Role of Platelets in Arterial Thrombosis
In healthy vasculature, circulating platelets are maintained in an inactive state by nitric oxide (NO) and prostacyclin released by endothelial cells lining the blood vessels. In addition, endothelial cells also express CD39 on their surface, a membrane-associated ecto-adenosine diphosphatase (ADPase) that degrades ADP released from activated platelets. When the vessel wall is damaged, release of these substances is impaired and subendothelial matrix is exposed. Platelets adhere to exposed collagen via α2β1 and glycoprotein (GP)V1 and to von Willebrand factor (vWF) via GPIbα and GPIIb/IIIa (αIIbβ3)—receptors that are constitutively expressed on the platelet surface. Adherent platelets undergo a change in shape, secrete ADP from their dense granules, and synthesize and release thromboxane A2. Released ADP and thromboxane A2, which are platelet agonists, activate ambient platelets and recruit them to the site of vascular injury (Fig. 118-2).
Coordinated role of platelets and the coagulation system in thrombogenesis. Vascular injury simultaneously triggers platelet activation and aggregation and activation of the coagulation system. Platelet activation is initiated by exposure of subendothelial collagen and von Willebrand factor (vWF), onto which platelets adhere. Adherent platelets become activated and release ADP and thromboxane A2, platelet agonists that activate ambient platelets and recruit them to the site of injury. When platelets are activated, glycoprotein IIb/IIIa on their surface undergoes a conformational change that enables it to ligate fibrinogen and mediate platelet aggregation. Coagulation is triggered by tissue factor exposed at the site of injury. Tissue factor triggers thrombin generation. As a potent platelet agonist, thrombin amplifies platelet recruitment to the site of injury. Thrombin also converts fibrinogen to fibrin, and the fibrin strands then weave the platelet aggregates together to form a platelet/fibrin thrombus.
Disruption of the vessel wall also exposes tissue factor–expressing cells to the blood. Tissue factor initiates coagulation. Activated platelets potentiate coagulation by binding clotting factors and supporting the assembly of activation complexes that enhance thrombin generation. In addition to converting fibrinogen to fibrin, thrombin also serves as a potent platelet agonist and recruits more platelets to the site of vascular injury.
When platelets are activated, GPIIb/IIIa, the most abundant receptor on the platelet surface, undergoes a conformational change that enables it to bind fibrinogen and, under high shear conditions, vWF. Divalent fibrinogen or multivalent vWF molecules bridge adjacent platelets together to form platelet aggregates. Fibrin strands, generated through the action of thrombin, then weave these aggregates together to form a platelet/fibrin mesh.
Antiplatelet drugs target various steps in this process. The commonly used drugs include aspirin, thienopyridines (clopidogrel, prasugrel, and ticlopidine), dipyridamole, and GPIIb/IIIa antagonists.
The most widely used antiplatelet agent worldwide is aspirin. As a cheap and effective antiplatelet drug, aspirin serves as the foundation of most antiplatelet strategies.
Aspirin produces its antithrombotic effect by irreversibly acetylating and inhibiting platelet cyclooxygenase (COX)-1 (Fig. 118-3), a critical enzyme in the biosynthesis of thromboxane A2. At high doses (∼1 g/d), aspirin also inhibits COX-2, an inducible COX isoform found in endothelial cells and inflammatory cells. In endothelial cells, COX-2 initiates the synthesis of prostacyclin, a potent vasodilator and inhibitor of platelet aggregation.
Site of action of antiplatelet drugs. Aspirin inhibits thromboxane A2 (TXA2) synthesis by irreversibly acetylating cyclooxygenase-1 (COX-1). Reduced TXA2 release attenuates platelet activation and recruitment to the site of vascular injury. Ticlopidine, clopidogrel, and prasugrel irreversibly block P2Y12, a key ADP receptor on the platelet surface; cangrelor and ticagrelor are reversible inhibitors of P2Y12. Abciximab, eptifibatide, and tirofiban inhibit the final common pathway of platelet aggregation by blocking fibrinogen and von Willebrand factor binding to activated glycoprotein (GP) IIb/IIIa. SCH530348 and E5555 inhibit thrombin-mediated platelet activation by targeting protease-activated receptor-1 (PAR-1), the major thrombin receptor on human platelets.
Aspirin is widely used for secondary prevention of cardiovascular events in patients with coronary artery, cerebrovascular, or peripheral vascular disease. Compared with placebo, aspirin produces a 25% reduction in the risk of cardiovascular death, MI, or stroke. Aspirin is also used for primary prevention in patients whose estimated annual risk of MI is >1%, a point where its benefits are likely to outweigh harms. This includes patients older than age 40 years with two or more major risk factors for cardiovascular disease or those older than age 50 years with one or more such risk factors. Aspirin is equally effective in men and women. In men, aspirin mainly reduces the risk of MI, while in women aspirin lowers the risk of stroke.
Aspirin is usually administered at doses of 75–325 mg once daily. Higher doses of aspirin are not more effective than lower aspirin doses, and some analyses suggest reduced efficacy with higher doses. Because the side effects of aspirin are dose-related, daily aspirin doses of 75–100 mg are recommended for most indications. When rapid platelet inhibition is required, an initial aspirin dose of at least 160 mg should be given.
Most common side effects are gastrointestinal and range from dyspepsia to erosive gastritis or peptic ulcers with bleeding and perforation. These side effects are dose-related. Use of enteric-coated or buffered aspirin in place of plain aspirin does not eliminate the risk of gastrointestinal side effects. The overall risk of major bleeding with aspirin is 1–3% per year. The risk of bleeding is increased when aspirin is given in conjunction with anticoagulants, such as warfarin. When dual therapy is used, low-dose aspirin should be given (75–100 mg daily). Eradication of Helicobacter pylori infection and administration of proton pump inhibitors may reduce the risk of aspirin-induced gastrointestinal bleeding in patients with peptic ulcer disease.
Aspirin should not be administered to patients with a history of aspirin allergy characterized by bronchospasm. This problem occurs in ∼0.3% of the general population but is more common in those with chronic urticaria or asthma, particularly in individuals with nasal polyps or chronic rhinitis. Hepatic and renal toxicity are observed with aspirin overdose.
Clinical aspirin resistance is defined as the failure of aspirin to protect patients from ischemic vascular events. This is not a helpful definition because it is made after the event occurs. Furthermore, it is not realistic to expect aspirin, which only blocks thromboxane A2–induced platelet activation, to prevent all vascular events.
Aspirin resistance has also been described biochemically as failure of the drug to produce its expected inhibitory effects on tests of platelet function, such as thromboxane A2 synthesis or arachidonic acid–induced platelet aggregation. However, the tests of platelet function used for diagnosis of biochemical aspirin resistance have not been well standardized. Furthermore, these tests are not proven to identify patients at risk of recurrent vascular events. In addition, resistance is not reversed by either giving higher doses of aspirin or adding other antiplatelet drugs. Thus, testing for aspirin resistance remains a research tool.
The thienopyridines include ticlopidine, clopidogrel, and prasugrel, drugs that target P2Y12, a key ADP receptor on platelets.
The thienopyridines are structurally related drugs that selectively inhibit ADP-induced platelet aggregation by irreversibly blocking P2Y12 (Fig. 118-3). Ticlopidine and clopidogrel are prodrugs that require metabolism by the hepatic cytochrome P450 (CYP) enzyme system to acquire activity. Consequently, when given in usual doses, their onset of action is delayed for several days. Although prasugrel also is a prodrug that requires metabolic activation, its onset of action is more rapid than that of ticlopidine or clopidogrel, and prasugrel produces greater and more predictable inhibition of ADP-induced platelet aggregation. These characteristics reflect the rapid and complete absorption of prasugrel from the gut and its more efficient activation pathways. Whereas nearly all of the absorbed prasugrel undergoes metabolic activation in the liver, only 15% of absorbed clopidogrel is activated; the remainder is inactivated by esterases.
Like aspirin, ticlopidine is more effective than placebo at reducing the risk of cardiovascular death, MI, and stroke in patients with atherosclerotic disease. Because of its delayed onset of action, ticlopidine is not recommended in patients with acute MI. Ticlopidine was used routinely as an adjunct to aspirin after coronary artery stenting and as an aspirin substitute in those intolerant to aspirin. Because clopidogrel is more potent than ticlopidine and has a better safety profile, clopidogrel has replaced ticlopidine.
When compared with aspirin in patients with recent ischemic stroke, MI, or peripheral arterial disease, clopidogrel reduced the risk of cardiovascular death, MI, and stroke by 8.7%. Therefore, clopidogrel is more effective than aspirin but is also more expensive. In some patients, clopidogrel and aspirin are combined to capitalize on their capacity to block complementary pathways of platelet activation. For example, the combination of aspirin plus clopidogrel is recommended for at least 4 weeks after implantation of a bare metal stent in a coronary artery and longer in those with a drug-eluting stent. Concerns about late in-stent thrombosis with drug-eluting stents have led some experts to recommend long-term use of clopidogrel plus aspirin for this indication.
The combination of clopidogrel and aspirin is also effective in patients with unstable angina. Thus, in 12,562 such patients, the risk of cardiovascular death, MI, or stroke was 9.3% in those randomized to the combination of clopidogrel and aspirin and 11.4% in those given aspirin alone. This 20% relative risk reduction with combination therapy was highly statistically significant. However, combining clopidogrel with aspirin increases the risk of major bleeding to about 2% per year. This bleeding risk persists even if the daily dose of aspirin is ≤100 mg. Therefore, the combination of clopidogrel and aspirin should only be used when there is a clear benefit. For example, this combination has not proven to be superior to clopidogrel alone in patients with acute ischemic stroke or to aspirin alone for primary prevention in those at risk for cardiovascular events.
Prasugrel was compared with clopidogrel in 13,608 patients with acute coronary syndromes who were scheduled to undergo a percutaneous coronary intervention. The incidence of the primary efficacy endpoint, a composite of cardiovascular death, MI, and stroke, was significantly lower with prasugrel than with clopidogrel (9.9% and 12.1%, respectively), mainly reflecting a reduction in the incidence of nonfatal MI. The incidence of stent thrombosis also was significantly lower with prasugrel than with clopidogrel (1.1% and 2.4%, respectively). However, these advantages were at the expense of significantly higher rates of fatal bleeding (0.4% and 0.1%, respectively) and life-threatening bleeding (1.4% and 0.9%, respectively) with prasugrel. Because patients older than age 75 years and those with a history of prior stroke or transient ischemic attack have a particularly high risk of bleeding, prasugrel should generally be avoided in older patients, and the drug is contraindicated in those with a history of cerebrovascular disease. Caution is required if prasugrel is used in patients weighing less than 60 kg or in those with renal impairment.
