Anticoagulation and antiplatelet therapy in acute coronary syndromes

Prasugrel: Faster metabolism to active drug

Prasugrel is an irreversible P2Y12 receptor antagonist (Figure 3) that is metabolized into its active metabolite faster and in a more predictable fashion than clopidogrel.²³

The TRITON-TIMI 38 study²⁴ included 13,608 ACS patients in whom an early invasive strategy was planned and who were pretreated with prasugrel or clopidogrel in addition to standard treatment. The rate of the primary efficacy end point of death, myocardial infarction, or stroke was 19% lower in the prasugrel group. In those who underwent percutaneous coronary intervention, the incidence of in-stent thrombosis was more than 50% lower in the prasugrel group regardless of whether bare metal stents or drug-eluting stents were used.

Greater platelet inhibition came at the price of a higher incidence of serious bleeding, particularly in the subgroups of patients who were over age 75, had a history of stroke or transient ischemic attack, or weighed less than 60 kg. Prasugrel is therefore contraindicated in patients with a history of transient ischemic attack or stroke. Some suggest that a 5-mg dose can be used with caution (rather than the usual 10-mg dose) in patients over age 75 years or those who have low body weight.

The TRILOGY-ACS trial²⁵ compared prasugrel and clopidogrel in medically managed patients with high-risk non-ST-elevation ACS. It found no difference in the rates of the primary end points of cardiovascular death, myocardial infarction, or stroke at 1 year. In the prespecified subset of patients over age 75 years, the rate of bleeding end points was no higher with prasugrel 5 mg once daily than with clopidogrel.

Prasugrel’s half-life is 7 hours, and its peak antiplatelet effect is within 30 minutes after an oral dose, compared with 4 hours with clopidogrel. Therefore, if a patient with non-ST-elevation ACS is going to go to the catheterization laboratory soon, he or she should not receive prasugrel beforehand, and should receive it later only if the results of angiography indicate that CABG will not be needed urgently. This is an important consideration when using prasugrel, as the rate of surgery-related bleeding was four times higher than with clopidogrel. If possible, this drug should be withheld for at least 7 days before CABG.

Ticagrelor, a direct P2Y12 receptor inhibitor

Ticagrelor, a reversible direct inhibitor of the P2Y12 receptor, inhibits adenosine diphosphate-mediated activation and aggregation (Figure 3). It has a median time to peak concentration of 1.3 to 2 hours and a half-life of 9 hours.

The PLATO trial²⁶ enrolled 18,624 patients with ACS who were given either ticagrelor or clopidogrel in addition to standard therapy. At 12 months, the composite primary end point of myocardial infarction, death, or stroke had occurred in 16% fewer patients receiving ticagrelor than in the clopidogrel group. Analyzed separately, there were 16% fewer myocardial infarctions, 21% fewer cardiovascular deaths, and 22% fewer deaths from any cause, regardless of whether an invasive or conservative strategy was used, and with or without prior clopidogrel use. Fewer cases of stent thrombosis occurred in the ticagrelor group, and the rate of major bleeding was the same.

In a prospectively defined subgroup analysis,²⁷ ticagrelor was beneficial only in patients who received lower doses of aspirin (< 100 mg daily): the hazard ratio for the primary end point was 0.79 (95% confidence interval [CI] 0.71–0.88) in ticagrelor recipients who received low-dose aspirin and 1.45 (95% CI 1.01–2.09) in those who received high-dose aspirin.

Although this analysis is underpowered and controversial, the current evidence suggests that when used in combination with ticagrelor, the aspirin dose should be 81 mg.

Ticagrelor was also associated with a 19% higher incidence of non-CABG- or procedure-related major bleeding, more nonfatal and fatal intracranial bleeding, a higher incidence of dyspnea, and significantly more ventricular pauses.

