ISCHEMIC HEART DISEASE (S BRENER, SECTION EDITOR)

Platelet Function and Inhibition in Ischemic Heart Disease

Annunziata Nusca & Giuseppe Patti

Published online: 5 May 2012# Springer Science+Business Media, LLC 2012

Abstract Platelets play an important role in the pathogen-esis of thrombosis, the most common cause for the devel-opment of acute coronary syndromes such as complicationsoccurring during percutaneous coronary intervention.Platelets act with a multiple step mechanism, in whichdifferent surface molecules are involved representing impor-tant therapeutic targets of antiplatelet agents. Despite clopi-dogrel efficacy which has been demonstrated in severalstudies, recurrent ischemic events remain considerably highin patients on treatment due to low clopidogrel responsive-ness, a phenomenon influenced by environmental, clinical,and genetic factors. New P2Y12 blockers such as prasugreland ticagrelor have been successfully introduced in clinicalpractice, whereas cangrelor, with a rapid offset and revers-ible platelet inhibition, may represent a useful bridgingtherapy in patients undergoing surgery. Moreover, thesimultaneous inhibition of thrombin platelet aggregationby protease-activated receptor inhibitors may be an adjunc-tive approach in patients with coronary artery disease.

Keywords Platelets . Antiplatelet agents . Percutaneouscoronary intervention . Clopidogrel . Clopidogrelresistance . High residual platelet reactivity . Point-of-careplatelet function assays . Prasugrel . Ticagrelor . Cangrelor .

Thrombin receptor antagonists . Platelet function .

Inhibition . Ischemic heart disease

Introduction

Platelets play an important role both in the pathogenesis ofatherosclerosis and subsequent thrombosis [1–3]. Followingfibrous cap’s rupture of the atherosclerotic plaque, prothrom-botic materials are exposed to the circulating blood leading tothe development of a platelet-rich thrombus over the disruptedlesion. Platelets contribute to acute thrombosis with a multiplestepmechanism [1, 2]: the first is adhesion to the endothelium.The interaction occurs between the constituents of the exposedsubendothelium, including collagen, von Willebrand factor,fibronectin, and specific platelet surface membrane receptors;thus, platelets overcome the high blood shear forces and attachthemselves to the target endothelium site. After attachment,platelets present an activation process with a specific confor-mational change that induces the onset of multiple internalsignaling networks. Adenosine diphosphate (ADP) andthrombin receptors on platelet surface are mainly responsiblefor this important phase. The ADP receptor (Fig. 1) binds tothe G-protein-coupled receptors P2Y1 (Gq/phospholipase C/Ca2 pathway, activating platelet shape change by inhibition ofphospholipase C), and P2Y12 (Gi/adenylyl cyclase pathway),reducing adenylyl cyclase activity, which leads to phosphoi-nositide 3 kinase activation and additional attachment of otherplatelets. Thrombin action is mediated by the protease-activated receptors (PARs), which are also G-protein-coupled receptors mediating paracrine and autocrine plateletpathways. Finally, platelet activation due to link of ADP andthrombin with specific platelet surface receptors is associatedwith granule release, expression of proinflammatory mole-cules such as P-selectin, and increased exposure of activatedglycoprotein IIb/IIIa, a receptor involved in a homotypicplatelet-platelet interaction with an αIIbβ3 receptor of anotherplatelet, definitely promoting platelet aggregation and devel-opment of a platelet-rich thrombus (Fig. 1) [1–3].

A. Nusca :G. Patti (*)Department of Cardiovascular Sciences,Campus Bio-Medico University of Rome,Via Alvaro del Portillo 200,00128 Rome, Italye-mail: g.patti@unicampus.it

A. Nuscae-mail: a.nusca@unicampus.it

Curr Cardiol Rep (2012) 14:457–467DOI 10.1007/s11886-012-0280-z

Thrombotic events represent the most common causefor the onset of acute coronary syndromes (ACS) andalso play a key role in the complications occurringimmediately after percutaneous coronary intervention(PCI), including recurrent ischemic events, periproce-dural myocardial infarction (PMI), or stent thrombosis.Their importance in ischemic heart diseases is indirectlyconfirmed by the clinical benefit due to antiplatelettherapy; in particular, the higher the risk profile inACS patients, the higher the prevention of ischemicadverse events provided by antiplatelet drugs. Moreover,clinical evidence suggests that a high platelet count oran elevated platelet reactivity despite specific treatmentmay be associated with an increased incidence of adversecardiovascular events in patients with coronary arterydisease [4–8].

Current Status of Antiplatelet Therapy

Aspirin, an inhibitor of the cyclooxygenase enzyme regulat-ing thrombosis via thromboxane A2 formation, is the oldestand most commonly used antiplatelet agent. Unfortunately,many patients continue to experience atherothromboticevents despite treatment with this drug. Thus, more potentantiplatelet agents have been studied and successfullyapplied in clinical practice in addition to aspirin therapy.Clopidogrel is a prodrug that requires a two-step hepaticcytochrome P (CYP) 450-dependent conversion to an activethiol metabolite that irreversibly binds the P2Y12 plateletreceptor inhibiting ADP-induced platelet aggregation. Useof clopidogrel has been associated with better safety profileand clinical efficacy than ticlopidine, a first-generation thie-nopyridine [9]; maximal inhibition after clopidogrel ranges

Fig. 1 Molecular mechanisms of action of different antiplatelet agents.AAarachidonic acid; ADCY3 adenylate cyclase type 3 enzyme; ADPadenosine diphosphate; AMP adenosine monophosphate; ATP adeno-sine triphosphate; cAMP cyclic adenosine monophosphate; COX-1cyclooxygenase 1; GP glycoprotein; Gs/Gg, GI, Gq—members of Gprotein family; IP3 inositol 1,4,5-trisphosphate; PAR protease-activated

receptor; PG prostaglandin; PGD2 prostaglandin D2; PGE2 prosta-glandin E2; PGG2 prostaglandin G2; PGH2 prostaglandin H2; PGI2prostacyclin; PLA2 phospholipase A2; P2Y12 and P2Y1-ADP plateletreceptors; TxA2 thromboxane A2; TxA2 R thromboxane A2 receptor;TxAS1 thromboxane synthase 1; VASP vasodilator-stimulated phos-phoprotein; VWF von Willebrand factor

458 Curr Cardiol Rep (2012) 14:457–467

from 40 % to 60 % and it is reached in 3 to 7 days after astandard 75-mg dose, in 8 to 12 h after a 300-mg loadingdose, and in 2 to 4 h after a 600-mg loading dose [10–12].

The beneficial effects of clopidogrel in addition to aspirinin patients with coronary artery disease have been wellestablished in several randomized trials. In the CURE (Clo-pidogrel in Unstable Angina to Prevent Recurrent Events)[13] trial, clopidogrel (300-mg loading dose, then 75 mg/day for a mean duration of 9 months) has been demonstratedto reduce the relative risk of cardiovascular death, nonfatalmyocardial infarction (MI), or stroke by 20 % versus aspirinalone in 12,562 patients with non-ST-segment elevation(NSTE)-ACS, with the greatest benefit observed in the rateof reinfarction (5.2 % vs 6.7 %). In the subgroup of patientsreceiving PCI (PCI-CURE study, N02658) [14], the benefitof clopidogrel was even higher, with 30 % relative riskreduction of death, MI, and repeat revascularization. Finally,the CREDO (Clopidogrel for the Reduction of Events Dur-ing Observation) [15] trial showed a 38 % risk reduction ofmajor adverse cardiac events (death, MI, urgent target vesselrevascularization [TVR]) at 1 month in patients undergoingelective or urgent PCI and pretreated with a 300 mg-loadingdose of clopidogrel versus initiation of clopidogrel at thetime of the procedure. On the basis of this evidence, the300-mg loading dose of clopidogrel given at least 6 h beforePCI plus aspirin had represented for many years the con-ventional antiplatelet strategy in patients with ACS, as wellas in those undergoing elective coronary angioplasty. How-ever, experimental and clinical studies showed that a higherclopidogrel loading regimen (600 mg) may provide a morerapid and strong platelet suppression versus the convention-al 300-mg dose, and this translates into significant reductionof early ischemic events.

