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Title:
New Anticoagulants; Basic Knowledge and Clinical Applications: Part I

Author(s):
Firouz Madadi MD, Mehrnoush Hassas Yeganeh MD*, Firouzeh Madadi MD, Hamid Reza Seyyed Hosseinzadeh MD

Affiliattion(s):
Shahid Beheshti University of Medical Sciences, Tehran, Iran

* Corresponding Author

Vol 1, Num 1, July 2014

 


   

Introduction

Venous thromboembolism, comprising deep vein thrombosis and pulmonary embolism, is one of the leading causes of mortality and morbidity. In the US, pulmonary embolism causes almost 250,000 deaths per year,(1) and it is estimated that 12% of deaths occurring annually in the European Union are associated with venous thromboembolism.(2) Hospitalized patients are at risk of developing venous thromboembolism, with most of them having one or more risk factors.(3)

Without thromboprophylaxis, the incidence of objectively confirmed deep vein thrombosis varies between 10% and 20% in patients in a general medical ward, between 15% and 40% in those undergoing major general, gynecologic, and urologic surgery or neurosurgery, and between 40% and 60% after hip or knee replacement surgery.(3)

Furthermore, pulmonary embolism is the most common preventable cause of hospital death, and the prevention of pulmonary embolism is the number one strategy to improve patient safety in hospitals.(3)

Venous thromboembolism should be considered as a chronic rather than an acute illness.(4) Patients with a first episode of venous thromboembolism are at increased risk of recurrence. The cumulative incidence of recurrence is approximately 18% after 2 years, 25% after 5 years, and 30% after 8 years.(5) Recurrent deep vein thrombosis is associated with the development of post-thrombotic syndrome,(6) with a frequency of 15-50% after symptomatic proximal deep vein thrombosis. Post-thrombotic syndrome-a long-term complication of deep vein thrombosis-is characterized by chronic, persistent pain, swelling, skin discoloration, and the potential to result in venous ulceration in the affected limb.(7) In most cases, post-thrombotic syndrome develops within 1-2 years after deep vein thrombosis.(8) Pulmonary embolism predisposes patients to chronic pulmonary hypertension. It has been found that symptomatic chronic thromboembolic pulmonary hypertension affects up to 4% of patients within 2 years after the first episode of symptomatic pulmonary embolism.(9)

Effective thromboprophylaxis can reduce the incidence of venous thromboembolism in high-risk patient populations, and providing adequate intensity and duration of anticoagulation for the treatment of deep vein thrombosis can prevent recurrence and, thus, its associated consequences. Evidence-based guidelines strongly recommend the use of anticoagulants for the prevention and treatment of venous thromboembolism.(3, 10)

The coagulation network

Activation of the coagulation network is a multi-phase process characterized by enzymatic sequential activation of a series of circulating inactive proteins, and their interaction with platelets.(Figure 1) Upon breaking of the vasculature, platelets adhere at the site of injury and subendothelial cells express a cell-surface molecule called tissue factor (TF), which, upon binding to factor VII/VIIa (FVII/VIIa), forms an activated complex that comprises TF and FVIIa. This phase is called initiation and is characterized by generation of small amounts of activated factors. Specifically, TF-FVIIa complex activates both factor IX (FIX) to FIXa and factor X (FX) to FXa. FIXa binds to platelets, where it plays a role in the later stages of hemostasis, but FXa forms a complex with factor Va (accelerin) (FVa) to convert a small amount of prothrombin (FII) to thrombin (FIIa).

The source of FVa for this reaction is probably the protein released from platelets which adhere to collagen of the subendothelial matrix by glycoprotein VI. Factor V circulates in plasma as a single-chain molecule with a plasma half-life of about 12 hours. Factor V is able to bind to activated platelets and is activated by thrombin. On activation, factor V is spliced in two chains that are noncovalently bound to each other by calcium. Factor V is active as a cofactor of the thrombinase complex. The activated factor X (FXa) enzyme requires calcium and activated factor V to convert prothrombin to thrombin on the cell surface membrane.(11)

In the subsequent step, called amplification, the thrombin formed in the initiation phase works as an amplifier by acting on platelets and proteins to facilitate platelet-driven thrombin generation. Platelet activation facilitates assembly of coagulation proteins on their surface, thus increasing their activity. In addition, thrombin can activate coagulation proteins with production of factor VIIIa (FVIIIa), FVa, FIXa and FXa. In a further step called propagation activated platelets and activated factors induce a burst of thrombin generation, which means that a large amount of prothrombin is converted to thrombin on the surface of activated platelets. Factor IXa binds FVIIIa on platelet membrane. This factor IXa-VIIIa complex activates FX on the platelet surface. Besides, this complex, in the presence of prothrombin, promotes thrombin generation. Factors Xa and Va, together with platelet membrane phospholipids, form the so called 'prothrombinase complex'.