Ticlopidine is given twice daily at a dose of 250 mg. The more potent clopidogrel is given once daily at a dose of 75 mg. Loading doses of clopidogrel are given when rapid ADP receptor blockade is desired. For example, patients undergoing coronary stenting are often given a loading dose of 300 mg, which affects inhibition of ADP-induced platelet aggregation in about 6 h. Loading doses of 600 or 900 mg produce an even more rapid effect. After a loading dose of 60 mg, prasugrel is given once daily at a dose of 10 mg. Patients older than age 75 years or weighing less than 60 kg should receive a lower daily prasugrel dose of 5 mg.
The most common side effects of ticlopidine are gastrointestinal. More serious are the hematologic side effects, which include neutropenia, thrombocytopenia, and thrombotic thrombocytopenic purpura. These side effects usually occur within the first few months of starting treatment. Therefore, blood counts must be carefully monitored when initiating therapy with ticlopidine. Gastrointestinal and hematologic side effects are rare with clopidogrel and prasugrel.
The capacity of clopidogrel to inhibit ADP-induced platelet aggregation varies among subjects. This variability reflects, at least in part, genetic polymorphisms in the CYP isoenzymes involved in the metabolic activation of clopidogrel. Most important of these is CYP2C19. Clopidogrel-treated patients with the loss-of-function CYP2C19*2 allele exhibit reduced platelet inhibition compared with those with the wild-type CYP2C19*1 allele and experience a higher rate of cardiovascular events. This is important because estimates suggest that up to 25% of whites, 30% of African Americans, and 50% of Asians carry the loss-of-function allele, which would render them resistant to clopidogrel. Even patients with the reduced function CYP2C19*3, *4, or *5 alleles may derive less benefit from clopidogrel than those with the full-function CYP2C19*1 allele. Concomitant administration of clopidogrel and proton pump inhibitors, which are inhibitors of CYP2C19, produces a small reduction in the inhibitory effects of clopidogrel on ADP-induced platelet aggregation. The extent to which this interaction increases the risk of cardiovascular events remains controversial.
In contrast to their effect on the metabolic activation of clopidogrel, CYP2C19 polymorphisms appear to be less important determinants of the activation of prasugrel. Thus, no association was detected between the loss-of-function allele and decreased platelet inhibition or increased rate of cardiovascular events with prasugrel. The observation that genetic polymorphisms affecting clopidogrel absorption or metabolism influence clinical outcomes raises the possibilities that pharmacogenetic profiling may be useful to identify clopidogrel-resistant patients and that point-of-care assessment of the extent of clopidogrel-induced platelet inhibition may help detect patients at higher risk for subsequent cardiovascular events. It is unknown whether administration of higher doses of clopidogrel to such patients will overcome this resistance. Instead, prasugrel or newer P2Y12 inhibitors may be better choices for these patients.
Dipyridamole is a relatively weak antiplatelet agent on its own, but an extended-release formulation of dipyridamole combined with low-dose aspirin, a preparation known as Aggrenox, is used for prevention of stroke in patients with transient ischemic attacks.
By inhibiting phosphodiesterase, dipyridamole blocks the breakdown of cyclic AMP. Increased levels of cyclic AMP reduce intracellular calcium and inhibit platelet activation. Dipyridamole also blocks the uptake of adenosine by platelets and other cells. This produces a further increase in local cyclic AMP levels because the platelet adenosine A2 receptor is coupled to adenylate cyclase (Fig. 118-4).
Mechanism of action of dipyridamole. Dipyridamole increases levels of cyclic AMP in platelets by (1) blocking the reuptake of adenosine and (2) inhibiting phosphodiesterase-mediated cyclic AMP degradation. By promoting calcium uptake, cyclic AMP reduces intracellular levels of calcium. This, in turn, inhibits platelet activation and aggregation.
Aggrenox is given twice daily. Each capsule contains 200 mg of extended-release dipyridamole and 25 mg of aspirin.
Because dipyridamole has vasodilatory effects, it must be used with caution in patients with coronary artery disease. Gastrointestinal complaints, headache, facial flushing, dizziness, and hypotension can also occur. These symptoms often subside with continued use of the drug.
Dipyridamole plus aspirin was compared with aspirin or dipyridamole alone, or with placebo, in patients with an ischemic stroke or transient ischemic attack. The combination reduced the risk of stroke by 22.1% compared with aspirin and by 24.4% compared with dipyridamole. A second trial compared dipyridamole plus aspirin with aspirin alone for secondary prevention in patients with ischemic stroke. Vascular death, stroke, or MI occurred in 13% of patients given combination therapy and in 16% of those treated with aspirin alone. Based on these data, Aggrenox was often used for stroke prevention. Another trial randomized 20,332 patients with noncardioembolic ischemic stroke to either Aggrenox or clopidogrel. The primary efficacy endpoint of recurrent stroke occurred in 9.0% of those given Aggrenox and in 8.8% of patients treated with clopidogrel. Although this difference was not statistically significant, the study failed to meet the prespecified margin to claim noninferiority of Aggrenox relative to clopidogrel. These results have dampened enthusiasm for the use of Aggrenox.
Because of its vasodilatory effects and the paucity of data supporting the use of dipyridamole in patients with symptomatic coronary artery disease, Aggrenox should not be used for stroke prevention in such patients. Clopidogrel is a better choice in this setting.
GPIIb/IIIA Receptor Antagonists
As a class, parenteral GPIIb/IIIa receptor antagonists have an established niche in patients with acute coronary syndromes. The three agents in this class are abciximab, eptifibatide, and tirofiban.
A member of the integrin family of adhesion receptors, GPIIb/IIIa is found on the surface of platelets and megakaryocytes. With about 80,000 copies per platelet, GPIIb/IIIa is the most abundant receptor. Consisting of a noncovalently linked heterodimer, GPIIb/IIIa is inactive on resting platelets. When platelets are activated, inside-outside signal transduction pathways trigger a conformational activation of the receptor. Once activated, GPIIb/IIIa binds adhesive molecules, such as fibrinogen and, under high shear conditions, vWF. Binding is mediated by the Arg-Gly-Asp (RGD) sequence found on the α chains of fibrinogen and on vWF, and by the Lys-Gly-Asp (KGD) sequence located within a unique dodecapeptide domain on the γ chains of fibrinogen. Once bound, fibrinogen and/or vWF bridge adjacent platelets together to induce platelet aggregation.
Although abciximab, eptifibatide, and tirofiban all target the GPIIb/IIIa receptor, they are structurally and pharmacologically distinct (Table 118-1). Abciximab is a Fab fragment of a humanized murine monoclonal antibody directed against the activated form of GPIIb/IIIa. Abciximab binds to the activated receptor with high affinity and blocks the binding of adhesive molecules. In contrast to abciximab, eptifibatide and tirofiban are synthetic small molecules. Eptifibatide is a cyclic heptapeptide that binds GPIIb/IIIa because it incorporates the KGD motif, whereas tirofiban is a nonpeptidic tyrosine derivative that acts as an RGD mimetic. Abciximab has a long half-life and can be detected on the surface of platelets for up to 2 weeks. Eptifibatide and tirofiban have shorter half-lives.
Table 118-1 Features of GPIIb/IIIA Antagonists |Favorite Table|Download (.pdf)
Table 118-1 Features of GPIIb/IIIA Antagonists
|Description||Fab fragment of humanized mouse monoclonal antibody||Cyclical KGD-containing heptapeptide||Nonpeptidic RGD mimetic|
|Specific for GPIIb/IIIa||No||Yes||Yes|
|Plasma half-life||Short (min)||Long (2.5 h)||Long (2.0 h)|
|Platelet-bound half-life||Long (days)||Short (s)||Short (s)|
In addition to targeting the GPIIb/IIIa receptor, abciximab also inhibits the closely related αvβ3 receptor, which binds vitronectin, and αMβ2, a leukocyte integrin. In contrast, eptifibatide and tirofiban are specific for GPIIb/IIIa. Inhibition of αvβ3 and αMβ2 may endow abciximab with anti-inflammatory and/or antiproliferative properties that extend beyond platelet inhibition.
All of the GPIIb/IIIa antagonists are given as an IV bolus followed by an infusion. Because they are cleared by the kidneys, the doses of eptifibatide and tirofiban must be reduced in patients with renal insufficiency.
In addition to bleeding, thrombocytopenia is the most serious complication. Thrombocytopenia is immune-mediated and is caused by antibodies directed against neoantigens on GPIIb/IIIa that are exposed upon antagonist binding. With abciximab, thrombocytopenia occurs in up to 5% of patients. Thrombocytopenia is severe in ∼1% of these individuals. Thrombocytopenia is less common with the other two agents, occurring in ∼1% of patients.
Abciximab and eptifibatide are used in patients undergoing percutaneous coronary interventions, particularly those with acute MI. Tirofiban is used in high-risk patients with unstable angina. Eptifibatide also can be used for this indication.
New agents in advanced stages of development include cangrelor and ticagrelor, direct-acting reversible P2Y12 antagonists, and SCH530348 (vorapaxar) and E5555 (atopaxar), orally active inhibitors of protease-activated receptor 1 (PAR-1), the major thrombin receptor on platelets (Fig. 118-3). Cangrelor is an adenosine analogue that binds reversibly to P2Y12 and inhibits its activity. The drug has a half-life of 3–6 min and is given intravenously as a bolus followed by an infusion. When stopped, platelet function recovers within 60 min. Trials comparing cangrelor with placebo during percutaneous coronary interventions or comparing cangrelor with clopidogrel after such procedures revealed little or no advantages of cangrelor. Consequently, identification of a role for cangrelor requires additional studies.