The ARMYDA-2 (Antiplatelet Therapy for Reduction ofMyocardial Damage During Angioplasty) [16] trial was thefirst randomized study of head-to-head comparison betweenpretreatment with 600-mg and 300-mg loading dose ofclopidogrel before coronary intervention in 255 patientswith stable or unstable coronary disease. Primary end pointof the ARMYDA-2 trial (30-day occurrence of death, MI,TVR) was detected in 4 % of patients in the high loadingdose and in 12 % of those in the conventional dose arm(P00.041); multivariable analysis showed a 52 % risk re-duction of PMI in patients pretreated with the 600-mgclopidogrel strategy (OR 0.48, 95 % CI, 0.15–0.97; P00.044). These results were then confirmed by a meta-analysisof 10 studies consisting of 1500 patients receiving differentclopidogrel loading doses (712 loaded with 300 mg, 11 with450 mg, 790 with 600 mg, and 54 with 900 mg) [17]; 600-mgload was superior to the 300-mg dose in preventing cardiacdeath or nonfatal MI (OR 0.54; 95 % CI, 0.32–0.90; P00.02),with no excess in major and minor bleeding. Accordingly, thelatest guidelines suggest a 600-mg load when PCI has to be

performed early (<6 h) after clopidogrel initiation [18]. Todate, no potential benefit of a loading dose greater than600 mg has been demonstrated in clinical and platelet functionstudies; in particular, in the ALBION (Assessment of the BestLoading Dose of Clopidogrel to Blunt Platelet Activation,Inflammation and Ongoing Necrosis) [19] and ISAR-CHOICE (Intracoronary Stenting and Antithrombotic Regi-men: Choose Between 3 High Oral Doses for ImmediateClopidogrel Effect) [20] studies, 900-mg loading dose doesnot provide a higher inhibition of platelet aggregation com-pared to 600 mg, probably due to limited clopidogrel absorp-tion and therefore limited availability of active metabolite.

During the last decade other crucial issues regardingoptimal clopidogrel strategy in the context of PCI have beeninvestigated. An additional loading dose of clopidogrelgiven to patients on chronic therapy with 75 mg/day wasassociated with significant benefit in higher-risk patientsenrolled in the randomized ARMYDA-4 study [21]; in aseries of 503 patients undergoing PCI on chronic clopidog-rel therapy, the incidence of 30-day major adverse cardiacevents (MACE) did not differ between patients who receivedreload with 600 mg of clopidogrel 4 to 8 h prior to PCI andthose who did not (6.7 % vs 8.8 %; P00.5). However, in thesubgroup of patients with ACS, a significant reduction inMACE rate was observed in the reload arm (6.4 % vs16.3 %; OR 0.34, 95 % CI, 0.32–0.90; RRR 66 %; P00.033). The ARMYDA-5 PRELOAD [22] study investigatedif 600 mg of clopidogrel in-laboratory loading, immediatelyafter diagnostic angiography and before PCI, may provideadequate antiplatelet effect and similar clinical protection fromperiprocedural ischemic complications versus pretreatmentwith 600 mg of clopidogrel given several hours preprocedure.This represents a relevant issue, as upstream preload with highclopidogrel dose might increase bleeding risk in patientsneeding urgent surgical revascularization after diagnostic cor-onary angiography. Results of this trial showed no significantreduction of 30-day MACE in patients randomized to up-stream 600-mg preload versus those receiving clopidogrelload in the catheterization laboratory after coronary angiogra-phy, but prior to PCI (8.8 % in-laboratory vs 10.3 % preload;P00.72), with no difference of bleeding complications in thetwo arms. Similar results were observed also in the PRAGUE-8 (Primary angioplasty vs thrombolysis for acute myocardialinfarction-8) trial [23] on patients with stable angina undergoingelective PCI.

Finally, an increased maintenance dose of clopidogrelmay also contribute to reduce platelet reactivity in patientsreceiving this drug after PCI. In the ISAR-CHOICE 2 (Intra-coronary Stenting and Antithrombotic Regimen: Choose aHigh Oral maintenance Dose for Intensified ClopidogrelEffect) study, von Beckerath et al. [24] observed thatadministration of 150-mg clopidogrel maintenance dosewas associated with more intense inhibition of ADP platelet

Curr Cardiol Rep (2012) 14:457–467 459

aggregation versus the standard 75-mg/day dose at 30 daysafter PCI; data from the OPTIMUS (Optimizing AntiplateletTherapy in Diabetes Mellitus) [25] study also demonstratedthat the 150-mg clopidogrel maintenance dose provokesstronger antiplatelet effects in patients with diabetes melli-tus. Finally, in a recent randomized study from theARMYDA group [26], 50 patients were randomly assignedafter PCI to receive standard (75 mg/day; n025) or high(150 mg/day; n025) clopidogrel maintenance dose for30 days; patients receiving the higher regimen showedhigher platelet inhibition by a point-of-care assay, betterflow-mediated vasodilation, and lower high-sensitive C-reactive protein levels; of note, the percentage of patientlow responders was significantly decreased in the 150-mgarm (12 % vs 32 %; P00.001).

The largest, randomized, prospective trial investigating theefficacy and safety of higher clopidogrel loading and mainte-nance doses is CURRENT-OASIS 7 (Clopidogrel and AspirinOptimal Dose Usage to Reduce Recurrent Events—SeventhOrganization to Assess Strategies in Ischemic Symptoms) [27].The trial, enrolling 25,086 patients with ACS undergoinginvasive strategy, had a 2-by-2 factorial model. In the firstcomponent of the factorial design, patients were randomlyassigned to double-dose regimen of clopidogrel (600-mg loadfollowed by a maintenance dose of 150 mg/day for 6 days andthen 75 mg/day thereafter) or to a standard-dose regimen (300-mg load followed by 75 mg daily thereafter). In the secondcomponent of the factorial design, patients randomly receivedhigher-dose aspirin or lower-dose aspirin. The primary out-come (death, MI, or stroke at 30 days) occurred in 4.4 % ofpatients assigned to standard-dose versus 4.2 % of those in thedouble-dose group, with a modest excess of bleeding in thelatter (2.5 % vs 2.0%; HR 1.24; 95 % CI, 1.05–1.46; P00.01).However, in the subgroup of patients undergoing PCI (n017,263) [28•], the double-dose regimen significantly reducedthe rate of the primary end point (3.9 % vs 4.5 %; HR 0.86,95 % CI, 0.74–0.99; P00.039) and stent thrombosis (0.7 % vs1.3 %; HR 0.54, 95 % CI, 0.39–0.74; P00.0001).