The burst of thrombin produced during the propagation phase leads to cleavage of fibrinogen with fibrin production and the assembly of fibrin strands, which stabilize the initial platelet plug. In addition to its role in cleaving fibrinogen, thrombin activates factor XI (FXI) on the platelet surface, which can in turn activate FIX to FIXa thus enhancing FXa generation, so contributing to the positive feedback loop to increase thrombin production. It is important to underline that high levels of thrombin generated during the propagation phase bind to fibrin where it is functionally active, but protected from inhibition by antithrombin (AT).(11)

  •  
    Figure 1: The coagulation cascade. This scheme emphasizes the understanding of 1, the importance of the tissue factor pathway in initiating clotting in vivo; 2, the interactions between pathways and 3, the pivotal role of thrombin in sustaining the cascade by feedback activation of coagulation factors. HMWK=high-molecular-weight kininogen; PK=prekallikrein; PL=phospholipid; PT=prothrombin; TF=tissue factor; Th=thrombin. From Schafer, 1994.
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    Traditional anticoagulants

    Heparins are indirect anticoagulants that bind to antithrombin, enhancing its ability to inhibit Factor Xa, thrombin, and other coagulation factors.(12)

    However, clot-bound thrombin is relatively protected from inhibition by heparin-antithrombin, possibly because the heparin-binding site is inaccessible when thrombin is bound to fibrin.(13) Unfractionated heparin binds to a number of plasma proteins, which contribute to its variable anticoagulant response. In addition, unfractionated heparin is associated with the risk of developing heparin-induced thrombocytopenia and osteoporosis. Consequently, rigorous and frequent coagulation monitoring is required.(14)

    Low molecular weight heparins, derived from unfractionated heparin by chemical or enzymatic depolymerization, exhibit more predictable anticoagulation and can be given at fixed doses without coagulation monitoring. However, both unfractionated heparin and low molecular weight heparins require parenteral administration, which limits their use in the outpatient setting. Although low molecular weight heparins can also cause heparin-induced thrombocytopenia, this risk is lower compared with unfractionated heparin.(14)

    Vitamin K antagonists, first introduced more than 60 years ago, were until recently the only orally active anticoagulants available for clinical use. They produce an anticoagulant effect by interfering with the γ-carboxylation of vitamin K-dependent coagulation Factors II, VII, IX, and X (Figure 2).(15) In addition to their slow onset of action, vitamin K antagonists are also challenging to use in clinical practice because they have a narrow therapeutic window, unpredictable pharmacokinetics and pharmacodynamics, and multiple food-drug and drug-drug interactions. Therefore, their use necessitates frequent coagulation monitoring and dose adjustment.(15)

    Conventional anticoagulant treatment comprises three stages: acute treatment (i.e. <5 days)-to stabilize the thrombus, prevent extension and possibly subsequent fatal pulmonary embolism; secondary prevention of recurrent venous thromboembolism (<3 months); and long-term maintenance treatment (>3 months/indefinite). It is apparent that traditional anticoagulants are all associated with drawbacks, and there is an increasing unmet need for new, better oral anticoagulant agents. In the search for an 'ideal anticoagulant' (Table 1), new oral agents that directly target Factor Xa (such as rivaroxaban, apixaban, edoxaban, betrixaban, and YM150) or thrombin (such as dabigatran etexilate and AZD0837) have shown promise.(16, 17, 18)

  •  
    Figure 2: Vitamin K 1 is reduced to vitamin KH2. The major warfarin-sensitive enzyme inthis reaction is the vitamin K oxide reductase mainly inhibited by the S-enantiomer of warfarin.
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    Why new anticoagulants?

    Immediately acting unfractionated heparins (UFHs), low molecular weight heparins (LMWHs), and slowly acting vitamin K antagonists (VKAs) (15) reduce the morbidity and mortality of patients at risk of recurrent venous thromboembolism (VTE) (10), cerebral and non-cerebral embolism, and coronary occlusion or reocclusion.(19) The use of these anticoagulants is limited by several drawbacks.

    Heparins and LMWHs have to be administered intravenously or subcutaneously and require dose adjustment guided by monitoring of the anticoagulant effect. LMWHs have to be administered subcutaneously, with the dose being adjusted in older patients and in renal impairment. Severe side effects such as heparin-induced thrombocytopenia or other drug-related side effects limit their administration.(20)

    The main downsides of VKAs are the requirement of regular dose adjustments by monitoring the anticoagulant effect, the low prevalence of international normalized ratio (INR) values within the therapeutic range (2-3),(21) the interactions with food and many drugs, severe intracranial and extracranial bleeding complications, and other severe side effects such as coumarin-induced hepatitis. The slow onset and offset of action of VKAs necessitate simultaneous administration of heparins and LMWHs during the induction of anticoagulation as well as during surgical interventions.

    New anticoagulants are being developed to overcome these drawbacks of the conventional anticoagulants and, thereby, help to improve patient care. In most instances, those novel anticoagulants are synthetic small molecules with specific modes of action. They are inhibitors of relevant coagulation enzymes for almost all of which new compounds are being developed. An advantage of these new anticoagulants is the fixed dosing without the need for anticoagulant monitoring and dose adjustment. This is important for the oral direct FXa and thrombin inhibitors because VKAs have to be dose adjusted according to the target range of the INR between 2 and 3.

    Direct thrombin inhibitors

    Thrombin plays a central role in the generation of a thrombus. Once formed, thrombin activates factors V, VIII and XI, which are involved in generating more thrombin, and it also activates factor XIII, a protein involved in fibrin cross-linking and clot stabilization. Thrombin's principal function is to convert soluble fibrinogen to insoluble fibrin, while also stimulating platelet activation. Thrombin can be inhibited directly or indirectly by the binding of thrombin-inhibiting drugs to one or two of its three domains: the active site and exosites 1 and 2 (Figure 3). Exosite 1 is the fibrin-binding site of thrombin and exosite 2 serves as the Heparin-binding domain.

