How does enoxaparin (a low molecular weight heparin) differ from unfractionated heparin?
Unfractionated heparin consists of a mixture of polysaccharide chains which range in molecular weight from 3000 to 30000 Daltons. Low molecular weight heparins are fragments of heparin of about 5000 Daltons produced by the controlled enzymatic or chemical depolymerization of unfractionated heparin chains.
Heparin exerts its anticoagulant activity by potentiating the action of antithrombin III, which acts to inhibit thrombin (factor IIa) and activated factor X (Xa). In contrast to unfractionated heparin which has equivalent activity on thrombin and Xa, low molecular weight heparin has proportionately more activity against Xa than against thrombin.
Low molecular weight heparin differs from unfractionated heparin by its higher bioavailability, longer plasma half life (2 to 4 hours following intravenous injection and 3 to 6 hours following subcutaneous injection), and dose-independent clearance. The greater bioavailability and more predictable pharmacokinetics of low-molecular weight heparin are a consequence of its reduced binding to plasma proteins, endothelium, and macrophages.
Binding to platelets is also less with low molecular weight heparin than with unfractionated heparin. Consequently, inhibition of platelet activity is decreased. Heparin-induced thrombocytopenia also occurs less frequently with low molecular weight heparin than with unfractionated heparin.
Low molecular weight heparins have proven more effective than unfractionated heparin for thromboprophylaxis following total knee and total hip arthroplasty as well as following general surgery.
REFERENCES
1. Weitz JI. Low-molecular weight heparins [review] New England Journal of Medicine 1997;337(10):688-98.
2. Horlocker-TT, Heit-JA. Low molecular weight heparin: biochemistry, pharmacology, perioperative prophylaxis regimens, and guidelines for regional anesthestic management [review]. Anesth Analg 1997;85:874-85.