PATHOGENESIS OF THROMBOSIS

The acute coronary syndromes form a spectrum of coronary disease from unstable angina to non-Q-wave and Q-wave myocardial infarction.⁷ They are caused by the development of thrombus on a ruptured coronary atherosclerotic plaque (fig 2).^2,8 This plaque consists of a core of lipid and collagen covered by a layer of connective tissue. Also present in the core are cholesterol-containing macrophages (foam cells), derived from monocytes that have crossed the endothelium from the arterial lumen. These cells produce large amounts of prothrombotic tissue factor together with several inflammatory cell mediators such as tumour necrosis factor-α and various interleukins.^3,9,10

The process of thrombosis starts when the atherosclerotic plaque tears and exposes the lipid-rich core to blood in the arterial lumen. Platelet adherence to the exposed subendothelium and collagen results in platelet activation and the release and local accumulation of soluble platelet agonists (thrombin, adenosine diphosphate (ADP), serotonin, and thromboxane A₂). This in turn causes further platelet aggregation, coronary artery vasoconstriction, and subsequent reduction in coronary artery blood flow.¹¹ On the surface of the activated platelet the integrin α_IIbβ₃ (glycoprotein IIb/IIIa) receptor undergoes a conformational change, allowing the platelets to bind fibrinogen tightly. Fibrinogen acts as a bivalent ligand and allows the formation of stable platelet aggregates through cross linking of α_IIbβ₃ receptors.¹² In this way α_IIbβ₃ mediates the so-called “final common pathway” of platelet aggregation. It has to be remembered that platelets not only promote thrombosis but also impede fibrinolysis by the secretion of factors such as plasminogen activation inhibitor 1.^13,14

Platelets are the “major player” in arterial thrombosis and therefore are attractive targets in the prevention and treatment of cardiovascular disease. Coronary thrombosis often precedes or complicates percutaneous coronary interventions (PCI), including percutaneous transluminal coronary angioplasty (PTCA) and intracoronary stent implantation, and refining antiplatelet therapy to cover these procedures, as well as to treat acute coronary syndromes, has been the objective of many clinical trials.

ASPIRIN

Bayer Co patented acetyl-salicylic acid in 1899 under the trade name of aspirin (“a” stood for acetyl and “spir” stood for spirasaure, the German word for salicylic acid).¹⁵ Aspirin was used initially as an analgesic and an antipyretic; however its effects on haemostasis were recognised as early as 1945.¹⁶ The antithrombotic action of aspirin depends on the irreversible inhibition of arachidonate cyclo-oxygenase activity in platelets, thereby reducing the extent of thromboxane A₂ formation that occurs after activation of phospholipase A₂ and release of arachidonic acid.¹⁷ Thromboxane A₂ is a strong platelet agonist that is an effective inducer of platelet granule secretion as well as platelet aggregation. However, phospholipase A₂ activation and arachidonic acid release seem to play a relatively minor part in the action of many platelet agonists, with the notable exception of collagen, such that the antiplatelet effects of aspirin are limited and overall aspirin may be considered a relatively weak antiplatelet agent.

The Antithrombotic Trialists Collaboration meta-analysis of 135 000 patients compared antiplatelet therapy versus control and showed that among “high risk” patients (those with acute or previous vascular disease or other predisposing condition) allocation to antiplatelet therapy reduced the combined outcome of non-fatal myocardial infarction, non-fatal stroke, or vascular death by about one quarter; non-fatal myocardial infarction was reduced by one third, non-fatal stroke by one quarter, and vascular mortality by one sixth.¹⁸ Available evidence suggests a daily aspirin dose of 75–150 mg for long term prevention of serious vascular events is recommended for high risk patients. In clinical situations where an immediate antithrombotic effect is required (such as acute myocardial infarction, stroke, or unstable angina) a loading dose of 300 mg is recommended.^18,19

The effect of aspirin on the outcome of patients with unstable angina has been assessed in four double blind placebo controlled trials.^20–23 The Veterans Administration Cooperative Study randomised 1266 men with unstable angina to aspirin 324 mg daily or placebo.²⁰ Death and recurrent non-fatal myocardial infarction at three months were significantly reduced in the aspirin group (12.1% v 5% in the placebo group, p<0.001). The other trials showed similar effects of aspirin and the RISC Study showed that aspirin at a dose of 75 mg daily significantly reduced death and myocardial infarction. Other studies have shown that aspirin reduces the incidence of myocardial infarction and sudden cardiac death in patients with stable angina, without influencing the severity or frequency of angina symptoms or the progression of coronary stenosis.^24–26 On the basis of these results, aspirin is standard treatment for eligible patients with unstable angina.

