIII was delivered to HA rats via adeno-associated virus (AAV)-mediated gene transfer or multiple intravenous protein injections.

Results And Conclusions: After 12 weeks of observation, both approaches significantly reduced bleeds per animal and increased the proportion of bleed-free animals compared with controls (43% vs 0%, respectively [AAV]; 75% vs 8%, respectively [injection]). Both approaches resulted in an anti-FVIII inhibitory response in 20% to 37% of treated animals, similar to HA patients. Inhibitory antibodies were refractory to clinical improvement (reduction of bleeds) only in the AAV-based prophylaxis. An integrated model-based analysis of data on FVIII exposure and bleeding events was performed. This predicted the bleeding risk at any given circulating FVIII activity. Specifically, 4.8 or 10 IU/dL FVIII (0.048 and 0.1 IU/mL, respectively) were predicted to reduce bleeding risk by 90% or 95%, respectively, compared with untreated controls. Our data establish the utility of the HA rat model in FVIII prophylaxis studies and describe how FVIII activity affects bleeding risk in this setting. These enable further studies on FVIII prophylaxis focusing on disease complications for an optimized treatment of HA patients.