The difference between clopidogrel responsiveness and posttreatment platelet reactivity

Serum Free Triiodothyronine and the Responsiveness to Clopidogrel in Patients Undergoing Elective Percutaneous Coronary Intervention

INTRODUCTION

Both thyroid dysfunction and low responsiveness to clopidogrel have been reported to be associated with increased cardiovascular risk. Our study aims at determining the relationship between free triiodothyronine (FT3) and low responsiveness to clopidogrel in patients undergoing elective percutaneous coronary intervention (PCI).

METHODS

Consecutive patients undergoing elective PCI were enrolled. All patients received a loading dose of 300 mg clopidogrel, and platelet function was assessed by thromboelastography at least 12 h later. Low responsiveness to clopidogrel was defined by an adenosine diphosphate-induced platelet-fibrin clot strength > 47 mm and adenosine diphosphate-induced platelet inhibition rate < 50%. Major adverse cardiovascular events (MACEs) were defined as a composite of cardiovascular death, non-fatal myocardial infarction, non-fatal stroke, and ischemia-driven revascularization.

RESULTS

Of 812 patients included in the study, 289 showed low responsiveness to clopidogrel. The FT3 level was significantly lower in low responders (4.61 ± 0.60 pmol/l versus 4.94 ± 4.66 pmol/l, p = 0.002). Moreover, the percentage of low responders was greater among patients with low FT3 level than among those without (56.1% versus 34.5%, p = 0.007). Logistic regression analysis showed that a FT3 level was independently associated with the risk of low responsiveness to clopidogrel (odds ratio 0.720, 95% confidence interval [CI] 0.533-0.973, p = 0.033). In patients with low responsiveness to clopidogrel, low FT3 was independently associated with increased risk of MACEs (adjusted hazard ratio 3.040, 95% CI 1.077-8.580, p = 0.036) at a median of 19-month follow-up.

CONCLUSIONS

Low FT3 was independently associated with increased risks of both low responsiveness to clopidogrel and cardiovascular events in patients undergoing elective PCI.

Physiologically-Based Pharmacokinetic-Pharmacodynamics Model Characterizing CYP2C19 Polymorphisms to Predict Clopidogrel Pharmacokinetics and Its Anti-Platelet Aggregation Effect Following Oral Administration to Coronary Artery Disease Patients With or Without Diabetes

Background and Objective: Clopidogrel (CLOP) is commonly used in coronary artery disease (CAD) patients with or without diabetes (DM), but these patients often suffer CLOP resistance, especially those with diabetes. This study was aimed to develop a physiologically-based pharmacokinetic-pharmacodynamic (PBPK-PD) model to describe the pharmacokinetics and pharmacodynamics of clopidogrel active metabolite (CLOP-AM) in CAD patients with or without DM.

Methods: The PBPK-PD model was first established and validated in healthy subjects and then in CAD patients with or without DM. The influences of CYP2C19, CYP2C9, CYP3A4, carboxylesterase 1 (CES1), gastrointestinal transit rates (K_t,i) and platelets response to CLOP-AM (k_irre) on predicted pharmacokinetics and pharmacodynamics were investigated, followed with their individual and integrated effects on CLOP-AM pharmacokinetics due to changes in DM status.

Results: Most predictions fell within 0.5–2.0 folds of observations, indicating successful predictions. Sensitivity analysis showed that contributions of interested factors to pharmacodynamics were CES1> k_irre> K_t,i> CYP2C19 > CYP3A4> CYP2C9. Mimicked analysis showed that the decreased exposure of CLOP-AM by DM was mainly attributed to increased CES1 activity, followed by decreased CYP2C19 activity.

Conclusion: The pharmacokinetics and pharmacodynamics of CLOP-AM were successfully predicted using the developed PBPK-PD model. Clopidogrel resistance by DM was the integrated effects of altered K_t,i, CYP2C19, CYP3A4, CES1 and k_irre.

Does high on-treatment platelet aggregability reflect poor individual response to clopidogrel?

