Therapeutic Efficacy of Anfibatide in a Murine Model of Thrombotic Thrombocytopenic Purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare but potentially fatal blood disorder, resulting from severe deficiency of plasma ADAMTS13 activity. Plasma infusion or exchange is the mainstay of treatment. However, complications associated with the administration of blood products and with the placement of a central venous line remain to be a major concern. To address the unmet clinical need, we test hypothesis that anfibatide, a novel snake venom-derived glycoprotein (GP) Ib antagonist, may be efficacious in treating TTP. Here, we show that purified anfibatide from snake venom inhibited murine platelet agglutination induced by botrocetin in the presence of recombinant murine VWF in a concentration-dependent manner. At 6-9 micro grams per ml (final concentrations), anfibatide nearly completely abolished botrocetin-induced murine platelet agglutination (Fig. 1A). As expected, anfibatide at the same concentrations showed little or no inhibitory effect on the protease-activating receptor 4 (PAR4) agonist (AYPGKF)-induced murine platelet aggregation. In vivo, an intravenously infused radiolabeled anfibatide exhibited a half-life of 5-7 hours depending on the initial loading dose. This information provided pharmacokinetic basis for therapeutic strategy for TTP in the murine model. An infusion of purified anfibatide at the dose of 60-90 ng per gram of body weight twice daily significantly reduced the rate of thrombocytopenia in Adamts13^-/- mice triggered by intravenous infusion of a bacterial toxin, i.e. shigatoxin-2 (Stx2) (250 pg per gram body weight). As shown, 5/5 (100%) Adamts13^-/- mice after being challenged with Stx2 and but treated with normal saline (control) exhibited severe thrombocytopenia (defined by 50% reduction of platelet counts from the baseline). Similarly, treatment of Adamts13^-/- mice with anfibatide at the dose of 30 ng per gram body weight twice daily had little or no effect in preventing thrombocytopenia. However, treatment of the same mice with anfibatide at doses of 60-90 ng per gram body weight twice daily resulted in significantly reduced rate of severe thrombocytopenia to 11%-37.5% (Fig. 1B). Our ongoing effort is to determine the efficacy of anfibatide in treatment of acquired TTP associated with inhibitors. We conclude that anfibatide when given at the optimal doses and interval can efficiently prevent Stx2-induced TTP syndrome in the murine model. These findings support a rationale for further development of anfibatide as a novel therapeutic for TTP in humans.