With the advent of thrombolytic therapy of myocardial infarction, it became clear that treatment with streptokinase was associated with marked platelet activation, as determined by increased thromboxane A₂ production.²⁷ This issue was addressed by the Second International Study Of Infarct Survival (ISIS-2) which involved 17 187 patients with acute myocardial infarction, randomised to one of four arms of therapy consisting of placebo, aspirin, streptokinase, or streptokinase plus aspirin.²⁸ Aspirin reduced mortality from 13.2% in the placebo group to 10.7% (23% relative risk reduction, p<0.00001) and streptokinase reduced mortality to 10.4% (25% relative risk reduction). However, the combination of aspirin and streptokinase produced the greatest benefit, reducing mortality to 8.0%, an overall relative risk reduction of 42%, thus establishing aspirin and thrombolysis as standard therapy for eligible patients with acute myocardial infarction.

Long term therapy with aspirin is associated with a significant increase in the incidence of gastrointestinal haemorrhage.²⁹ No evidence exists that reducing the dose or using modified release formulations reduces the incidence of gastrointestinal haemorrhage.³⁰

THIENOPYRIDINES

This group comprises ticlopidine and clopidogrel, which act by causing irreversible blockade of ADP binding to one of its receptors on the platelet surface, the P2Y₁₂ receptor. Both drugs are effective orally, with about 80%–90% absorption. They require metabolism by the hepatic cytochrome P450 enzyme system to acquire their antiplatelet activity.³¹

Ticlopidine has been established as an alternative to aspirin in the prevention of recurrent cerebral ischaemia and stroke but its use has been limited in view of its side effects, including thrombotic thrombocytopenic purpura and potentially fatal severe neutropenia.³² Due to these side effects and its slow onset of action it has been replaced by clopidogrel, which is felt to be safer and, when a loading dose is employed, faster acting. The CAPRIE trial showed that clopidogrel was slightly more effective than aspirin in reducing ischaemic complications (ischaemic stroke, myocardial infarction, or vascular death) in patients with atherosclerotic disease³³ but overall, the safety and tolerability of aspirin and clopidogrel were similar. The CAPRIE Study therefore established clopidogrel as an alternative antiplatelet to aspirin for secondary prevention across a wide spectrum of patients with vascular disease.

Ticlopidine in addition to aspirin has been shown to reduce thrombotic complications after intracoronary stent deployment.³⁴ In the CLASSICS trial, three regimens were compared in 1020 patients after coronary stent implantation, consisting of aspirin 325 mg once a day plus ticlopidine 250 mg twice a day, aspirin plus clopidogrel 75 mg once a day and aspirin plus clopidogrel 300 mg loading dose followed by 75 mg once a day.³⁵ This study confirmed that clopidogrel, with or without the loading dose, is much better tolerated than ticlopidine and showed no significant difference in major adverse cardiac events between the three groups (0.9% ticlopidine, 1.5% clopidogrel 75 mg group, 1.2% clopidogrel 300/75 mg group).