INTRODUCTION

On-treatment platelet aggregability represents the major form of functional assessment for patients treated with P2Y₁₂ receptor antagonists, with "high" on-treatment platelet aggregability (HTPA) predicting thrombotic risk. However HTPA reflects a variable combination of pre-treatment hyperaggregability and poor response to P2Y₁₂ antagonists. We have previously shown that integrity of platelet adenylate cyclase/cAMP signaling, assessed with PGE₁, is a strong predictor of individual responses to clopidogrel. We therefore sought to determine the extent to which HTPA reflects impaired platelet responsiveness to clopidogrel.

METHODS

Using data from our previous investigations of acute and sub-acute effects of clopidogrel, we analyzed the relationship between on-treatment aggregability and acute/steady state responsiveness to clopidogrel, utilizing ADP, the thromboxane A₂ mimetic U46619, and thrombin receptor-activating peptide (TRAP) as pro-aggregants. The relationship between anti-aggregatory response to PGE₁ and both on-treatment and pre-treatment aggregability was also examined.

RESULTS AND CONCLUSIONS

With all 3 pro-aggregants, (1) response to clopidogrel after 4 h, as measured by ΔADP response, exhibits a strong inverse relationship with on-treatment aggregation, with a similarly inverse relationship between pre-treatment PGE₁ response and on-treatment aggregability; (2) there is a weaker inverse relationship between clopidogrel response and pre-treatment platelet aggregability, and a significant inverse relationship between pre-treatment PGE₁ response and pre-treatment platelet aggregability. Furthermore, pre-treatment PGE₁ response also predicts on-treatment platelet aggregability in response to ADP at steady state. Thus, HTPA largely represents clopidogrel resistance.

Residual platelet reactivity is preferred over platelet inhibition rate in monitoring antiplatelet efficacy: insights using thrombelastography

Although thrombelastography (TEG) has been widely implemented in the clinical setting of endovascular intervention, consensus on the optimal parameter for defining high ischemic risk patients is lacking due to the limited data about the relationship between various TEG parameters and clinical outcomes. In this article, we report a post hoc analysis of a prospective, single-center cohort study, including 447 patients with acute coronary syndrome (ACS). Arachidonic acid (AA)- or adenosine diphosphate (ADP)-induced platelet-fibrin clot strength (MAAA or MAADP) was indicative of the net residual platelet reactivity after the treatment with aspirin or clopidogrel, respectively. AA% or ADP% was indices of the relative platelet inhibition rate on AA or ADP pathway. We found that each parameter alone was predictive of the risk of 6-month ischemic event, even after adjusting for confounding factors. However, the association between AA% and clinical outcome disappeared when further adjusted for MAAA. Likewise, inclusion of MAADP changed the significant relation between ADP% and clinical outcome. MAADP > 47.0 mm and MAAA > 15.1 mm were identified as the optimal cutoffs by receiver operating characteristic analysis. High MAAA (HR = 3.963; 95% CI: 1.152-13.632; P = 0.029) and high MAADP (HR = 5.185; 95% CI: 2.228-12.062; P < 0.001) were independent predictors when both were included in multivariable Cox regression hazards model. Interestingly, an even higher risk was found for the coexisting high MAAA and high MAADP (HR = 7.870; 95% CI: 3.462-17.899; P < 0.001). We conclude that when performing TEG to predict clinical efficacy, residual platelet reactivity has superiority over platelet inhibition rate as a measure of thrombotic risk in patients treated with aspirin and clopidogrel after ACS.

Association of ABCB1 Gene Polymorphisms and Clopidogrel Responsiveness in Iranian Patients undergoing Percutaneous Coronary Intervention

Clopidogrel is an antiplatelet agent currently used for preventing stent thrombosis. Despite certain clinical benefits of clopidogrel in patients undergoing percutaneous coronary intervention (PCI), adequate antiplatelet effect has not been obtained in some patients. The present study was designed to investigate the potential association of ABCB1 (ATP-Binding Cassette, Subfamily B, member1) gene polymorphism, and clopidogrel responsiveness in Iranian patients after PCI. Sixty-seven patients were included in the study. Blood samples were taken from patients at baseline, 2 h after administration of 600-mg loading dose of clopidogrel, 24 h and 30 days after PCI. Platelet aggregation was measured by light transmittance aggregometry (LTA) with two levels of adenosine diphosphate (ADP) concentrations (5 and 20 µM). ABCB1 genotyping was performed by restriction fragment length polymorphism-polymerase chain reaction (RFLP-PCR). The allelic frequencies of wild type, heterozygote, and homozygote genotypes of ABCB1 were 20.9%, 74.6%, and 4.5%, respectively. There was no significant association between polymorphism of ABCB1 and clopidogrel non-responsiveness (P > 0.05) in various situations. No significant difference was observed for demographic characteristics. Genetic and demographic factors had no significant effect on the platelet activity of clopidogrel in an Iranian population.

