Deep vein thrombosis and pulmonary embolism are collectively referred to as venous thromboembolism, which is the third leading cause of cardiovascular-associated death, after myocardial infarction and stroke. Deep vein thrombosis occurs most often in the large veins of the legs. Pulmonary embolism is a complication of deep vein thrombosis that can occur if part of the thrombus breaks away, travels to the lungs and lodges in a pulmonary artery, resulting in the disruption of blood flow. Thrombi that form in veins are rich in fibrin and trapped red blood cells and are referred to as red clots (as opposed to the platelet-rich thrombi that form in arteries, which are referred to as white clots). Deep vein thrombosis mainly occurs as a result of changes in the composition of the blood that promote thrombosis, changes that reduce or abolish blood flow, and/or changes to the vessel wall24
. Both genetic and environmental factors can increase the risk of thromboembolism25,26
. In inherited venous thrombotic disease, there can be increased activity or abundance of proteins that promote coagulation and/or decreased abundance of proteins that inhibit coagulation. For example, a specific point mutation (present in about 5% of Caucasians) in the gene encoding factor V results in a variant that is resistant to inactivation by the anticoagulant protease activated protein C and therefore leads to increased clotting27
. Acquired risk factors for venous thromboembolism include cancer, obesity and major surgery 25,26
. Increased amounts of circulating tissue factor, which sits at the apex of the coagulation cascade (), might also trigger venous thrombosis28–30
Anticoagulants are used to treat a wide variety of conditions that involve arterial or venous thrombosis, including prevention of venous thromboembolism and long-term prevention of ischaemic stroke in patients with atrial fibrillation. The two main classes of anticoagulant drug are vitamin K antagonists and heparins, which target multiple proteases in the coagulation cascade ( and Box 2
). As is the case for antiplatelet drugs, the main side effect of anticoagulant therapy is bleeding. Which targets are best for anticoagulant therapy and whether the anticoagulant drugs under development will have better therapeutic windows than the existing drugs are topics of intense debate31,32
. Recent data from the RECORD 1 clinical trial show that the anticoagulant rivaroxaban holds promise33
. Rivaroxaban is an orally available inhibitor of activated factor X (factor Xa, a component of the coagulation cascade), and it reduced the incidence of venous thromboembolic events in patients undergoing total hip replacement — from 3.7% in those administered a low-molecular-weight heparin (enoxaparin) to 1.1% (ref. 33
). This translates to a 70% reduction in risk without an increase in bleeding.
Box 2. Anticoagulant therapy
Anticoagulant drugs reduce the activity of various proteases in the coagulation cascade () by directly inhibiting them, by inhibiting their post-translational modification or by increasing the activity of an anticoagulant. The advantages and disadvantages of the main types of anticoagulant are described below.
Vitamin K antagonists
Vitamin K antagonists are used for long-term anticoagulant therapy. These inhibitors, introduced more than 50 years ago, are the only orally active anticoagulants in clinical use today. They function by inhibiting the enzyme vitamin K epoxide reductase, which uses vitamin K to modify several coagulation proteins (factor VII, factor IX, factor X and prothrombin) post-translationally. Warfarin is the most commonly prescribed vitamin K antagonist; about 1% of the US population is currently being treated with this drug. Despite careful monitoring, the incidence of major bleeding is about 1–3% of warfarin-treated patients per year53
. The activity of warfarin is affected by diet and by genetic make-up: polymorphisms in the gene that encodes vitamin K epoxide reductase and in the cytochrome P450 gene CYP2C9
account for up to 50% of the interindividual variability of warfarin dosing54
. In August 2007, the US Food and Drug Administration announced a label change for warfarin, advising that pharmacogenetic tests for polymorphisms in these two genes could improve the accuracy of dosing.