The CURE (Clopidogrel in Unstable Angina to Prevent Recurrent Events) study investigated the effect of clopidogrel combined with aspirin in the treatment of patients with non-ST elevation acute coronary syndromes.³⁶ A total of 12 562 patients presenting within 24 hours of onset of symptoms were randomly assigned to receive clopidogrel (300 mg immediately followed by 75 mg once daily) or placebo in addition to aspirin for three to 12 months. The primary outcome—a composite of death from cardiovascular causes, non-fatal myocardial infarction or stroke—occurred in 9.3% of the patients in the clopidogrel group and 11.4% of the patients in the placebo group (relative risk 0.8; p<0.001). The secondary endpoint, a composite of the primary endpoint and refractory ischaemia, was also reduced (relative risk 0.86, p<0.001). The benefits of clopidogrel, observed across various subgroups, were apparent as early as the first 24 hours after randomisation, indicating that the oral loading dose was rapidly effective. The addition of clopidogrel to aspirin was, however, associated with an increased risk of major bleeding, particularly gastrointestinal haemorrhage. This, in part, reflects one of the limitations of combining other oral antithrombotic agents with aspirin in view of the propensity of aspirin to cause gastric erosions and ulceration with associated compromise to haemostasis.

The PCI CURE Study was designed to test whether treatment with clopidogrel, in addition to aspirin, before PCI and continued beyond the standard course of four weeks after PCI is superior to placebo in preventing major ischaemic events.³⁷ Altogether 2658 patients who were recruited into the CURE Study and underwent PCI in response to refractory ischaemia or adverse events were examined; 1313 were assigned to clopidogrel and 1345 to placebo. A total of 1730 PCI procedures were performed during the initial hospital stay and 928 after discharge. Patients were pre-treated with clopidogrel for a median of 10 days before PCI. After PCI most (>80%) patients in both groups received open label thienopyridine for about four weeks, after which the study drug was restarted for a mean of about seven months. Fewer patients in the clopidogrel group had myocardial infarction or refractory ischaemia before PCI.

The number of patients with the primary endpoint of cardiovascular death, myocardial infarction, or urgent revascularisation was significantly lower in the clopidogrel than in the placebo group. Since the vast majority of patients received open label thienopyridine treatment for the four weeks after the procedure, the improvement in the primary endpoint was presumably caused by pre-treatment with clopidogrel prior to PCI. The results implied that in patients with non-ST elevation acute coronary syndrome in which an invasive strategy was planned, clopidogrel started on admission before the procedure and continued long afterwards was beneficial in reducing both early and late complications.

The efficacy of clopidogrel treatment both pre-PCI and subsequently was also recently addressed by the CREDO (Clopidogrel for the Reduction of Events During Observation) Study.³⁸ Long term clopidogrel therapy was associated with a relative reduction in the combined risk of death, myocardial infarction, or stroke. Overall no risk reduction in death, myocardial infarction, or target vessel revascularisation from pre-treatment with clopidogrel was observed. From a subgroup analysis, however, it appeared that there was a reduction in the combined endpoint (death, myocardial infarction, or urgent revascularisation) if clopidogrel was administered more than six hours before the procedure.

Numerous trials have been initiated to study the benefits of combining aspirin and clopidogrel for other indications, such as stroke prevention (the MATCH study) or stable ischaemic heart disease (CHARISMA). Clopidogrel therapy only yields partial blockade of the P2Y₁₂ receptor,^39,40 and there is much interest in the development of more effective strategies of P2Y₁₂ receptor antagonism in order potentially to enhance the clinical benefits seen with clopidogrel.

DIPYRIDAMOLE

Dipyridamole inhibits adenosine uptake in erythrocytes and endothelial cells. This increases plasma adenosine levels, which means that there is more available for binding to the adenosine receptor on the platelet. Adenosine activates the release of adenylate cyclase, which converts cyclic adenosine triphosphate (cATP) to cyclic adenosine monophosphate (cAMP). Dipyridamole also blocks the enzyme cyclic guanine monophosphate (cGMP) phosphodiesterase, thereby inhibiting the breakdown of cGMP. Raised levels of cAMP and cGMP within platelets have antiaggregatory effects.

In patients with ischaemic heart disease, dipyridamole in combination with aspirin has not been found to be better than aspirin alone.⁴¹ In patients with cerebrovascular disease however, the European Stroke Prevention Study 2 (ESPS-2) demonstrated that the co-prescription of modified release dipyridamole (400 mg daily) and aspirin (50 mg daily) was more effective in preventing stroke than either drug alone.⁴² A meta-analysis, of 25 trials comparing dipyridamole plus aspirin with aspirin alone, showed that the addition of aspirin was associated with only a non-significant further reduction in serious vascular events.¹⁸ The current feeling is that there is not yet sufficient evidence to justify adoption of aspirin and dipyridamole as first line treatment for the secondary prevention of stroke.