Platelet aggregation in response to ADP is highly variable in normal donors and patients on anti-platelet medication

BACKGROUND

P2Y12 inhibitors are indicated in patients following percutaneous coronary intervention. Several studies have demonstrated that high on treatment platelet reactivity is correlated with outcomes yet prospective studies of guided therapy have failed to show benefit. There is a paucity of studies on the platelet aggregation response to ADP before P2Y12 therapy is started. The aim of this study was to characterize platelet responses to 20 μM ADP by light transmission aggregometry (LTA) in a homogenous population.

METHODS

Platelet aggregation was assessed in 201 patients on dual antiplatelet therapy, 98 patients on aspirin alone and 47 normal, healthy volunteers free from anti-platelet medication.

RESULTS

Consensus guidelines suggest that a platelet aggregation response in response to the agonist ADP of <57% is an adequate therapeutic response to P2Y12 inhibition. Seven healthy donors and 38 patients taking aspirin only had aggregation responses below 57%.

CONCLUSIONS

The results of our study demonstrate that 15% of normal donors and 38% of patients taking aspirin only would be classified as having a therapeutic response to P2Y12 inhibition using current guidelines.

Contribution of whole platelet aggregometry to the endovascular management of unruptured aneurysms: an institutional experience

BACKGROUND

Stent-assisted coiling of intracranial aneurysms is an efficient alternative treatment to surgical clipping but requires prolonged antiplatelet therapy. Some patients are non-responsive to aspirin and/or clopidogrel.

OBJECTIVE

To analyze the implications of this assessment using the 'whole blood aggregometry (WBA) by impedance' technique.

MATERIALS AND METHODS

The Southwestern Tertiary Aneurysm Registry was reviewed between 2002 and 2012 for patients with unruptured aneurysms treated with stent-assisted coiling. The study population was divided into patients who were tested preoperatively for platelet responsiveness to aspirin and clopidogrel ('tested' patients) and those who were not ('non-tested'). Where necessary, tested patients received additional doses of antiplatelet drugs to achieve adequate platelet inhibition. Endpoints included the incidence of non-responsiveness, the rates of thrombotic and hemorrhagic complications, and the rates of permanent morbidity and mortality.

RESULTS

A total of 266 patients fulfilled our selection criteria: 114 non-tested patients who underwent 121 procedures, and 152 tested patients who underwent 171 procedures. The two groups did not vary significantly in patient age, gender, and aneurysms location. Aspirin non-responsiveness was detected in 3 patients (1.75%) and clopidogrel non-responsiveness in 21 patients (12.3%). Non-tested patients had an 11.6% rate of thrombotic complications with a 4.1% permanent morbidity or mortality rate versus 2.3% and 0.6% in tested patients (p=0.0013). The incidence of hemorrhagic complications was similar between the two groups.

CONCLUSIONS

Preoperative platelet inhibition testing using WBA can be useful to assess and correct antiaggregant non-responsiveness, and may reduce postoperative mortality and permanent morbidity.

Clopidogrel Resistance: Myth or Reality?

Platelets have a central role in the development of arterial thrombosis and subsequent cardiovascular events. An appreciation of this complex process has made antiplatelet therapy the cornerstone of cardiovascular disease management. Dual antiplatelet therapy with aspirin and clopidogrel has been approved for the secondary prevention of cardiovascular events and is currently part of the postpercutaneous coronary intervention treatment regimen. However, subacute stent thrombosis continues to occur in 1% to 2% of patients despite dual antiplatelet therapy. Studies have shown interindividual variations in response to clopidogrel, where a cohort of patients seems to be resistant to the antithrombotic effects of clopidogrel. Furthermore, there is an apparent link between clopidogrel resistance and clinical outcomes. Currently, there is neither a universally accepted definition of clopidogrel resistance nor an agreement on the phenomenon's mechanism. This review highlights the origins of clopidogrel resistance, the current problems that exist with its definition, and discusses the future implications and relevant challenges it poses for the clinician.