    Traditional anticoagulants such as unfractionated heparin (UFH) and low-molecular weight heparin (LMWH) inhibit free thrombin in an indirect manner by binding simultaneously to antithrombin and exosite 2, thereby forming a heparin-thrombin-antithrombin complex. However, heparins also can simultaneously bind to both fibrin and thrombin and act as a bridge between them, thereby enhancing thrombin's affinity for fibrin and increasing the concentration of thrombin-bound fibrin. This fibrin-heparin-thrombin complex occupies both thrombin exosites but leaves the active site ezymatically protected from inactivation as the heparin-antithrombin complex cannot bind to fibrin-bound thrombin, resulting in further thrombus growth.(1, 22)

    In addition to its inability to neutralize fibrin-bound thrombin, other limitations of heparin include its binding to various plasma proteins, creating an unpredictable dose-dependent anticoagulant response, need for routine dose-adjustments and anticoagulant monitoring and heparin-induced thrombocytopenia (HIT).(23) Direct thrombin inhibitors (DTIs) bind directly to thrombin and do not require a cofactor such as antithrombin to exert their effect. DTIs can inhibit both soluble thrombin and fibrin-bound thrombin.(23) Other key advantages include a more predictable anticoagulant effect compared with heparins because of their lack of binding to other plasma proteins(24) an anti-platelet effect (25) and the absence of immune-mediated thrombocytopenia.(26)

    Both parenteral and oral direct thrombin inhibitors have been investigated for prophylaxis and treatment of venous thromboembolism (VTE), prevention of thromboembolic complications in patients with HIT or at risk for HIT and undergoing percutaneous coronary intervention (PCI), acute coronary syndromes (ACS) with and without percutaneous transluminal coronary angioplasty (PTCA), secondary prevention of coronary events after ACS and nonvalvular atrial fibrillation.(27, 28) Currently, four parenteral DTIs are approved for use as anticoagulants in the United States: lepirudin, desirudin, bivalirudin and agratroban. Several oral compounds are currently under investigation, with dabigatran etexilate furthest along in development.

    Parenteral direct thrombin inhibitors

  •  
    Figure 3: Schematics of the interaction of thrombin with six different anticoagulants. Unfractionated heparin (UFH) - UFH mediates its affect through binding to antithrombin and enhancing its reactivity with the enzymatic site of thrombin. UFH requires an additional 13 saccharides that bind to the heparin binding site tomaximize its interactionwiththrombin.UFH is thus an indirect,parenteral inhibitor of thrombin.Lowmolecularweight heparin (LMWH) - LMWHlacks the longer chains of UFH and has less binding to thrombin thus decreasing its ability to neutralize thrombin.LMWH is thus an indirect, parenteral inhibitor of thrombin. Lepirudin/Desirudin - Lepirudin and desirudin bind directly and strongly to both the active enzymatic site and exosite 1 of thrombin to inhibit its activity. Lepirudin and desirudin are direct, parenteral inhibitors of thrombin. Argatroban - Argatroban is a small molecule that binds reversibly to the active enzymatic site of thrombin. Argatroban is a direct, parenteral inhibitor of thrombin. Bivalirudin - Bivalirudin is a modified form of hirudin with four glycine residues connecting two amino acids sequences important for binding to thrombin. It is a more reversible inhibitor of thrombin than is lepirudin, and is a direct, parenteral inhibitor of thrombin. Dabigatran - Dabigatran is a small molecule that binds reversibly to the active enzymatic site of thrombin. (Exosite 1= Fibrin Binding Site, Exosite 2= Heparin Binding Site)
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    Recombinant hirudins: lepirudin and desirudin

    Lepirudin and desirudin are derivatives of hirudin, a peptide originally isolated from the salivary glands of medicinal leeches (29) that were developed by recombinant technology in Saccharomyces cerevisiae.(26) Both recombinant hirudins (r-hirudins) are bivalent direct thrombin inhibitors that bind simultaneously to the active site and exosite 1 domain on thrombin, an interaction that increases their specificity for thrombin.(26) They also have the highest affinity for thrombin as they rapidly form essentially irreversible, 1:1 stoichiometric complexes .(30)

    Compared with hirudin their affinity is 10 times weaker for thrombin, however, they are still considered the most potent of all the thrombin inhibitors.(26, 29) The plasma half-life of the r-hirudins after intravenous (I.V.) injection and subcutaneous (s.c.) administration is 60 and 120 min, respectively.(31)

    Lepirudin is licensed for the treatment of thrombosis complicating HIT. It is given as an intravenous infusion with or without a bolus, and its dosing is dependent on body weight. As lepirudin is renally excreted, dose adjustments are required in patients with renal impairment.(14) Significant limitations to the use of lepirudin are its narrow therapeutic window and potential for increased bleeding events.(26) Furthermore, formation of antihirudin antibodies following treatment of HIT in 40% of patients treated with lepirudin has been reported. These immunogenic complexes may delay the renal excretion of lepirudin and cause drug accumulation.(32, 33)

    Therefore, dose adjustment based on aPTT is required during treatment. Although rare, anaphylaxis can also occur if patients with hirudin-induced antibodies are re-exposed to hirudin.(33) Currently,there is no specific antidote to reverse the effects of the recombinant hirudins or any of the other direct thrombin inhibtitors.(34)

    Desirudin is the only fixed-dose subcutaneously administered DTI approved by the Federal Drug Administration (FDA) for postoperative prevention of VTE in patients undergoing elective hip replacement surgery.(35) In March 2010 it became available in the United States. Two randomized, double-blind, multi-center clinical studies compared the efficacy and safety of desirudin (15 mg s.c. twice daily injections) with unfractionated heparin (UFH) (5000 units s.c. three times daily) and enoxaparin (40 mg s.c. daily) for the prophylaxis of DVT in patients undergoing major orthopaedic surgeries. Desirudin was superior to both heparin anticoagulants (P < 0.001 for both) after 8-12 days of treatment, while showing a similar safety profile.(36, 37)