Ticagrelor is an orally active, reversible inhibitor of P2Y12. The drug is given twice daily and it not only has a more rapid onset and offset of action than clopidogrel but also produces greater and more predictable inhibition of ADP-induced platelet aggregation. When compared with clopidogrel in patients with acute coronary syndromes, ticagrelor produced a greater reduction in the primary efficacy endpoint, a composite of cardiovascular death, MI, and stroke at 1 year (9.8% and 11.7%, respectively; p = .001). This difference reflected a significantly greater reduction in cardiovascular death (4.0% and 5.1%, respectively; p = .001) and MI (5.8% and 6.9%, respectively; p = .005) with ticagrelor than with clopidogrel. Rates of stroke were similar with ticagrelor and clopidogrel (1.5% and 1.3%, respectively) and there were no differences in the rates of major bleeding. When minor bleeding was added to the major bleeding results, however, ticagrelor showed an increase relative to clopidogrel (16.1% and 14.6%, respectively; p = 0.008). Ticagrelor also was superior to clopidogrel in the acute coronary syndrome patients who underwent percutaneous coronary interventions or aortocoronary bypass surgery. Although not yet licensed, ticagrelor is the first new antiplatelet drug to demonstrate a greater reduction in cardiovascular death than clopidogrel in patients with acute coronary syndromes.
SCH530348, an orally active inhibitor of PAR-1, is under investigation as an adjunct to aspirin or aspirin plus clopidogrel. Two large phase III trials are underway. E5555, a second oral PAR-1 antagonist, is earlier in development.
There are both parenteral and oral anticoagulants. Currently available parenteral anticoagulants include heparin, low-molecular-weight heparin (LMWH), and fondaparinux, a synthetic pentasaccharide. The only available oral anticoagulants are the vitamin K antagonists, of which warfarin is the agent most often used in North America.
Dabigatran etexilate, an oral thrombin inhibitor, and rivaroxaban, an oral Factor Xa inhibitor, are licensed in Europe and Canada for short-term thromboprophylaxis after elective hip or knee replacement surgery. Dabigatran etexilate was licensed in the United States and Canada as an alternative to warfarin for stroke prevention in patients with atrial fibrillation.
Heparin is a sulfated polysaccharide and is isolated from mammalian tissues rich in mast cells. Most commercial heparin is derived from porcine intestinal mucosa and is a polymer of alternating d-glucuronic acid and N-acetyl-d-glucosamine residues.
Heparin acts as an anticoagulant by activating antithrombin (previously known as antithrombin III) and accelerating the rate at which antithrombin inhibits clotting enzymes, particularly thrombin and factor Xa. Antithrombin, the obligatory plasma cofactor for heparin, is a member of the serine protease inhibitor (serpin) superfamily. Synthesized in the liver and circulating in plasma at a concentration of 2.6 ± 0.4 μM, antithrombin acts as a suicide substrate for its target enzymes.
To activate antithrombin, heparin binds to the serpin via a unique pentasaccharide sequence that is found on one-third of the chains of commercial heparin (Fig. 118-5). The remainder of the heparin chains that lack this pentasaccharide sequence have little or no anticoagulant activity. Once bound to antithrombin, heparin induces a conformational change in the reactive center loop of antithrombin that renders it more readily accessible to its target proteases. This conformational change enhances the rate at which antithrombin inhibits Factor Xa by at least two orders of magnitude but has little effect on the rate of thrombin inhibition by antithrombin. To catalyze thrombin inhibition, heparin serves as a template that binds antithrombin and thrombin simultaneously. Formation of this ternary complex brings the enzyme in close apposition to the inhibitor, thereby promoting the formation of a stable covalent thrombin-antithrombin complex.
Mechanism of action of heparin, low-molecular-weight heparin (LMWH), and fondaparinux, a synthetic pentasaccharide. A. Heparin binds to antithrombin via its pentasaccharide sequence. This induces a conformational change in the reactive center loop of antithrombin that accelerates its interaction with factor Xa. To potentiate thrombin inhibition, heparin must simultaneously bind to antithrombin and thrombin. Only heparin chains composed of at least 18 saccharide units, which corresponds to a molecular weight of 5400, are of sufficient length to perform this bridging function. With a mean molecular weight of 15,000, all of the heparin chains are long enough to do this. B. LMWH has greater capacity to potentiate factor Xa inhibition by antithrombin than thrombin because, with a mean molecular weight of 4500–5000, at least half of the LMWH chains are too short to bridge antithrombin to thrombin. C. The pentasaccharide only accelerates factor Xa inhibition by antithrombin because the pentasaccharide is too short to bridge antithrombin to thrombin.
Only pentasaccharide-containing heparin chains composed of at least 18 saccharide units (which correspond to a molecular weight of 5400) are of sufficient length to bridge thrombin and antithrombin together. With a mean molecular weight of 15,000, and a range of 5000–30,000, almost all of the chains of unfractionated heparin are long enough to effect this bridging function. Consequently, by definition, heparin has equal capacity to promote the inhibition of thrombin and factor Xa by antithrombin and is assigned an anti-factor Xa to anti-factor IIa (thrombin) ratio of 1:1.
Heparin causes the release of tissue factor pathway inhibitor (TFPI) from the endothelium. A factor Xa–dependent inhibitor of tissue factor–bound factor VIIa, TFPI may contribute to the antithrombotic activity of heparin. Longer heparin chains induce the release of more TFPI than shorter chains.
Heparin must be given parenterally. It is usually administered SC or by continuous IV infusion. When used for therapeutic purposes, the IV route is most often employed. If heparin is given SC for treatment of thrombosis, the dose of heparin must be high enough to overcome the limited bioavailability associated with this method of delivery.
In the circulation, heparin binds to the endothelium and to plasma proteins other than antithrombin. Heparin binding to endothelial cells explains its dose-dependent clearance. At low doses, the half-life of heparin is short because it binds rapidly to the endothelium. With higher doses of heparin, the half-life is longer because heparin is cleared more slowly once the endothelium is saturated. Clearance is mainly extrarenal; heparin binds to macrophages, which internalize and depolymerize the long heparin chains and secrete shorter chains back into the circulation. Because of its dose-dependent clearance mechanism, the plasma half-life of heparin ranges from 30 to 60 min with bolus IV doses of 25 and 100 U/kg, respectively.
Once heparin enters the circulation, it binds to plasma proteins other than antithrombin, a phenomenon that reduces its anticoagulant activity. Some of the heparin-binding proteins found in plasma are acute-phase reactants whose levels are elevated in ill patients. Others, such as high-molecular-weight multimers of vWF, are released from activated platelets or endothelial cells. Activated platelets also release platelet factor 4 (PF4), a highly cationic protein that binds heparin with high affinity. The large amounts of PF4 found in the vicinity of platelet-rich arterial thrombi can neutralize the anticoagulant activity of heparin. This phenomenon may attenuate heparin's capacity to suppress thrombus growth.
Because the levels of heparin-binding proteins in plasma vary from person to person, the anticoagulant response to fixed or weight-adjusted doses of heparin is unpredictable. Consequently, coagulation monitoring is essential to ensure that a therapeutic response is obtained. This is particularly important when heparin is administered for treatment of established thrombosis because a subtherapeutic anticoagulant response may render patients at risk for recurrent thrombosis, whereas excessive anticoagulation increases the risk of bleeding.
Monitoring the Anticoagulant Effect
Heparin therapy can be monitored using the activated partial thromboplastin time (aPTT) or anti-factor Xa level. Although the aPTT is the test most often employed for this purpose, there are problems with this assay. aPTT reagents vary in their sensitivity to heparin, and the type of coagulometer used for testing can influence the results. Consequently, laboratories must establish a therapeutic aPTT range with each reagent-coagulometer combination by measuring the aPTT and anti-factor Xa level in plasma samples collected from heparin-treated patients. For most of the aPTT reagents and coagulometers in current use, therapeutic heparin levels are achieved with a two- to threefold prolongation of the aPTT.
Anti-factor Xa levels also can be used to monitor heparin therapy. With this test, therapeutic heparin levels range from 0.3 to 0.7 units/mL. Although this test is gaining in popularity, anti-factor Xa assays have yet to be standardized, and results can vary widely between laboratories.
Up to 25% of heparin-treated patients with venous thromboembolism require >35,000 units/d to achieve a therapeutic aPTT. These patients are considered heparin resistant. It is useful to measure anti-factor Xa levels in heparin-resistant patients because many will have a therapeutic anti-factor Xa level despite a subtherapeutic aPTT. This dissociation in test results occurs because elevated plasma levels of fibrinogen and factor VIII, both of which are acute-phase proteins, shorten the aPTT but have no effect on anti-factor Xa levels. Heparin therapy in patients who exhibit this phenomenon is best monitored using anti-factor Xa levels instead of the aPTT. Patients with congenital or acquired antithrombin deficiency and those with elevated levels of heparin-binding proteins may also need high doses of heparin to achieve a therapeutic aPTT or anti-factor Xa level. If there is good correlation between the aPTT and the anti-factor Xa levels, either test can be used to monitor heparin therapy.
For prophylaxis, heparin is usually given in fixed doses of 5000 units SC two or three times daily. With these low doses, coagulation monitoring is unnecessary. In contrast, monitoring is essential when the drug is given in therapeutic doses. Fixed-dose or weight-based heparin nomograms are used to standardize heparin dosing and to shorten the time required to achieve a therapeutic anticoagulant response. At least two heparin nomograms have been validated in patients with venous thromboembolism and reduce the time required to achieve a therapeutic aPTT. Weight-adjusted heparin nomograms have also been evaluated in patients with acute coronary syndromes. After an IV heparin bolus of 5000 units or 70 units/kg, a heparin infusion rate of 12–15 units/kg per hour is usually administered. In contrast, weight-adjusted heparin nomograms for patients with venous thromboembolism use an initial bolus of 5000 units or 80 units/kg, followed by an infusion of 18 units/kg per hour. Thus, patients with venous thromboembolism appear to require higher doses of heparin to achieve a therapeutic aPTT than do patients with acute coronary syndromes. This may reflect differences in the thrombus burden. Heparin binds to fibrin, and the fibrin content of extensive deep-vein thrombi is greater than that of small coronary thrombi.
Heparin manufacturers in North America have traditionally measured heparin potency in USP units, with a unit defined as the concentration of heparin that prevents 1 mL of citrated sheep plasma from clotting for 1 h after calcium addition. In contrast, manufacturers in Europe measure heparin potency with anti-Xa assays using an international heparin standard for comparison. Because of problems with heparin contamination with oversulfated chondroitin sulfate, which the USP assay system does not detect, North American heparin manufacturers now use the anti-Xa assay to assess heparin potency. Although use of international units in place of USP units results in a 10% reduction in heparin doses, this change is unlikely to affect patient care because heparin has been dosed in international units in Europe for many years. Furthermore, heparin monitoring ensures a therapeutic anticoagulant response in high-risk situations, such as cardiopulmonary bypass surgery or percutaneous coronary intervention.
