Anticoagulant Activity Of Black Snake (Elapidae: Pseudechis ) Venoms: Mechanisms, Potency, And Antivenom E Fficacy

Venoms from Pseudechis species (Australian black snakes) within the Elapidae family are rich in anticoagulant PLA 2 toxins, with the exception of one species ( P. porphyriacus ) that possesses procoagulant mutated forms of the clotting enzyme Factor Xa. Previously the mechanism of action of the PLA 2 toxins ? anticoagulant toxicity was said to be due to inhibition of Factor Xa, but this statement was evidence free. We conducted a series of an- ticoagulation assays to elucidate the mechanism of anticoagulant action produced by P. australis venom. Our results revealed that, rather than targeting FXa, the PLA 2 toxins inhibited the prothrombinase complex, with FVa -alone or as part of the prothrombinase complex -as the primary target; but with signi ficant thrombin inhibition also noted. In contrast, FXa, and other factors inhibited only to a lesser degree were minor targets. We quanti fied coagulotoxic e ffects upon human plasma caused by all nine anticoagulant Pseudechis species, in- cluding nine localities of P. australis across Australia, and found similar anticoagulant potency across all Pseudechis species, with greater potency in P. australis and the undescribed Pseudechis species in the NT. In addition, the northern localities and eastern of P. australis were signi ficantly more potent than the central, western, and southern localities. All anticoagulant venoms responded well to Black Snake Antivenom, except P. colletti which was poorly neutralised by Black Snake Antivenom and also Tiger Snake Antivenom (the prescribed antivenom for this species). However, we found LY315920 (trade name: Varespladib), a small molecule inhibitor of PLA 2 proteins, exhibited strong potency against P. colletti venom. Thus, Varespladib may be a clinically viable treatment for anticoagulant toxicity exerted by this species that is not neutralised by available antivenoms. Our results provide insights into coagulotoxic venom function, and suggest future in vivo work be conducted to progress the development of a cheaper, first -line treatment option to treat PLA 2 -rich snake venoms globally.