    Desirudin is also currently under investigation as a potential anticoagulant for patients with HIT with or without thrombosis. The PREVENT-HIT study is a small, randomized, open-label trial comparing the clinical efficacy, safety and economic utility of fixed-dose s.c. desirudin against argatroban.(38) The study is now completed and results will be soon published. Desirudin, like lepirudin has been investigated for its anticoagulant efficacy and prevention of adverse outcomes in patients with acute coronary syndromes with or without percutaneous coronary interventions. Results from both the HELVETICA and GUSTO-IIb trials demonstrated a significant reduction in the incidence of death or MI with desirudin compared with unfractionated heparin, particularly in the most unstable patients.(39,40) However, desirudin was associated with an increased incidence of major bleeding events.

    After s.c. administration, desirudin reaches maximum plasma concentrations after 1-3 h, has a terminal half-life of 2 h and is predominantly (80-90%) renally excreted. Advantages of s.c. desirudin include the lack of weight-based dose calculations and need for routine monitoring. In the setting of severe renal insufficiency (CLCR <30 ml min-1), dose reduction and monitoring with aPTT are strongly recommended.(26,32) Results from a recent pharmacokinetics study suggest that dosage adjustments and aPTT monitoring are unnecessary in patients with moderate renal impairment (CLCR 31-60 ml min-1).(41)

    Bivalirudin

    Bivalirudin is an engineered 20-amino acid, synthetic, bivalent analogue of hirudin with a thrombin inhibition activity nearly 800 times weaker than that of hirudin.(42) Unlike the recombinant hirudins, the binding of bivalirudin to thrombin is reversible because once bound, it is slowly cleaved by thrombin. As a result, thrombin activity is only transiently inhibited and the enzymatic activity of the thrombin site is restored. This reversible relationship between bivalirudin and thrombin may contribute to its decreased bleeding risk and improved safety profile when compared with r-hirudins.(23, 43)

    Bivalirudin is given intravenously, has an immediate onset of action with therapeutic activated clotting times (ACT) achieved within 5 min after initiating therapy, and a half-life of 25 min, all characteristics that are favourable for a PCI setting.(26, 32) Bivalirudin is mainly cleared by proteolytic cleavage and hepatic metabolism.(44) However, 20% of the dose is renally eliminated and dose adjustments are necessary in patients with moderate renal insufficiency.(45, 46) Bivalirudin is contraindicated in patients with severe renal impairment.(26)

    Bivalirudin has been extensively investigated in various clinical trials for its efficacy in reducing death, myocardial infarction (MI) or repeat vascularization in patients with ACS undergoing PCI. Reviews of these studies are available elsewhere.(32, 47, 48) The Bivalirudin Angioplasty Study showed that bivalirudin had a better efficacy in preventing these primary outcomes as well as a lower bleeding rate when compared with UFH in over 4000 patients undergoing PTCA for unstable or post-infarct angina.(49) This led to the 2000 FDA-approval of bivalirudin as an alternative anticoagulant to heparin in patients undergoing PTCAs.

    In 2005, the FDA expanded its approval of bivalirudin to include provisional use of concomitant glycoprotein IIb/IIIa inhibitors (GPI) for patients undergoing elective or urgent PCI procedures.(50) This decision was based on data from the Randomized Evaluation of PCI Linking Angiomax to Reduced Clinical Events (REPLACE-2) study, which demonstrated a non-inferiority of bivalirudin to UFH (each with provisional GPI) in regards to the combined primary endpoint (mortality, MI, urgent revascularization or severe bleeding), and with significantly less bleeding.(51)

    Additional studies have evaluated the use of bivalirudin in patients with ST-elevation MI (STEMI),(52) and in HIT patients undergoing PCI or cardiopulmonary bypass surgery.(53-55) Results from the ATBAT trial showed bivalirudin to have a safe and effective anticoagulant effect during PCI procedures in patients with HIT/HITTS.(53) Soon after, the FDA expanded its approval of bivalirudin to include its use as an alternative to heparin in HIT patients with or without thrombosis undergoing PCI.

    Argatroban

    Argatroban is a small (527 Da), univalent DTI that noncovalently and reversibly binds to the active site on thrombin.(26) Argatroban is licensed in the United States for the prophylaxis or treatment of thrombosis in patients with HIT and for anticoagulation in patients with a history of HIT or at risk of HIT undergoing PCI. It is given as an intravenous infusion with a starting dose of 2 mg kg-1 min-1 and does not require a bolus injection.(14) Steady-state plasma concentrations are reached in 10 h, and the plasma half-life of argatroban is approximately 45 min. As argatroban is hepatically metabolized and predominantly excreted through the biliary system, dose adjustments are necessary in patients with hepatic but not renal impairment.

    The aPTT and ACT can be used to monitor its effect; dosing is titrated to maintain an aPTT of 1.5-3 times that of baseline. Argatroban prolongs thrombin-dependent coagulation tests and thus prolongs the prothrombin time (PT) and INR. When used together with warfarin, the INR is prolonged greater than that of warfarin alone. Separate dosing and monitoring guidelines should be followed. Argatroban therapy can be discontinued when the INR during concomitant use of warfarin and argatroban is greater than 4.(26, 32)

    Oral direct thrombin inhibitors

    Oral IIa inhibitors represent a new era of anticoagulation for the prevention and treatment of venous and selected arterial thromboembolisms. Ximelagatran is the oral double prodrug of melagatran and was the first oral direct thrombin inhibitor developed. Studies demonstrated it to be as effective as the traditional anticoagulants in the prevention and treatment of venous and arterial thrombosis and secondary prevention of cardiovascular events post MI.

