Oral antiplatelet agents in ACS: from pharmacology to clinical differences

Advances in antiplatelet technologies to improve cardiovascular disease morbidity and mortality: a review of ticagrelor

Antiplatelet therapy is widely used with proven benefit for the prevention of further ischemic cardiac complications in patients with acute coronary syndrome. Treatment guidelines for acute coronary syndrome and percutaneous coronary intervention now recommend the use of oral antiplatelet agents including ticagrelor, prasugrel, or clopidogrel in combination with aspirin to comprise dual antiplatelet therapy for the prevention of recurrent ischemic events. The limitations of conventional antiplatelet therapy with clopidogrel or prasugrel include the potential for low response to clopidogrel identified through platelet reactivity or genetic testing, increased risk of bleeding with prasugrel, or slower return to normal platelet activity in patients who received either prasugrel or clopidogrel prior to emergent or planned surgical procedures. This review will discuss the pharmacokinetic and pharmacodynamic properties of ticagrelor in comparison to conventional P2Y12 receptor inhibitors and its utility in patients identified as low responders to clopidogrel. Completed clinical studies and substudies comparing ticagrelor to clopidogrel and ongoing clinical trials evaluating ticagrelor in acute coronary syndrome patients will also be reviewed.

Investigation of drug-drug interactions between clopidogrel and fluoxetine

Drug-drug interactions may contribute to the variability of the response of clopidogrel. Several hypotheses have been proposed concerning the potential modification of clopidogrel pharmacokinetics and pharmacodynamics by fluoxetine. This open-label crossover study assessed the effect of fluoxetine on the pharmacological activity of clopidogrel in healthy volunteers. Eight healthy male volunteers received a single 600-mg loading dose of clopidogrel followed by 20 mg of fluoxetine on 4 days and then 20 mg of fluoxetine plus 600 mg of clopidogrel on the fifth day. Eleven blood samples were withdrawn after clopidogrel administration to determine plasma concentrations of clopidogrel active metabolite (CAM) and platelet function. Platelet aggregation was measured by light transmittance aggregometry (LTA) and platelet reactivity index by flow cytometric vasodilator-stimulated phosphoprotein (VASP) analysis. The areas under the curve and maximum plasma concentrations of CAM were, respectively, 20.6 and 25.3% lower after co-administration of fluoxetine compared with administration of clopidogrel alone. The percentage maximum platelet aggregation values in the presence of 5 μM and 10 μM adenosine diphosphate, measured by LTA, were, respectively, 13.9 and 22.4% lower after fluoxetine co-administration. The platelet reactivity index measured by the flow cytometric VASP method was 36.8% lower when clopidogrel was administered in conjunction with fluoxetine.

Current antiplatelet options for NSTE-ACS patients

Non-ST elevation (NSTE) myocardial infarction and unstable angina are the most common clinical presentations of acute coronary syndrome (ACS). Platelet activation is central to the pathogenesis of NSTE-ACS and consensus guidelines that advocate early revascularization supported by intensive antiplatelet therapy. This review examines the drugs used concurrently with aspirin as dual antiplatelet therapy in the NSTE-ACS setting. Clopidogrel represented an important therapeutic advance. However, variations in platelet response and a relatively slow onset of action compromise outcomes with clopidogrel. Evidence reviewed in this article shows that in NSTE-ACS patients, ticagrelor and prasugrel are more effective than clopidogrel and are relatively well tolerated, with an acceptable and manageable bleeding risk. The literature suggests several differences between ticagrelor and prasugrel that should allow clinicians to better tailor treatment to the patient. Head-to-head comparisons are now needed to compare directly the risks and benefits of ticagrelor and prasugrel in NSTE-ACS. Further studies also need to address other outstanding issues such as the benefits and risks of prasugrel pre-treatment and to stratify efficacy and tolerability according to diabetes mellitus (DM) and other co-morbidities. In the meantime, the issues discussed in this review should enhance clinicians' ability to optimize and individualize NSTE-ACS treatment, thereby further reducing the morbidity and mortality associated with this common cardiovascular condition.

Antiplatelet therapy in patients with ST-elevation myocardial infarction undergoing myocardial revascularisation: beyond clopidogrel

BACKGROUND

Despite revascularisation, outcomes among patients presenting with ST-elevation myocardial infarction (STEMI) remain suboptimal.

SCOPE

This review compares clopidogrel, ticagrelor and prasugrel as antiplatelet strategies with a particular focus on STEMI. Medline and Google Scholar were searched for relevant terms and citations from these articles were also assessed.

FINDINGS

While clopidogrel represented an important therapeutic advance, variations in platelet response and a relatively slow onset of action compromise outcomes in some patients. Ticagrelor and prasugrel are more effective than clopidogrel, although essentially only one large study supports each drug. Nevertheless, a detailed examination of the evidence reveals several issues that may influence the decision to prescribe ticagrelor instead of prasugrel and vice versa. Arguably, prasugrel could be the preferred strategy in STEMI, reflecting the drugs' efficacy in clopidogrel-naïve patients, the most common group in clinical practice. Conversely, ticagrelor may be a better option than clopidogrel in clopidogrel-pretreated patients showing a mortality benefit irrespective of clopidogrel pre-treatment. The clinical benefits offered by prasugrel and ticagrelor need to be offset against the increased cost and we suggest an algorithm for using these new compounds in the primary percutaneous coronary intervention (PCI) setting. The risk of bleeding associated with prasugrel is similar to that of clopidogrel and ticagrelor following exclusion of at-risk patients. Nevertheless, prasugrel may be especially appropriate for STEMI patients undergoing PCI who are considered to be at high risk of ischaemia. Conversely, ticagrelor's short half-life, while potentially a limitation during maintenance therapy, may reduce bleeding risk if the patient undergoes CABG during the same hospital admission, although confirmatory studies are needed.

CONCLUSION

Future studies also need to address several other outstanding issues, such as the subsequent approach if patients do not undergo PCI, and to overcome limitations in and differences between the primary studies. In particular, head-to-head comparisons need to compare directly the risks and benefits of ticagrelor and prasugrel in STEMI patients. These caveats notwithstanding, ticagrelor and prasugrel markedly improve the prognosis for patients with STEMI.