Limitations of Clopidogrel Use and Clinical Impactof High Residual Platelet Reactivity

Despite those favorable findings, up to 15 % of ACSpatients continue to suffer from ischemic events, whichmight be at least in part related to the high inter-individualvariability in clopidogrel responsiveness [6, 29–32]. Thisphenomenon is surely multifactorial and largely influencedby environmental, clinical, and genetic factors [33, 34].Older age, elevated body mass index, diabetes, ACS atpresentation, drug–drug interactions, and patient compli-ance have been invoked to explain a low response to clopi-dogrel (ie, high on-treatment platelet reactivity); moreover,

genetic polymorphisms influencing drug absorption, varia-tions in biotransformation rate into active metabolite, andlinkage to P2Y12 receptor have been also involved. Clopi-dogrel is converted into the active metabolite in a two-stepoxidative process catalyzed by different CYP enzymes [35].The genes encoding for these enzymes are polymorphic andthere is evidence that some allelic variants of these enzymesgreatly influence clopidogrel response and are linked to anincreased risk of adverse cardiovascular events. This is thecase of the CYP2C19 variant, an enzyme that actively con-tributes to both oxidative steps generating the active clopi-dogrel metabolite. The CYP2C19 gene is located onchromosome 10 and at least 25 genetic variants of this genehave been described [35]. Among these, two loss-of-function variant alleles, CYP2C19*2 and *3, represent themajority of the defective genotypes. In a large observationalcohort of 1477 ACS patients receiving clopidogrel in theTRITON-TIMI 38 (Trial to Assess Improvement in Thera-peutic Outcomes by Optimizing Platelet Inhibition withPrasugrel-Thrombolysis in Myocardial Infarction) [36, 37]trial, CYP2C19 loss-of-function allele carriers had a higherrate of the primary end point (death from cardiovascularcauses, MI, stroke) versus noncarrier patients (12.1 % vs8.0 %; HR 1.53, 95 % CI, 1.07–2.19, P00.01).

In consideration of the great emphasis related to the con-cept of clopidogrel nonresponsiveness, an important issueconcerns techniques for identifying low-responder patients,laboratory definitions of high residual platelet reactivity, andcorrelation between poor response to clopidogrel and adverseevents during follow-up [38]. Light transmittance aggregom-etry (LTA) is considered the gold standard to evaluate clopi-dogrel responsiveness. However, it is not routinely available,requires special training, and complex sample preparation;moreover, it is not specific, because LTA also measures ag-gregation due to binding of ADP to P2Y1 platelet receptor,which is not inhibited by clopidogrel [39]. Vasodilator-stimulated phosphoprotein phosphorylation by flow cytome-try (VASP) is more specific, but also in this case logisticproblems, such as need for long sample preparations, skilledpersonnel, and elevated costs, may limit its routine use [39]. Inthe last years, point-of-care assays for platelet function mea-surement have become available, including the VerifyNow(Accumetrics, Inc. San Diego, CA) system, a rapidturbidimetry-based optical detection device. It measures therate of ADP-mediated platelet aggregation by changes in lighttransmission and does not require any sample preparation orspecific laboratory training. A close correlation betweenresults of LTA and absolute P2Y12 reaction unit (PRU) valueby VerifyNow has been reported in patients undergoing PCI[39]. Finally, a relatively new point-of-care system is repre-sented by multiple electrode aggregometry (MEA), assessedby the Multiplate Analyzer (Verum Diagnostica GmbH,Munich, Germany). This is also a rapid and standardized

460 Curr Cardiol Rep (2012) 14:457–467

method for the assessment of platelet function based on theprinciples of impedance aggregometry; it is a simple, whole-blood test, without need of centrifugation, which is signifi-cantly correlated with LTA and VerifyNow [40].

Several studies evaluated the clinical impact of residualplatelet reactivity, differently measured by all these techni-ques, on short- and long-term prognosis of patients withischemic heart disease undergoing PCI. In the ARMYDA-PRO (ARMYDA-Platelet Reactivity Predicts Outcome) [8]study, high platelet reactivity after clopidogrel administra-tion, measured by the VerifyNow P2Y12 assay at the time ofintervention, was associated with higher incidence of 30-dayMACE after PCI (20 % in patients with high residualplatelet reactivity vs 3 % in those without, P00.034); thisoutcome was mainly driven by an increased incidence ofPMI in the first group. The receiver operating characteristic(ROC) curve analysis indicated an optimal cutoff of ≥240PRU to discriminate patients at higher risk of adverseevents. This threshold was similar to that observed by Priceet al. [41] in another series of 380 patients undergoing PCI;in this study, a PRU cutoff ≥235 appeared to be predictive ofcardiovascular death or stent thrombosis during follow-upafter drug-eluting stent implantation. Recently, a large col-laborative patient-level meta-analysis [42••] from six studiesevaluated the impact of platelet reactivity measured by theVerifyNow assay on adverse cardiovascular events in 3059patients undergoing PCI. The primary end point (death, MI,and stent thrombosis) occurred more frequently in highquartiles of PRU (5.8 % in quartile I, 6.9 % in quartile II,10.9 % in quartile III, and 15.8 % in quartile IV; P<0.001).Every 10-unit increase in PRU was associated with a 4 %relative elevation in the primary end point rates (HR: 1.04;95 % CI, 1.03–1.06; P<0.0001), and a cutoff ≥230 wasassociated with a greater than twofold higher risk of MACE(HR: 2.10; 95 % CI, 1.62–2.73; P<0.0001).

On the basis of this evidence, an emerging approach toincrease efficacy of antiplatelet therapy might be to performan individual “tailored” strategy, according to the measure-ment of platelet reactivity by platelet function assays. Aplatelet reactivity-guided therapy was first evaluated byBonello et al. [43], who investigated the concept of adjust-ing clopidogrel loading dose according to platelet functionmonitoring by VASP index. Despite the small sample size,use of VASP-guided clopidogrel dosing significantly im-proved clinical outcome after PCI in patients with lowresponse to clopidogrel already treated with 600-mg load,without increase in the risk of major bleeding. Recently, theGRAVITAS (Gauging Responsiveness with a VerifyNowAssay: Impact of Thrombosis and Safety) [44••] trial inves-tigated whether high-dose clopidogrel is superior tostandard-dose therapy for the prevention of cardiovascularevents after PCI in patients with high on-treatment reactivityaccording to the VerifyNow test. In this study, 2214 patients

on clopidogrel with PRU greater than 230 after PCI wererandomly assigned to high-dose (600-mg reload followedthereafter by 150-mg/day maintenance dose for 6 months)versus standard-dose clopidogrel (no additional loading dosefollowed by 75-mg/day regimen). However, 6-month resultsof this study demonstrated a similar rate in the primary endpoint (death from cardiovascular causes, nonfatal MI, or stentthrombosis) between the two treatment arms (2.3 % vs 2.3 %;HR 1.01, 95 % CI, 0.58–1.76; P00.97), without excess inbleeding events in the high-dose group.

