Antithrombotic, indirect and selective synthetic factor Xa inhibitor; Fondaparinux is a synthetic and specific inhibitor of activated Factor X (Xa) with no animal sourced components. The anti-thrombotic activity of fondaparinux is the result of antithrombin III (ATIII) mediated selective inhibition of Factor Xa. By binding selectively to ATIII, fondaparinux potentiates (about 300 times) the innate neutralisation of Factor Xa by antithrombin. Neutralisation of Factor Xa interrupts the blood coagulation cascade and inhibits both thrombin formation and thrombus development.  Fondaparinux does not inactivate thrombin (activated Factor II) and has no effects on platelet aggregation. It does not cross-react with sera from patients with heparin-induced thrombocytopenia. At the recommended dose, it does not affect fibrinolytic activity or bleeding time. At equivalent anti-thrombotic doses, an experimental bleeding model in rats demonstrates that fondaparinux induces less bleeding than unfractionated heparin.