    The approval of ximelagatran in Europe for prevention of venous thromboembolism in major elective orthopaedic surgeries marked a milestone in the advancement of oral anticoagulants as it had been almost 60 years prior since the introduction of oral vitamin K antagonists. However, ximelagatran was removed from the European market approximately 20 months later and was never approved in North America after studies showed that therapy greater than 35 days was associated with a risk of hepatotoxicity.(56)

    Like ximelagatran, dabigatran etexilate is an orally active double prodrug that is rapidly converted to dabigatran, a low-molecular weight molecule that acts as a specific, potent and reversible direct thrombin inhibitor. It is currently the most studied and promising of the oral direct thrombin inhibitors. Key clinical advantages of this drug include a rapid onset of action, lack of interaction with cytochrome P450 enzymes or with other food and drugs, excellent safety profile, lack of need for routine monitoring, broad therapeutic window and a fixed-dose administration.

    The efficacy and safety of dabigatran etexilate against current standard anticoagulant therapy have been evaluated for multiple indications such as primary VTE prevention after hip and knee surgeries, treatment of acute deep vein thrombosis (DVT) and/or pulmonary embolism (PE) and their secondary prevention, prevention of stroke and systemic embolism in patients with non-valvular atrial fibrillation (AF) and secondary prevention of cardiac events in patients with ACS. Unlike ximelagatran, long-term use of dabigatran has not been associated with liver toxicity. Dabigatran etexilate was approved in Canada and Europe in 2008 for the prevention of venous thromboembolism after elective total hip replacement and/or total knee replacement. In October 2010, the US FDA advisory committee approved dabigatran etexilate for stroke prevention in patients with AF.

    Metabolism, pharmacokinetics and pharmacodynamics of oral DTIs

    Dabigatran etexilate and dabigatran

    Dabigatran (initially referred to as BIBR 953) is a selective, reversible, direct thrombin inhibitor given as dabigatran etexilate, an orally absorbable prodrug, since dabigatran itself is a strongly polar molecule that is not absorbed from the gut.

    Dabigatran is a small (472 Da) peptidomimetic that exert its direct thrombin inhibition by binding to its active site via ionic interactions. Dabigatran was synthesized as a derivative of the peptide-like, benzamidine-based thrombin inhibitor AA-[alpha]-naphthylsulphonylglycyl-4-amidinophenylalanine piperidine ([alpha]-NAPAP). It rapidly and reversibly inhibits both clot-bound and free thrombin in a concentration dependent manner with an inhibition constant (Ki) of 4.5 nmol L-1. It also exhibits high specificity for thrombin over other serine proteases. Direct thrombin inhibitors, such as dabigatran, the hirudins, and argatroban, do not require a cofactor, which differentiates them from the indirect coagulation inhibitors like the heparins, other glycosaminoglycans, and the synthetic pentasaccharide that must form a complex with plasma antithrombin before they can accelerate inhibition of thrombin and/or factor Xa. The inhibitor prevents access to the active site of thrombin by forming a salt bridge between its amidine group and Asp 189 and through hydrophobic interactions.(57) Like melagatran and argatroban, dabigatranis a univalent inhibitor that interacts with the active site of thrombin alone, whereas hirudin, lepirudin,and desirudin also bind to a substrate recognition site (exosite 1).(58)

    As a highly polar and charged molecule, dabigatran has poor intestinal absorption and no bioavailability after oral absorption. The conversion of the carboxylate group of dabigatran into an ester group and the masking of the amidinium moiety as a carbamate ester led to the development of dabigatran etexilate, a highly lipophilic and gastrointestinally absorbed double prodrug. After oral administration, dabigatran etexilate is rapidly converted to active dabigatran, via two intermediates (BIBR 951 and BIBR 1087) after cleavage of both lipophilic groups by serine esterases (Figure 4).(59)

    The absolute bioavailability of dabigatran after oral absorption is low (6-7%) and is independent of the dose of the prodrug.(59) As dose-escalation studies have shown that dabigatran plasma concentrations increase in a dose proportional manner,(60) relatively high doses of dabigatran etexilate are necessary to achieve adequate plasma concentrations. Dabigatran etexilate is also optimally and more consistently absorbed in an acidic environment. To achieve this goal, a new capsule formulation containing multiple small pellets containing a tartaric acid core and coated with dabigatran etexilate was developed. The advantage of this modification is that dabigatran etexilate brings its optimal pH environment to the site of absorption (i.e. gastrointestinal tract). Therefore, variations in intrinsic gastric pH do not significantly affect the extent of its absorption, even in the presence of proton pump inhibitors.(61)

    Another significant characteristic of this prodrug is the lack of involvement by the P450 (CYP) isoenzymes or other oxidoreductases in the metabolism of dabigatran etexilate to dabigatran. Rather, ubiquitous esterases present in plasma are involved in the conversion. As in vitro studies also show that dabigatran does not inhibit the cytochrome P450 enzymes, the potential for drug-drug interactions is low.(59) After absorption of dabigatran etexilate, bioconversion to dabigatran occurs in enterocytes, hepatocytes and the portal vein. Twenty percent of dabigatran is conjugated to glucuronic acid to yield pharmacologically activated glucuronide conjugates.(61)

    Of note, the absorption and metabolism of dabigatran etexilate has also been tested in patients with moderate hepatic impairment. Although the bioconversion rate of dabigatran etexilate to its active form was slightly slower than that in matched healthy control subjects, the mean values for the area under the plasma concentration-time curves (AUCs), terminal half-life and renal clearance were comparable between the two groups. In addition, results from several clot-based assays were also similar between the healthy controls and patients with hepatic insufficiency.(62) These findings demonstrate that moderate hepatic impairment does not affect the efficacy or safety profile of dabigatran, and that these patients can be given unadjusted doses of dabigatran etexilate.