New P2Y12 Antagonists and the Balance BetweenThrombotic and Hemorrhagic Risk

Pharmacological limitations of clopidogrel and the substan-tial inter-individual variability in the response may reduceclinical efficacy of this drug, especially in the context ofhigh-risk patients with coronary artery disease. Thus, novel,more potent and with a rapid onset of antiplatelet action,P2Y12 receptors antagonists were introduced and investi-gated in clinical studies. These new drugs appeared to havesuperior pharmacological profiles and to provide greaterplatelet inhibition than clopidogrel. However, if maximizingantiplatelet therapy significantly reduces the ischemic riskof patients, on the other side it may provoke a higher rate ofbleeding complications. Of note, hemorrhagic risk hasemerged as an important issue that must be carefully con-sidered in patients undergoing “aggressive” antithrombotictherapy, especially for the recent data concerning the prog-nostic implications of bleeding, including effects on mortal-ity. Eikelboom et al. [45] analyzed individual patient datafrom a large dataset involving more than 30,000 patientsfrom three studies: the OASIS (Organization to AssessIschemic Syndromes) registry, OASIS-2, and the CURE.In this analysis, 783 (2.3 %) patients developed majorbleeding, with an increased fivefold incidence of deathduring the first 30 days (12.8 % vs 2.5 %; P<0.0001; HR5.37, 95 % CI, 3.97–7.26, P<0.0001). In the ACUITY(Acute Catheterization and Urgent Intervention Triage Strat-egy) [46] trial, 1-month mortality was greater than sixfoldhigher in patients with versus those without major bleeding(7.3 % vs 1.2 %, P<0.0001). Moreover, in this study patientswith major bleeding also had significantly higher incidence ofischemic events (MI, unplanned revascularization, and stentthrombosis) versus patients without this complication; thismay be explained by the need for a premature discontinuationof antiplatelet drugs when a major bleeding occurs.

In consideration of the strong clinical impact of hemor-rhagic events, identifying patients at higher risk of thesecomplications represents an important objective in clinicalpractice. Besides the formulation of several risk scores basedon patients’ characteristics, two recent studies demonstrated

Curr Cardiol Rep (2012) 14:457–467 461

an association between low residual platelet reactivity andhigher risk of bleeding, suggesting the usefulness of point-of-care assays also in this setting. Sibbing et al. [47] usedMEA to evaluate platelet aggregation in a population of 2533patients, in whom bleeding rate was increased in patients withhigh degree of clopidogrel responsiveness; in turn, incidenceof stent thrombosis was elevated in patients with low responseto this drug. In the ARMYDA-BLEEDS [48] study, 310patients were prospectively enrolled and clopidogrel respon-siveness evaluated by the VerifyNow assay immediatelybefore PCI. Patients in the lowest PRU quartile had a higherincidence of major bleeding at 1 month (10.1 %) versus thosein the highest quartile (1.4 %, P00.05). ROC analysis identi-fied a PRU value ≤189 as an optimal cutoff point to predictbleeding outcome, with sensitivity of 87 % and specificity of70 % (area under the curve 0.76; 95 % CI, 0.66–0.87, P00.001). The ARMYDA-BLEEDS, by defining the lowerthreshold for bleeding (PRU≤189) represents the “pendant”of the ARMYDA-PRO [8], in which a cutoff point of PRU≥240 was identified as a threshold associated with increasedischemic risk; thus, a favorable PRU range (190–239 PRU)may potentially represent the optimal therapeutic window forclopidogrel treatment. In particular, the incidence of ischemicand bleeding events according to PRU values follows a cur-vilinear distribution, in which, below a certain safety thresholdof PRU, ischemic events are not further reduced, to theexpense of increasing bleeding, and above an efficacythreshold, bleeding is not reduced, but ischemic events may besignificantly increased.

The new antiplatelet agents we are going to discuss aredrugs currently at different stages of clinical development;they are distinguished by pharmacodynamics features, routeof administration, and reversibility.

Prasugrel is a third-generation thienopyridine. After inges-tion, prasugrel is hydrolyzed in the gastrointestinal systeminto an intermediary metabolite, a prodrug requiring hepaticconversion. However, this process needs only one CYP450-dependent oxidative step to generate the active metabolite andthis difference explains the more rapid onset of action com-pared to clopidogrel, the greater inhibition of platelet aggre-gation, the lower incidence of nonresponders, and the lesserinfluence of genetic polymorphisms [49]. It is an irreversiblereceptor blocker; its peak concentration is reached after30 min and 60 % to 70 % of platelet inhibition is usuallyachieved after 2 to 4 h after drug administration [50]. Theclinical efficacy of prasugrel was initially evaluated in thephase II PRINCIPLE-TIMI 44 (Prasugrel in Comparison toClopidogrel for Inhibition of Platelet Activation and Aggre-gation—Thrombolysis in Myocardial Infarction 44) [51]study: patients undergoing planned PCI were randomized toprasugrel (60-mg loading dose and 10 mg thereafter) or clo-pidogrel (600-mg load and 150 mg thereafter). Use of prasu-grel was associated with enhanced platelet inhibition versus

clopidogrel, with nonsignificant increase in bleeding events.These results were confirmed in the more recent phase IIITRITON-TIMI 38 [52] study, which compared the efficacyand safety of prasugrel (60-mg loading dose, 10-mg dailymaintenance dose) and clopidogrel (300-mg loading dose,75-mg daily maintenance dose) in 13,608 patients with ACSundergoing PCI; of note, the administration of the assigneddrug was essentially made after knowing coronary anatomy(ie, after diagnostic angiography). Over a median follow-up of14.5 months, patients pretreated with prasugrel showed sig-nificantly lower incidence of the primary end point (cardio-vascular death, MI, or stroke) versus the clopidogrel group(9.9 % vs 12.1 %, P<0.001; 19 % risk reduction; numberneeded to treat 46); this difference was mainly entirely due tothe reduction in the occurrence of nonfatalMI. Of note, a 52%risk reduction of stent thrombosis was found in the prasugrelarm. However, the greater antiplatelet inhibition obtained withprasugrel needs to be weighed against an increase in hemor-rhagic complications: the incidence of non-coronary arterybypass graft (CABG)-related TIMI major bleeding was sig-nificantly higher in the prasugrel group (2.4 % vs 1.8 % inclopidogrel; P00.03). A further analysis from this study sug-gested a strong net benefit of prasugrel in patients with diabe-tes mellitus or ST-segment elevation MI (both conditionscharacterized by high basal platelet reactivity) (ie, reductionof ischemic events without increased bleeding); conversely, amore pronounced risk of bleeding with this drug was observedin patients with previous history of cerebrovascular events,age≥75 years, or body weight less than 60 kg.

Ticagrelor is an oral, reversible, short-acting, non-thienopyridine P2Y12 antagonist. One of the advantagesof this molecule is that has a direct action and is lessdependent on metabolic activation. The maximum plateletinhibition is reached in 1 to 3 h and its half-life is 6 to 13 h[50]. In platelet aggregation studies, ticagrelor inhibitedplatelet aggregation more effectively than clopidogrel, withlower degree of inter-individual response variability [53]. Inthe DISPERSE-2 (Dose Confirmation Study AssessingAntiplatelet Effects of Ticagrelor Versus Clopidogrel innon-ST-Segment Elevation Acute Coronary Syndrome)[54] phase II trial, 990 patients with NSTE-ACS wererandomized to receive 90 to 180 mg of ticagrelor twice aday or clopidogrel 75 mg once a day for up to 12 weeks.Comparable rates of major and minor bleeding (primary endpoint) were observed in the two arms, with a lower inci-dence of MI in the ticagrelor group (secondary end point).However, the most interesting results regarding ticagrelorefficacy derive from the phase III PLATO (Platelet Inhibi-tion and Patient Outcomes) [55] trial, a double-blind, ran-domized investigation comparing ticagrelor (180-mgloading dose, 90 mg twice daily thereafter) and clopidogrel(300- to 600-mg loading dose, 75 mg daily thereafter) in18,624 patients admitted for ACS. At 12 months, incidence