Heparin has pharmacokinetic and biophysical limitations (Table 118-2). The pharmacokinetic limitations reflect heparin's propensity to bind in a pentasaccharide-independent fashion to cells and plasma proteins. Heparin binding to endothelial cells explains its dose-dependent clearance, whereas binding to plasma proteins results in a variable anticoagulant response and can lead to heparin resistance.
Table 118-2 Pharmacokinetic and Biophysical Limitations of Heparin |Favorite Table|Download (.pdf)
Table 118-2 Pharmacokinetic and Biophysical Limitations of Heparin
|Poor bioavailability at low doses||Binds to endothelial cells and macrophages|
|Dose-dependent clearance||Binds to macrophages|
|Variable anticoagulant response||Binds to plasma proteins whose levels vary from patient to patient|
|Reduced activity in the vicinity of platelet-rich thrombi||Neutralized by platelet factor 4 released from activated platelets|
|Limited activity against factor Xa incorporated in the prothrombinase complex and thrombin bound to fibrin||Reduced capacity of heparin-antithrombin complex to inhibit factor Xa bound to activated platelets and thrombin bound to fibrin|
The biophysical limitations of heparin reflect the inability of the heparin-antithrombin complex to (1) inhibit factor Xa when it is incorporated into the prothrombinase complex, the complex that converts prothrombin to thrombin, and (2) to inhibit thrombin bound to fibrin. Consequently, factor Xa bound to activated platelets within platelet-rich thrombi has the potential to generate thrombin, even in the face of heparin. Once this thrombin binds to fibrin, it too is protected from inhibition by the heparin-antithrombin complex. Clot-associated thrombin can then trigger thrombus growth by locally activating platelets and amplifying its own generation through feedback activation of factors V, VIII, and XI. Further compounding the problem is the potential for heparin neutralization by the high concentrations of PF4 released from activated platelets within the platelet-rich thrombus.
The most common side effect of heparin is bleeding. Other complications include thrombocytopenia, osteoporosis, and elevated levels of transaminases.
The risk of heparin-induced bleeding increases with higher heparin doses. Concomitant administration of drugs that affect hemostasis, such as antiplatelet or fibrinolytic agents, increases the risk of bleeding, as does recent surgery or trauma. Heparin-treated patients with serious bleeding can be given protamine sulfate to neutralize the heparin. Protamine sulfate, a mixture of basic polypeptides isolated from salmon sperm, binds heparin with high affinity, and the resultant protamine-heparin complexes are then cleared. Typically, 1 mg of protamine sulfate neutralizes 100 units of heparin. Protamine sulfate is given IV. Anaphylactoid reactions to protamine sulfate can occur, and drug administration by slow IV infusion is recommended to reduce the risk.
Heparin can cause thrombocytopenia. Heparin-induced thrombocytopenia (HIT) is an antibody-mediated process that is triggered by antibodies directed against neoantigens on PF4 that are exposed when heparin binds to this protein. These antibodies, which are usually of the IgG isotype, bind simultaneously to the heparin-PF4 complex and to platelet Fc receptors. Such binding activates the platelets and generates platelet microparticles. Circulating microparticles are prothrombotic because they express anionic phospholipids on their surface and can bind clotting factors and promote thrombin generation.
The clinical features of HIT are illustrated in Table 118-3. Typically, HIT occurs 5–14 days after initiation of heparin therapy, but it can manifest earlier if the patient has received heparin within the past 3 months. It is rare for the platelet count to fall below 100,000/μL in patients with HIT, and even a 50% decrease in the platelet count from the pretreatment value should raise the suspicion of HIT in those receiving heparin. HIT is more common in surgical patients than in medical patients and, like many autoimmune disorders, occurs more frequently in females than in males.
Table 118-3 Features of Heparin-Induced Thrombocytopenia |Favorite Table|Download (.pdf)
Table 118-3 Features of Heparin-Induced Thrombocytopenia
|Thrombocytopenia||Platelet count of ≤100,000/μL or a decrease in platelet count of ≥50%|
|Timing||Platelet count falls 5–10 days after starting heparin|
|Type of heparin||More common with unfractionated heparin than low-molecular-weight heparin|
|Type of patient||More common in surgical patients and patients with cancer than general medical patients. More common in women than in men|
|Thrombosis||Venous thrombosis more common than arterial thrombosis|
HIT can be associated with thrombosis, either arterial or venous. Venous thrombosis, which manifests as DVT and/or PE, is more common than arterial thrombosis. Arterial thrombosis can manifest as ischemic stroke or acute MI. Rarely, platelet-rich thrombi in the distal aorta or iliac arteries can cause critical limb ischemia.
The diagnosis of HIT is established using enzyme-linked assays to detect antibodies against heparin-PF4 complexes or with platelet activation assays. Enzyme-linked assays are sensitive but can be positive in the absence of any clinical evidence of HIT. The most specific diagnostic test is the serotonin release assay. This test is performed by quantifying serotonin release when washed platelets loaded with labeled serotonin are exposed to patient serum in the absence or presence of varying concentrations of heparin. If the patient serum contains the HIT antibody, heparin addition induces platelet activation and serotonin release.
Management of HIT is outlined in Table 118-4. Heparin should be stopped in patients with suspected or documented HIT, and an alternative anticoagulant should be administered to prevent or treat thrombosis. The agents most often used for this indication are parenteral direct thrombin inhibitors, such as lepirudin, argatroban, or bivalirudin, or factor Xa inhibitors, such as fondaparinux.
Table 118-4 Management of Heparin-Induced Thrombocytopenia |Favorite Table|Download (.pdf)
Table 118-4 Management of Heparin-Induced Thrombocytopenia
|Stop all heparin|
|Give an alternative anticoagulant, such as lepirudin, argatroban, bivalirudin, or fondaparinux|
|Do not give platelet transfusions|
|Do not give warfarin until the platelet count returns to its baseline level. If warfarin is administered, give vitamin K to restore the INR to normal|
|Evaluate for thrombosis, particularly deep-vein thrombosis|
Patients with HIT, particularly those with associated thrombosis, often have evidence of increased thrombin generation that can lead to consumption of protein C. If these patients are given warfarin without a concomitant parenteral anticoagulant to inhibit thrombin or thrombin generation, the further decrease in protein C levels induced by the vitamin K antagonist can trigger skin necrosis. To avoid this problem, patients with HIT should be treated with a direct thrombin inhibitor or fondaparinux until the platelet count returns to normal levels. At this point, low-dose warfarin therapy can be introduced, and the thrombin inhibitor can be discontinued when the anticoagulant response to warfarin has been therapeutic for at least 2 days.
Treatment with therapeutic doses of heparin for >1 month can cause a reduction in bone density. This complication has been reported in up to 30% of patients given long-term heparin therapy, and symptomatic vertebral fractures occur in 2–3% of these individuals.
Heparin causes bone loss both by decreasing bone formation and by enhancing bone resorption. Thus, heparin affects the activity of both osteoblasts and osteoclasts.
Elevated Levels of Transaminases
Therapeutic doses of heparin frequently cause modest elevation in the serum levels of hepatic transaminases, without a concomitant increase in the level of bilirubin. The levels of transaminases rapidly return to normal when the drug is stopped. The mechanism of this phenomenon is unknown.
Consisting of smaller fragments of heparin, LMWH is prepared from unfractionated heparin by controlled enzymatic or chemical depolymerization. The mean molecular weight of LMWH is 5000, one-third the mean molecular weight of unfractionated heparin. LMWH has advantages over heparin (Table 118-5) and has replaced heparin for most indications.
Table 118-5 Advantages of LMWH over Heparin |Favorite Table|Download (.pdf)
Table 118-5 Advantages of LMWH over Heparin
|Better bioavailability and longer half-life after subcutaneous injection||Can be given subcutaneously once or twice daily for both prophylaxis and treatment|
|Dose-independent clearance||Simplified dosing|
|Predictable anticoagulant response||Coagulation monitoring is unnecessary in most patients|
|Lower risk of heparin-induced thrombocytopenia||Safer than heparin for short- or long-term administration|
|Lower risk of osteoporosis||Safer than heparin for extended administration|
Like heparin, LMWH exerts its anticoagulant activity by activating antithrombin. With a mean molecular weight of 5000, which corresponds to about 17 saccharide units, at least half of the pentasaccharide-containing chains of LMWH are too short to bridge thrombin to antithrombin (Fig. 118-5). However, these chains retain the capacity to accelerate factor Xa inhibition by antithrombin because this activity is largely the result of the conformational changes in antithrombin evoked by pentasaccharide binding. Consequently, LMWH catalyzes factor Xa inhibition by antithrombin more than thrombin inhibition. Depending on their unique molecular weight distributions, LMWH preparations have anti-factor Xa to anti-factor IIa ratios ranging from 2:1 to 4:1.
Although usually given SC, LMWH also can be administered IV if a rapid anticoagulant response is needed. LMWH has pharmacokinetic advantages over heparin. These advantages reflect the fact that shorter heparin chains bind less avidly to endothelial cells, macrophages, and heparin-binding plasma proteins. Reduced binding to endothelial cells and macrophages eliminates the rapid, dose-dependent, and saturable mechanism of clearance that is a characteristic of unfractionated heparin. Instead, the clearance of LMWH is dose-independent and its plasma half-life is longer. Based on measurement of anti-factor Xa levels, LMWH has a plasma half-life of ∼4 h. LMWH is cleared almost exclusively by the kidneys, and the drug can accumulate in patients with renal insufficiency.
LMWH exhibits about 90% bioavailability after SC injection. Because LMWH binds less avidly to heparin-binding proteins in plasma than heparin, LMWH produces a more predictable dose response, and resistance to LMWH is rare. With a longer half-life and more predictable anticoagulant response, LMWH can be given SC once or twice daily without coagulation monitoring, even when the drug is given in treatment doses. These properties render LMWH more convenient than unfractionated heparin. Capitalizing on this feature, studies in patients with venous thromboembolism have shown that home treatment with LMWH is as effective and safe as in-hospital treatment with continuous IV infusions of heparin. Outpatient treatment with LMWH streamlines care, reduces health care costs, and increases patient satisfaction.
In the majority of patients, LMWH does not require coagulation monitoring. If monitoring is necessary, anti-factor Xa levels must be measured because most LMWH preparations have little effect on the aPTT. Therapeutic anti-factor Xa levels with LMWH range from 0.5 to 1.2 units/mL when measured 3–4 h after drug administration. When LMWH is given in prophylactic doses, peak anti-Factor Xa levels of 0.2–0.5 units/mL are desirable.
