How does fondaparinux work?

Fondaparinux (Arixtra®) differs from standard heparin and LMWH in several ways. In particular, it does not come from animal sources. Instead, this drug is a synthetic version of the active site of the heparin molecule and is an even smaller molecule than LMWH. Heparin blocks both thrombin and factor Xa, whereas LMWH has relatively more activity against factor Xa and relatively less activity against thrombin. By contrast, fondaparinux acts exclusively against factor Xa—it does not affect thrombin. Fondaparinux does not require monitoring.