α_IIBβ₃ ANTAGONISTS

The integrin α_IIbβ₃ (glycoprotein IIb/IIIa) is a receptor located on the platelet membrane that mediates platelet aggregation.⁴³ These receptors recognise an arginine-glycine-aspartic acid (RGD) sequence contained in adhesive molecules such as fibrinogen and von Willebrand factor. When platelets become activated, α_IIbβ₃ is converted into a functional receptor, binding these adhesive proteins⁴⁴ and allowing platelets to aggregate and form a haemostatic plug. Therefore, antagonists of α_IIbβ₃ block the final common pathway of platelet aggregation. Three classes of α_IIbβ₃ inhibitor have been developed: murine-human chimeric antibodies, such as abciximab; synthetic peptide forms, such as eptifibatide; and synthetic non-peptide (peptidomimetic) forms, such as tirofiban.⁴⁵

The antibody abciximab (ReoPro) has a particularly high avidity for α_IIbβ₃ and binds to, and exchanges between, platelets for as long as two weeks, although most of the antithrombotic effect disappears within 12–24 hours after termination of intravenous abciximab infusion. On the other hand, synthetic α_IIbβ₃ inhibitors inhibit ex vivo platelet aggregation for only a few hours after the end of an intravenous infusion. Eptifibatide (Integrilin) is a synthetic, conformationally constrained, cyclic heptapeptide, fashioned after the integrin antagonist barbourin. Barbourin is a small protein inhibitor of α_IIbβ₃ isolated from the venom of the Southeastern pygmy rattlesnake Sistruvus m barbouri. It is a highly competitive inhibitor of α_IIbβ₃ with a half life of 90–120 minutes and is excreted by the kidneys. The degree of platelet inhibition is related to the drug dose and plasma concentration.^45–47 Tirofiban (Aggrastat) is the most developed peptidomimetic compound; it is an analogue of tyrosine that is a highly potent antagonist of fibrinogen binding to α_IIbβ₃. This also has a short half life of 90–120 minutes and relies on renal clearance.⁴⁵

Drugs from these three groups have been evaluated in the following clinical situations: PCI with and without stenting, medical management of acute coronary syndromes, and acute myocardial infarction both as an adjunct to lytic therapy and for primary PCI. A large and varied group of trials have been carried out (see tables 1–5).

Intravenous α_IIbβ₃ receptor antagonists in PCI (see table 1)

The intravenous use of α_IIbβ₃ antagonists has been shown to be effective in patients undergoing PCI (EPIC, EPILOG, and CAPTURE) and stenting (EPISTENT).^48–51 The benefits are additional to those achieved with other antiplatelet agents and are most prominent with abciximab. In the IMPACT II trial negative results were found with eptifibatide in high and low risk patients undergoing PCI.⁵² The results were felt to be due to inadequate platelet inhibition with the dose used, and in ESPRIT (where a higher dose was used) eptifibatide was demonstrated to be highly effective in PCI with stenting.⁵³ TARGET was a head to head comparison of tirofiban versus abciximab in patients with acute coronary syndromes undergoing PCI. The findings demonstrated that tirofiban offered less protection from major ischaemic events than did abciximab with no significant differences in the rates of major bleeding complications or transfusion.⁵⁴ However, there is evidence that the levels of platelet inhibition during the first four hours after receiving a bolus of tirofiban were suboptimal in this study in contrast to the predictable and generally optimal effects of abciximab in the first few hours after bolus administration.⁵⁵

There is now compelling evidence for the routine use of α_IIbβ₃ antagonists as an adjunct to aspirin and heparin in patients undergoing PCI.