Indications for LMWH monitoring include renal insufficiency and obesity. LMWH monitoring in patients with a creatinine clearance of ≤50 mL/min is advisable to ensure that there is no drug accumulation. Although weight-adjusted LMWH dosing appears to produce therapeutic anti-factor Xa levels in patients who are overweight, this approach has not been extensively evaluated in those with morbid obesity. It may also be advisable to monitor the anticoagulant activity of LMWH during pregnancy because dose requirements can change, particularly in the third trimester. Monitoring should also be considered in high-risk settings, such as in patients with mechanical heart valves who are given LMWH for prevention of valve thrombosis, and when LMWH is used in treatment doses in infants or children.
The doses of LMWH recommended for prophylaxis or treatment vary depending on the LMWH preparation. For prophylaxis, once-daily SC doses of 4000–5000 units are often used, whereas doses of 2500–3000 units are given when the drug is administered twice daily. For treatment of venous thromboembolism, a dose of 150–200 units/kg is given if the drug is administered once daily. If a twice-daily regimen is employed, a dose of 100 units/kg is given. In patients with unstable angina, LMWH is given SC on a twice-daily basis at a dose of 100–120 units/kg.
The major complication of LMWH is bleeding. Meta-analyses suggest that the risk of major bleeding is lower with LMWH than with unfractionated heparin. HIT and osteoporosis are less common with LMWH than with unfractionated heparin.
Like the situation with heparin, bleeding with LMWH is more common in patients receiving concomitant therapy with antiplatelet or fibrinolytic drugs. Recent surgery, trauma, or underlying hemostatic defects also increase the risk of bleeding with LMWH.
Although protamine sulfate can be used as an antidote for LMWH, protamine sulfate incompletely neutralizes the anticoagulant activity of LMWH because it only binds the longer chains of LMWH. Because longer chains are responsible for catalysis of thrombin inhibition by antithrombin, protamine sulfate completely reverses the anti-factor IIa activity of LMWH. In contrast, protamine sulfate only partially reverses the anti-factor Xa activity of LMWH because the shorter pentasaccharide-containing chains of LMWH do not bind to protamine sulfate. Consequently, patients at high risk for bleeding may be more safely treated with continuous IV unfractionated heparin than with SC LMWH.
The risk of HIT is about fivefold lower with LMWH than with heparin. LMWH binds less avidly to platelets and causes less PF4 release. Furthermore, with lower affinity for PF4 than heparin, LMWH is less likely to induce the conformational changes in PF4 that trigger the formation of HIT antibodies.
LMWH should not be used to treat HIT patients because most HIT antibodies exhibit cross-reactivity with LMWH. This in vitro cross-reactivity is not simply a laboratory phenomenon because there are case reports of thrombosis when HIT patients are treated with LMWH.
The risk of osteoporosis is lower with long-term LMWH than with heparin. For extended treatment, therefore, LMWH is a better choice than heparin because of the lower risk of osteoporosis and HIT.
A synthetic analogue of the antithrombin-binding pentasaccharide sequence, fondaparinux differs from LMWH in several ways (Table 118-6). Fondaparinux is licensed for thromboprophylaxis in general medical or surgical patients and in high-risk orthopedic patients and as an alternative to heparin or LMWH for initial treatment of patients with established venous thromboembolism. The drug is not yet licensed in the United States as an alternative for heparin or LMWH in patients with acute coronary syndromes.
Table 118-6 Comparison of LMWH and Fondaparinux |Favorite Table|Download (.pdf)
Table 118-6 Comparison of LMWH and Fondaparinux
|Number of saccharide units||15–17||5|
|Catalysis of factor Xa inhibition||Yes||Yes|
|Catalysis of thrombin inhibition||Yes||No|
|Bioavailability after subcutaneous administration (%)||90||100|
|Plasma half-life (h)||4||17|
|Induces release of tissue factor pathway inhibitor||Yes||No|
|Neutralized by protamine sulfate||Partially||No|
As a synthetic analogue of the antithrombin-binding pentasaccharide sequence found in heparin and LMWH, fondaparinux has a molecular weight of 1728. Fondaparinux binds only to antithrombin (Fig. 118-5) and is too short to bridge thrombin to antithrombin. Consequently, fondaparinux catalyzes factor Xa inhibition by antithrombin and does not enhance the rate of thrombin inhibition.
Fondaparinux exhibits complete bioavailability after SC injection. With no binding to endothelial cells or plasma proteins, the clearance of fondaparinux is dose independent and its plasma half-life is 17 h. The drug is given SC once daily. Because fondaparinux is cleared unchanged via the kidneys, it is contraindicated in patients with a creatinine clearance <30 mL/min and should be used with caution in those with a creatinine clearance <50 mL/min.
Fondaparinux produces a predictable anticoagulant response after administration in fixed doses because it does not bind to plasma proteins. The drug is given at a dose of 2.5 mg once daily for prevention of venous thromboembolism. For initial treatment of established venous thromboembolism, fondaparinux is given at a dose of 7.5 mg once daily. The dose can be reduced to 5 mg once daily for those weighing <50 kg and increased to 10 mg for those >100 kg. When given in these doses, fondaparinux is as effective as heparin or LMWH for initial treatment of patients with DVT or PE and produces similar rates of bleeding.
Fondaparinux is used at a dose of 2.5 mg once daily in patients with acute coronary syndromes. When this prophylactic dose of fondaparinux was compared with treatment doses of enoxaparin in patients with non-ST-segment elevation acute coronary syndromes, there was no difference in the rate of cardiovascular death, MI, or stroke at 9 days. However, the rate of major bleeding was 50% lower with fondaparinux than with enoxaparin, a difference that likely reflects the fact that the dose of fondaparinux was lower than that of enoxaparin. In acute coronary syndrome patients who require percutaneous coronary interventions, there is a risk of catheter thrombosis with fondaparinux, unless adjunctive heparin is given.
Fondaparinux does not cause HIT because it does not bind to PF4. In contrast to LMWH, there is no cross-reactivity of fondaparinux with HIT antibodies. Consequently, fondaparinux appears to be effective for treatment of HIT patients, although large clinical trials supporting its use are lacking.
The major side effect of fondaparinux is bleeding. There is no antidote for fondaparinux. Protamine sulfate has no effect on the anticoagulant activity of fondaparinux because it fails to bind to the drug. Recombinant activated factor VII reverses the anticoagulant effects of fondaparinux in volunteers, but it is unknown whether this agent will control fondaparinux-induced bleeding.
Parenteral Direct Thrombin Inhibitors
Heparin and LMWH are indirect inhibitors of thrombin because their activity is mediated by antithrombin. In contrast, direct thrombin inhibitors do not require a plasma cofactor; instead, these agents bind directly to thrombin and block its interaction with its substrates. Approved parenteral direct thrombin inhibitors include lepirudin, argatroban, and bivalirudin (Table 118-7). Lepirudin and argatroban are licensed for treatment of patients with HIT, whereas bivalirudin is approved as an alternative to heparin in patients undergoing percutaneous coronary interventions, including those with HIT.
Table 118-7 Comparison of the Properties of Lepirudin, Bivalirudin, and Argatroban |Favorite Table|Download (.pdf)
Table 118-7 Comparison of the Properties of Lepirudin, Bivalirudin, and Argatroban
|Site(s) of interaction with thrombin||Active site and exosite 1||Active site and exosite 1||Active site|
|Plasma half-life (min)||60||25||45|
A recombinant form of hirudin, lepirudin is a bivalent direct thrombin inhibitor that interacts with both the active site and exosite 1, the substrate-binding site, on thrombin. For rapid anticoagulation, lepirudin is given by continuous IV infusion, but the drug can be given SC for thromboprophylaxis. Lepirudin has a plasma half-life of 60 min after IV infusion and is cleared by the kidneys. Consequently, lepirudin accumulates in patients with renal insufficiency. A high proportion of lepirudin-treated patients develop antibodies against the drug. Although these antibodies rarely cause problems, in a small subset of patients, they can delay lepirudin clearance and enhance its anticoagulant activity. Serious bleeding has been reported in some of these patients.
Lepirudin is usually monitored using the aPTT, and the dose is adjusted to maintain an aPTT that is 1.5–2.5 times the control. The aPTT is not an ideal test for monitoring lepirudin therapy because the clotting time plateaus with higher drug concentrations. Although the ecarin clotting time provides a better index of lepirudin dose than the aPTT, the ecarin clotting time has yet to be standardized.
A univalent inhibitor that targets the active site of thrombin, argatroban is metabolized in the liver. Consequently, this drug must be used with caution in patients with hepatic insufficiency. Argatroban is not cleared via the kidneys, so this drug is safer than lepirudin for HIT patients with renal insufficiency.
Argatroban is administered by continuous IV infusion and has a plasma half-life of ∼45 min. The aPTT is used to monitor its anticoagulant effect, and the dose is adjusted to achieve an aPTT 1.5–3 times the baseline value, but not to exceed 100 s. Argatroban also prolongs the international normalized ratio (INR), a feature that can complicate the transitioning of patients to warfarin. This problem can be circumvented by using the levels of factor X to monitor warfarin in place of the INR. Alternatively, argatroban can be stopped for 2–3 h before INR determination.
A synthetic 20-amino-acid analogue of hirudin, bivalirudin is a divalent thrombin inhibitor. Thus, the N-terminal portion of bivalirudin interacts with the active site of thrombin, whereas its C-terminal tail binds to exosite 1, the substrate-binding domain on thrombin. Bivalirudin has a plasma half-life of 25 min, the shortest half-life of all the parenteral direct thrombin inhibitors. Bivalirudin is degraded by peptidases and is partially excreted via the kidneys. When given in high doses in the cardiac catheterization laboratory, the anticoagulant activity of bivalirudin is monitored using the activated clotting time. With lower doses, its activity can be assessed using the aPTT.
Studies comparing bivalirudin with heparin suggest that bivalirudin produces less bleeding. This feature plus its short half-life make bivalirudin an attractive alternative to heparin in patients undergoing percutaneous coronary interventions. Bivalirudin also has been used successfully in HIT patients who require percutaneous coronary interventions.
Current oral anticoagulant practice dates back almost 60 years to when the vitamin K antagonists were discovered as a result of investigations into the cause of hemorrhagic disease in cattle. Characterized by a decrease in prothrombin levels, this disorder is caused by ingestion of hay containing spoiled sweet clover. Hydroxycoumarin, which was isolated from bacterial contaminants in the hay, interferes with vitamin K metabolism, thereby causing a syndrome similar to vitamin K deficiency. Discovery of this compound provided the impetus for development of other vitamin K antagonists, including warfarin.