Platelet-Mediated Thrombosis

The pivotal role that platelets play in thrombosis and resultant ischemic event occurrences in patients with high-risk coronary artery disease is well established. This role provides the fundamental basis for the current wide implementation of dual antiplatelet therapy with aspirin and a P2Y 12 receptor inhibitor. The development of user friendly point-of-care methods to assess platelet reactivity to adenosine diphosphate has increased the frequency of platelet function testing in clinical practice. Recent large observational studies have established an independent relation between the results of point-of-care platelet function testing and clinical event occurrence in patients undergoing coronary artery stenting. However, prospective, randomized trials have failed to demonstrate that personalized antiplatelet therapy based on point-of-care assessment of platelet function is effective in reducing ischemic event occurrences. Important limitations were associated with these trials. In addition, the concept of a therapeutic window of P2Y 12 receptor reactivity with an upper threshold associated with ischemic event occurrence and a lower threshold associated with bleeding has also been proposed. In the absence of strong prospective evidence to support personalized antiplatelet therapy, clinical decision making about antiplatelet therapy rests on the large body of observational data and the fundamental importance of platelet physiology in catastrophic event occurrence in patients with high-risk coronary artery disease.

Impact of an integrated treatment algorithm based on platelet function testing and clinical risk assessment: results of the TRIAGE Patients Undergoing Percutaneous Coronary Interventions To Improve Clinical Outcomes Through Optimal Platelet Inhibition study

Assessment of platelet reactivity alone for thienopyridine selection with percutaneous coronary intervention (PCI) has not been associated with improved outcomes. In TRIAGE, a prospective multicenter observational pilot study we sought to evaluate the benefit of an integrated algorithm combining clinical risk and platelet function testing to select type of thienopyridine in patients undergoing PCI. Patients on chronic clopidogrel therapy underwent platelet function testing prior to PCI using the VerifyNow assay to determine high on treatment platelet reactivity (HTPR, ≥230 P2Y12 reactivity units or PRU). Based on both PRU and clinical (ischemic and bleeding) risks, patients were switched to prasugrel or continued on clopidogrel per the study algorithm. The primary endpoints were (i) 1-year major adverse cardiovascular events (MACE) composite of death, non-fatal myocardial infarction, or definite or probable stent thrombosis; and (ii) major bleeding, Bleeding Academic Research Consortium type 2, 3 or 5. Out of 318 clopidogrel treated patients with a mean age of 65.9 ± 9.8 years, HTPR was noted in 33.3 %. Ninety (28.0 %) patients overall were switched to prasugrel and 228 (72.0 %) continued clopidogrel. The prasugrel group had fewer smokers and more patients with heart failure. At 1-year MACE occurred in 4.4 % of majority HTPR patients on prasugrel versus 3.5 % of primarily non-HTPR patients on clopidogrel (p = 0.7). Major bleeding (5.6 vs 7.9 %, p = 0.47) was numerically higher with clopidogrel compared with prasugrel. Use of the study clinical risk algorithm for choice and intensity of thienopyridine prescription following PCI resulted in similar ischemic outcomes in HTPR patients receiving prasugrel and primarily non-HTPR patients on clopidogrel without an untoward increase in bleeding with prasugrel. However, the study was prematurely terminated and these findings are therefore hypothesis generating.

