Thrombosis and Haemostasis

Oral anticoagulant therapy has been shown to be effective for several indications. The optimal intensity of anticoagulation for each indication, however, is largely unknown. To determine this optimal intensity, randomised clinical trials are conducted in which two target levels of anticoagulation are compared. This approach is inefficient, since the choice of the target levels will be arbitrary. Moreover, the achieved intensity is not taken into account.

We propose a method to determine the optimal achieved intensity of anticoagulation. This method can be applied within a clinical trial as an “efficacy-analysis”, but also on data gathered in day-to-day patient care.

In this method, INR-specific incidence rates of events, either thromboembolic or hemorrhagic, are calculated. The numerator of the incidence rate is based on data on the INR at the time of the event. The denominator consists of the person-time at each INR value, summed over all patients, and is calculated from all INR measurements of all patients during the follow-up interval. This INR-specific person-time is calculated with the assumption of a linear increase or decrease between two consecutive INR determinations. Since the incidence rates may be substratified on covariates, efficient assessment of the effects of other factors (e.g. age, sex, comedication) by multivariate regression analysis becomes possible.

This method allows the determination of the optimal pharmacological effects of anticoagulation, which can form a rational starting point for choosing the target levels in subsequent clinical trials.

Based on our work and that of many other workers, we have developed a model of coagulation in vivo. Many workers have demonstrated mechanisms by which cells can influence the coagulation process. Nonetheless, the prevailing view of hemostasis remains that the protein coagulation factors direct and control the process with cells serving primarily to provide a phosphatidylserine containing surface on which the procoagulant complexes are assembled. By contrast, we propose a model in which coagulation is regulated by properties of cell surfaces. This model emphasizes the importance of specific cellular receptors for the coagulation proteins. Thus, cells with similar phosphatidylserine content can play very different roles in hemostasis depending on their complement of surface receptors. We propose that coagulation occurs not as a “cascade”, but in three overlapping stages: 1) initiation, which occurs on a tissue factor bearing cell; 2) amplification, in which platelets and cofactors are activated to set the stage for large scale thrombin generation; and 3) propagation, in which large amounts of thrombin are generated on the platelet surface. This cell based model explains some aspects of hemostasis that a protein-centric model does not.

Dabigatran etexilate is an oral, reversible direct thrombin inhibitor that is approved in the EU and several other countries for the prevention of venous thromboembolism after elective hip and knee replacement, and is in advanced clinical development for other thromboembolic disorders. Dabigatran has a predictable pharmacokinetic profile, allowing for a fixed-dose regimen without the need for routine coagulation monitoring. In certain clinical situations such as serious bleeding into critical organs (e.g. intracerebral bleeding), potential overdose and emergency surgery, clinicians will need to make an assessment of the anticoagulant status of a patient receiving dabigatran before deciding on future management strategies. If available, thrombin clotting time (TT), ecarin clotting time (ECT) and TT determined by Hemoclot→ thrombin inhibitor assay are sensitive tests to evaluate the anticoagulant effects of dabigatran. Prothrombin time (INR) is less sensitive than other assays and cannot be recommended. The activated partial thromboplastin time (aPTT) can provide a useful qualitative assessment of anticoagulant activity but is less sensitive at supratherapeutic dabigatran levels. There are limited data for activated clotting time (ACT). Overall, the aPTT and TT are the most accessible qualitative methods for determining the presence or absence of anticoagulant effect. Although there is no specific antidote to antagonise the anticoagulant effect of dabigatran, due to its short duration of effect drug discontinuation is usually sufficient to reverse any excessive anticoagulant activity. In case of potential overdose, the feasibility of early administration of activated charcoal and subsequent charcoal filtration are undergoing preclinical evaluation. Dabigatran can also be dialysed in patients with renal impairment. In instances of life-threatening bleeding, where conventional measures have failed or are unavailable, other non-specific prohaemo -static agents such as recombinant activated factor VII and prothrombin complex concentrates can be considered.

Disclosure statement: All authors are employees of Boehringer Ingelheim.

Figure 1 has been corrected in this version of the article.

Information was obtained by questionnaire about 215 nonhemophilic patients who developed inhibitors against factor VIII (antihemophilic factor). The majority of the patients were over 50 years of age, and approximately equal numbers of males and females were reported. Rheumatoid arthritis was present in 8% of the cases, 7% occurred during pregnancy or the post-partum period, and in several there was an association with allergy to penicillin, asthma, “auto-immune” diseases, or malignancy. In 46% of cases, no underlying disorders were identified. Major bleeding was observed in 87 % of patients, and in 22%, death was attributed either directly or indirectly to the presence of the inhibitor.