A water-soluble vitamin K antagonist initially developed as a rodenticide, warfarin is the coumarin derivative most often prescribed in North America. Like other vitamin K antagonists, warfarin interferes with the synthesis of the vitamin K–dependent clotting proteins, which include prothrombin (factor II) and factors VII, IX, and X. The synthesis of the vitamin K–dependent anticoagulant proteins, proteins C and S, is also reduced by vitamin K antagonists.
All of the vitamin K–dependent clotting factors possess glutamic acid residues at their N termini. A posttranslational modification adds a carboxyl group to the γ-carbon of these residues to generate γ-carboxyglutamic acid. This modification is essential for expression of the activity of these clotting factors because it permits their calcium-dependent binding to negatively charged phospholipid surfaces. The γ-carboxylation process is catalyzed by a vitamin K–dependent carboxylase. Thus, vitamin K from the diet is reduced to vitamin K hydroquinone by vitamin K reductase (Fig. 118-6). Vitamin K hydroquinone serves as a cofactor for the carboxylase enzyme, which in the presence of carbon dioxide replaces the hydrogen on the γ-carbon of glutamic acid residues with a carboxyl group. During this process, vitamin K hydroquinone is oxidized to vitamin K epoxide, which is then reduced to vitamin K by vitamin K epoxide reductase.
Mechanism of action of warfarin. A racemic mixture of S- and R-enantiomers, S-warfarin is most active. By blocking vitamin K epoxide reductase, warfarin inhibits the conversion of oxidized vitamin K into its reduced form. This inhibits vitamin K–dependent γ-carboxylation of factors II, VII, IX, and X because reduced vitamin K serves as a cofactor for a γ-glutamyl carboxylase that catalyzes the γ-carboxylation process, thereby converting prozymogens to zymogens capable of binding calcium and interacting with anionic phospholipid surfaces. S-warfarin is metabolized by CYP2C9. Common genetic polymorphisms in this enzyme can influence warfarin metabolism. Polymorphisms in the C1 subunit of vitamin K reductase (VKORC1) also can affect the susceptibility of the enzyme to warfarin-induced inhibition, thereby influencing warfarin dosage requirements.
Warfarin inhibits vitamin K epoxide reductase (VKOR), thereby blocking the γ-carboxylation process. This results in the synthesis of vitamin K–dependent clotting proteins that are only partially γ-carboxylated. Warfarin acts as an anticoagulant because these partially γ-carboxylated proteins have reduced or absent biologic activity. The onset of action of warfarin is delayed until the newly synthesized clotting factors with reduced activity gradually replace their fully active counterparts.
The antithrombotic effect of warfarin depends on a reduction in the functional levels of factor X and prothrombin, clotting factors that have half-lives of 24 and 72 h, respectively. Because of the delay in achieving an antithrombotic effect, initial treatment with warfarin is supported by concomitant administration of a rapidly acting parenteral anticoagulant, such as heparin, LMWH, or fondaparinux, in patients with established thrombosis or at high risk for thrombosis.
Warfarin is a racemic mixture of R and S isomers. Warfarin is rapidly and almost completely absorbed from the gastrointestinal tract. Levels of warfarin in the blood peak about 90 min after drug administration. Racemic warfarin has a plasma half-life of 36–42 h, and more than 97% of circulating warfarin is bound to albumin. Only the small fraction of unbound warfarin is biologically active.
Warfarin accumulates in the liver where the two isomers are metabolized via distinct pathways. CYP2C9 mediates oxidative metabolism of the more active S isomer (Fig. 118-6). Two relatively common variants, CYP2C9*2 and CYP2C9*3, encode an enzyme with reduced activity. Patients with these variants require lower maintenance doses of warfarin. Approximately 25% of Caucasians have at least one variant allele of CYP2C9*2 or CYP2C9*3, whereas those variant alleles are less common in African Americans and Asians (Table 118-8). Heterozygosity for CYP2C9*2 or CYP2C9*3 decreases the warfarin dose requirement by 20–30% relative to that required in subjects with the wild-type CYP2C9*1/*1 alleles, whereas homozygosity for the CYP2C9*2 or CYP2C9*3 alleles reduces the warfarin dose requirement by 50–70%.
Table 118-8 Frequencies of CYP2C9 Genotypes and VKORC1 Haplotypes in Different Populations and Their Effect on Warfarin Dose Requirements |Favorite Table|Download (.pdf)
Table 118-8 Frequencies of CYP2C9 Genotypes and VKORC1 Haplotypes in Different Populations and Their Effect on Warfarin Dose Requirements
|Genotype/haploptye||Caucasians||African Americans (A/A)||Asians (A)||Dose reduction compared with wild-type|
Consistent with their decreased warfarin dose requirement, subjects with at least one CYP2C9 variant allele are at increased risk for bleeding. Compared with individuals with no variant alleles, the relative risks for warfarin-associated bleeding in CYP2C9*2 or CYP2C9*3 carriers are 1.91 and 1.77, respectively.
Polymorphisms in VKORC1 also can influence the anticoagulant response to warfarin. Several genetic variations of VKORC1 are in strong linkage disequilibrium and have been designated as non-A haplotypes. VKORC1 variants are more prevalent than variants of CYP2C9. Asians have the highest prevalence of VKORC1 variants, followed by Caucasians and African Americans (Table 118-8). Polymorphisms in VKORC1 likely explain 30% of the variability in warfarin dose requirements. Compared with VKORC1 non-A/non-A homozygotes, the warfarin dose requirement decreases by 25 and 50% in A halotype heterozygotes and homozygotes, respectively. These findings prompted the Food and Drug Administration to amend the prescribing information for warfarin to indicate that lower initiation doses should be considered for patients with CYP2C9 and VKORC1 genetic variants. In addition to genotype data, other pertinent patient information has been incorporated into warfarin dosing algorithms. Although such algorithms help predict suitable warfarin doses, it remains unclear whether better dose identification improves patient outcome in terms of reducing hemorrhagic complications or recurrent thrombotic events.
In addition to genetic factors, the anticoagulant effect of warfarin is influenced by diet, drugs, and various disease states. Fluctuations in dietary vitamin K intake affect the activity of warfarin. A wide variety of drugs can alter absorption, clearance, or metabolism of warfarin. Because of the variability in the anticoagulant response to warfarin, coagulation monitoring is essential to ensure that a therapeutic response is obtained.
Warfarin therapy is most often monitored using the prothrombin time, a test that is sensitive to reductions in the levels of prothrombin, factor VII, and factor X. The test is performed by adding thromboplastin, a reagent that contains tissue factor, phospholipid, and calcium, to citrated plasma and determining the time to clot formation. Thromboplastins vary in their sensitivity to reductions in the levels of the vitamin K–dependent clotting factors. Thus, less sensitive thromboplastins will trigger the administration of higher doses of warfarin to achieve a target prothrombin time. This is problematic because higher doses of warfarin increase the risk of bleeding.
The INR was developed to circumvent many of the problems associated with the prothrombin time. To calculate the INR, the patient's prothrombin time is divided by the mean normal prothrombin time, and this ratio is then multiplied by the international sensitivity index (ISI), an index of the sensitivity of the thromboplastin used for prothrombin time determination to reductions in the levels of the vitamin K–dependent clotting factors. Highly sensitive thromboplastins have an ISI of 1.0. Most current thromboplastins have ISI values that range from 1.0 to 1.4.
Although the INR has helped to standardize anticoagulant practice, problems persist. The precision of INR determination varies depending on reagent-coagulometer combinations. This leads to variability in the INR results. Also complicating INR determination is unreliable reporting of the ISI by thromboplastin manufacturers. Furthermore, every laboratory must establish the mean normal prothrombin time with each new batch of thromboplastin reagent. To accomplish this, the prothrombin time must be measured in fresh plasma samples from at least 20 healthy volunteers using the same coagulometer that is used for patient samples.
For most indications, warfarin is administered in doses that produce a target INR of 2.0–3.0. An exception is patients with mechanical heart valves, where a target INR of 2.5–3.5 is recommended. Studies in atrial fibrillation demonstrate an increased risk of cardioembolic stroke when the INR falls to <1.7 and an increase in bleeding with INR values >4.5. These findings highlight the fact that vitamin K antagonists have a narrow therapeutic window. In support of this concept, a study in patients receiving long-term warfarin therapy for unprovoked venous thromboembolism demonstrated a higher rate of recurrent venous thromboembolism with a target INR of 1.5–1.9 compared with a target INR of 2.0–3.0.
Warfarin is usually started at a dose of 5–10 mg. Lower doses are used for patients with CYP2C9 or VKORC1 polymorphisms, which affect the pharmacodynamics or pharmacokinetics of warfarin and render patients more sensitive to the drug. The dose is then titrated to achieve the desired target INR. Because of its delayed onset of action, patients with established thrombosis or those at high risk for thrombosis are given concomitant treatment with a rapidly acting parenteral anticoagulant, such as heparin, LMWH, or fondaparinux. Initial prolongation of the INR reflects reduction in the functional levels of factor VII. Consequently, concomitant treatment with the parenteral anticoagulant should be continued until the INR has been therapeutic for at least 2 consecutive days. A minimum 5-day course of parenteral anticoagulation is recommended to ensure that the levels of prothrombin have been reduced into the therapeutic range with warfarin.
Because warfarin has a narrow therapeutic window, frequent coagulation monitoring is essential to ensure that a therapeutic anticoagulant response is obtained. Even patients with stable warfarin dose requirements should have their INR determined every 2–3 weeks. More frequent monitoring is necessary when new medications are introduced because so many drugs enhance or reduce the anticoagulant effects of warfarin.
Like all anticoagulants, the major side effect of warfarin is bleeding. A rare complication is skin necrosis. Warfarin crosses the placenta and can cause fetal abnormalities. Consequently, warfarin should not be used during pregnancy.
At least half of the bleeding complications with warfarin occur when the INR exceeds the therapeutic range. Bleeding complications may be mild, such as epistaxis or hematuria, or more severe, such as retroperitoneal or gastrointestinal bleeding. Life-threatening intracranial bleeding can also occur.