462 Curr Cardiol Rep (2012) 14:457–467

of the composite end point of cardiovascular death, MI, andstroke was significantly reduced in the ticagrelor group(9.8 % vs 11.7 % in the clopidogrel arm; HR 0.84; 95 %CI, 0.77–0.92; P<0.001). The benefit was maintained con-sidering single components of the composite end point,including cardiovascular mortality (4.5 % vs 5.9 %; P<0.001) and MI (5.8 % vs 6.9 %; P00.005). No significantdifference in the rates of overall major bleeding was alsoobserved (11.6 % vs 11.2 %; P00.43), but ticagrelor wasassociated with higher incidence of major bleeding notrelated to CABG (4.5 % vs 3.8 %; P00.03). However, it isnot completely clear about the lack of ischemic benefit ofticagrelor in the subgroup of patients from the United States.Of note, a clear mortality benefit was demonstrated withticagrelor in the subgroup of patients undergoing CABG andin those treated medically after the index event. Finally,patients treated with ticagrelor developed more frequentlyside effects such as dyspnea (13.8 % vs 7.8 % in clopidog-rel; P<0.001) or early ventricular pauses ≥3 seconds (5.8 %vs 3.6 %; P00.01). On the other side, the results of PLATOrevealed an impressive advantage of ticagrelor especially oncardiovascular mortality (107 more lives saved), a findingthat had not been observed with oral antiplatelet agents otherthan aspirin; thus, mechanisms other than antithromboticeffects have been hypothesized to explain this better out-come with ticagrelor (ie, increased levels of adenosine thatpotentially improve myocardial contractility and vasculartone). To date, no study has directly compared prasugreland ticagrelor; however, a recent study-level meta-analysis[56] performed an indirect comparison between these twodrugs considering three large randomized trials (32,893patients from DISPERSE-2, PLATO, and TRITON-TIMI38). The pooled data showed no significant difference inthe risk of the composite ischemic end point (death, MI,stroke) between prasugrel and ticagrelor, with the risk ofprobable and definitive thrombosis significantly reduced inthe prasugrel group (P00.02), even if at the price of higherrisk of any TIMI major bleeding (P00.007).

Cangrelor is an intravenous non-thienopyridine adeno-sine triphosphate analogue, direct-acting, selective, revers-ible blocker of the P2Y12 receptor. Cangrelor rapidlyreaches steady-state plasma levels and platelet aggregationinhibition within 30 min after infusion onset; its plasmatichalf-life is short, being approximately 9 min, and plateletfunction normalizes within 30 to 60 min after drug discon-tinuation [57]. In in-vitro studies, cangrelor further de-creased platelet inhibition in patients receiving long-termtreatment with clopidogrel [57]. However, in the CHAMPI-ON (Cangrelor Versus Standard Therapy to Achieve Opti-mal Management of Platelet Inhibition)-PCI trial [58] (8877patients with stable and unstable syndromes), cangrelor(bolus of 30 μg/kg and an intravenous infusion of 4 μg/kg/minute, starting within 30 min before PCI and continued

for at least 2 h or until the conclusion of the procedure) was notsuperior to clopidogrel in reducing the composite primary endpoint (death, MI, and TVR at 48 h), which occurred in 7.5% ofpatients in the cangrelor group versus 7.1 % of those receivingclopidogrel (OR01.05; 95%CI, 0.88–1.24;P00.59); a higherincidence of major bleeding (ACUITY criteria) was demon-strated in the cangrelor arm (3.6 % vs 2.9 %; P00.06). Also inthe CHAMPION-PLATFORM [59] study, cangrelor was notassociated with increased benefit versus clopidogrel. In thisstudy, 5362 patients were randomized to cangrelor or placebo(bolus and infusion) for the duration of the angioplasty proce-dure. The primary end point (death, MI, or ischemia-drivenrevascularization at 48 h) occurred in 7 % in the cangrelorversus 8 % in the placebo group (OR00.87; 95 % CI, 0.71–1.07; P00.17). Acute stent thrombosis was reduced in thecangrelor arm (0.2 % vs 0.6 %; P00.02), but this may beexplained by the study design using drug initiation before PCIin the cangrelor arm and clopidogrel initiation after the proce-dure in the control group.

On the basis of these results and in consideration of therapid onset/offset and reversible platelet inhibition obtainedwith cangrelor, other potential uses of such drugs have beeninvestigated, in particular a “bridging” strategy to bypasssurgery in patients treated with antiplatelet therapy to avoidrecurrent events after an ACS or after stent implantationwaiting for the operation; this was evaluated in the recentlypublished BRIDGE [60••] trial, in which 210 ACS patientsundergoing CABG were randomized to receive cangrelorinfusion (0.75 μg/kg/minute for at least 48 h and discon-tinued 1–6 h before surgery) or placebo. The proportion ofpatients with low levels of platelet reactivity (PRU<240 byVerifyNow) was higher in the cangrelor group (98.8 % vs19.0 % in placebo; P<0.001), without increase in CABG-related bleeding (11.8 % vs 10.4 %; P00.763).

Elinogrel is a reversible and competitive P2Y12 receptorantagonist. It does not need metabolic activation and is avail-able for intravenous and oral administration. The safety andtolerability of elinogrel have been investigated in theINNOVATE-PCI trial [61], a phase II randomized, double-blind, clopidogrel-controlled trial; this trial was not poweredfor ischemic end points and results are not yet fully published.

Thrombin Receptor Antagonists

As thrombin is a potent platelet activator, inhibition of PARsmay represent a new and interesting approach for effectiveantiplatelet drugs; moreover, the simultaneous inhibition ofmultiple platelet activation pathways may significantly im-prove antithrombotic strategy in high-risk patients with coronaryartery disease [62].

Four PARs have been identified (PAR-1, -2, -3, and –4);of those, only PAR-1 and -4 are expressed in human

Curr Cardiol Rep (2012) 14:457–467 463

platelets (Fig. 1). These two receptors can trigger plateletsecretion and aggregation, even if PAR-1 is considered theprincipal thrombin receptor also activated by low concen-trations of thrombin, whereas PAR-4 requires higher con-centrations of agonist. Currently, two different moleculeshave been investigated and tested in phase III clinical trials:vorapaxar (SCH530348) and atopaxar (E5555) [62].

Vorapaxar is a synthetic tricyclic 3-phenylpyridine an-alogue of himbacine, a natural product that has beenmodified as a crystalline salt for drug development. Itis a small molecule, orally active, with high affinity, thatcompetitively blocks thrombin-mediated platelet activa-tion and aggregation, without interfering with thrombin-mediated coagulation cascade activation [62, 63]. Vora-paxar is rapidly absorbed, with a rapid onset of plateletinhibition (2 h) and a calculated half-life up to 311 h.This drug is metabolized by CYP450 3A4; thus, co-administration of other drugs interfering with the CYPactivity may potentially influence efficacy of vorapaxar[63]. The safety of vorapaxar was evaluated in a multi-center, randomized trial, TRA-PCI (Thrombin ReceptorAntagonist-Percutaneous Coronary Intervention) [64]; thisstudy enrolled 1030 patients undergoing nonurgent PCIor coronary angiography with planned PCI. Patients wererandomized in a 3:1 model to placebo or vorapaxar ontop of standard of care antiplatelet therapy (aspirin andclopidogrel). The study drug was administered for atleast 1 h before PCI using three different loading doses(10, 20, or 40 mg) and continued during follow-up (0.5-,1.0-, or 2.0-mg daily doses) for 2 months. Incidence ofthe primary end point (TIMI major or minor bleedings)was 2 % in the 10-mg, 3 % in the 20-mg, 4 % in the40-mg group, and 3 % in placebo (P00.578). Incidenceof MACE was not different in the four arms; moreover,aggregometry data showed a higher proportion of patientswith at least 80 % inhibition of thrombin receptor agonistpeptide-induced platelet aggregation in the vorapaxargroups. The recently published TRACER (Thrombin Re-ceptor Antagonist-Cardiovascular Event Reduction) [65]study was focused on efficacy of vorapaxar in reducingrecurrent ischemic events in patients with ACS (n012,944). Patients randomly received in a 1:1 ratio vora-paxar (40-mg loading dose followed by a 2.5-mg main-tenance dose thereafter) or placebo. A composite endpoint of cardiovascular death, MI, stroke, recurrent ische-mia with rehospitalization, and urgent coronary revascu-larization occurred in 18.5 % of patients in the vorapaxargroup versus 19.9 % of those in the placebo arm (HR00.92; 95 % CI, 0.85–1.01; P00.07). Rate of moderate orsevere bleeding (Global Use of Strategies to Open Oc-cluded Coronary Arteries [GUSTO] and TIMI definitions)was increased in the vorapaxar group (7.2 % vs 5.2 %;HR 1.35; 95 % CI, 1.16–1.58; P<0.001), with a higher

incidence of intracranial hemorrhage in patients receivingthe study drug.

