Response: Response to “Clinical relevance of brown snake (Pseudonaja spp) factor V escaping hemostatic regulation”

We thank Isbister and colleagues for their interest in our paper detailing the procoagulant properties of venom-derived factor V (pt-FV) from the common brown snake, Pseudonaja textilis.¹  In their letter, they contend that the presence of pt-FV is not a contributing factor to the coagulopathy seen clinically. The studies in reference suggest that the final extent of coagulopathy after envenomation by either the brown or tiger snake (no venom FV) was comparable.^2-4  Masci et al made a similar observation investigating the time course of snake bite envenomation by these snakes.^5,6  To a certain extent the observation that there are no differences in the final coagulopathy and time course of recovery is not surprising. Laboratory measurements (> 3 INR, undetectable fibrinogen, elevated D-dimer) were initially made more than 4 hours after the snake bite, when there has been ample time to exhaust endogenous clotting factors.³  Furthermore, the rate of recovery (based on INR) after antivenom treatment is expected to correlate with resynthesis of the consumed clotting factors. Notably, no reference was made to prothrombin levels after envenomation.³  We speculate that this information could provide greater insight into the robustness of the brown snake versus tiger snake venom, as indicated by Masci et al.^5,6 

While the final extent of the coagulopathy may indeed be independent of venom FV, we would argue that the rate of development will not be the same. This is an important point since pt-FV-FXa (pseutarin C)⁷  is expected to act very quickly, as pt-FV is constitutively active, functions in the absence of anionic membranes, and is resistant to activated protein C.¹  These factors are expected to have a measureable effect on the rate and intensity of the initial development of the coagulopathy in humans and, more importantly, for effective envenomation of their natural prey. In fact, this is consistent with further human clinical research referred by Isbister and colleagues.⁴  In this study, presented in abstract form, they observe a discernible delay in the development of coagulopathy after tiger snake envenomation compared with that of brown snake in 112 patients.⁴  An immediate fall in the concentration of fibrinogen, FV, and FVIII was observed after the brown snake bite, whereas there was a delay of 1 to 2 hours for that upon tiger snake envenomation. The presence of constitutively active pt-FV in the brown snake venom allows for rapid activation of prothrombin after envenomation, causing immediate clotting. In contrast, tiger snake venom does not have an active FV component; therefore, the observed delay in coagulopathy is likely due to the activation of several clotting factors, including human FV. Interestingly, members of our team have shown that pt-FV on its own is a potent procoagulant in mice and is an effective anti-bleeding agent in a mouse tail excision model (Patent Collaboration Treaty patent: P.P.M. and J.d.J., “Hemostasis-Modulating Compositions and Uses Therefore”; Publication number: PCT/AU/2008/001866 and WO/2009/079690; World Intellectual Property Office; Filing date December 18, 2008), further supporting the effectiveness of pt-FV.

Finally, although we accept that studies carried out in vitro must be viewed with caution when extrapolating to what happens in vivo, we nevertheless believe that our work¹  is entirely consistent with the outcome of envenomation by the brown snake. Rather than looking at toxin research in an isolated system as a “pitfall,” we submit that both in vitro biochemical work and clinical studies, done carefully, can complement each other.