The importance of factor Xa in the initiation and propagation of coagulation, as well as its pluripotential cellular properties, has been discussed previously. Considered collectively, the effects exhibited by this particular protease make it an attractive target for pharmacologic inhibition. Factor Xa inhibition can be classified and subcategorized as follows: indirect (antithrombin [AT]-dependent), nonselective (e.g., unfractionated heparin); indirect, semi-selective (e.g., low-molecular-weight heparin); indirect, selective (e.g., fondaparinux); and direct, selective (e.g., DX-9065a [in early stage of development]). Oral factor Xa inhibitors are in phase II testing. Fondaparinux (Arixtra) is a synthetic pentasaccharide that requires AT for selective fXa binding (Petitou et al., 1991). Unlike heparin compounds, fondaparinux does not inhibit thrombin directly nor does it interact with platelets (or platelet-derived proteins). After subcutaneous administration in healthy volunteers, fondaparinux was nearly 100% bioavailable, and absorption was rapid (Cmax within 2 hours) (Donat et al., 2002). Clearance is through renal mechanisms with a terminal halflife of 17 ± 3 hours (slightly longer in elderly volunteers). Overall, drug clearance is 25% lower in patients with mild renal impairment (creatinine clearance [CrCl] 50–80 mL/min), approximately 40% lower in patients with moderate renal impairment (CrCl 30–50 mL/min), and 55% lower in patients with severe renal impairment (CrCl <30 mL/min). OASIS-5, the largest trial performed to date in non-ST segment elevation acute coronary syndromes (ACS), randomized 20,078 patients to fondaparinux (2.5 mg subcuaneous once daily) or enoxaparin (1 mg/kg twice daily) for 2 to 8 days. The primary outcome (composite of death, MI, and refractory ischemia on day 9) was 5.9% and 5.8%, respectively. Major bleeding rates were 2.1% and 4.0%, respectively (hazard ratio 0.53; p<.001). By 6-month follow-up, the endpoints of death, death/MI, stroke, and composite of death/MI/stroke were significantly reduced in those receiving fondaparinux. Catheter thrombosis (during PCI) was 3-fold higher in fondaparinux-treated patients compared to those receiving enoxaparin (1.3% vs. 0.5%, respectively) (European Society of Cardiology presentation, September 2005). The clinical use of fondaparinux for venous thromboembolism prophylaxis will be discussed in Chapter 22.