Clopidogrel Response Variability: Review of the Literature and Practical Considerations

Dual antiplatelet therapy with aspirin and a P2Y12 inhibitor is standard therapy following acute coronary syndrome and percutaneous coronary intervention. Despite the use of potent antiplatelet agents, vascular events continue to occur. Lack of response to clopidogrel therapy has been widely investigated using various methods of platelet function testing. These studies have consistently found an association between poor clopidogrel response and an increased risk of vascular events. Strategies to overcome this problem include higher clopidogrel doses or the use of an alternative P2Y12 agent. To date, the majority of studies investigating tailored antiplatelet therapy have failed to show any reduction in clinical events likely due to the low-risk population studied. Despite this lack of benefit from altering therapy, platelet function testing may be done in certain patient populations. Patients at high risk of deleterious outcomes from stent thrombosis may be an appropriate patient population for platelet function testing to ensure adequate response to therapy. In addition, emerging data suggests a potential role for platelet function testing to assess for bleeding risk. The purpose of this article is to review the key studies demonstrating response variability to clopidogrel therapy, strategies to overcome variability, and practical considerations for the clinician.

Comparison of the effect of CYP2C19 polymorphism on clinical outcome between acute coronary syndrome and stable angina

AIM

CYP2C19 polymorphism modulates platelet reactivity in coronary artery disease patients with stent implants. However, the impact of the CYP2C19 genotype on clopidogrel response and clinical outcome has not been fully understood to date.

METHODS

We enrolled 518 consecutive patients with acute coronary syndrome (ACS) (n=214) and stable angina (SA) (n=304). All patients received stent implants followed by dual antiplatelet therapy of aspirin and clopidogrel. We determined CYP2C19 phenotype, measured platelet reactivity, and assessed the risk of cardiovascular events.

RESULTS

During a median follow-up of 894 days, the rate of cardiovascular events was higher in patients of the ACS group than the SA group (ACS: 20.1%, SA: 12.5%, p=0.015). The mean platelet reactivity was significantly higher in the CYP2C19 loss-of-function allele carriers of the two groups (ACS, non-carriers: 3909±1836AUmin, carriers: 4854±1594AUmin, respectively, p<0.01; SA, 3606±1579AUmin, 4381±1373AUmin, ±SD, p<0.01). In the ACS group, cardiovascular events were higher in the loss-of-function allele carriers (24.6%) versus non-carriers (11.1%, p<0.05), but no such difference was noted in the SA group (carriers: 14.8%; non-carriers: 7.9%, p=0.078). Furthermore, landmark analysis from 30 days did not show differences in ACS group (carriers: 14.8%, non-carriers: 11.1%, p=0.315). Multivariate Cox proportional hazards analysis identified the presence of loss-of-function allele as an independent predictor of cardiovascular events (hazard ratio, 2.1, 95% CI, 1.194-3.587, p=0.010).

CONCLUSIONS

The impact of CYP2C19 loss-of-function gene on clinical outcome is more powerful in early phase of ACS compared with SA.

Impact of CYP2C19 polymorphism on clinical outcome following coronary stenting is more important in non-diabetic than diabetic patients

OBJECTIVE

The aim of this study was to examine the impact of CYP2C19 genotype on clinical outcome in coronary artery disease (CAD) patients with or without diabetes mellitus (DM).

METHODS

CYP2C19 polymorphism and DM are associated with increased risk of cardiovascular events during antiplatelet therapy following stent implantation. Platelet reactivity during clopidogrel therapy and CYP2C19 polymorphism were measured in 519 CAD patients (males 70%, age 69 years) treated with stent placement. Patients were divided into two groups; DM (n=249), and non-DM (n=270), and clinical events were evaluated according to the carrier state, which included at least one CYP2C19 loss-of-function allele.

RESULTS

The level of platelet reactivity and incidence of cardiovascular events were significantly different between Carriers and non-Carriers of the non-DM (platelet reactivity: 4501+/-1668 versus 3691+/-1714AU min, P<0.01; events, 32/178 versus 2/92, P<0.01, respectively), however, there was no difference in clinical outcome in the DM group (events, 34/168 versus 14/81, respectively, P=0.57). Multivariate analysis identified CYP2C19 loss-of-function allele carriage as an independent predictor of cardiovascular events in non-DM, but not in DM (non-DM, HR 7.180, 95% CI, 1.701 to 30.298, P=0.007; DM, HR 1.374, 95% CI, 0.394 to 4.792, P=0.618).

CONCLUSION

The impact of CYP2C19 polymorphism on clinical outcome seems to be more significant in non-DM compared with DM in patients with coronary stents.

