The emerging role of low-molecular-weight heparin in cardiovascular medicine

Anticoagulant drugs: an update

Thromboembolic disorders continue to be a major cause of morbidity and mortality, resulting in an increased need for anticoagulant therapy. In recent years, new anticoagulant drugs have been developed at a rapid pace, prompted by the recognition of many undesirable properties of currently used agents, and by a greater knowledge of the active enzymatic sites of clotting factors. Furthermore, the structure of a thrombus is better understood, so that newer drugs can inhibit thrombin or Factor Xa not only on the surface of a thrombus, as in the case of heparin, but also the fibrin-bound thrombin or Factor Xa within the thrombus. These agents are usually small molecules synthesized on the basis of their inhibition of specific active sites in the respective coagulation factors. They possess many improved characteristics, such as greater efficacy and safety, oral administration, reliable pharmacokinetics, less need for laboratory monitoring and minimal interactions with other drugs and diet. Prominent among these are lepuridin (Refludan, Pfizer), fondaparinux (Arixtra, Sanofi-Synthelabo) and ximelagatran (Exanta, Astra Zeneca). However, these new drugs are still far from fulfilling the desired objectives. Most of them possess some but not all of the needed properties. Furthermore, many do not have specific antidotes for immediate reversal of their pharmacologic actions, and all are much more costly than conventional agents. Development of newer agents with properties closer to that of the ideal drug remains a challenge.

Models of blood coagulation

Our research aims to provide quantitatively transparent, biologically realistic descriptions of the processes involved in hemostasis which will permit predictions of the behavior of the coagulation system in normal and pathologic states. We use four models of coagulation: (1) numerical approximations of the tissue factor (Tf) pathway of thrombin generation based upon mechanism and dynamics; (2) Tf activation of the "blood coagulation proteome" from isolated cells and proteins; (3) Tf activated contact pathway inhibited whole blood in vitro; and (4) blood shed from standardized microvascular wounds in vivo. The results from these models are integrated in interactive assessments aimed at achieving convergence of biochemical rigor and biological authenticity. Microvascular injury is the most biologically secure but least accessible to mechanistic study. Numerical models while quantitatively transparent are biologically limited. By the integrated analyses of all four models, we establish observations which require inclusion or discovery of new parameters to achieve mechanistically interpretable biological reality. Discoveries made in this fashion have included thrombin's role in the initiation phase, TFPI/ATIII/APC synergy interactions, rfVIIa in fVII deficiency, the roles of fVIII and fIX in the Tf reaction, and the cleavage of fIX by fXa membrane. Ideally, our results will provide descriptions which predict the behavior of the biological blood coagulation system under normal and pathologic conditions.

Direct thrombin inhibitors: pharmacology and clinical relevance

Although heparin has been a cornerstone of treatment for the prevention of thrombosis, it is limited by its adverse effects and unpredictable bioavailability. Direct thrombin inhibitors are a novel class of drugs that have been developed as an effective alternative mode of anticoagulation in patients who suffer from heparin-induced thrombocytopaenia, and for the management of thromboembolic disorders and acute coronary syndromes. The main disadvantages of the direct thrombin inhibitors are the lack of an antidote or readily available clinical monitoring. The mechanism of action, the properties of direct thrombin inhibitors and their potential to replace currently available anticoagulants are reviewed.

Current anticoagulant therapy--unmet clinical needs

Heparin and the vitamin K antagonist warfarin have been in clinical use for more than 50 years. However, both are associated with several well-documented drawbacks that limit their use. Warfarin can be administered orally, making it the agent of choice for long-term management of thromboembolic conditions, but frequent coagulation monitoring is necessary because of its unpredictable anticoagulant effect--the result, in part, of food and drug interactions-and its narrow therapeutic window. Heparin and low-molecular-weight heparin (LMWH) can be administered parenterally only. Coagulation monitoring is also required with heparin although not with LMWH, due to reduced levels of plasma protein binding. In the last 10 years, in the quest to develop new agents that are at least as effective as those currently available, with improved safety and greater ease of use, anticoagulants that target almost every step in the coagulation pathway have been developed. These include inhibitors of the factor VIIa (FVIIa)/tissue factor complex, FIXa inhibitors, direct and antithrombin-dependent FXa inhibitors, agents that enhance the protein C anticoagulant pathway, and direct thrombin inhibitors (DTIs) that inhibit the activity of thrombin. Of the new agents, three DTIs-hirudin, bivalirudin, and argatroban-and the synthetic pentasaccharide (Arixtra) are approved for clinical use. Three other new agents-activated protein C (APC), tissue factor pathway inhibitor (TFPI), and the oral DTI ximelagatran (Exanta, AstraZeneca)-have been evaluated in Phase III studies. The mechanism of action and properties of these new anticoagulants and their potential to replace those in current use will be reviewed here.