In 11 of 31 patients receiving no therapy other than supportive transfusions of blood or factor VIII concentrate, the inhibitor disappeared after being present for an average duration of 14 months. Corticosteriods were thought to be effective in abolishing the inhibitor in 22 of 45 patients in whom these were the only drugs administered. Twenty-eight patients received azathioprine as well as corticosteriods; in 19, the inhibitor declined or disappeared during treatment. Finally, 80 patients were treated with cyclophosphamide; in 37 there was a favorable outcome. Inhibitors in children and post-partum patients were more likely to disappear spontaneously or with steroid therapy, whereas those in patients with rheumatoid arthritis or other “autoimmune” disorders required treatment with alkylating agents. However, before any specific therapy can be recommended for this disorder, prospective trials of potential therapeutic agents should be conducted in selected subgroups.

Plasmas from 16 patients that were found to be positive both for anticardiolipin antibodies (ACA) and lupus anticoagulants (LA) were incubated with liposomes that contained anionic phospholipids. In 11 of these plasmas, ACA could be cosedimented with the liposomes in a dose-dependent manner, whereas LA activity of the remaining supernatant was unaffected. LA activity of purified total IgG from 6 patients was measured in three different coagulation tests, using normal plasmas from different species. Prolongation of the aPTT, KCT and dRW clotting times was observed only with normal plasma from human origin, not with bovine, rat or sheep plasma.

Highly purified coagulation factors Xa, Va and prothrombin, both of human and bovine origin, were used to establish for two patient IgG's the effect of LA on the rate of thrombin formation in the presence and absence of lipid vesicles composed of 20 mole% phosphatidylserine and 80 mole% phosphatidylcholine. A strong and dose dependent inhibition by LA was observed only when human prothrombin was used as substrate in the prothrombinase complex in the presence of lipids. No inhibition was found when bovine prothrombin was used as substrate. The inhibitory effect observed in the presence of human prothrombin was independent of the source of factors Xa and Va, and was not found in the absence of lipid. Preliminary binding studies suggest that LA only associate with a lipid surface, provided that human prothrombin and calcium ions are present. These data indicate that LA are not directed to phospholipids alone, but presumably recognize an epitope which becomes exposed upon Ca2+-mediated binding of human prothrombin to phospholipids.

The incidence of venous thromboembolism exceeds 1 per 1000; over 200,000 new cases occur in the United States annually. Of these, 30% die within 30 days; one-fifth suffer sudden death due to pulmonary embolism. Despite improved prophylaxis, the incidence of venous thromboembolism has been constant since 1980. Independent risk factors for venous thromboembolism include increasing age, male gender, surgery, trauma, hospital or nursing home confinement, malignancy, neurologic disease with extremity paresis, central venous catheter/ transvenous pacemaker, prior superficial vein thrombosis, and varicose veins; among women, risk factors include pregnancy, oral contraceptives, and hormone replacement therapy. About 30% of surviving cases develop recurrent venous thromboembolism within ten years. Independent predictors for recurrence include increasing age, obesity, malignant neoplasm, and extremity paresis. About 28% of cases develop venous stasis syndrome within 20 years. To reduce venous thromboembolism incidence, improve survival, and prevent recurrence and complications, patients with these characteristics should receive appropriate prophylaxis.

Heparin-induced thrombocytopenia (HIT) is a drug-induced, immunoglobulin-mediated thrombocytopenic disorder that is important for at least three reasons. First, it is a relatively common drug-induced immunohematologic adverse reaction. Second, it is frequently complicated by life- and limb-threatening thrombotic complications. And third, there remains uncertainty about the optimal treatment approach for these patients. Recently, there has emerged increasing consensus on such important issues as the frequency, pathogenesis, and diagnostic testing, which we will summarize here. Further, a greater appreciation of the activation of the coagulation pathways in this syndrome indicate a rationale to treatment approaches that emphasize thrombin inhibition (eg. danaparoid sodium; hirudin and its analogues).

Vascular remodeling, defined as lasting structural changes in the vessel wall in response to hemodynamic stimuli, plays a role in many (patho)physiological processes requiring cell migration and degradation of extracellular matrix (ECM). Two proteolytic systems, the fibrinolytic (plasminogen/plasmin) and matrix metalloproteinase (MMP) systems can degrade most ECM components. The availability of mice models with deficiency of main components of both systems has allowed to study their contribution to vascular remodeling in several biological processes. In mouse models of atherosclerosis, urokinase-mediated plasmin generation plays a role in activation of several macrophage-derived MMPs (MMP-3, -9, -12 and -13), triggering elastolysis and collagenolysis, resulting in media destruction and aneurysm formation. Neointima formation after vascular injury, a process that depends on smooth muscle cell migration, is reduced in mice with plasminogen or urokinase deficiency and enhanced in mice with deficiency of TIMP-1 (type 1 tissue inhibitor of MMPs). Also in allograft transplant arteriosclerosis and in abdominal aortic aneurysm both proteolytic systems contribute to matrix degradation. In a mouse model of myocardial infarction, urokinase deficiency protects totally and MMP-9 deficiency partially against cardiac rupture, but these animals suffer cardiac failure. Thus, the plasminogen/plasmin and MMP systems, in concert, contribute to vascular remodeling in the setting of cardiovascular disease.