Mechanisms of action of low molecular weight heparins and heparinoids

Isoform composition of antithrombin in a covalent antithrombin-heparin complex

Antithrombin (AT) circulates in two isoforms, alpha- (90-95%) and beta-AT (5-10%). AT inhibits clotting factors such as thrombin and factor Xa, a reaction catalyzed by heparin. Heparin has been used in many clinical situations but suffers from limitations such as a short intravenous half-life, bleeding risk, and the inability to inhibit thrombin bound to fibrin clots. In order to overcome some of heparin's limitations, we prepared a covalent AT-heparin complex (ATH) that has increased intravenous half-life, reduced bleeding risk, and can directly inhibit clot-bound thrombin. However, structural analysis is required to further develop this promising antithrombotic agent. It was found that the proportion of isoforms in ATH (55% alpha-AT, and 45% beta-AT) was significantly different than that in the commercial AT starting material (80% alpha-AT and 20% beta-AT). Further analysis of the rate of heparin-catalyzed inhibition of thrombin by AT isoforms prepared from ATH revealed that the beta-variant reacted approximately 2-fold faster.

Favourable influence of low molecular weight heparin in mitigating the peroxidative membrane damage induced by a cytotoxic agent and an atherogenic diet

The present study is aimed to demonstrate the protective effect of a heparin derivative, low molecular weight heparin (LMWH) against erythrocyte membrane injury. Two models serve to induce membrane lipid peroxidative damage, namely a potent cytotoxic agent, adriamycin and a hypercholesterolemic atherogenic diet. Two groups of male Wistar rats (140 +/- 10 g) received a single intravenous injection of adriamycin (ADR, 7.5 mg/kg), while two other groups were fed an atherogenic diet comprising a supplementation of 4% cholesterol, 1% cholic acid and 0.5% thiouracil (CCT diet) for 2 weeks. For each of the above two groups, LMWH (Troparin; 300 microg/day per rat subcutaneously) treatment commenced on day 8 and continued for a week. One group was maintained as the normal control group, and another group that received only LMWH treatment was designated as the LMWH drug control group. Erythrocyte membrane was isolated and assayed for its cholesterol levels, lipid peroxidation and ATPases activity. The activities of antioxidant enzymes were assessed in the haemolysate. The findings of the study were that both adriamycin and the atherogenic diet produced elevated membrane cholesterol levels and lipid peroxidation. The membrane ATPases suffered loss in activity. Accentuated oxidative stress was marked by rise in the activities of antioxidant enzymes (SOD, catalase and GPx). LMWH intervention reverted these changes thereby normalizing the membrane composition and function. The membrane protective effect of LMWH is illuminated by this work.

Randomized trial of different regimens of heparins and in vivo thrombin generation in acute deep vein thrombosis

Low-molecular-weight and unfractionated heparins are frequently used to treat venous thromboembolism, but it is not known whether they are equally effective in inhibiting in vivo generation of thrombin. In this multicenter trial, 1048 patients were randomized to intravenous unfractionated heparin (group A), twice daily low-molecular-weight heparin (reviparin) for 1 week (group B), or once daily reviparin for 4 weeks (group C). All patients received vitamin K antagonists. Blood samples withdrawn at the baseline and at weeks 1 and 3 were analyzed using markers of in vivo thrombin generation and other coagulation parameters. During the first 3 weeks symptomatic recurrent deep vein thrombosis-pulmonary embolism (DVT/PE) occurred in 17 (4.5%) of 375 patients in group A compared with 4 (1.0%) of 388 patients in group B, and 9 (2.4%) of 374 patients in group C. Forty percent of patients in group A, 53.4% in group B, and 53.5% in group C showed 30% or greater reduction in thrombus size assessed by venography. Patients in group B had significantly greater reduction in D-dimer, prothrombin fragments 1 and 2 (F1 + 2), endogenous thrombin potential (ETP), and thrombin-antithrombin (TAT) complexes compared to groups A and C. Greater release of tissue factor pathway inhibitor (TFPI) and reduction in levels of thrombin activatable fibrinolysis inhibitor (TAFI) and fibrinogen were significantly more pronounced in group C patients. Reviparin administered twice daily plus vitamin K antagonist is more effective in inhibiting in vivo thrombin generation compared to intravenous unfractionated heparin plus vitamin K antagonist, and reviparin once daily produced significantly higher TFPI release and greater reduction in TAFI and fibrinogen levels.

Mechanisms for the anticoagulant effects of synthetic antithrombins

The important roles of thrombin in the development and propagation of thrombosis are well recognized. In addition to being the enzyme for clotting fibrinogen (the major protein component of blood clots), thrombin accelerates its own generation by activating factor V, factor VIII, factor XI and platelets. It accelerates the stabilization of clots by activating factor XIII to factor XIIIa, the enzyme which crosslinks fibrin. There are probably two major pathways for regulating the availability of thrombin in vivo: inactivation of thrombin (by antithrombin III/vessel wall heparan sulfate and perhaps by other endogenous antithrombins) and the inactivation of factor Va and factor VIIIa by activated protein C. Factor Va and factor VIIIa accelerate the production of thrombin. However, when thrombin becomes bound to fibrin (in clots or possibly on cell surfaces), the ability of antithrombin III/heparin to inactivate thrombin is then reduced significantly. Impairment by fibrin of thrombin inhibition by antithrombin III may account in part for the inability of unfractionated heparin to prevent post-operative deep vein thrombosis in up to 20% of patients who undergo major elective orthopaedic surgery, and may also explain the need for oral anticoagulants after unfractionated and low molecular weight heparins are used to initiate the treatment of established deep vein thrombi. The ineffectiveness of the antithrombin III/heparin pathway for inhibiting thrombin under some circumstances has been a contributory factor for the development, evaluation and identification of other inhibitors of thrombin which are more able than antithrombin III/heparin to inactivate thrombin when the enzyme is bound to fibrin. The focus of this review is to detail how these synthetic agents, by directly or indirectly inactivating thrombin, can also effectively inhibit prothrombin activation in vitro.