Medical indications and considerations for future clinical decision making

There are many well-known drawbacks associated with the currently used antithrombotic agents, warfarin, heparin, and low-molecular-weight heparins (LMWHs). Because heparins can be administered only parenterally, their application is limited. Though warfarin can be administered orally, its unpredictable anticoagulant effect means that it must be regularly monitored. Ximelagatran (Exanta, AstraZeneca) is a novel, oral direct thrombin inhibitor (oral DTI) that is rapidly converted to its active form, melagatran, upon administration. The antithrombotic effects of melagatran have been demonstrated. Following the oral administration of ximelagatran, melagatran has stable and reproducible pharmacokinetic and pharmacodynamic properties that enable ximelagatran to be administered orally, twice daily, according to a fixed-dose regimen, with no need for routine coagulation monitoring. In view of its favourable profile, a clinical trial programme has been designed to evaluate the efficacy and tolerability of ximelagatran compared with standard therapies, for the prophylaxis and treatment of venous thromboembolism (VTE), the prevention of stroke in patients with atrial fibrillation (AF), and the prevention of cardiovascular events in patients with previous acute coronary syndromes. These studies show that oral ximelagatran is well tolerated at doses of up to 60 mg, twice daily (bid), and that it is as effective as standard therapy for the prevention of thromboembolic events in patients undergoing hip or knee replacement surgery, for the treatment of clinically verified acute deep vein thrombosis (DVT), and in patients with nonvalvular AF who have a moderate to high risk of stroke. The protocols and results of some of these studies--and a study that investigates the use of ximelagatran in combination with aspirin for the management of acute coronary artery disease--are described in this paper.

A novel approach to thrombin inhibition

The inability of the heparin-antithrombin complex to inhibit fibrin-bound thrombin limits the utility of heparin for treatment of arterial thrombosis. In contrast with heparin, melagatran, a direct thrombin inhibitor (DTI), is equally effective at inhibiting fluid-phase thrombin and thrombin bound to fibrin. This reflects the ability of melagatran, a reversible, active site-directed DTI, to access the active site of thrombin even when it is fibrin-bound. Because bivalent DTIs, such as hirudin, compete with fibrin for access to the fibrin-binding site on thrombin, bivalent DTIs produce less inhibition of fibrin-bound thrombin than free thrombin when given in low doses. Consequently, higher doses of hirudin are needed for complete inhibition of fibrin-bound thrombin resulting in a steep dose-response curve. This phenomenon, combined with the fact that hirudin irreversibly inhibits thrombin, may account for hirudin's narrow therapeutic window. In a rabbit arterial thrombosis prevention and ear bleeding model, melagatran produced less bleeding than hirudin when the two agents were given at doses that produced nearly complete inhibition of thrombosis. The more favourable benefit-to-risk profile of melagatran in this model likely reflects better access to fibrin-bound thrombin and the reversible nature of the melagatran/thrombin complex. The theoretical advantages of melagatran may endow it with mechanistic advantages over heparin, and safety advantages over hirudin. Large-scale clinical trials are under way to evaluate the utility of an orally active prodrug form of melagatran for prevention and treatment of venous and arterial thrombosis.

The use of low-molecular-weight heparins in outpatient oral surgery for patients receiving anticoagulation therapy

BACKGROUND

When planning oral surgery, dentists occasionally will have patients who first need to have their anticoagulation regimen altered. To minimize the side effects and not adversely affect the patient's health, therapeutic anticoagulation should be interrupted for as short a time as possible. Low-molecular-weight heparins, or LMWHs, recently have emerged as an alternative in the management of patients whose anticoagulant status should not be modified for lengthy periods.

CASE DESCRIPTION

A 72-year-old man, who had a history of deep venous thrombosis, needed to have 19 teeth extracted and an alveoloplasty performed. An LMWH was substituted for warfarin a few days before surgery, and it was withheld from the patient for only a few hours the day of the surgery.

