Effect of recombinant factor VIIa on the hemostatic defect in dogs with hemophilia A, hemophilia B, and von Willebrand disease.

Abstract

Recombinant factor VIa (rF.VIIa) is a twochain procoagulant enzyme (Mr, =50,000) active only when complexed with tissue factor in the extrinsic clotting system. We administered human rF.VIIa to hemophilic and von Willebrand disease (vWD) dogs to determine its hemostatic effectiveness and survival in the circulation. Hemophilia A dogs lacking factor VIII demonstrated an immediate increase in plasma rF.VIIa and prompt stoppage of hemorrhage at bleeding time (BT) sites. In seven studies in two dogs, the range of dose of rF.VIIa was 50-220 ,ug/kg, with an apparent 7-to 11-fold increase in plasma factor VII and a mean recovery in plasma of 34%. The til2 was 2.8 ± 0.5 hr. The BT was normalized except in an animal given the minimum dose. In four studies in two hemophilia B dogs lacking factor IX, BT was normalized. The elevation in plasma factor VII was by a factor of 8-30, with a mean recovery of rF.VIIa in plasma of 44%. In two studies in a homozygous vWD dog lacking von Willebrand factor, which is needed for platelet function, BT was not corrected even though large doses of rF.VIIa were given. The human rF.VIIa protein was immunogenic for dogs. These studies indicate that factor VIa corrects the hemostatic defect in dogs with hemophilia A and B, diseases primarily of the intrinsic clotting system, but does not correct the hemostatic defect in vWD.

Successful treatment of canine hemophilia by continuous expression of canine FVIIa

“…This large animal model has been a good predictor for efficacy in hemophilia using a range of products. [12][13][14][15] However, because FVIIa is a protease (in contrast to previous gene transfer approaches using zymogen FVIII or FIX), the issue of safety, especially with respect to thrombosis, is pertinent. Hence, our study focused on demonstrating both efficacy and safety, and we report 3 major findings in hemophilic dogs: (1) canine FVII can be engineered for secretion in a biologically active, 2-chain form; (2) continuous expression of FVIIa at levels of approximately 2 g/mL (ϳ40 nM) can result in a marked, long-term improvement in the bleeding diathesis, despite only partial correction of in vitro hemostatic parameters; and (3) at these expression levels and administered AAV8 vector doses, we observed no hepatotoxicity or increased risk of thrombosis.…”

“…by guest www.bloodjournal.org From dose would be expected given the similarity of these 2 hemophilia models in response to rhFVIIa infusions. 12 The fact that AAVmediated, low vector dose FVIIa gene transfer may offer some clinical benefit certainly warrants further investigation in the dog model because it will be safe and may have potential application in prophylaxis. Such experiments in dogs, which are currently underway, will help define the lowest expression levels that result in a measurable clinical improvement.…”

“…Based on this result, we infused progressively higher doses of AAV8-cFVIIa (3-or 6-fold) in HA dogs (Table 1). Both HA and HB dogs respond similarly to infusions of rhFVIIa (Brinkhous et al 12 ; T.C.N., personal communication, August 2008) and thus were expected to perform similarly after AAV8-cFVIIa gene transfer. The AAV8-cFVIIa-treated HA dogs exhibited an initial reduction in WBCT and PT because of prophylactic normal canine plasma infusions (daily for the first 4-5 days after vector administration, Figure 2C and D, respectively).…”

“…11 In addition, it has been a good predictor of efficacy in hemophilia treatments. [12][13][14][15] Viral-mediated gene transfer of factor VIII (FVIII) or factor IX (FIX) in this model demonstrated that the antigenicity of the transgene product remains a concern 16,17 because these animals may not be fully tolerant to wild-type FVIII or FIX. On the other hand, continuous expression of murine FVIIa in immunocompetent hemophilic mice after gene transfer did not result in a transgene-specific immune response, as evidenced by the long-term expression.…”

Long-term expression of murine activated factor VII is safe, but elevated levels cause premature mortality

“…The effect of continuously expressed FVIIa on frequency of joint bleeds will perhaps be better assessed in a canine model of hemophilia, where observation of animals over many years has established a baseline number of readily diagnosed bleeds, including joint bleeds, in untreated animals (26). Improvement of hemostasis in a large-animal model has previously been a strong predictor of efficacy of products in human subjects (27)(28)(29).…”