Atopaxar is another PAR-1 antagonist that completed thephase II clinical development. Atopaxar is a small molecule,chemically identified as 1-(3-tert-butyl-4-methoxy-5-mor-pholinophenyl)-2-(5,6-diethoxy-7-fluoro-1-imino-1,3-dihy-dro-2h-isoindol-2yl)-ethanone hydrobromide; it has aslower onset of action (3–5 h) and lower half-life (23 h)compared to vorapaxar [62, 63]. Two recent phase II trialsinvestigated the safety and efficacy of atopaxar in patientswith stable coronary artery disease (LANCELOT [Lessonsfrom Antagonizing the Cellular Effect of Thrombin-CAD])[66] and acute coronary syndromes (LANCELOT-ACS)[67]. In the first study, 720 patients were randomized in adouble-blind fashion to receive three dosing regimen of thedrug (50, 100, and 200 mg daily) or placebo for 24 weeks.Bleeding rate was superior in the atopaxar group versusplacebo, both considering CURE and TIMI definitions(3.9 % vs 0.6 %, P00.03, and 10.3 % vs 6.8 %, P00.17,respectively). MACE were numerically lower in the ato-paxar group, and higher platelet inhibition was observed inthe groups treated with atopaxar. A similar study design wasused for LANCELOT-ACS: 603 patients were randomizedto three doses of atopaxar (400-mg load followed by 50,100, or 200 mg daily) or placebo; incidence of CURE majoror minor bleeding did not differ between the atopaxar andplacebo groups (3.08 % vs 2.17 %, P00.63), and incidenceof the efficacy end point (cardiovascular death, MI, stroke,or recurrent ischemia) was also similar.

Conclusions

Platelets play a key role in the development of throm-botic events in patients with coronary artery disease.However, recurrent ischemic events remain considerablyhigh with clopidogrel, and this may also be attributed tolow clopidogrel responsiveness. Clinical alternativeapproaches to overcome this phenomenon are increasingloading and maintenance clopidogrel doses, reloading inpatients already on chronic therapy, and using newer andmore potent P2Y12 blockers such prasugrel and ticagre-lor; inhibition of other platelet aggregation pathways suchas thrombin receptor antagonists might also have a role.However, if maximizing antiplatelet therapy significantlyattenuates ischemic events in patients with coronary ar-tery disease, on the other side, it may also increasebleeding events. Given the important prognostic role ofboth ischemic and hemorrhagic events in patients withcoronary artery disease, the strategy of choosing theappropriate antiplatelet strategy by individualizing andpersonalizing such therapy according to both risks mayrepresent the optimal practice approach.

464 Curr Cardiol Rep (2012) 14:457–467

Disclosure Conflicts of interest: A. Nusca: none; G. Patti: has boardmembership with Daiichi-Sankyo/Lilly; and has received payment fordevelopment of educational presentations including service on speakers’bureaus from BMS/Sanofi Aventis.

References

Papers of particular interest, published recently, have beenhighlighted as:• Of importance•• Of major importance

1. Davì G, Patrono C. Platelet activation and atherothrombosis. NEngl J Med. 2007;357:2482–94.

2. Vorchheimer DA, Becker R. Platelet in atherothrombosis. MayoClinic Proc. 2006;81(1):59–68.

3. Kottke-Marchant K. Importance of platelet and platelet response inacute coronary syndromes. Cleve Clin J Med. 2009;76(1):S2–7.

4. Mueller C, Neumann FJ, Hochholzer W, et al. The impact ofplatelet count on mortality in unstable angina/non-ST-segmentelevation myocardial infarction. Am Heart J. 2006;151:1214.e1–7.

5. Fuchs I, Frossard M, Spiel A, et al. Platelet function in patientswith acute coronary sindrome (ACS) predicts recurrent ACS. JThomb Haemost. 2006;4:2547–52.

6. Angiolillo DJ, Bernardo E, Sabatè M, et al. Impact of plateletreactivity on cardiovascular outcomes in patients with type 2diabetes mellitus and coronary artery disease. J Am Coll Cardiol.2007;50:1541–7.

7. Buonamici P, Marcucci R, Migliorini A, et al. Impact of plateletreactivity after clopidogrel administration on drug-eluting stentthrombosis. J Am Coll Cardiol. 2007;49(24):2312–7.

8. Patti G, Nusca A, Mangiacapra F, et al. Point-of-care measurementof clopidogrel responsiveness predicts clinical outcome in patientsundergoing percutaneous coronary intervention. Results of theARMYDA-PRO (Antiplatelet Therapy for Reduction of MYocardialDamage During Angioplasty—Platelet Reactivity Predicts Outcome)study. J Am Coll Cardiol. 2008;52:1128–33.

9. Tran H, Anand SS. Oral antiplatelet therapy in cerebrovasculardisease, coronary artery disease, and peripheral arterial disease.JAMA. 2004;292:1867–74.

10. Thebault JJ, Kiefer G, Cariou R. Single-dose pharmacodynamicsof clopidogrel. Semin Thromb Hemost. 1999;25:3–8.

11. Muller I, Seyfarth M, Rudiger S, et al. Effect of high loading doseof clopidogrel on platelet function in patients undergoing coronarystent placement. Heart. 2001;85:92–3.

12. Leimbach ME, Peyrou V, Marzec UM, et al. Single dose clopi-dogrel inhibition of platelet adenosine receptor function in patientswith atherosclerotic coronary artery disease (abstr). Circulation.1999;100:1-681P.

13. Yusuf S, Zhao F, Mehta SR, et al. Effects of clopidogrel in additionof aspirin in patients with acute coronary syndromes without ST-segment elevation. New Engl J Med. 2001;345:494–502.

14. Mehta SR, Yusuf S, Peters RJ, Clopidogrel in Unstable angina toprevent Recurrent Events trial (CURE) Investigators, et al. Effectsof pretreatment with clopidogrel and aspirin followed by long-termtherapy in patients udergoing percutaneous coronary intervention:the PCI-CURE Study. Lancet. 2001;358:527–33.

15. Steinhubl SR, Berger PB, Mann III JT, CREDO Investigators, etal. Clopidogrel for the Reduction of Events During Observation.Early and sustained dual oral antiplatelet therapy following percu-taneous coronary intervention: a randomized controlled trial.JAMA. 2002;288:2411–20.

16. Patti G, Colonna G, Pasceri V, et al. Randomized trial of highloading dose of clopidogrel for reduction of periprocedural myo-cardial infarction in patients undergoing coronary intervention:results from the ARMYDA-2 (Antiplatelet therapy for Reductionof MYocardial Damage during Angioplasty) study. Circulation.2005;111(16):2099–106.

