Fondaparinux is a synthetic pentasaccharide factor Xa inhibitor. Apart from the O-methyl group at the reducing end of the molecule, the identity and sequence of the five monomeric sugar units contained in fondaparinux is identical to a sequence of five monomeric sugar units that can be isolated after either chemical or enzymatic cleavage of the polymeric glycosaminoglycans heparinand heparin sulfate (HS). Within heparin and heparin sulfate this monomeric sequence is thought to form the high-affinity binding site for the anti-coagulant factor antithrombin III (ATIII). Binding of heparin/HS to ATIII has been shown to increase the anti-coagulant activity of antithrombin III 1000 fold. In contrast to heparin, fondaparinux does not inhibit thrombin. Fondaparinux sodium is an antithrombotic anticoagulant, a Factor Xa inhibitor. Fondaparinux sodium is chemically related to low molecular weight heparins. Its pentasaccharide structure corresponds to the antithrombin III (ATIII) binding site of heparin. Fondaparinux sodium binding at this site potentiates the natural inhibitory effect of ATIII against factor Xa by a factor of approximately 300, which results in inhibition of thrombin generation.