Intravenous α_IIbβ₃ antagonists in acute coronary syndromes (see table 2)

The role of α_IIbβ₃ antagonists in the treatment of acute coronary syndromes, independent of the use of coronary revascularisation, was tested in PRISM, PRISM-PLUS, PURSUIT, PARAGON, and GUSTO IV-ACS.^56–60 There was overall benefit in the groups treated with α_IIbβ₃ antagonists; however, the results of GUSTO IV-ACS were disappointing and revealed no benefit of abciximab when added to aspirin and heparin. In TACTICS an early invasive strategy was compared to an early conservative strategy, and the primary endpoint was significantly reduced in the early invasive group.⁶¹

While there is clear benefit for adjunctive treatment with α_IIbβ₃ antagonists in patients with acute coronary syndromes before revascularisation, their role in the purely medical management of these patients is less certain, particularly since many physicians are currently using the combination of aspirin, clopidogrel, and low molecular weight heparin for the management of these patients, which is an advance in practice since trials such as PURSUIT and PRISM.

Intravenous α_IIbβ₃ antagonists for primary PTCA in acute myocardial infarction (see table 3)

The use of α_IIbβ₃ antagonists has been assessed in primary PTCA for acute myocardial infarction in ADMIRAL, RAPPORT, and CADILLAC.^62–64 Abciximab was shown to improve the results of primary angioplasty for the treatment of acute myocardial infarction in the ADMIRAL trial. In RAPPORT, abciximab was associated with a significant reduction in early major adverse cardiac events. In the CADILLAC trial, PTCA alone was compared to PTCA plus abciximab therapy, stenting alone, and stenting plus abciximab therapy. There was no long term benefit from abciximab after primary stenting, although abciximab did reduce the rates of subacute thrombosis and recurrent ischaemia leading to repeated revascularisation of the target vessel during the first few weeks after PTCA or stenting.

On the basis of these trials, there is no definite indication for α_IIbβ₃ antagonists as an adjunct to primary PTCA as it has been demonstrated to have variable effects.

Intravenous α_IIbβ₃ antagonists as adjuncts to lytic therapy in acute myocardial infarction (see table 4)

Combining a α_IIbβ₃ antagonist with a thrombolytic has been evaluated in the management of acute myocardial infarction. The combination of half dose recombinant tissue-type plasminogen activator (rt-PA) and abciximab used in the TIMI 14 angiographic patency study resulted in some of the highest rates of optimal (TIMI-3) flow seen among trials of pharmacological reperfusion.⁶⁵

The large GUSTO V outcome study compared reteplase with half dose reteplase plus abciximab in 16 588 patients. The results were disappointing, with no significant difference in 30 day mortality rates. The reduction in the number of the secondary endpoints in the combination group, including reinfarction and the need for urgent revascularisation was counterbalanced by an increase in bleeding complications and no improvement in the survival rates at one year.⁶⁶

More investigation of the use of α_IIbβ₃ antagonists with lytic therapy in acute myocardial infarction is required before a definite decision on their role can be made.

Oral α_IIbβ₃ antagonists (see table 5)

To date, large trials of oral α_IIbβ₃ antagonists (sibrafiban, orbofiban, and xemilofiban), have reported no obvious benefit and there are excess bleeding complications in those assigned these drugs. Both the Orbofiban in Patients with Acute Coronary Syndromes Thrombolysis in Myocardial Infarction 16 trial (OPUS-TIMI 16) and Sibrafiban Versus Aspirin to Yield Maximum Protection from Ischaemic Heart Events Post-Acute Coronary Syndromes 2 (SYMPHONY 2) trial demonstrated statistically significant increased mortality in the treatment groups.^67,68 Why there should be a detrimental effect compared to intravenous α_IIbβ₃ antagonists remains unclear. The increase in mortality may relate to a prothrombotic effect, unfavourable pharmacokinetics, or indeed there may be paradoxical platelet activation with oral agents. Understanding why there is apparently an adverse effect from the oral agents may lead to the design of better agents and dosing regimens.

Overall the development and use of α_IIbβ₃ antagonists have been crucial in the management of cardiovascular disease. Their benefit when used in conjunction with PCI is undoubted and now is accepted practice. Their use in the management of acute coronary syndromes without mandatory PCI has a lesser effect but has been proven in the majority of trials. Their use must be combined with risk stratification and other antiplatelet agents.

Further development of these agents for the treatment of acute coronary syndromes including acute myocardial infarction is required.