    Renal excretion is the primary elimination pathway for dabigatran in humans. Eighty-five percent of the dose is excreted by renal clearance, almost all as unchanged dabigatran.(63) The remainder undergoes conjugation with glucuronic acid to form acylglucuronides, which are excreted via the bile. Both the bioavailability and extent of exposure to dabigatran are elevated in renal insufficiency (CLCR < 50 ml min-1) and appropriate dose reduction of dabigatran etexilate is required. Although dabigatran is dialyzable, it is contraindicated in patients with severe renal impairment (CLCR < 30 ml min-1).(64)

    After oral administration of the prodrug, dabigatran etexilate is rapidly converted to dabigatran, with peak plasma concentrations (Cmax) achieved within 1.5-2 h in healthy volunteers regardless of age or gender.(59, 64, 65) After absorption, a rapid distribution phase occurs followed by a prolonged elimination time. The volume of distribution of dabigatran is 60-70 L, indicating moderate volume distribution.(64) The mean plasma terminal half-life of dabigatran in healthy young and elderly volunteers is 12-14 h, independent of dose, and accordingly steady-state concentrations are attained within 3 days with multiple dosing and without evidence of significant accumulation.(60, 66)

    The overall exposure is increased by 40-60% in healthy elderly volunteers compared with younger subjects primarily secondary to reduced renal clearance and slower elimination of dabigatran.(66) Prolongation of blood coagulation parameters (aPTT, PT, TT, ECT) occurs in parallel with increasing concentrations of dabigatran, and peak clotting times coincide with the Cmax of dabigatran (2 h after oral administration). Similarly, 12 h after oral administration of dabigatran (time of first half-life), the prolongation of blood coagulation returned to 50% of the maximum effect.(60, 64)

    As would be expected from a direct thrombin inhibitor, dabigatran prolongs the thrombin clotting time(TCT), PT, aPTT, and ecarin clotting time (ECT) of plasma from humans, rats, rabbits, dogs, and rhesus monkeys and also inhibits thrombin generation inhuman plasma.(67) The ECT has been a preferred measure of anticoagulant effect for r-hirudin and other direct thrombin inhibitors; ecarin is a metalloprotease enzyme obtained from venom of the saw-scaledviper (Echiscarinatus) that generates meizothrombin from prothrombin.(60,68) Dabigatran prevents thrombin-induced platelet aggregation but not platelet aggregation by arachidonic acid, collagen, or adenosine diphosphate.(67)

    The thrombin time (TT) is most responsive to dabigatran in the clinically relevant plasma concentration range whereas the aPTT and prothrombin time (PT) are least so. The ECT assay may be the most useful test for measuring thrombin inhibition as it has been reported as having adequate sensitivity and precision with predictable and reproducible results.(61, 69) Currently under development is a diluted thrombin time assay (Hemoclot® Thrombin Inhibitor Assay), which uses dabigatran standards for quantitative measurement of DTI activity in plasma. A direct relationship between dabigatran concentration and clotting time (from 30-75s) has been reported.(69)

  •  
    Figure 4: Chemical structures of the double prodrug, Dabigatran etexilate and its active form, dabigatran.
  •    

    Factors Interfering With Absorption

    Food can prolong the time to peak plasma dabigatran concentrations by 2 h without having an effect on its extent of absorption (AUC).(65) Absorption of dabigatran etexilate is influenced by gastric pH as affected by proton pump inhibitors, food, the postoperative state, and also by drugs that inhibit or induce activity of the cell efflux transporter P-glycoprotein (P-gp). Dabigatran etexilate has a low aqueous solubility that is further reduced by increased pH,(70) as is observed in patients taking the gastric proton pump inhibitor pantoprazole. Twice daily pretreatment with 40 mg pantoprazole for 48 h in a crossover study reduced geometric mean levels of Cmax and AUC after 150 mg dabigatran etexilate by 40% and by 32% in healthy male volunteers aged 18 to 55 years.(65) Bioavailability (steady-state AUC) was also reduced by 20% to 40% in a parallel group study in which older volunteers, aged >65 years, took 40 mg pantoprazole with 150 mg dabigatran etexilate bid for 6 days; pantoprazole raised the gastric pH from 2.2 to 5.9, and pH correlated with AUC.

    Small corresponding changes in Cmax, ECT, and aPTT were not believed to have clinical importance.(66) Taking 150 mg dabigatran etexilate after a high-fat, high-calorie breakfast prolonged the time taken to reach Cmax from 2 h to 4 h in the crossover study described above, although Cmax and total drug exposure remained unchanged.(65)

    Absorption of doses taken 4 to 8 h after a hip replacement was slowed and reduced, compared with 2 to 10 days later, such that time to reach the peak plasma concentration was delayed to 6 h and both Cmax and AUC were greatly diminished; the changes were attributed to early effects of surgery on GI motility and gastric acidity.(71)

    Other Drug-Drug Interactions: Important drug-drug interactions most often result from changes in drug metabolism that are due to induction or inhibition of CYP3A4 and other enzymes of the microsomal cytochrome P450 complex or from changes in drug bioavailability mediated by the adenosine diphosphate-dependent cell efflux transporter, P-glycoprotein (P-gp).(72) Potential drug interactions with dabigatran etexilate have been explored in studies in which volunteers received dabigatran etexilate together with drugs known to provoke such mechanisms. Many drugs may interact through more than one pathway.