Importance of measurement of platelet reactivity to ADP in patients with coronary artery disease: an historical account

The pivotal roles of platelets in physiological hemostasis and pathological thrombosis at the site of plaque rupture are well established. The latter roles provide the fundamental basis for the most widely implemented pharmacologic management of coronary artery disease--dual antiplatelet therapy with aspirin to inhibit platelet thromboxane A2 generation, and a P2Y12 receptor inhibitor to prevent adenosine diphosphate (ADP)-induced platelet activation. Although suboptimal pharmacodynamic efficacy, also described as high on-treatment platelet reactivity to ADP, has been associated with greater risk for post-stenting ischemic event occurrence, enhanced responsiveness is associated with higher risk for bleeding in selected patients. In this review article, we aim to provide an historical account of the one and a half century long journey starting with the first description of platelets through the first report of ex vivo measurement of ADP-induced platelet aggregation, the first demonstration of an association between ADP-induced platelet aggregation and post-stenting ischemic event occurrence, and finally to the most recent description of a 'therapeutic window' concept for P2Y12 receptor inhibitor therapy.

The Role of Platelet Function Testing in Risk Stratification and Clinical Decision-Making

Clopidogrel (a widely used second-generation thienopyridine) therapy is associated with an unpredictable pharmacodynamic response whereby approximately 1 in 3 patients will have a high on-treatment platelet reactivity to adenosine diphosphate. High on-treatment platelet reactivity is an established risk factor for ischemic event occurrence in patients undergoing percutaneous coronary intervention. Platelet function testing may have a role in monitoring therapeutic efficacy when clopidogrel is the chosen agent and in safety when more potent drugs are used, especially in patients with high bleeding risk. At this time, it seems most reasonable to assess platelet function in high-risk clopidogrel-treated patients.

PON1 Q192R genotype influences clopidogrel responsiveness by relative platelet inhibition instead of on-treatment platelet reactivity

BACKGROUND

paraoxonase-1 (PON1) was recently identified as the crucial enzyme for clopidogrel bioactivation, with PON1 Q192R (rs662) polymorphism determining the clopidogel antiplatelet efficacy. However, subsequent studies showed controversies over the findings. This study aimed to evaluate the impact of PON1 Q192R in parallel to that of CYP2C19*2 (rs4244285) on clopidogrel responsiveness in a cohort of Chinese patients with unstable angina pectoris.

MATERIAL AND METHODS

One hundred and eighty Chinese-Han patients diagnosed with unstable angina pectoris and treated with clopidogrel were consecutively recruited. Clopidogrel responsiveness, measured by relative platelet inhibition {RI=[(pretreatment aggregation-posttreatment aggregation at 5days)/(pretreatment aggregation)] x100%}, was assessed in relation to PON1 Q192R and CYP2C19*2 genotypes. RI values were stratified into four quartiles, with patients in quartile 1 defined as individuals of clopidogrel non-responsiveness. The contributions of PON1 Q192R and CYP2C19*2 to on-treatment platelet reactivity (OTPR) at 5days maintenance dose of clopidogrel were also evaluated.

RESULTS

For PON1 Q192R genotypes, RI values were significantly lower in patients with QR and RR alleles than in patients with QQ alleles (p=0.01). OTPR values at 5days maintenance dose of clopidogrel were similar across all the PON1 Q192R genotypes (p=0.41). PON1 192 QR and RR conferred increased risks for clopidogrel non-responsiveness [OR 3.64; 95% CI (1.21-10.92), p=0.02]. For CYP2C19*2 genotypes, compared to CYP2C19*1/*1 wild type carriers, CYP2C19*2 carriers showed a significantly higher OTPR (p=0.009), and a trend for lower RI values (p=0.06). An increased risk for clopidogrel non-responsiveness was found in patients with CYP2C19*2 genotype [OR 2.02; 95% CI (1.03-3.96), p=0.04].

CONCLUSIONS

Both PON1 Q192R and CYP2C19*2 genotypes influence clopidogrel responsiveness, with the impact of PON1 Q192R mainly on relative platelet inhibition instead of OTPR of clopidogrel.