To minimize the risk of bleeding, the INR should be maintained in the therapeutic range. In asymptomatic patients whose INR is between 3.5 and 4.5, warfarin should be withheld until the INR returns to the therapeutic range. If the INR is >4.5, a therapeutic INR can be achieved more rapidly by administration of low doses of sublingual vitamin K. A vitamin K dose of 1 mg is usually adequate for patients with an INR between 4.9 and 9, whereas 2–3 mg can be used for those with an INR >9. Higher doses of vitamin K can be administered if more rapid reversal of the INR is required or if the INR is excessively high. Although vitamin K administration results in a more rapid reduction in the INR compared with simply holding the warfarin, there is no evidence that vitamin K administration reduces the risk of hemorrhage.
Patients with serious bleeding need more aggressive treatment. These patients should be given 10 mg of vitamin K by slow IV infusion. Additional vitamin K should be given until the INR is in the normal range. Treatment with vitamin K should be supplemented with fresh-frozen plasma as a source of the vitamin K–dependent clotting proteins. For life-threatening bleeds, or if patients cannot tolerate the volume load, prothrombin complex concentrates can be used.
Warfarin-treated patients who experience bleeding when their INR is in the therapeutic range require investigation into the cause of the bleeding. Those with gastrointestinal bleeding often have underlying peptic ulcer disease or a tumor. Similarly, investigation of hematuria or uterine bleeding in patients with a therapeutic INR may unmask a tumor of the genitourinary tract.
A rare complication of warfarin, skin necrosis usually is seen 2–5 days after initiation of therapy. Well-demarcated erythematous lesions form on the thighs, buttocks, breasts, or toes. Typically, the center of the lesion becomes progressively necrotic. Examination of skin biopsies taken from the border of these lesions reveals thrombi in the microvasculature.
Warfarin-induced skin necrosis is seen in patients with congenital or acquired deficiencies of protein C or protein S. Initiation of warfarin therapy in these patients produces a precipitous fall in plasma levels of proteins C or S, thereby eliminating this important anticoagulant pathway before warfarin exerts an antithrombotic effect through lowering of the functional levels of factor X and prothrombin. The resultant procoagulant state triggers thrombosis. Why the thrombosis is localized to the microvasculature of fatty tissues is unclear.
Treatment involves discontinuation of warfarin and reversal with vitamin K, if needed. An alternative anticoagulant, such as heparin or LMWH, should be given in patients with thrombosis. Protein C concentrates or recombinant activated protein C can be given to protein C–deficient patients to accelerate healing of the skin lesions; fresh-frozen plasma may be of value for those with protein S deficiency. Occasionally, skin grafting is necessary when there is extensive skin loss.
Because of the potential for skin necrosis, patients with known protein C or protein S deficiency require overlapping treatment with a parenteral anticoagulant when initiating warfarin therapy. Warfarin should be started in low doses in these patients, and the parenteral anticoagulant should be continued until the INR is therapeutic for at least 2–3 consecutive days.
Warfarin crosses the placenta and can cause fetal abnormalities or bleeding. The fetal abnormalities include a characteristic embryopathy, which consists of nasal hypoplasia and stippled epiphyses. The risk of embryopathy is highest if warfarin is given in the first trimester of pregnancy. Central nervous system abnormalities can also occur with exposure to warfarin at any time during pregnancy. Finally, maternal administration of warfarin produces an anticoagulant effect in the fetus that can cause bleeding. This is of particular concern at delivery when trauma to the head during passage through the birth canal can lead to intracranial bleeding. Because of these potential problems, warfarin is contraindicated in pregnancy, particularly in the first and third trimesters. Instead, heparin, LMWH, or fondaparinux can be given during pregnancy for prevention or treatment of thrombosis.
Warfarin does not pass into the breast milk. Consequently, warfarin can safely be given to nursing mothers.
Patients with a lupus anticoagulant or those who need urgent or elective surgery present special challenges. Although observational studies suggested that patients with thrombosis complicating the antiphospholipid antibody syndrome required higher intensity warfarin regimens to prevent recurrent thromboembolic events, two randomized trials showed that targeting an INR of 2.0–3.0 is as effective as higher intensity treatment and produces less bleeding. Monitoring warfarin therapy can be problematic in patients with antiphospholipid antibody syndrome if the lupus anticoagulant prolongs the baseline INR.
If patients receiving long-term warfarin treatment require an elective invasive procedure, warfarin can be stopped 5 days before the procedure to allow the INR to return to normal levels. Those at high risk for recurrent thrombosis can be bridged with once- or twice-daily SC injections of LMWH when the INR falls to <2.0. The last dose of LMWH should be given 12–24 h before the procedure, depending on whether LMWH is administered twice or once daily. After the procedure, treatment with warfarin can be restarted.
New oral anticoagulants that target thrombin or factor Xa are under development. These drugs have a rapid onset of action and have half-lives that permit once- or twice-daily administration. Designed to produce a predictable level of anticoagulation, these new oral agents are given in fixed doses without routine coagulation monitoring. Therefore, these drugs are more convenient to administer than warfarin.
Dabigatran etexilate, an oral thrombin inhibitor, and rivaroxaban, an oral factor Xa inhibitor, are licensed in Europe and Canada for short-term thromboprophylaxis after elective hip or knee replacement surgery. Phase III trials with apixaban, another oral factor Xa inhibitor, also have been completed in patients undergoing major orthopedic surgery (Table 118-9).
Table 118-9 Comparison of the Features of New Oral Anticoagulants in Advanced Stages of Development |Favorite Table|Download (.pdf)
Table 118-9 Comparison of the Features of New Oral Anticoagulants in Advanced Stages of Development
|Time to peak (h)||3||3||2|
|Renal excretion (%)||65||25||80|
The RE-LY trial shows the promise of these new agents for long-term indications. This trial compared two different dose regimens of dabigatran etexilate (110 mg or 150 mg twice daily) with warfarin (dose-adjusted to achieve an INR between 2 and 3) for stroke prevention in 18,113 patients with nonvalvular atrial fibrillation. The annual rates of the primary efficacy outcome, stroke or systemic embolism, were 1.7% with warfarin, 1.5% with the lower dose dabigatran regimen, and 1.1% with the higher dose regimen. Thus, the lower dose dabigatran regimen was noninferior to warfarin, while the higher dose regimen was superior. Annual rates of major bleeding were 3.4% with warfarin compared with 2.7% and 3.1% with the lower and higher dose dabigatran regimens, respectively. Thus, the lower dose dabigatran regimen was associated with significantly less major bleeding than warfarin, while the rate of major bleeding with the higher dose regimen was not significantly different from that with warfarin. Rates of intracerebral bleeding were significantly lower with both doses of dabigatran than with warfarin, as were rates of life-threatening bleeding. There was no evidence of hepatotoxicity with dabigatran.
Based on the results of the RE-LY trial, dabigatran etexilate has been licensed in the United States and Canada for stroke prevention in patients with atrial fibrillation. The 150 mg twice daily dose of dabigatran is recommended for most patients. In the United States, a 75 mg twice daily dose is recommended for patients with a creatinine clearance of 30 to 50 mL/min, while in Canada, the 110 mg twice daily dose is recommended for those over the age of 80 years or for patients at high risk of bleeding. The drug is contraindicated in patients with a creatinine clearance less than 15 mL/min.
Dabigatran etexilate also was compared with warfarin in 2539 patients with acute venous thromboembolism. Patients were initially treated with heparin or LMWH and then randomized to a 6-month course of dabigatran (150 mg twice daily) or warfarin, which was dose-adjusted to achieve an INR of 2–3. The primary endpoint, a composite of recurrent venous thromboembolism or fatal pulmonary embolism, occurred in 2.4% of patients given dabigatran and in 2.1% of those treated with warfarin. Major bleeding occurred in 1.6 and 1.9% of patients given dabigatran and warfarin, respectively. Based on the results of this trial, unmonitored fixed-dose dabigatran appears to be noninferior to warfarin for treatment of patients with venous thromboembolism. Taken together with the results of the RE-LY trial, these findings suggest that the new oral anticoagulants will gradually replace warfarin.
Role of Fibrinolytic Therapy
Fibrinolytic drugs can be used to degrade thrombi and are administered systemically or can be delivered via catheters directly into the substance of the thrombus. Systemic delivery is used for treatment of acute MI, acute ischemic stroke, and most cases of massive PE. The goal of therapy is to produce rapid thrombus dissolution, thereby restoring antegrade blood flow. In the coronary circulation, restoration of blood flow reduces morbidity and mortality rates by limiting myocardial damage, whereas in the cerebral circulation, rapid thrombus dissolution decreases the neuronal death and brain infarction that produce irreversible brain injury. For patients with massive PE, the goal of thrombolytic therapy is to restore pulmonary artery perfusion.
Peripheral arterial thrombi and thrombi in the proximal deep veins of the leg are most often treated using catheter-directed thrombolytic therapy. Catheters with multiple side holes can be utilized to enhance drug delivery. In some cases, intravascular devices that fragment and extract the thrombus are used to hasten treatment. These devices can be used alone or in conjunction with fibrinolytic drugs.
Currently approved fibrinolytic agents include streptokinase; acylated plasminogen streptokinase activator complex (anistreplase); urokinase; recombinant tissue-type plasminogen activator (rtPA), which is also known as alteplase or activase; and two recombinant derivatives of rtPA, tenecteplase and reteplase. All of these agents act by converting the proenzyme, plasminogen, to plasmin, the active enzyme (Fig. 118-7). Plasmin then degrades the fibrin matrix of thrombi and produces soluble fibrin degradation products.
The fibrinolytic system and its regulation. Plasminogen activators convert plasminogen to plasmin. Plasmin then degrades fibrin into soluble fibrin degradation products. The system is regulated at two levels. Type 1 plasminogen activator inhibitor (PAI-1) regulates the plasminogen activators, whereas α2-antiplasmin serves as the major inhibitor of plasmin.
Endogenous fibrinolysis is regulated at two levels. Plasminogen activator inhibitors, particularly the type 1 form (PAI-1), prevent excessive plasminogen activation by regulating the activity of tPA and urokinase-type plasminogen activator (u-PA). Once plasmin is generated, it is regulated by plasmin inhibitors, the most important of which is α2-antiplasmin. The plasma concentration of plasminogen is twofold higher than that of α2-antiplasmin. Consequently, with pharmacologic doses of plasminogen activators, the concentration of plasmin that is generated can exceed that of α2-antiplasmin. In addition to degrading fibrin, unregulated plasmin can also degrade fibrinogen and other clotting factors. This process, which is known as the systemic lytic state, reduces the hemostatic potential of the blood and increases the risk of bleeding.
