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Abstract

Cardiovascular disease is a leading cause of death and pathologic coagulation plays an integral role in the development, propagation, and intervention of cardiovascular disease. The 2 classic anticoagulants, heparin and vitamin K antagonists, though having served humanity for nearly a century, are both inconvenient and nonspecific. Through both direct and indirect roles, thrombin is essential to coagulation, and makes for a very attractive target in medical intervention of pathologic thrombosis. This article will review the nature of direct thrombin inhibitors, current indications, and ongoing trials.

Keywords

thrombosis pathophysiology

Cardiovascular disease is the leading cause of death in the United States, accounting for approximately one-third of all deaths.¹ Pathologic coagulation plays an integral role in the development, propagation, and intervention of cardiovascular disease. It is therefore not surprising that anticoagulants are among the most prescribed medications and account for a substantial portion of overall health care spending.

Mechanism of Thrombus Formation

The classic explanation for thrombosis, as defined by Virchow in 1860, is reflected by abnormalities in blood flow, blood constituents, and vessel walls. When updated with modern understanding of coagulation biology, this can be redefined as turbulence and stenosis, coagulation or fibrinolytic defects, and endothelial injury.² Mechanical intervention focuses on the former, while medical therapies would tend to focus on the latter.

Using the techniques of molecular biology, over 100 hemostasis proteins, in conjunction with platelets and vascular endothelium, have been identified in the coagulation cascade.³ The initial step in coagulation is exposure of subendothelial collagen in the vessel wall. This leads to rapid platelet binding via von Willebrand Factor (vWF) and platelet receptor glycoprotein (Gp) Ib forming a temporary “plug.”⁴ Platelets subsequently become activated and release the contents of their granules. This leads to a morphologic change in the platelet structure, with translocation of negatively-charged phosphatidylserine residues to the external leaflet of the phospholipid bilayer, which will then serve as a binding surface for the next step in the coagulation cascade, that of fibrin-deposition.⁵

Tissue factor (TF) is normally found in the subendothelial space, protected from circulating Factor VII by intact endothelium. When endothelial disruption occurs, such as rupture of an atherosclerotic plaque, tissue factor is released into the circulation, binding with Factor VII, which causes a 10 000 000-fold increase in enzymatic activity.⁶ If vessel injury is limited to endothelial activation (no rupture), circulating TF in the blood is activated and can bind Factor VII, thereby initiating coagulation. TF-VIIa then activates Factors IX and X. Small amounts of thrombin are generated, leading to activation of Factors V and VIII, and subsequent amplification of thrombin production predominately through the prothrombinase and tenase complexes.⁷ Factors Xa, Va, and prothrombin (II) form the prothrombinase complex on the platelet surface and, in the presence of calcium, form activated thrombin.⁸ Factors IXa and VIIIa, with calcium, form the tenase complex, which leads to further generation of Factor Xa.⁹ An additional “intrinsic” pathway exists, utilizing high-molecular weight kininogen, kallikrein, and factors XII, XI, and IX to activate factor X, which then ultimately shares a common pathway in the formation of thrombin. However, the true physiological nature of the intrinsic pathway has yet to be elucidated and may play a more significant role in extracorporeal bypass circuits.¹⁰

Thrombin is central to coagulation, as it cleaves fibrinogen into fibrin strands, which then bind and cross link (via factor XIII) the platelet plug, making it less susceptible to thrombolysis.¹¹ In addition, thrombin is capable of feedback amplification of factors V, VIII, XI, and XII. Through both direct and indirect roles, thrombin is essential to coagulation and makes for a very attractive target in medical intervention of pathologic thrombosis.

Disadvantages of Current Anticoagulants

The classic anticoagulant, heparin, was discovered in 1916 by McLean and remains to this day one of the most effective and widely used compounds. However, heparin does have some distinct disadvantages. First, heparin is poorly absorbed from the gastrointestinal tract, requiring intravenous administration, usually in a continuous fashion.¹² Heparin undergoes substantial protein binding, resulting in variable dose-response curves. The heparin-antithrombin complex is unable to bind its usual exosite 1 in fibrin-bound thrombin; therefore, bound thrombin continues to promote systemic thrombogenesis without inhibition from heparin.^13,14 Lastly, heparin binds platelet factor 4, resulting in immune-mediated platelet activation and the well-known syndrome of heparin-induced thrombocytopenia/thrombosis (HIT).¹⁵

Low-molecular-weight heparins (LMWH) were developed to address many of these issues. However, LMWH is similarly unable to inactivate fibrin or clot bound thrombin and development of HIT syndrome due to cross-reactivity of antibodies remains quite real.

