Standardization and Clinical Use of LMW Heparin

Antithrombotic efficacy of low-molecular-weight heparin in deep arterial injury

Low-molecular-weight heparin subfractions more specifically inhibit factor Xa than thrombin, and they may have advantages over unfractionated heparin in arterial thrombosis. The antithrombotic efficacy of four dosages of a low-molecular-weight heparin (CY216 at 100, 200, 400, or 500 Institute Choay units/kg) was compared with unfractionated calcium heparin (100 US Pharmacopeia units/kg) and placebo during deep arterial injury produced by balloon dilatation of the carotid artery in the pig. The acute thrombotic end points were 111In-labeled platelet and 125I-labeled fibrinogen/fibrin deposition and macroscopic mural thrombosis; these were related to the anti-factor Xa and antithrombin effects of the heparin preparations. Platelet deposition in segments with deep arterial injury was 42 +/- 28, 22 +/- 5, 29 +/- 12, 9 +/- 2, 9 +/- 2, and 11 +/- 3 x 10(6)/cm2 (mean +/- SEM) for pigs treated with placebo, with 100, 200, 400, and 500 units/kg CY216, and with 100 units/kg unfractionated heparin, respectively. Fibrinogen/fibrin deposition was 35 +/- 8, 19 +/- 2, 19 +/- 4, 21 +/- 3, 14 +/- 4, and 12 +/- 3 molecules x 10(12)/cm2, respectively; deposition was significantly reduced in pigs given 100 units/kg unfractionated heparin compared with placebo (p less than 0.05). Mural thrombosis was present in 74%, 45%, 30%, 14%, 5%, and 9% of deeply injured arterial segments, respectively (p = 0.02). Plasma anti-factor Xa activity and prolongation of the activated partial thromboplastin time (aPTT) with 100 units/kg unfractionated heparin were similar to that produced by 200 units/kg and 500 units/kg CY216, respectively. Thus, low-molecular-weight heparin, which predominantly inhibits factor Xa activity, was only moderately effective at reducing platelet thrombus deposition. It was less effective than 100 units/kg unfractionated heparin, except at high dosages, producing similar prolongation of the aPTT and the thrombin time.(ABSTRACT TRUNCATED AT 250 WORDS)

The anticoagulant activity of heparin: measurement and relationship to chemical structure

For many years the anticoagulant activity of heparin has been estimated by coagulation assays, in which the prolongation of clotting times by heparin is measured under various conditions. More recently, assays have been developed which measure the inhibitory action of heparin on isolated coagulation enzymes, notably Factor Xa and thrombin, using specific amidolytic peptide substrates. The anticoagulant activity of heparin arises primarily from its ability to bind to antithrombin III (AT III), altering the conformation and enhancing the activity of this major protease inhibitor. Passage of heparin through an immobilised AT III column yields two fractions: a high affinity fraction with 300-350 iu mg-1 anticoagulant activity, comprising one-third of the total, and a low affinity fraction with an activity of less than 10 iu mg-1, comprising the remaining two-thirds. Studies in several laboratories have demonstrated that a specific pentasaccharide sequence is required for AT III binding. The authors have shown that the presence or absence of this sequence can be detected by high-field proton NMR, thus providing a semi-quantitative method for a functionally important group. A second major influence on anticoagulant activity is molecular weight distribution. Studies in the authors' laboratory on a series of fractions of 5000-35,000 showed that whereas anticoagulant activity in APTT clotting assays decreased with decreasing molecular weight (Mr), activity in anti-Xa assays was maintained or increased in the low Mr fractions. However, in vivo studies showed that high affinity fragments with anti-Xa activity only were poor antithrombotic agents. It appears that the presence of the AT III binding site alone is not sufficient for full antithrombotic activity; an extra length of polysaccharide chain of at least 15 residues is required. Molecular weight distribution is readily assessed by HPLC, although the lack of suitable reference materials hampers assignment of absolute molecular weights. Important determinants of anticoagulant activity can now be assessed by physicochemical techniques but, at present, these techniques are not precise enough to replace anticoagulant assays as predictors of in vivo behaviour.

Pharmacology and special clinical applications of low-molecular-weight heparins

In this overview, the rationale of the development of low-molecular-weight (LMW) heparins and their toxicological, anticoagulant, fibrinolytic, lipolytic, and protamine interactions are summarized. Clinical experiences are reviewed on the benefit of LMW heparin for anticoagulation in patients with bleeding and other complications on conventional anticoagulants and during pregnancy. It is concluded that animal experiments have demonstrated the safety of LMW heparins, that the pharmacologic profile is improved compared with normal heparin, and that the simple and safe applicability of LMW heparins gives rise to new indications for the long-term prophylaxis of thromboembolism.