Latest evidence in personalized antiplatelet therapy in patients with acute coronary syndromes undergoing percutaneous coronary intervention

In patients with acute coronary syndromes undergoing percutaneous coronary intervention, the combination of aspirin and clopidogrel, a P2Y12 adenosine diphosphate (ADP) receptor antagonist, is the gold standard of antiplatelet therapy. Two more potent P2Y12 ADP receptor antagonists are now available. Pharmacodynamic studies have revealed a large interindividual variability in the biological response to clopidogrel that is primarily related to variable active metabolite generation, depending on clinical factors, drug-drug interactions, and genetic polymorphisms. Several assays to measure platelet function are available and have revealed a high prevalence of high on-treatment platelet reactivity (HTPR). Patients exhibiting HTPR after a clopidogrel loading dose have a higher risk of thrombotic recurrence after percutaneous coronary intervention. A recent consensus has defined HTPR for the main platelet assays available (using receiver operating characteristic curve analysis) to define the optimal cutoff value for each assay in order to predict thrombotic recurrences. In this article, we present several lines of evidence that suggest a therapeutic window of platelet reactivity inhibition with P2Y12 ADP receptor antagonists. Such a paradigm shift is supported by the results of the Platelet Inhibition and Patient Outcomes (PLATO) trial and the Trial to Assess Improvement in Therapeutic Outcomes by Optimizing Platelet Inhibition with Prasugrel-Thrombolysis in Myocardial Infarction (TRITON-TIMI) 38, which showed the superiority of ticagrelor and prasugrel on thrombotic events compared with clopidogrel; however, these 2 medications had an increased bleeding rate. With the results of these trials, in addition to the evidence of a therapeutic window with P2Y12 ADP receptor antagonists, we summarize the potential of platelet reactivity monitoring and pharmacogenomics to tailor therapy.

The pattern of platelet response to clopidogrel in Iranian patients after percutaneous coronary intervention

Despite certain clinical benefit in using clopidogrel in patients undergoing percutaneous coronary intervention (PCI), some patients do not attain adequate antiplatelet effects. In this study, the authors investigated the response to clopidogrel in Iranian patients after PCI. Patients who were candidates for elective PCI were enrolled in this study. All patients had received aspirin 80 to 325 mg daily for ≥1 week before PCI. Blood samples were taken from patients at baseline, 2 hours after taking a 600-mg loading dose of clopidogrel, and 24 hours and 30 days after stenting. Platelet aggregation was measured by light transmittance aggregometry with adenosine diphosphate (5 and 20 μM) and arachidonic acid (500 and 5000 μg/mL). One hundred twelve patients were included (79 men, 33 women). Maximal and minimal clopidogrel nonresponsiveness occurred at 2 hours (26%) and 48 hours (13%) after taking 600 mg clopidogrel, respectively. Pretreatment platelet reactivity had no effects on posttreatment platelet reactivity. Moreover, clopidogrel responsiveness did not correlate with pretreatment reactivity. Patients' demographic and procedural characteristics had no significant effect on clopidogrel responsiveness. The frequency of clopidogrel nonresponsiveness in this study was similar to other studies. However, clopidogrel required more than 2 hours for induction of its maximal antiplatelet effect in this study.

Platelet function profile post-clopidogrel therapy in patients with type 2 diabetes undergoing coronary stent implantation

Platelet dysfunction contributes to the increased risk of thromboischemic complications after percutaneous coronary intervention (PCI), particularly in type 2 diabetes. Little is known about the effects of glycemic control on platelet reactivity. We assessed adenosine diphosphate-induced platelet aggregation and flow cytometric expression of P-selectin in 90 patients (56 diabetic and 34 nondiabetic patients) undergoing coronary stent implantation after administration of clopidogrel as a potential predictor of poststent complications and its relation to glycemic control. Posttreatment platelet reactivity was significantly elevated in diabetic compared with nondiabetic participants and was associated with smoking, hypercholesterolemia, overweight, and cardiovascular ischemic events. A linear relationship was found between hemoglobin A1c in diabetic patients and platelet reactivity. Both methods (standard aggregometry and P-selectin expression) used for assessment of platelet function were positively correlated. Low responsiveness to clopidogrel detected by posttreatment platelet reactivity is a risk factor for ischemic events after PCI in diabetic patients.