The endogenous fibrinolytic system is geared to localize plasmin generation to the fibrin surface. Both plasminogen and tPA bind to fibrin to form a ternary complex that promotes efficient plasminogen activation. In contrast to free plasmin, plasmin generated on the fibrin surface is relatively protected from inactivation by α2-antiplasmin, a feature that promotes fibrin dissolution. Furthermore, C-terminal lysine residues, exposed as plasmin degrades fibrin, serve as binding sites for additional plasminogen and tPA molecules. This creates a positive feedback that enhances plasmin generation. When used pharmacologically, the various plasminogen activators capitalize on these mechanisms to a lesser or greater extent.
Plasminogen activators that preferentially activate fibrin-bound plasminogen are considered fibrin-specific. In contrast, nonspecific plasminogen activators do not discriminate between fibrin-bound and circulating plasminogen. Activation of circulating plasminogen results in the generation of unopposed plasmin that can trigger the systemic lytic state. Alteplase and its derivatives are fibrin-specific plasminogen activators, whereas streptokinase, anistreplase, and urokinase are nonspecific agents.
Unlike other plasminogen activators, streptokinase is not an enzyme and does not directly convert plasminogen to plasmin. Instead, streptokinase forms a 1:1 stoichiometric complex with plasminogen. Formation of this complex induces a conformational change in plasminogen that exposes its active site (Fig. 118-8). This conformationally altered plasminogen then converts additional plasminogen molecules to plasmin.
Mechanism of action of streptokinase. Streptokinase binds to plasminogen and induces a conformational change in plasminogen that exposes its active site. The streptokinase/plasmin(ogen) complex then serves as the activator of additional plasminogen molecules.
Streptokinase has no affinity for fibrin, and the streptokinase-plasminogen complex activates both free and fibrin-bound plasminogen. Activation of circulating plasminogen generates sufficient amounts of plasmin to overwhelm α2-antiplasmin. Unopposed plasmin not only degrades fibrin in the occlusive thrombus but also induces a systemic lytic state.
When given systemically to patients with acute MI, streptokinase reduces mortality. For this indication, the drug is usually given as an IV infusion of 1.5 million units over 30–60 min. Patients who receive streptokinase can develop antibodies against the drug, as can patients with prior streptococcal injection. These antibodies can reduce the effectiveness of streptokinase.
Allergic reactions occur in ∼5% of patients treated with streptokinase. These may manifest as a rash, fever, chills, and rigors. Although anaphylactic reactions can occur, these are rare. Transient hypotension is common with streptokinase and has been attributed to plasmin-mediated release of bradykinin from kininogen. The hypotension usually responds to leg elevation and administration of IV fluids and low doses of vasopressors, such as dopamine or norepinephrine.
To generate this drug, streptokinase is combined with equimolar amounts of Lys-plasminogen, a plasmin-cleaved form of plasminogen with a Lys residue at its N terminus. The active site of Lys-plasminogen that is exposed upon combination with streptokinase is then masked with an anisoyl group. After IV infusion, the anisoyl group is slowly removed by deacylation, giving the complex a half-life of ∼100 min. This allows drug administration via a single bolus infusion.
Although it is more convenient to administer, anistreplase offers few mechanistic advantages over streptokinase. Like streptokinase, anistreplase does not distinguish between fibrin-bound and circulating plasminogen. Consequently, it too produces a systemic lytic state. Likewise, allergic reactions and hypotension are just as frequent with anistreplase as they are with streptokinase.
When anistreplase was compared with alteplase in patients with acute MI, reperfusion was obtained more rapidly with alteplase than with anistreplase. Improved reperfusion was associated with a trend toward better clinical outcomes and reduced mortality rate with alteplase. These results and the high cost of anistreplase have dampened the enthusiasm for its use.
Urokinase is a two-chain serine protease derived from cultured fetal kidney cells with a molecular weight of 34,000. Urokinase converts plasminogen to plasmin directly by cleaving the Arg560-Val561 bond. Unlike streptokinase, urokinase is not immunogenic and allergic reactions are rare. Urokinase produces a systemic lytic state because it does not discriminate between fibrin-bound and circulating plasminogen.
Despite many years of use, urokinase has never been systemically evaluated for coronary thrombolysis. Instead, urokinase is often employed for catheter-directed lysis of thrombi in the deep veins or the peripheral arteries. Because of production problems, the availability of urokinase is limited.
A recombinant form of single-chain tPA, alteplase has a molecular weight of 68,000. Alteplase is rapidly converted into its two-chain form by plasmin. Although single- and two-chain forms of tPA have equivalent activity in the presence of fibrin, in its absence, single-chain tPA has tenfold lower activity.
Alteplase consists of five discrete domains (Fig. 118-9); the N-terminal A chain of two-chain alteplase contains four of these domains. Residues 4 through 50 make up the finger domain, a region that resembles the finger domain of fibronectin; residues 50 through 87 are homologous with epidermal growth factor, whereas residues 92 through 173 and 180 through 261, which have homology to the kringle domains of plasminogen, are designated as the first and second kringle, respectively. The fifth alteplase domain is the protease domain; it is located on the C-terminal B chain of two-chain alteplase.
Domain structures of alteplase (tPA), tenecteplase (TNK-tPA), desmoteplase (b-PA), and reteplase (r-PA). The finger (F), epidermal growth factor (EGF), first and second kringles (K1 and K2, respectively), and protease (P) domains are illustrated. The glycosylation site (Y) on K1 has been repositioned in tenecteplase to endow it with a longer half-life. In addition, a tetra-alanine substitution in the protease domain renders tenecteplase resistant to PAI-1 inhibition. Desmoteplase differs from alteplase and tenecteplase in that it lacks a K2 domain. Reteplase is a truncated variant that lacks the F, EGF, and K1 domains.
The interaction of alteplase with fibrin is mediated by the finger domain and, to a lesser extent, by the second kringle domain. The affinity of alteplase for fibrin is considerably higher than that for fibrinogen. Consequently, the catalytic efficiency of plasminogen activation by alteplase is two to three orders of magnitude higher in the presence of fibrin than in the presence of fibrinogen. This phenomenon helps to localize plasmin generation to the fibrin surface.
Although alteplase preferentially activates plasminogen in the presence of fibrin, alteplase is not as fibrin-selective as was first predicted. Its fibrin specificity is limited because like fibrin, (DD)E, the major soluble degradation product of cross-linked fibrin, binds alteplase and plasminogen with high affinity. Consequently, (DD)E is as potent as fibrin as a stimulator of plasminogen activation by alteplase. Whereas plasmin generated on the fibrin surface results in thrombolysis, plasmin generated on the surface of circulating (DD)E degrades fibrinogen. Fibrinogenolysis results in the accumulation of fragment X, a high-molecular-weight clottable fibrinogen degradation product. Incorporation of fragment X into hemostatic plugs formed at sites of vascular injury renders them susceptible to lysis. This phenomenon may contribute to alteplase-induced bleeding.
A trial comparing alteplase with streptokinase for treatment of patients with acute MI demonstrated significantly lower mortality with alteplase than with streptokinase, although the absolute difference was small. The greatest benefit was seen in patients age <75 years with anterior MI who presented <6 h after symptom onset.
For treatment of acute MI or acute ischemic stroke, alteplase is given as an IV infusion over 60–90 min. The total dose of alteplase usually ranges from 90 to 100 mg. Allergic reactions and hypotension are rare, and alteplase is not immunogenic.
Tenecteplase is a genetically engineered variant of tPA and was designed to have a longer half-life than tPA and to be resistant to inactivation by PAI-1. To prolong its half-life, a new glycosylation site was added to the first kringle domain (Fig. 118-9). Because addition of this extra carbohydrate side chain reduced fibrin affinity, the existing glycosylation site on the first kringle domain was removed. To render the molecule resistant to inhibition by PAI-1, a tetra-alanine substitution was introduced at residues 296–299 in the protease domain, the region responsible for the interaction of tPA with PAI-1.
Tenecteplase is more fibrin-specific than tPA. Although both agents bind to fibrin with similar affinity, the affinity of tenecteplase for (DD)E is significantly lower than that of tPA. Consequently, (DD)E does not stimulate systemic plasminogen activation by tenecteplase to the same extent as tPA. As a result, tenecteplase produces less fibrinogenolysis than tPA.
For coronary thrombolysis, tenecteplase is given as a single IV bolus. In a large phase III trial that enrolled >16,000 patients, the 30-day mortality rate with single-bolus tenecteplase was similar to that with accelerated-dose tPA. Although rates of intracranial hemorrhage were also similar with both treatments, patients given tenecteplase had fewer noncerebral bleeds and a reduced need for blood transfusions than those treated with tPA. The improved safety profile of tenecteplase likely reflects its enhanced fibrinspecificity.
Reteplase is a recombinant tPA derivative and is a single-chain variant that lacks the finger, epidermal growth factor, and first kringle domains (Fig. 118-9). This truncated derivative has a molecular weight of 39,000. Reteplase binds fibrin more weakly than tPA because it lacks the finger domain. Because it is produced in Escherichia coli, reteplase is not glycosylated. This endows it with a plasma half-life longer than that of tPA. Consequently, reteplase is given as two IV boluses, which are separated by 30 min. Clinical trials have demonstrated that reteplase is at least as effective as streptokinase for treatment of acute MI, but the agent is not superior to tPA.
Several new drugs are under investigation. These include desmoteplase (Fig. 118-9), a recombinant form of the full-length plasminogen activator isolated from the saliva of the vampire bat, and alfimeprase, a truncated form of fibrolase, an enzyme isolated from the venom of the southern copperhead snake. Clinical studies with these agents have been disappointing. Desmoteplase, which is more fibrin-specific than tPA, was investigated for treatment of acute ischemic stroke. Patients presenting 3–9 h after symptom onset were randomized to one of two doses of desmoteplase or to placebo. Overall response rates were low and no different with desmoteplase from with placebo. Mortality rate was higher in the desmoteplase arms.
Alfimeprase is a metalloproteinase that degrades fibrin and fibrinogen in a plasmin-independent fashion. In the circulation, alfimeprase is inhibited by α2-macroglobulin. Consequently, the drug must be delivered via a catheter directly into the thrombus. Studies of alfimeprase for treatment of peripheral arterial occlusion or for restoration of flow in blocked central venous catheters were stopped due to lack of efficacy. The disappointing results with desmoteplase and alfimeprase highlight the challenges of introducing new fibrinolytic drugs.