    Because cytochrome P450 enzymes have almost no role in the metabolism of dabigatran and are not affected by dabigatran in vitro, this becomes an unlikely mechanism for drug-drug interactions,(63) and volunteer studies confirm the lack of a clinically important interaction with atorvastatin (a substrate for CYP3A4 and substrate/inhibitor of P-gp) and diclofenac (a substrate for CYP2C9 and uridineglucuronyltransferase 2b7, and also a substrate and weak inhibitor of UGT1A).(61) When 22 volunteers aged 43±15 years took 80 mg atorvastatin together with 150 mg dabigatran bid for 4 days in an open-labelcrossover study, the steady-state AUC of dabigatran was reduced by 18%, whereas the Cmax and AUC of atorvastatin Increased by 15% and 23%.(73) Cmax and AUC of dabigatran remained unchanged in a similar study of 24 volunteers aged 18 to 55 years who took one 50-mg dose of diclofenac after 4 days of bid dosing with 150 mg dabigatran etexilate, whereas Cmax of diclofenac and its main metabolite decreased by 11% to 17%.(74) The changes were believed to be small and clinically unimportant.

    In vitro studies find that dabigatran etexilate (but not dabigatran) is a substrate for P-glycoprotein (P-gp, MDR1) with a medium affinity when tested using the Caco-2 cell-line1, which makes it a potential target for P-gp-related drug interactions.(75) The bioavailability of P-gp substrates like dabigatran may be raised or reduced through inhibition or induction of P-gp: the P-gp inhibitors include amiodarone, verapamil, ketoconazole, quinidine, and clarithromycin, whereas P-gp inducers include rifampicin and St. John's wort (Hypericum perforatum).(75)

    In formal interaction studies with amiodarone, a first dose of 600 mg raised the AUC and Cmax of dabigatran by about 50% and 60%, an interaction that may persist for some weeks after stopping amiodarone due to the long half-life of this drug.(73) The effects of verapamil depend on its dosing schedule and drug formulation.(73) The first dose of an immediate release formulation, when given 1 hour before 150 mg dabigatran etexilate, increased the Cmax and AUC of dabigatran by about 180% and 150%, but these elevations were reduced to about 60% and 50% after repeated dosing and to about 90% and 70% when taking an extended-release formulation. The interaction became negligible (increases of 10% in Cmax and 20% for AUC) if verapamil was taken 2 h after dabigatran etexilate when dabigatran absorption was essentially complete. Twice-daily coadministration of 500 mg clarithromycin increased the AUC and Cmax of dabigatran by about 19% and 15%, respectively.

    The strong P-glycoprotein inhibitors quinidine and ketoconazole are contraindicated when taking dabigatran etexilate because they markedly increase exposure to dabigatran.(73) The approved product information advises caution when considering coadministration of strong P-gp inducers like rifampicin or St. John's wort, which may significantly decrease the Cmax and AUC.(73)

    The P-gp substrate digoxin is used to probe P-gp mediated drug-drug interactions. After 4 days of dosing with once-daily digoxin plus bid 150 mg dabigatran etexilate, there was little effect on the pharmacokinetics of either drug in a three-way crossover study of 23 healthy volunteers aged 18 to 65 years.(76)

    Drug interactions that may change the Cmax and AUC of dabigatran have not been correlated with clinical outcomes. Very large increases of Cmax d AUC, like those of quinidine and systemic ketoconazole, are likely to raise the bleeding risk. Moderate increases, like those provoked by amiodarone or verapamil, may become important if combined with old age or reduced renal clearance. The concern about strong P-gp inducers like rifampicin is their potential to decrease drug exposure and therefore reduce efficacy.

    However the pharmacokinetic profile of dabigatran is not affected by the use of co-medications, such as opioids,(65) diclofenac,(77) atorvastatin (73) or digoxin.(28,61) A reduced dose of dabigatran etexilate is recommended when co-administered with amiodarone or other strong P-glycoprotein (P-gp) inhibitors like verapamil, clarithromycin and others. Dabigatran is contraindicated with quinidine use. Other potent P-gp inducers such as rifampicin may reduce the systemic exposure of dabigatran.(78, 79)

    Dabigatran Etexilate and Antiplatelet Drugs:

    The added bleeding risk when platelet function inhibitors like aspirin and clopidogrel are taken during anticoagulant therapy is compounded for aspirin and other nonsteroidalanti inflammatory drugs by the increased likelihood of peptic ulceration due to interference with prostaglandin-mediated cytoprotection of the gastrointestinal mucosa.(80) These mechanisms are independent from any pharmacokinetic drug-drug interactions (none was demonstrated between dabigatran etexilate and diclofenac).(74) Aspirin increased the bleeding rate when added to ximelagatran in patients with AF and to warfarin in patients with AF, a prosthetic heart valve, coronary artery disease, or peripheral vascular disease.(81,82,83)