Factor Xa inhibitors rarely cause HIT syndrome, although fondaparinux-induced HIT has been previously reported.¹⁶ Newer, active site-directed Factor Xa inhibitors, such as rivaroxaban, do not cross-react with HIT antibodies and are unlikely to develop HIT syndrome.¹⁷ While Factor Xa inhibitors do inhibit thrombin generation, active thrombin, produced through alternative pathways, is free to promote amplification of coagulation and platelet aggregation.¹⁸

Vitamin K antagonists are the most widely used oral anticoagulants today; warfarin was the 11th most prescribed drug in 1999.^19,20 They exhibit their effect through inhibition of vitamin K-dependent carboxylation of factors II, VII, IX, and X. These drugs have a very narrow therapeutic window, and multiple foods and drugs, as well as certain physiological states, can interact with their metabolism, often with disastrous consequences. Frequent monitoring is therefore required to ensure appropriate level of anticoagulation.

As a result of the many drawbacks listed above, a search for alternative forms of anticoagulation has been ongoing. As previously mentioned, the central role of thrombin makes it an ideal target for interruption.

In 1884, Haycraft demonstrated that a substance produced by leeches prevented coagulation. The anticoagulant hirudin was isolated from the extracts of the medicinal leech Hirudo medicinalis. The characterization of this new drug class, direct thrombin inhibitors, was of limited clinical use due to inadequate supply. With the advent of genetic engineering, recombinant hirudin has opened the door to a whole new group of synthetic drugs and clinical interest in this drug class is once again strong. Direct thrombin inhibitors (DTI’s) can be described as hirudin derivatives, hirulogs, or synthetically produced small-molecule direct thrombin inhibitors.¹⁹

Hirudin Derivatives

The original direct thrombin inhibitor, hirudin, is available in a recombinant form, lepirudin (Refludan) and desirudin (Revasc, Iprivask). Desirudin is the first subcutaneous DTI approved in the United States These are bivalent direct thrombin inhibitors, meaning they block both the active site and exosite I (antithrombin binding site) in an irreversible 1:1 stoichiometric complex.^21,22

Hirudin

The Direct Thrombin Inhibitor Trialists’ Collaborative Group was a meta-analysis of 11 randomized trials involving hirudin and other DTI’s versus heparin in the treatment of patients with acute coronary syndrome.²³ This study had significant heterogeneity, but there was a 15% reduction in composite outcome of death or myocardial infarction in patients with acute coronary syndrome (ACS) treated with DTI instead of heparin at end of treatment and at 30 days. This was primarily driven by a reduction in myocardial infarction, as there was no overall change in death rate. Hirudin had a significantly higher incidence of bleeding than heparin. Based on these findings, hirudin is not recommended for treatment of acute coronary syndrome nor does it have any other current clinical indications.²¹

Lepirudin

Three prospective cohort studies were performed (HAT 1,2,3) comparing lepirudin to historical controls in the management of heparin-induced thrombocytopenia without current thrombotic complications.²⁴ Patients on lepirudin had only 4.4% incidence of new thrombotic events versus 14.9% for historical controls. Major bleeding was significantly more common in the lepirudin group (14.3% vs 8% for controls); this is thought to be secondary to an approved dosing regimen that is too high, with subsequent recommendations to halve the initial dose (0.05-0.10 mg/kg per hour).^25,26 Interestingly, 39.4% of patients in the lepirudin group developed antihirudin antibodies. There were no differences in outcomes, but occurrence of fatal anaphylaxis with re-treatment (interval <3 months) has been reported elsewhere.²⁷ Lepirudin has an FDA-approved indication for treatment of HIT.