The anticoagulant properties of unfractionated heparin were first described in 1916. Since then, it has become evident that heparin binds to the protein antithrombin and markedly increases the ability of this protein to inhibit factor Xa and thrombin (). Unfractionated heparin is currently used for cardiovascular surgery and for the prevention of venous thromboembolism. Fractionated heparin, in the form of low-molecular-weight heparins, was introduced more than 15 years ago. These molecules also target both factor Xa and thrombin, but their administration results in a lower incidence of bleeding than does unfractionated heparin (1.4% for low-molecular-weight heparins versus 2.3% for unfractionated heparin)55
. Synthetic pentasaccharides, such as fondaparinux and idraparinux, have been designed with a structure based on the antithrombin-binding sequence of heparin56
. Owing to their small size, these drugs target factor Xa but not thrombin in an antithrombin-dependent manner (as they are too short to stabilize the interaction between antithrombin and thrombin). A complication of administering unfractionated heparin is the syndrome of heparin-induced thrombocytopenia, which is associated with high rates of both arterial thrombosis and venous thrombosis. More specifically, heparin administration can result in the generation of antibodies specific for heparin–platelet-factor-4 complexes; these antibodies can then activate platelets, generating thrombin and leading to thrombosis57
. The incidence of heparin-induced thrombocytopenia is reduced when low-molecular-weight heparins are used, and thrombocytopenia is rarely observed when synthetic pentasaccharides are used.
Direct inhibitors of factor Xa and thrombin
Direct thrombin inhibitors, such as lepirudin and desirudin, are used for anticoagulant therapy and for the treatment of patients with heparin-induced thrombocytopenia. Several orally administered agents are in development, including: the thrombin inhibitor dabigatran, which is as effective as the low-molecular-weight heparin enoxaparin at reducing the risk of venous thromboembolism after hip-replacement surgery and has a similar safety profile58
; and the factor-Xa inhibitor rivaroxaban, which has a favourable balance of efficacy and safety for preventing venous thromboembolism after major orthopaedic surgery33,59
. Several other orally administered direct inhibitors of factor Xa are also in the pipeline40
. Further studies are required to determine whether oral inhibitors of thrombin or factor Xa can replace the use of heparins and warfarin for both short-term and long-term anticoagulant therapy.
When targeting factors in the coagulation cascade, it is important to consider that the sequential activation of factors by proteolytic cleavage results in an amplification of each step. Therefore, a drug that targets an upstream component of the cascade, such as tissue factor, might be more potent than one that targets a downstream component, such as thrombin. However, the tissue factor and factor VIIa complex, which initiates the coagulation cascade, is essential for haemostasis, and inhibition of this complex can lead to considerable bleeding34
. Indeed, gene-knockout experiments in mice have shown that tissue factor, as well as factor VII, factor X and prothrombin, are essential for haemostasis and for life35
It is also important to consider that the coagulation cascade can be subdivided into three pathways (): the extrinsic pathway (tissue factor and factor VIIa), which is the primary activator of the cascade; the intrinsic pathway (factor XIIa, factor XIa, factor IXa and factor VIIIa), which amplifies the cascade; and the common pathway (factor Xa, factor Va and thrombin), which generates thrombin and fibrin. In contrast to the critical nature of the extrinsic pathway, mice can survive without components of the intrinsic pathway35
. Humans deficient in factor VIII, factor IX or factor XI have mild haemostatic defects, whereas those deficient in factor XII have normal haemostasis36
. Intrinsic-pathway components might therefore be usefully targeted for therapy. Factor XIIa is of particular interest in this regard. A recent study with factor-XII-deficient mice confirmed that factor XIIa is not required for haemostasis; however, it was shown to be important for thrombosis and thus seems an inviting target for antithrombotic therapy37
. Factor IXa, part of the intrinsic pathway, has also been proposed as a target38
. Despite the possibility that the risk of bleeding is lower after inhibition of components of the intrinsic pathway than of the common coagulation pathway, most pharmaceutical companies have chosen to focus on inhibition of factor Xa and thrombin39,40
(). This might be because inhibition of the intrinsic pathway is expected to have less impact on ongoing thrombosis than would inhibition of the downstream proteases.
An important concern about antithrombotic drugs is how to reverse their effects in the event of bleeding. A new approach that addresses this concern uses aptamers, which are composed of modified oligonucleotides. The first aptamer developed as an anticoagulant was targeted to thrombin and was shown to inhibit the activity of clot-bound thrombin and to reduce the formation of platelet-rich arterial thrombi41
. More recently, an RNA aptamer that inhibits factor IXa has been developed42
. By elegant design, an ‘antidote’ oligonucleotide was also developed, to reverse the anticoagulant activity of the inhibitory aptamer rapidly in the event of bleeding43
. The factor-IXa aptamer–antidote pair was well tolerated in a phase Ia clinical trial with healthy volunteers44
. In another approach using oligonucleotides, antisense therapy has been used to block not the activity of the target but its production (in this case, targeting prothrombin)45