    The added effects on bleeding when aspirin is combined with dabigatran etexilate were explored in the Prevention of Embolic and Thrombotic Events in Patients With Persistent Atrial Fibrillation (PETRO) trial, a phase 2, parallel-group, randomized, dose ranging safety study in 502 patients with nonvalvular AF who also had coronary artery disease and/or one or more other risk factors for systemic embolism. The patients received 12 weeks of treatment with open label warfarin alone (target INR 2-3) or with blinded 50 mg, 150 mg, or 300 mg dabigatran etexilate bid plus daily aspirin (81 mg or 325 mg) or a placebo, using a 3 33 factorial design to allocate study treatments to patient groups of unequal size. Aspirin increased the chances of major or clinically significant nonmajor bleeding in patients given a supratherapeutic dabigatran dose of 300 mg bid, in whom the bleeding rate was 20% with an aspirin dose of 325 mg/d (six of 30), 14.7% if the dose was 81 mg/d (five of 34), and 5.7% (six of 105) in patients given the aspirin placebo. The trends reached statistical significance when the two aspirin groups were pooled. Bleeding risk was not apparently raised when aspirin was added to 50-mg or 150-mg doses of dabigatran, but the sample size was too small to exclude clinically important effects.(84)

    Low doses of aspirin (<100 mg/d) were permitted in the Randomized Evaluation of Long-term Anticoagulant Therapy (RE-LY) (nonvalvular AF) and Dabigatran in the Treatment of Venous Thromboembolism (RE-MEDY) (VTE) phase 3 studies of dabigatran etexilate, which compared bid doses of 110 mg and/or 150 mg with warfarin, but subgroup analyses of bleeding risk are not yet available.(85,86)

    Product information recommends against the combination of dabigatran etexilate with clopidogrel and other thienopyridines, given alone or as dual antiplatelet therapy with aspirin.

    Monitoring Anticoagulant Intensity

    There is no evidence relevant to the possible clinical benefits from laboratory testing, since the phase 3 studies evaluated fixed doses and their reports have not examined clinical outcomes in relation to drug levels or clotting test results. The intent has been to recommend standard doses for most patients, although first principles suggest that laboratory-assisted dose adjustment of this mainly renally excreted drug could add clinical value in selected populations, as in elderly subjects with reduced renal function,(87) it seems unlikely that routine monitoring would yield any wide clinical benefit.(88) Perhaps the most likely role for laboratory testing may be in treated patients who bleed or develop thrombosis, need an acute invasive procedure, or could have taken an overdose.

    In the setting of major bleeding, or if urgent or emergent surgery is required, a normal TCT rules out the presence of dabigatran. TCT tests are available routinely in many laboratories.

    Reversal of Drug Effect

    There is insufficient clinical experience to firmly guide the management of major bleeding, suspected overdose, urgently needed surgery, or urgent invasive diagnostic or therapeutic procedures in patients who are taking this new drug. Pharmacokinetic modeling does, however, indicate how long it takes for drug effects to dissipate after stopping dabigatran etexilate before an elective intervention, although conclusions about the time taken before a return to normal hemostasis remain tentative pending well-documented information about clinical outcomes.

    The half-life of dabigatran suggests that drug levels and drug effects should decrease by about 50% at 12 to 18 h after the most recent dose, and the trough levels to 25% of their previous steady state by 24 h after stopping dabigatran etexilate, so long as creatinine clearance exceeds 50 mL/min. The level at which it is safe to undertake surgery or an invasive procedure is unknown. Moderately severe renal dysfunction (creatinine clearance of 30-50 mL/min) extends the half-life to about 18 h, in which case, or if the surgical bleeding risk is critically high (as with intracranial surgery), it may be better to delay elective procedures until 2 to 4 days after stopping the drug. Measuring the TCT or aPTT should help to estimate the residual level of dabigatran.(69)

    In addition to immediately stopping drug administration, the clinical management of major bleeding would require early volume and RBC replacement, urgent assessment for cause, and any local measures that may be required until the bleeding stops (pressure, cautery, suture, or other interventions). It is believed that maintaining an adequate diuresis could help to protect the renal excretion of dabigatran. Although product information for dabigatran etexilate mentions the use of fresh frozen plasma to help control bleeding, this seems unlikely to influence the drug effects. Thus plasma should only be administered in the setting of a documented dilutional coagulopathy.

    Dabigatran has no antidote, and the management of life-threatening bleeding remains empirical. Indirect evidence from animal models and in vitro studies suggests that recombinant factor VIIa or a prothrombin complex concentrate may bypass the anticoagulant effects of high dabigatran concentrations.(69) It may also be relevant that hemodialysis removed 62% of circulating dabigatran within 2 h and 68% within 4 h in an open-label study of 12 subjects with end-stage renal failure who received 50 mg dabigatran etexilate.(69,89) In vitro mixing experiments suggest that early administration of activated charcoal might reduce the absorption of dabigatran etexilate.(69)



    Firouz Madadi MD
    Orthopaedic Surgeon, Associate Professor, Shahid Beheshti University of Medical Sciences, Tehran, Iran
    fmadadi@yahoo.com

       

    Mehrnoush hassas Yeganeh MD
    Paediatric Rheumatologist, Assistant Professor, Shahid Beheshti Medical University, Tehran, Iran
    Mehrnoushyeganeh@gmail.com

       

    Firouzeh Madadi MD
    Medical student and researcher, Tehran, Iran
    Fmadadi33@gmail.com

       

    Hamid Reza Seyyed Hosseinzadeh MD
    Orthopaedic surgeon, Associate professor, Shahid Beheshti Medical Univerity, Tehran, Iran
    hosseinzadehmd@yahoo.com

       
     

    Acknowledgements:
    None declared.

     
     

    Financial disclosure:
    None declared.

     
     

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