Desirudin

Two randomized controlled trials have been performed comparing desirudin to enoxaparin or unfractionated heparin in prevention of postoperative VTE in orthopedic patients.^28,29 Desirudin (15 mg SQ BID) was found to be superior to enoxaparin (40 mg SQ QD) and UFH (5000 u SQ TID), with overall incidence of VTE 18.4% vs 25.5% and 13% vs 23%, respectively. A significant advantage in rate of proximal DVT was also observed. Bleeding rates were similar. Desirudin is approved in the European Union for VTE prophylaxis. Multiple phase III trials are currently ongoing in the United States.³⁰

Peg-hirudin

PEG-Hirudin is a hybrid of native hirudin which, when coupled with polyethylene glycol (PEG), has been shown to significantly prolong plasma half-life.³¹ This new combination has been shown to inhibit arterial thrombosis in ex vivo human models.³² A phase II clinical trial was completed comparing PEG-Hirudin (SPP200) with unfractionated heparin in dialysis grafts, but no results have been published at this time.³³ Currently, this drug remains experimental and there are no current indications or recommendations.

Hirulogs

Bivalirudin

The hirulogs consist of bivalirudin (Angiomax), a synthetic analog of hirudin. Bivalirudin, like the hirudins, is also a bivalent direct thrombin inhibitor but, unlike the hirudins, bivalirudin is cleaved by thrombin after binding, resulting in a transient, reversible thrombin inhibition.^34,35

Bivalirudin has been extensively studied in the setting of acute coronary syndrome. A randomized trial published in 2002 (HERO-2) compared either bivalirudin or unfractionated heparin plus streptokinase in the treatment of ST-elevation myocardial infarction (MI).³⁶ No difference in primary outcome (30-day mortality) was noted, although a secondary outcome, that of reinfarction within 96 hours, was noted to be 30% less in the bivalirudin group. Bleeding rates were equal between the 2 groups.

A randomized trial performed in 2003 (REPLACE-2) compared heparin with planned glycoprotein IIb/IIIa inhibition to bivalirudin with provisional glycoprotein IIb/IIIa inhibition in patients undergoing elective percutaneous coronary intervention.³⁷ Bivalirudin was found to be noninferior to heparin with regard to the major endpoints and was associated with significantly reduced major bleeding (2.4% vs 4.1%).

The Acute Catheterization and Urgent Intervention Triage Study (ACUITY) was a randomized trial comparing heparin plus IIb/IIIa inhibition, bivalirudin plus IIb/IIIa inhibition, and bivalirudin alone in the treatment of stable acute coronary syndrome in patients undergoing early intervention.³⁸ Bivalirudin alone was found to be noninferior to the other 2 regimens with regard to the composite ischemia endpoint but had significantly reduced major bleeding (3.0% vs 5.7%).

The Harmonizing Outcomes with Revascularization and Stents in Acute Myocardial Infarction (HORIZONS-AMI) trial compared heparin with IIb/IIIa inhibition versus bivalirudin alone in the treatment of acute myocardial infarction.³⁹ Bivalirudin had significantly reduced 30-day rates of death from cardiac causes (1.8% vs 2.9%) and all causes (2.1% vs 3.1%), primarily due to a lower rate of bleeding (4.9% vs 8.3%).

The sum of these trials would indicate that, in the setting of acute coronary syndrome with early intervention, bivalirudin is associated with better outcomes, primarily through a lower incidence of bleeding. Glycoprotein IIb/IIIa inhibitors should be added if periprocedural complications are encountered.³⁷ There are currently at least 25 active phase II and III trials involving bivalirudin in the setting of acute coronary syndrome ongoing in the United States, indicating much more knowledge is yet to come.⁴⁰

Small-Molecule Direct Thrombin Inhibitors

Argatroban

The small-molecule direct thrombin inhibitors are synthetic, univalent inhibitors that bind thrombin at the active site in a reversible fashion.⁴¹ Argatroban has previously been studied in cohort trials versus historical controls for the treatment of HIT.^42,43 These studies found no difference in the endpoints of all-cause death or all-cause amputation over the 37-day study period. However, new thrombosis was significantly reduced with argatroban in the isolated HIT (5.8% vs 23%) and HIT with thrombosis (13.1% vs 34.8%) treatment arms. Bleeding rates were similar between the 2 groups. Argatroban is the only FDA-approved small-molecule DTI in the United States for the treatment of HIT.