CLINICAL IMPLICATIONS

LMWHs are administered on an outpatient basis and do not require hospitalization, as does unfractionated heparin. As a result, they are more cost-effective and offer greater convenience than heparin therapy. Depending on the procedure and the degree to which patients are medically compromised, dentists may not feel comfortable treating patients who continuously receive anticoagulation therapy. As a result, patients' physicians may prescribe LMWH injections to be administered by patients, family members or caregivers to more safely manage the patients' care during oral surgery. As part of the health care team, dentists must be familiar with LMWH and its use to help guide patients safely through treatment.

Low-molecular-weight heparin for the treatment of patients with mechanical heart valves

BACKGROUND

The interruption of oral anticoagulant (OAC) administration is sometimes indicated in patients with mechanical heart valves, mainly before noncardiac surgery, non-surgical interventions, and pregnancy. Unfractionated heparin (UH) is currently the substitute for selected patients. Low-molecular-weight heparin (LMWH) offers theoretical advantages over UH, but is not currently considered in clinical guidelines as an alternative to UH in patients with prosthetic valves.

HYPOTHESIS

The aim of the present study was to review the data accumulated so far on the use of LMWH in this patient population and to discuss its applicability in common practice.

METHODS

For this paper, the current medical literature on LMWH in patients with mechanical heart valves was extensively reviewed.

RESULTS

There were eight series and six case reports. None of the studies was randomized, and only one was prospective. Data to establish the thromboembolic risk were incomplete. After excluding case reports, the following groups were constructed: (a) short-term administration, after valve insertion (n = 212); (b) short-term, perioperative (noncardiac)/periprocedural (n = 114); (c) long-term, due to intolerance to OAC (n = 16); (d) long-term, in pregnancy (n = 10). The incidence rate of thromboembolism was 0.9% for all the studies and 0.5, 0, 20, and 0% in groups a, b, c, and d, respectively; for hemorrhage, the overall rate was 3.4% (3.8, 2.6, 10, and 0% for the respective groups).

CONCLUSIONS

In patients with mechanical heart valves, short-term LMWH therapy compares favorably with UH. Data on mid- and long-term LMWH administration in these patients are sparse. Further randomized studies are needed to confirm the safety and precise indications for the use of LMWH in patients with mechanical heart valves.

Are activated clotting times helpful in the management of anticoagulation with subcutaneous low-molecular-weight heparin?

BACKGROUND

Enoxaparin has recently been shown to be superior to unfractionated heparin in patients with unstable angina/non-ST-elevation myocardial infarction. Theoretical advantages of low-molecular-weight heparin versus unfractionated heparin include a higher ratio of anti-Xa to anti-IIa activity (3:1 for enoxaparin), a more predictable dose response that precludes the need for frequent monitoring, and the convenience of subcutaneous administration. Both activated partial thromboplastin time and activated clotting time (ACT) are used to monitor anticoagulation with heparin, and ACTs are now standard during percutaneous coronary intervention (PCI) with heparin. At doses of up to 90 mg, subcutaneous enoxaparin leads to a modest dose-related increase in activated partial thromboplastin time, but the effect on ACT is unknown.

METHODS

Thrombolysis In Myocardial Infarction (TIMI) 11A was a multicenter, dose-ranging trial to evaluate the safety and tolerability of subcutaneous enoxaparin in patients with unstable angina/non-ST-elevation myocardial infarction. We obtained peak (mean 4.3 hours after enoxaparin) and trough (mean 11.5 hours after enoxaparin) anti-Xa levels and ACTs for 26 patients in the TIMI 11A trial.

RESULTS

Despite doses of enoxaparin in the range of 89 +/- 19 mg every 12 hours and significant increases in anti-Xa levels even at trough, there was no change in the ACT measured by HemoTec and only a small increase with Hemachron. The correlation of peak Hemachron ACT with peak anti-Xa levels was poor (R = 0.5, P =.08).

CONCLUSIONS

In contrast to heparin, ACTs are not useful for assessment of anticoagulation with subcutaneous enoxaparin and should not be relied on in patients receiving enoxaparin who require acute PCI. Studies to determine the optimal dose, safety, and efficacy of enoxaparin in patients undergoing PCI are underway.

