Lack of Substantial Antithrombotic Effect of Low Circulating Coagulation Factor VII Levels in a Baboon Model of Acute Vascular Graft Thrombosis

Localized pathologic thrombin generation by the tissue factor/factor VII (TF/FVII)-dependent extrinsic pathway of blood coagulation has been suggested to be the key initiator of thrombosis following blood vessel injury and exposure of blood to the subendothelial matrix that contains coagulation/platelet activator matrix proteins, such as collagen and TF. Inherited FVII deficiency (<1% FVII activity) causes severe symptomatic coagulopathy. Yet, there are no data to suggest that it also protects against thrombosis; in fact, the contrary may be true (Giansily-Blaziot et al, Thromb Res. 2012;130:S47). It has been reported that active site-blocked FVIIa (FVIIai) at doses that do not affect the prothrombin time (PT) help restore cyclic flow variations in stenotic carotids of rabbits without affecting clotting times (Cirillo et al., J Thromb Haemost. 2003;1:992). However, in a previous primate study, we could not demonstrate the antithrombotic effect of FVIIai at a dose that caused significant prolongation of the bleeding time in our established baboon model of acute vascular graft thrombosis (Gruber et al., Thromb Res. 2007;119:121-7). The present study was designed to determine the effect of severe yet incomplete experimental FVII deficiency in the same standardized primate model of thrombosis that we used for evaluating the effect of FVIIai. First, baseline (negative control) graft thrombosis was evaluated in baboon studies (N=6), followed by establishing the efficacy of the low molecular weight heparin (LMWH), enoxaparin, at a single 1 mg/kg dose, as positive control. LMWH at this dose was moderately but demonstrably antithrombotic, as assessed by measuring the inhibition of fibrin and platelet deposition in TF- or collagen-coated vascular grafts. Reducing circulating FVII activity levels by 95% to 99% after a 2 week long pre-treatment with a FVII antisense oligonucleotide (FVII ASO) caused significant and sustained (>30 days) systemic anticoagulation, as evidenced by PT increase that reached 2.0-2.6 fold baseline. Surprisingly, fibrin deposition was not affected, and platelet deposition in the thrombogenic devices (N=8) was also not inhibited on collagen-coated grafts, and only marginally inhibited on TF. Treatment with vitamin K antagonists that reduces the level of vitamin K-dependent coagulation enzymes, including FVII, also prolong the PT. Warfarin-induced doubling of the PT is antithrombotic and moderately safe. Yet, in this study where FVII ASO selectively reduced FVII levels, the observed and significant PT prolongation was not associated with anti-fibrin or anti-platelet effects on collagen, and showed an unremarkable anti-platelet effect on TF. We conclude that if the TF/FVII pathway drives the propagation of large vessel thrombi under flow in primates indeed, very low FVII activity (≈1%) must be sufficient to sustain thrombogenesis, supporting the clinical data on deep vein thrombosis, routinely observed in FVII deficiency. Since FVII activity cannot be entirely eliminated without bleeding risks, these data suggest that targeting the TF/FVII pathway with FVII ASO or inhibitors may not be an entirely sound antithrombotic treatment approach.