Low molecular weight heparins: background and pharmacology.

Low molecular weight heparins (LMWHs) are a class of drugs designed to improve on the therapeutic effects and complications of unfractionated heparin (UH). The two drug types share many structural properties and have multiple mechanisms of action in common. The relative potencies with which the mechanisms are expressed differ between LMWH and UH and among specific LMWHs. The pharmacokinetics of LMWHs and UH are often compared according to the results of plasma activity against purified factor Xa, although the correlation between ;;anti-Xa'' assays and the actual mechanisms of action of the drugs is questionable. Animal models of thrombosis give some information regarding the antithrombotic efficacy of different LMWHs compared with UH and with other LMWHs, but the results are varied and may not be applicable to human thrombosis. Similarly, clinical measurements of clot suppression have yielded conflicting results when the two types of anticoagulants have been directly compared. Numerous clinical outcome studies support the conclusion that subcutaneous regimens of LMWH (and of UH) are at least as effective and safe as intravenous UH for the treatment of clinically stable patients with venous thromboembolism. However, the superiority of one subcutaneous regimen over the others can be established only by the performance of well-planned comparative clinical trials.