Anticoagulation Therapy

Mechanism of Action

Antithrombin acts as a plasma cofactor that is essential for heparin's anticoagulant activity, inhibiting multiple activated coagulation enzymes, including thrombin, Factor Xa, and Factors IXa, XIa, and XIIa 1, 2
The binding of heparin to antithrombin produces a conformational change in antithrombin's arginine reactive center, allowing it to bind covalently to the active serine center of coagulation enzymes, as supported by the American College of Cardiology and the American College of Chest Physicians 2, 3, 1
The heparin-antithrombin complex inhibits several coagulation enzymes, with thrombin being approximately 10-fold more sensitive to inhibition than Factor Xa 2, 3, 1

Heparin binds to lysine sites on antithrombin through a specific pentasaccharide sequence, producing a conformational change in antithrombin, and only about one-third of administered heparin binds to antithrombin, but this fraction is responsible for most of heparin's anticoagulant effect 1, 2
After binding to coagulation enzymes, heparin dissociates from the complex and can be reused, with thrombin inhibition requiring heparin molecules with at least 18 saccharides to bind simultaneously to both thrombin and antithrombin 1, 2
Only approximately one-third of administered heparin contains the high-affinity pentasaccharide sequence required for binding to antithrombin, which is critical for its function, as stated by the American College of Cardiology and the American College of Chest Physicians 2, 3, 1

The American Heart Association and American Stroke Association clearly differentiate between anticoagulants (including heparin) and antiplatelet agents (such as aspirin, ticlopidine, clopidogrel, dipyridamole) 4
Guidelines specifically categorize heparin, low molecular weight heparins, and heparinoids as anticoagulants, distinct from antiplatelet agents, and recommend aspirin as the antiplatelet agent of choice in acute ischemic stroke, while noting that heparin is not recommended for decreasing stroke-related morbidity or mortality 3, 4

Heparin can bind to platelets and, depending on experimental conditions, can either induce or inhibit platelet aggregation, with high-molecular-weight heparin fractions having greater effects on platelet function than low-molecular-weight fractions 2, 3
Heparin-induced bleeding may occur through mechanisms independent of its anticoagulant effect, such as interactions with platelets and endothelial cells 1
The anticoagulant response to heparin is nonlinear at therapeutic doses due to its complex pharmacokinetics, and heparin binding to plasma proteins can reduce its anticoagulant activity at low concentrations, contributing to variability in anticoagulant response 1
Heparin's heterogeneity, with a molecular weight range of 3,000-30,000 Da, contributes to its complex pharmacology and variable anticoagulant response in clinical practice 2, 3, 1