Ximelagatran

Ximelagatran was the first, and only, oral anticoagulant to be introduced since warfarin appeared in the 1950s. This prodrug, which is metabolized to the active metabolite melagatran, was extensively studied in randomized trials for treatment of atrial fibrillation, myocardial infarction, VTE prophylaxis, and VTE treatment comprising nearly 23 000 patients.^44–53 Data was quite promising, and approval for short-term use was granted in the European Union. However, long-term data revealed elevated alanine aminotransferase (ALT) concentrations greater than 3 times the upper normal limit in 7.9% of study patients (vs 1.2% for control patients).⁵⁴ This prompted the FDA to refuse approval. Ultimately, there were 112 deaths in ximelagatran-treated patients, of which 8% exhibited signs of liver failure.⁵⁵ Astra Zeneca removed the drug from the world market in February 2006.

AZD

AZD0837 is the follow-up compound of ximelagatran, also produced by Astra Zeneca. A recent, unpublished phase IIb trial of 955 patients compared 4 different dosages of AZD0837 to warfarin in stroke prevention of patients with atrial fibrillation and at least 1 additional risk factor.⁵⁶ Total bleeding events were similar or less in all AZD0837 groups. Frequency of ALT elevations greater than 3-fold in the AZD0837 groups were also similar to the warfarin group (2.3% vs 1.6%, respectively), indicating previous liver side effects may have been resolved. An extended-release formulation has been developed, giving rise to the possibility of once-daily dosing. A phase III clinical trial involving treatment of chronic atrial fibrillation is currently underway.⁵⁷

Dabigatran

Dabigatran etexilate (Pradaxa) is an oral synthetic DTI which is rapidly absorbed, has peak onset within 0.5 to 2 hours, and a half-life of 8 to 17 hours (depending on previous doses).⁵⁸ The phase II Boehringer Ingelheim Study in Thrombosis II (BISTRO II) trial was a dosage-finding study comparing dabigatran to enoxaparin in VTE prophylaxis after orthopedic surgery. The results of this study indicated a daily total dose of dabigatran in the range of 100 to 300 mg had equal efficacy and safety to enoxaparin with regard to the major outcomes of DVT incidence and bleeding.⁵⁹

The REVOLUTION program is a worldwide phase III collection of randomized clinical trials studying dabigatran in VTE prophylaxis after orthopedic surgery (RE-NOVATE, RE-MODEL, RE-MOBILIZE), VTE treatment (RE-SOLVE, RE-COVER, RE-MEDY), and atrial fibrillation (RE-LY). The initial VTE prophylaxis, VTE treatment (RE-COVER), and atrial fibrillation trials have been completed, with results from the remaining trials pending.⁶⁰

The Prevention of Venous Thromboembolism after Total Hip Replacement (RE-NOVATE) trial compared dabigatran in doses of 150 and 220 mg daily (started postoperatively) to enoxaparin in the European fashion of 40 mg SQ daily (started preoperatively). Analysis of the final data determined dabigatran (in either dose) was not inferior to enoxaparin in prevention of VTE and all-cause mortality, with similar safety profiles.⁶¹

The Thromboembolism Prevention After Knee Surgery (RE-MODEL) study was nearly identical to RE-NOVATE but instead looked at patients with total knee replacement (TKR).⁶² The same dosages and primary endpoints (total VTE, all-cause mortality, major bleeding) were once again used, and dabigatran (at both dosages) was determined to be noninferior to enoxaparin. The incidence of alanine aminotransferase (ALT) elevation greater than 3-fold was low, and not significantly different among all 3 groups.

The Dabigatran versus Enoxaparin in Preventing Venous Thromboembolism Following Total Knee Arthroplasty (RE-MOBILIZE) trial differed from the other REVOLUTION orthopedic studies in that it compared enoxaparin using the North American dose of 30 mg SQ BID.⁶³ With regard to the major outcomes (total VTE and all-cause death), dabigatran 220 mg (31.1%) and 150 mg (33.7%) was found to be inferior to enoxaparin (25.3%). Interestingly, however, the incidence of symptomatic VTE and VTE-related death were similar among all groups. Bleeding rates were also similar. Enoxaparin was dosed 50% higher in this study, and both dabigatran and enoxaparin were given a mean of 5 days longer. There are no studies in the literature comparing the efficacy of enoxaparin in European versus North American protocols; thus, one could wonder if the results of the RE-MOBILIZE trial are clinically relevant (the superiority of enoxaparin was based significantly on distal, subclinical findings of VTE) or did the RE-NOVATE/RE-MODEL trials underscore the true efficacy of enoxaparin? It is hopeful that the other current trials investigating VTE treatment and long-term proxphylaxis (RE-SOLVE, RE-MEDY) will shed further light on this subject.