Anticoagulation: the present and future

Thrombin is a central bioregulator of coagulation and is therefore a key target in the therapeutic prevention and treatment of thromboembolic disorders, including deep vein thrombosis and pulmonary embolism. The current mainstays of anticoagulation treatment are heparins, which are indirect thrombin inhibitors, and coumarins, such as warfarin, which modulate the synthesis of vitamin K-dependent proteins. Although efficacious and widely used, heparins and coumarins have limitations because their pharmacokinetics and anticoagulant effects are unpredictable, with the risk of bleeding and other complications resulting in the need for close monitoring with their use. Low-molecular-weight heparins (LMWHs) provide a more predictable anticoagulant response, but their use is limited by the need for subcutaneous administration. In addition, discontinuation of heparin treatment can result in a thrombotic rebound due to the inability of these compounds to inhibit clot-bound thrombin. Direct thrombin inhibitors (DTI) are able to target both free and clot-bound thrombin. The first to be used was hirudin, but DTIs with lower molecular weights, such as DuP 714, PPACK, and efegatran, have subsequently been developed, and these agents are better able to inhibit clot-bound thrombin and the thrombotic processes that take place at sites of arterial damage. Such compounds inhibit thrombin by covalently binding to it, but this can result in toxicity and nonspecific binding. The development of reversible noncovalent DTIs, such as inogatran and melagatran, has resulted in safer, more specific and predictable anticoagulant treatment. Oral DTIs, such as ximelagatran, are set to provide a further breakthrough in the prophylaxis and treatment of thrombosis.

P-selectin, carcinoma metastasis and heparin: novel mechanistic connections with therapeutic implications

Metastasis is a multistep cascade initiated when malignant cells penetrate the tissue surrounding the primary tumor and enter the bloodstream. Classic studies indicated that blood platelets form complexes around tumor cells in the circulation and facilitate metastases. In other work, the anticoagulant drug heparin diminished metastasis in murine models, as well is in preliminary human studies. However, attempts to follow up the latter observation using vitamin K antagonists failed, indicating that the primary mechanism of heparin action was unrelated to its anticoagulant properties. Other studies showed that the overexpression of sialylated fucosylated glycans in human carcinomas is associated with a poor prognosis. We have now brought all these observations together into one mechanistic explanation, which has therapeutic implications. Carcinoma cells expressing sialylated fucosylated mucins can interact with platelets, leukocytes and endothelium via the selectin family of cell adhesion molecules. The initial organ colonization of intravenously injected carcinoma cells is attenuated in P-selectin-deficient mice, in mice receiving tumor cells pretreated with O-sialoglycoprotease (to selectively remove mucins from cell surfaces), or in mice receiving a single dose of heparin prior to tumor cell injection. In each case, we found that formation of a platelet coating on cancer cells was impeded, allowing increased access of leukocytes to the tumor cells. Several weeks later, all animals showed a decrease in the extent of established metastasis, indicating a long-lasting effect of the short-term intervention. The absence of obvious synergism amongst the three treatments suggests that they all act via a common pathway. Thus, a major mechanism of heparin action in cancer may be inhibition of P-selectin-mediated platelet coating of tumor cells during the initial phase of the metastatic process. We therefore suggest that heparin use in cancer be re-explored, specifically during the time interval between initial visualization of a primary tumor until just after definitive surgical removal.

Medical therapy of unstable angina and non-Q-wave myocardial infarction

Management of acute coronary syndromes has been the focus of increased interest in recent years. This has come about with the recognition that the majority of patients who present to the hospital with chest pain have unstable angina or non-Q-wave myocardial infarction (MI). Further, sensitive biochemical markers of myocardial necrosis, such as troponin and creatine kinase, have improved early diagnosis. Markers of inflammation such as C-reactive protein (CRP), although not in wide clinical practice, may provide an early and important marker of prognosis. The current approach to management of acute coronary syndromes is careful risk stratification so as to select appropriate medical therapies and to guide the clinician to appropriate interventions such as angiography or percutaneous coronary intervention (PCI). Established therapies such as aspirin, heparin, intravenous nitrates, and, in selected patients, beta blockers or calcium antagonists, are being used concomitantly with, or are being supplanted by, newer therapies such as low-molecular-weight heparins and glycoprotein IIb/IIIa inhibitors. The role of hydroxymethyl glutaryl coenzyme A reductase inhibitors (statins) in patients with acute coronary syndromes is being investigated and shows promise.