The interaction of ancrod with human platelets

Ancrod, a serine protease purified from the venom of Agkistrodon rhodostoma , has been used as a therapeutic anticoagulant for a number of indications, including replacement of heparin in patients with heparin-induced thromboc ytopenia. Ancrod has similar fibrinolytic activity to thrombin, but ancrod specifically cleaves only the alpha chain of fibrinogen, producing the char acteristic fibrinopeptides A. AP and AY. Because ancrod has been used in pa tients with heparin-induced thrombocytopenia, it is important to ensure tha t ancrod does not directly affect the platelets and potentially increase th e hemostatic effect. The effect of ancrod on platelets has not been well es tablished, and there is not agreement in published studies. Additionally, s ome of the studies are over 15 veers old and pre-date sensitive assays such as glycoprotein analysis. For these reasons, we investigated the interacti on of ancrod with human platelets using direct and indirect, functional and biochemical techniques. Incubation of platelets with ancrod alone did not induce platelet aggregation or the release of dense-granule contents. Pre-i ncubation of platelets with ancrod did not augment or inhibit the maximal a ggregation achieved with thrombin, nor did it affect the amount of serotoni n release from dense granules caused by activation by thrombin. Studies of ancrod-treated platelets using monoclonal antibody-mediated radioimmunoprec ipitation demonstrated that high concentrations of ancrod did not cause mea surable cleavage of either the glycoproteins Ib-IX or IIB-IIIa. Incubation of radiolabeled platelets with ancrod-treated plasma also had no effect on the platelet glycoproteins, indicating that ancrod does not indirectly affe ct the major surface receptors. Direct binding studies using radiolabeled a ncrod did not demonstrate specific binding to the platelet surface. Togethe r these studies indicate that ancrod does not directly affect nor bind to p latelets in vitro. The hypocoagulant state and subsequent platelet function defect resulting from the use of ancrod appears to be limited to the remov al of fibrinogen from the circulation.