Although ticagrelor carries no black-box warning about its use in patients with prior stroke or transient ischemic attack, the number of such patients in PLATO was small. Thus, caution should still be used in these patients.²⁸

Ticagrelor should preferably be discontinued 5 days before CABG.

Glycoprotein IIb/IIIa inhibitors: Eptifibatide, tirofiban, abciximab

Glycoprotein IIb/IIIa inhibitors are intravenous agents that act by inhibiting fibrinogen-and von Willebrand factor-mediated platelet-to-platelet cross-linkage, the final pathway of platelet aggregation (Figure 3).

Use of these agents in ACS has been decreasing, as evidence supporting their use was largely established before the era of dual antiplatelet therapy.

A meta-analysis29 of 46,374 patients with non-ST-elevation ACS found that routinely adding a glycoprotein IIb/IIIa inhibitor “upstream” as a third agent in patients receiving dual antiplatelet therapy bought only a modest (11%) reduction in death or myocardial infarction at 30 days, at the price of a 23% increase in major bleeding and no decrease in the overall rate of death. Roughly 70% of the patients were receiving dual antiplatelet therapy before cardiac catheterization.

These agents can be considered in high-risk ACS patients, such as those with ST-segment changes or elevated troponin concentrations, and in diabetic patients, on the assumption that these patients likely have a high intracoronary thrombus burden and are at higher risk of microvascular embolization.^6,30 They can also be considered at the time of primary percutaneous coronary intervention in selected patients receiving heparin.⁷

Eptifibatide

Eptifibatide is a small-molecule, short-acting glycoprotein IIb/IIIa inhibitor with a half-life of 2.5 hours. Its inhibition of platelet aggregation is reversible by stopping the drug infusion and is thought to be a result of dissociation of the drug from platelets.

The PURSUIT trial³¹ studied 10,948 patients presenting with non-ST-elevation ACS randomized to placebo, eptifibatide in a 180-μg/kg bolus followed by a 2.0-μg/kg/min infusion, or eptifibatide in a 180-μg/kg bolus followed by a 1.3-μg/kg/min infusion. Both eptifibatide groups had a 1.5% absolute reduction in the incidence of the primary end point of death or myocardial infarction, a benefit that was apparent at 96 hours and that persisted through 30 days. Bleeding was more common in the eptifibatide groups, but there was no increase in the rate of hemorrhagic stroke.

The ACUITY trial³² found that early use of eptifibatide or tirofiban had no effect on the primary outcome. (See the section below on bivalirudin for more information about the ACUITY trial.)

PARENTERAL ANTICOAGULANTS

Unfractionated heparin: A declining role

Heparin binds to antithrombin and induces a conformational change, causing rapid inhibition of factor IIa (thrombin), factor IXa, and factor Xa, thus preventing further thrombus propagation (Figure 4). An intravenous bolus of 60 units/kg produces a time to peak of 5 to 10 minutes and a half-life of 30 to 60 minutes.

Heparin can be reversed by giving protamine sulfate (1 mg per 100 units of heparin). For ACS, it is given in a bolus of 60 units/kg not exceeding 4,000 units, followed by an infusion of 12 units/kg/hour, with monitoring of the activated partial thromboplastin time every 6 hours with a goal value of 50 to 70 seconds or 1.5 to 2.5 times control.

Side effects include thrombocytopenia, heparin-induced thrombocytopenia (a distinct condition), and bleeding.

The use of unfractionated heparin was tested in ACS in the early 1990s. Oler et al³³ performed a meta-analysis of six randomized trials and found a 33% lower rate of death in patients treated with heparin in addition to aspirin in ACS, as well less reported ischemic pain.

Advantages of unfractionated heparin are that it has stood the test of time, is inexpensive, and can be rapidly reversed. The disadvantages are that it can have serious side effects, including heparin-induced thrombocytopenia, and is more likely to cause bleeding than the newer intravenous anticoagulants discussed below. Thus, its position as the main anticoagulant in ACS is being challenged.