17. Lotrionte M, Biondi-Zoccai GG, Agostoni P, et al. Meta-analysisappraising high clopidogrel loading in patients undergoing percuta-neous coronary intervention. Am J Cardiol. 2007;100(8):1199–206.

18. Task Force on Myocardial Revascularization of the EuropeanSociety of Cardiology (ESC) and the European Association forCardio-Thoracic Surgery (EACTS). Guidelines on myocardial re-vascularization. Eur Heart J. 2010;31(20):2501–55.

19. Montalescot G, Sideris G, Meuleman C, et al. A randomizedcomparison of high clopidogrel loading doses in patients withnon-ST-segment elevation acute coronary syndromes: the ALBI-ON (Assessment of the Best Loading Dose of Clopidogrel to BluntPlatelet Activation, inflammation and Ongoing Necrosis) trial. JAm Coll Cardiol. 2006;48:931–8.

20. von Beckerath N, Taubert D, Pogatsa-Murray G, et al. Absorption,metabolization, and antiplatelet effects of 300-, 600-, and 900-mgloading doses of clopidogrel: results of the ISAR-CHOICE (Intra-coronary Stenting and Antithrombotic Regimen: Choose Between3 High Oral Doses for Immediate Clopidogrel Effect) trial. Circu-lation. 2005;112:2946–50.

21. Di Sciascio G, Patti G, Pasceri V, on behalf of the ARMYDA-4RELOAD investigators, et al. Clopidogrel reloading in patientsundergoing percutaneous coronary intervention on chronic clopi-dogrel therapy. Results of the ARMYDA-4 RELOAD (Antiplatelettherapy for Reduction of MYocardial Damage during Angioplasty)randomized trial. Eur Heart J. 2010;31(11):1337–43.

22. Di Sciascio G, Patti G, Pasceri V, on behalf of the ARMYDA-5PRELOAD investigators, et al. Effectiveness of in-lab high doseclopidogrel loading vs routine preload in patients undergoingpercutaneous coronary intervention. Results of the ARMYDA-5PRELOAD (Antiplatelet therapy for Reduction of MYocardialDamage during Angioplasty) randomized trial. J Am Coll Cardiol.2010;56(7):550–7.

23. Widimsky P, Motovská Z, Simek S, PRAGUE-8 Trial Investiga-tors, et al. Clopidogrel pre-treatment in stable angina: for allpatients >6 h before elective coronary angiography or only forangiographically selected patients a few minutes before PCI? Arandomized multicentre trial PRAGUE-8. Eur Heart J. 2008;29(12):1495–503.

24. von Beckerath N, Kastrati A, Wieczorek A, et al. A double-blind,randomized study on platelet aggregation in patients treated with adaily dose of 150 or 75 mg of clopidogrel for 30 days. Eur Heart J.2007;28:1814–9.

25. Angiolillo DJ, Shoemaker SB, Desai B, et al. Randomized com-parison of a high clopidogrel maintenance dose in patients withdiabetes mellitus and coronary artery disease: results of the Opti-mizing Antiplatelet Therapy in Diabetes Mellitus (OPTIMUS)study. Circulation. 2007;115:708–16.

26. Patti G, Grieco D, Dicuonzo G, et al. High versus standard clopi-dogrel maintenance dose after percutaneous coronary interventionand effects on platelet inhibition, endothelial function, and inflam-mation results of the ARMYDA-150 mg (antiplatelet therapy forreduction of myocardial damage during angioplasty) randomizedstudy. J Am Coll Cardiol. 2011;57(7):771–8.

27. Mehta SR, Bassand JP, Chrolavicius S, CURRENT-OASIS 7 Inves-tigators, et al. Dose comparisons of clopidogrel and aspirin in acutecoronary syndromes. N Engl J Med. 2010;363(10):930–42.

28. • Mehta SR, Tanguay JF, Eikelboom JW, CURRENT-OASIS 7trial investigators, et al. Double-dose versus standard-dose clopi-dogrel and high-dose versus low-dose aspirin in individuals un-dergoing percutaneous coronary intervention for acute coronary

Curr Cardiol Rep (2012) 14:457–467 465

syndromes (CURRENT-OASIS 7): a randomised factorial trial.Lancet. 2010;376(9748):1233–43. This subgroup analysis of theCURRENT-OASIS 7 is the largest randomized trial confirming thebenefit of doubling the dose of clopidogrel for the first week inpatients with ACS, who are planned to receive a stent.

29. Matetzky S, Shenkman B, Guetta V, et al. Clopidogrel resistance isassociated with increased risk of recurrent atherothrombotic eventsin patients with acute myocardial infarction. Circulation. 2004;109(25):3171–5.

30. Bliden KP, DiChiara J, Tantry US, et al. Increased risk in patientswith high platelet aggregation receiving chronic clopidogrel ther-apy undergoing percutaneous coronary intervention. J Am CollCardiol. 2007;49:657–66.

31. Gurbel PA, Bliden KP, Samara W, et al. Clopidogrel effect onplatelet reactivity in patients with stent thrombosis: results of theCREST Study. J Am Coll Cardiol. 2005;46:1827–32.

32. Muller I, Besta F, Schulz C, et al. Prevalence of clopidogrel non-responders among patients with stable angina pectoris scheduledfor elective coronary stent placement. Thromb Haemost.2003;89:783–7.

33. Gurbel PA, Bliden KP, Hiatt BL, O’Connor CM. Clopidogrel forcoronary stenting: response variability, drug resistance, and theeffect of pretreatment platelet reactivity. Circulation.2003;107:2908–13.

34. Angiolillo DJ, Fernandez-Ortiz A, Bernardo E, et al. Variability inindividual responsiveness to clopidogrel: clinical implications,management, and future perspectives. J Am Coll Cardiol.2007;49(14):1505–16.

35. Yin T, Miyata T. Pharmacogenomics of clopidogrel: evidence andperspectives. Thromb Res. 2011;128(4):307–16.

36. Mega JL, Close SL, Wiviott SD, et al. Cytochrome P450 geneticpolymorphisms and the response to prasugrel: relationship to phar-macokinetic, pharmacodynamic, and clinical outcomes. Circula-tion. 2009;119(19):2553–60.

37. Mega JL, Close SL, Wiviott SD, et al. Cytochrome p-450 poly-morphisms and response to clopidogrel. N Engl J Med. 2009;360(4):354–62.

38. Gurbel PA, Becker RC, Mann KG, et al. Platelet function moni-toring in patients with coronary artery disease. J Am Coll Cardiol.2007;50:1822–34.

39. Michelson AD. Methods for the measurement of platelet function.Am J Cardiol. 2009;103:20A–6A.

40. Freynhofer MK, Brozovic I, Bruno V, et al. Multiple electrodeaggregometry and vasodilator stimulated phosphoprotein-phosphorylation assay in clinical routine for prediction of postpro-cedural major adverse cardiovascular events. Thromb Haemost.2011;106(2):230–9.

41. Price MJ, Endemann S, Gollapudi RR, et al. Prognostic signifi-cance of post-clopidogrel platelet reactivity assessed by a point-of-care assay on thrombotic events after drug-eluting stent implanta-tion. Eur Heart J. 2008;29(8):992–1000.

42. •• Brar SS, ten Berg J, Marcucci R, et al. Impact of plateletreactivity on clinical outcomes after percutaneous coronary inter-vention. A collaborative meta-analysis of individual participantdata. J Am Coll Cardiol. 2011;58:1945–54. This meta-analysisdefinitely confirmed the strong prognostic impact of high residualplatelet reactivity evaluated by the VerifyNow assay on outcome ofpatients undergoing coronary stenting.