A meta-analysis of the RE-MODEL, RE-NOVATE, and RE-MOBILIZE trials, using data from only the 220 mg once daily dabigatran dose, revealed no significant differences in major endpoints of total VTE, all-cause mortality, or bleeding.⁶⁴ Heterogeneity between the trials could not be ruled out, however, possibly for reasons previously mentioned.

RE-COVER was a randomized, double-blind, noninferiority trial comparing dabigatran versus warfarin in the treatment of acute venous thromboembolism.⁶⁵ Patients were first administered parenteral anticoagulation for a median of 9 days, followed by either dabigatran (150 mg twice daily) or warfarin (sufficient to achieve an international normalized ratio of 2.0 to 3.0) for 6 months. Dabigatran was determined to be noninferior to warfarin with regard to primary outcomes (6-month incidence of recurrent, symptomatic VTE, and related deaths). Safety end points (bleeding events, acute coronary syndromes, other adverse events, and liver-function tests) were similar as well. No laboratory monitoring was required for the dabigatran treatment arm.

Dabigatran has been approved in the European Union for short-term VTE prophylaxis at a dose of 220 mg daily, with half of the initial dose to be given 1 to 4 hours after surgery. No monitoring is required. Dabigatran is predominantly excreted renally (80%): dose reductions must be made for moderate renal insufficiency (CrCl 30-50 mL/min) and patients over the age of 75, and, like enoxaparin, is contraindicated in patients with severe or end-stage renal disease (CrCl <30 mL/min).

The Randomized Evaluation of Long-Term Anticoagulation Therapy (RE-LY) trial was a large clinical trial involving 18 113 patients that randomized atrial fibrillation patients with at least 1 risk-factor to 1 of 2 blinded dabigatran dosages (110 or 150 mg PO BID) or warfarin (open-label).⁶⁶ Major outcomes were stroke and systemic embolism. The primary safety outcome was major hemorrhage. The 110 mg dose of dabigatran was noninferior to warfarin with regard to major outcomes (1.53% vs 1.69%), and was associated with lower rates of major hemorrhage (2.71% vs 3.36%). The 150 mg dose of dabigatran was actually superior to warfarin with regard to major outcomes (1.11% vs 1.69%) but had a similar rate of hemorrhage (3.11% vs 3.36%). Thus, the higher dosage has better efficacy, and the lower dosage is safer; neither require monitoring. There was no difference in elevation of liver enzymes among the groups. Further subgroup analysis is needed to determine which dose should be administered to which group of patients.

Disadvantages of Direct Thrombin Inhibitors

Approximately 11% of patients taking dabigatran complained of dyspepsia; this ultimately led to a higher second-year dropout rate (21%) than warfarin (16.6%).⁶⁷ Rates of myocardial infarction were significantly higher in the dabigatran groups, which have previously been shown in studies with ximelagatran, and the clinical implications of this are unclear at this time.⁵² There is a significant interaction with P-glycoprotein inhibitors (verapamil, amiodarone, quinidine), which lead to elevated serum dabigatran concentrations. In contrast, an acidic environment is needed for absorption, and proton pump inhibitors reduce bioavailability by more than 20%, although the clinical relevance of this is unclear.⁵⁷

Reversal

At this time, there is no specific antidote to direct thrombin inhibition. Animal and ex-vivo studies would indicate that recombinant Factor VIIa can at least partially reverse the bleeding time, but further research in this area is needed.^68,69 An advantage of direct thrombin inhibition, as opposed to warfarin, is the greatly shortened half-life; the major anticoagulant effects should disappear 12 to 24 hours after the last dose.²¹

Monitoring

The best method of monitoring direct thrombin inhibition is unclear.^70–72 Recombinant hirudins and argatroban can be followed by the activated partial thromboplastin time (aPTT). Activated clotting time (ACT) can be used with bivalirudin, although reports suggest the ecarin clotting time (ECT) may be more accurate.^73,74

Conclusion

After a combined 150 years of uncontested reign, heparin and warfarin may finally have a worthy, or even superior, replacement. Approved clinical indications are still few, and cost will no doubt have to be considered, but it is without doubt that soon patients will have more options than ever before.

Footnotes

The author(s) declared no conflicts of interest with respect to the authorship and/or publication of this article.

The author(s) received no financial support for the research and/or authorship of this article.

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