CONCLUSION

Ischaemic heart disease is the biggest killer in the Western world and therefore any treatment that improves its outcome may save large numbers of lives. In recent years much attention has focused on inhibiting platelet aggregation in order to prevent thrombotic occlusion. Currently many agents are available but there is room and evidence for the development of others. Aspirin remains the cornerstone of therapy, although its adverse effects on gastric mucosa contribute to haemostatic compromise. Clopidogrel is beneficial in prevention as well as medical and interventional therapy of acute coronary syndromes. Dipyridamole has not been proven as an effective agent in cardiovascular disease, although it is of benefit in cerebrovascular disease. The α_IIbβ₃ antagonists are effective intravenous agents in PCI but their role in the conservative (non-interventional) management of acute coronary syndromes is more controversial.

Platelets not only interact with each other but also leucocytes and it appears increasingly likely that therapy targeted on this interaction could be effective. Antiplatelet therapy is now an accepted weapon in the arsenal of treatment for vascular disease but further research and developments will increase its efficacy.

MULTIPLE CHOICE QUESTIONS (ANSWERS AT END OF REFERENCES)

In each of the following select the one correct option.

1. Which of the following is not a platelet agonist?

2. ADP

3. Collagen

4. Serotonin

5. Nitric oxide

6. Aspirin exerts its antiplatelet effect by:

7. Blocking the final common pathway of platelet activation

8. Irreversibly inhibiting platelet cyclo-oxygenase

9. Reversible inhibition of platelet ADP receptors

10. Inhibiting the conversion of cAMP to 5′AMP

11. Regarding aspirin therapy, which one of the following is true:

12. Reducing the daily dose from 300 mg to 75 mg has been shown to decrease gastrointestinal complications

13. Aspirin has a very strong antiplatelet effect

14. Aspirin is given with streptokinase in acute myocardial infarction to prevent pyrexia

15. The Antithrombotic Trialists Collaboration confirmed the overall benefit of antiplatelet therapy for secondary prevention in high risk patients

16. Regarding the thienopyridines which one of the following is true:

17. Ticlopidine is as safe as clopidogrel

18. In the CAPRIE trial clopidogrel was found to be not as effective as aspirin for secondary prevention in patients with atherosclerotic disease

19. Clopidogrel acts at the platelet P2Y₁₂ receptor

20. Clopidogrel does not require hepatic metabolism

21. Regarding the CURE Study which one of the following is true:

22. There was an increase in major bleeding in the clopidogrel arm of the trial

23. Patients with acute coronary syndromes were randomly assigned clopidogrel or placebo within four weeks of presentation

24. Patients with ST elevation acute coronary syndromes were recruited

25. Which of the following is true?

26. Dipyridamole should be used in all patients with ischaemic heart disease

27. Dipyridamole exerts its antiplatelet effect by blocking cyclo-oxygenase in platelets

28. Dipyridamole has been shown to have a significant benefit in the secondary prevention of stroke

29. Which one of the following statements about abciximab is true?

30. Abciximab is a monoclonal antibody

31. Abciximab is a non-peptide inhibitor

32. Abciximab’s antiplatelet effect ceases immediately when the infusion is stopped

33. Abciximab is isolated from snake venom

34. Glycoprotein IIb/IIIa receptor antagonists were studied in PCI in which one of the following trials?

35. GUSTO IV

36. PURSUIT

37. PRISM

38. EPIC

39. Which one of the following was not a trial of oral glycoprotein IIb/IIIa receptor antagonists?

40. OPUS

41. SYMPHONY

42. EXCITE

43. CADILLAC

44. Which one of the following statements is true?

45. All trials of glycoprotein IIb/IIIa receptor antagonists as adjuncts to lytic therapy in acute myocardial infarction have shown benefit

46. Intravenous glycoprotein IIb/IIIa receptor antagonists are effective in patients undergoing PCI

47. Oral glycoprotein IIb/IIIa receptor antagonists are now accepted treatment for acute coronary syndromes

ANSWERS

1. D; 2. B; 3. D; 4. C; 5. A; 6. C; 7. A; 8. D; 9. D; 10. B.