43. Bonello L, Camoin-Jau, Arques, et al. Adjusted clopidogrel loadingdoses according to vasodilator-stimulated phosphoprotein phosphor-ylation index decrease rate of major adverse cardiovascular events inpatients with clopidogrel resistance: a multicenter randomized pro-spective study. J Am Coll Cardiol. 2008;51:1404–11.

44. •• Price MJ, Berger PB, Teirstein PS, GRAVITAS Investigators, etal. Standard- vs high-dose clopidogrel based on platelet functiontesting after percutaneous coronary intervention: the GRAVITAS

randomized trial. JAMA. 2011;305(11):1097–105. The GRAVITAStrial is the first study to determine whether tailored antiplatelettherapy using the Accumetrics VerifyNow P2Y12 assay reducesMACE after drug-eluting stent implantation.

45. Eikelboom JW, Mehta SR, Anand SS, et al. Adverse impact ofbleeding on prognosis in patients with acute coronary syndromes.Circulation. 2006;114(8):774–82.

46. Manoukian SV, Feit F, Mehran R, et al. Impact of major bleedingon 30-day mortality and clinical outcomes in patients with acutecoronary syndromes: an analysis from the ACUITY Trial. J AmColl Cardiol. 2007;49(12):1362–8.

47. Sibbing D, Schulz S, Braun S, et al. Antiplatelet effects of clopi-dogrel and bleeding in patients undergoing coronary stent place-ment. J Thromb Haemost. 2010;8(2):250–6.

48. Patti G, Pasceri V, Vizzi V, et al. Usefulness of platelet response toclopidogrel by point-of-care testing to predict bleeding outcomesin patients undergoing percutaneous coronary intervention (fromthe Antiplatelet Therapy for Reduction of Myocardial DamageDuring Angioplasty-Bleeding Study). Am J Cardiol. 2011;107(7):995–1000.

49. Wallentin L. P2Y(12) inhibitors: differences in properties andmechanisms of action and potential consequences for clinicaluse. Eur Heart J. 2009;30(16):1964–77.

50. Damman P, Woudstra P, Kuijt WJ, et al. P2Y12 platelet inhibitionin clinical practice. J Thromb Thrombolysis. 2012;33:143–53.

51. Wiviott SD, Trenk D, Frelinger AL, PRINCIPLE-TIMI 44 Inves-tigators, et al. Prasugrel compared with high loading- andmaintenance-dose clopidogrel in patients with planned percutaneouscoronary intervention: the Prasugrel in Comparison to Clopi-dogrel for Inhibition of Platelet Activation and Aggregation-Thrombolysis in Myocardial Infarction 44 trial. Circulation.2007;116(25):2923–32.

52. Wiviott SD, Braunwald E, McCabe CH, TRITON-TIMI 38 Inves-tigators, et al. Prasugrel versus clopidogrel in patients with acutecoronary syndromes. N Engl J Med. 2007;357(20):2001–15.

53. Husted S, Emanuelsson H, Heptinstall S, et al. Pharmacodynamics,pharmacokinetics, and safety of the oral reversible P2Y12 antago-nist AZD6140 with aspirin in patients with atherosclerosis: adouble-blind comparison to clopidogrel with aspirin. Eur Heart J.2006;27:1038–47.

54. Cannon CP, Husted S, Harrington RA, DISPERSE-2 Investigators,et al. Safety, tolerability, and initial efficacy of AZD6140, the firstreversible oral adenosine diphosphate receptor antagonist, com-pared with clopidogrel, in patients with non-ST-segment elevationacute coronary syndrome: primary results of the DISPERSE-2trial. J Am Coll Cardiol. 2007;50:1844–51.

55. Wallentin L, Becker RC, Budaj A, PLATO Investigators, et al.Ticagrelor versus clopidogrel in patients with acute coronary syn-dromes. N Engl J Med. 2009;361(11):1045–57.

56. Biondi-Zoccai G, Lotrionte M, Agostoni P, et al. Adjusted indirectcomparison meta-analysis of prasugrel versus ticagrelor forpatients with acute coronary syndromes. Int J Cardiol. 2011;150(3):325–31.

57. Storey RF, Wilcox RG, Heptinstall S. Comparison of the pharma-codynamic effects of the platelet ADP receptor antagonists clopi-dogrel and AR-C69931MX in patients with ischaemic heartdisease. Platelets. 2002;13(7):407–13.

58. Harrington RA, Stone GW, McNulty S, et al. Platelet inhibitionwith cangrelor in patients undergoing PCI. N Engl J Med.2009;361(24):2318–29.

59. Bhatt DL, Lincoff AM, Gibson CM, CHAMPION PLATFORMInvestigators, et al. Intravenous platelet blockade with cangrelorduring PCI. N Engl J Med. 2009;361(24):2330–41.

60. •• Angiolillo DJ, Firstenberg MS, Price MJ, BRIDGE Investiga-tors, et al. Bridging antiplatelet therapy with cangrelor in patientsundergoing cardiac surgery: a randomized controlled trial. JAMA.

466 Curr Cardiol Rep (2012) 14:457–467

2012;307(3):265–74. The BRIDGE trial was the first study dem-onstrating the efficacy and safety of a “bridging” intravenousadministration of an antiplatelet agent such as cangrelor betweendiscontinuing oral thienopyridines and up to 6 hours before car-diac surgery.

61. Leonardi S, Rao SV, Harrington RA, et al. Rationale and design ofthe randomized, double-blind trial testing INtraveNous and Oraladministration of elinogrel, a selective and reversible P2Y(12)-receptor inhibitor, versus clopidogrel to eVAluate Tolerability andEfficacy in nonurgent Percutaneous Coronary Interventionspatients (INNOVATE-PCI). Am Heart J. 2010;160(1):65–72.

62. Tello-Montoliu A, Tomasello SD, Ueno M, Angiolillo DJ. Anti-platelet therapy: thrombin receptor antagonists. Br J Clin Pharmacol.2011;72(4):658–71.

63. Ueno M, Ferreiro JL, Angiolillo DJ. Mechanism of action andclinical development of platelet thrombin receptor antagonists.Expert Rev Cardiovasc Ther. 2010;8(8):1191–200.

64. Becker RC, Moliterno DJ, Jennings LK, et al. Safety and tolera-bility of SCH 530348 in patients undergoing non-urgent percuta-neous coronary intervention: a randomised, double-blind, placebo-controlled phase II study. Lancet. 2009;373(9667):919–28.

65. Tricoci P, Huang Z, Held C, TRACER Investigators, et al.Thrombin-receptor antagonist vorapaxar in acute coronary syn-dromes. N Engl J Med. 2012;366(1):20–33.

66. Wiviott SD, Flather MD, O’Donoghue ML, LANCELOT-CADInvestigators, et al. Randomized trial of atopaxar in the treatmentof patients with coronary artery disease: the lessons from antago-nizing the cellular effect of Thrombin–Coronary Artery DiseaseTrial. Circulation. 2011;123(17):1854–63.

67. O’Donoghue ML, Bhatt DL, Wiviott SD, LANCELOT-ACSInvestigators, et al. Safety and tolerability of atopaxar in thetreatment of patients with acute coronary syndromes: the lessonsfrom antagonizing the cellular effects of Thrombin–Acute CoronarySyndromes Trial. Circulation. 2011;123(17):1843–53.

Curr Cardiol Rep (2012) 14:457–467 467