CHEST Supplement Parenteral Anticoagulants ANTITHROMBOTICTHERAPY AND PREVENTION OFTHROMBOSIS, 9TH ED: ACCP GUIDELINES This article focuses on parenteral anticoagulants in current use. These agents can be divided into indirect anticoagulants whose activity is mediated by plasma cofactors and direct anticoagulants that do not require plasma cofactors to express their activity. The indirect parenteral anticoagulants in current use include heparin, low-molecular-weight-heparins (LMWHs), fondaparinux, and danaparoid. These drugs have little or no intrinsic anticoagulant activity, and exert their anticoagulant activity by potentiating antithrombin (AT), an endogenous inhibitor of var- ious activated clotting factors. The parenteral direct anticoagulants in current use all target thrombin. These agents include recombinant hirudins, bivaliru- din, and argatroban. 1.0 Indirect Parenteral Anticoagulants More than 90 years ago, McLean1 discovered that heparin has anticoagulant properties. Brinkhous and associates2 then demonstrated that heparin requires a plasma cofactor to express its anticoagulant activity. This article describes the pharmacology of approved parenteral anticoagulants. These include the indirect anticoagulants, unfractionated heparin (UFH), low-molecular-weight heparins (LMWHs), fondaparinux, and danaparoid, as well as the direct thrombin inhibitors hirudin, biva- lirudin, and argatroban. UFH is a heterogeneous mixture of glycosaminoglycans that bind to antithrombin via a unique pentasaccharide sequence and catalyze the inactivation of thrombin, factor Xa, and other clotting enzymes. Heparin also binds to cells and plasma proteins other than antithrombin causing unpredictable pharmacokinetic and pharmacodynamic properties and trig- gering nonhemorrhagic side effects, such as heparin-induced thrombocytopenia (HIT) and osteo- porosis. LMWHs have greater inhibitory activity against factor Xa than thrombin and exhibit less binding to cells and plasma proteins than heparin. Consequently, LMWH preparations have more predictable pharmacokinetic and pharmacodynamic properties, have a longer half-life than heparin, and are associated with a lower risk of nonhemorrhagic side effects. LMWHs can be administered once daily or bid by subcutaneous injection, without coagulation monitoring. 