Clopidogrel "resistance": where are we now?

Antiplatelet therapy with aspirin and clopidogrel in PCI patients, though effective, is still associated with thrombotic complications. These are multifactorial in origin, but partially attributable to "clopidogrel resistance." However, how best to identify and manage "clopidogrel resistance" remains unclear. Targeting therapeutic changes specifically at those individuals with poor response to clopidogrel is likely to be a solution. A "one size fits all" approach to clopidogrel dosing is probably flawed. This review will explore (1) the definition and mechanisms of clopidogrel resistance, (2) assessment of clopidogrel resistance by (i) platelet function testing and (ii) genetic testing, (3) the management of "clopidogrel resistance," and (4) newer antiplatelet agents, and evolving stent technology. A pubmed literature review was performed using the keywords "clopidogrel", "resistance", "poor response", "adverse events", "platelet function tests", and "genetic tests". In looking at new agents, keywords "prasugrel", "cangrelor", "ticagrelor""Elinogrel", and "P2Y12 receptor antagonists" were used. Third, a search was performed looking at "stent design", "IVUS", "bioabsorbable stents", and "stent apposition". Whilst new P2Y12 receptor antagonists and improved stent technology may reduce thrombotic events in the future, there is still a need for clopidogrel. There is good evidence that poor response to clopidogrel is associated with adverse outcome. Platelet function tests probably provide more clinically useful data than genetic tests, but the question of how best to identify and manage variability in response to clopidogrel demands further research.

Clopidogrel response variability and the advent of personalised antiplatelet therapy. A bench to bedside journey

Platelet-mediated thrombosis is a dreaded clinical event and is the primary cause of acute coronary syndromes and post-percutaneous intervention (PCI) ischaemic events. There has been a long standing interest in the ex vivo quantification of platelet reactivity to assess the risk of thrombosis. Early studies demonstrated platelet activation and heightened platelet reactivity in acute coronary syndromes and after PCI. However, a demonstration that heightened reactivity actually precipitated the ischaemic event was lacking. Our knowledge of platelet receptor physiology and the advent of novel inhibitors have significantly advanced the field. The P2Y12 receptor has been shown to play a pivotal role in the amplification of platelet activation by multiple agonists and its inhibition has resulted in improved clinical outcomes. The most widely used drug to block P2Y12 receptor, clopidogrel is associated with resistance in selected patients and these patients have been shown to be at increased risk for post-PCI ischaemic event occurrence in multiple studies. Importantly, a threshold of high platelet reactivity has been demonstrated, and beyond this threshold ischaemic events occur precipitously. Based on the current evidence, it is rational to quantify the intensity of the ADP-P2Y12 interaction in the patient at the greatest risk for thrombosis-the PCI patient. However, there is only evidence from small clinical trials demonstrating the clinical efficacy of changing an antiplatelet regimen based on an ex vivo platelet function measurement. Moreover, there are numerous patients with vulnerable coronary anatomy that have not yet experienced plaque rupture; the prognostic role of a measurement of platelet reactivity in the latter group has never been studied. Large-scale trials are ongoing that will investigate the role of personalised antiplatelet therapy in the PCI patient.

Oral antiplatelet therapy after acute coronary syndrome and percutaneous coronary intervention: balancing efficacy and bleeding risk

PURPOSE

The benefits and risk of bleeding associated with oral antiplatelet agents used for acute coronary syndrome (ACS) or in patients who have undergone percutaneous coronary intervention (PCI) are discussed.

SUMMARY

Over the past decade, significant advances have been made with the use of oral antiplatelet agents in ACS patients and in those undergoing PCI and stenting. Dual antiplatelet therapy with clopidogrel and aspirin has been considered the gold standard for reducing cardiovascular events in these patients. However, use of clopidogrel has limitations, including variable patient response. These limitations can affect patient outcomes achieved with clopidogrel, leading to concerns regarding its use. Subsequently, more potent oral antiplatelet agents have been developed, including prasugrel and ticagrelor. Prasugrel is an oral thienopyridine with greater potency and less antiplatelet variability than clopidogrel. Ticagrelor, another oral antiplatelet agent that has shown greater platelet inhibition than clopidogrel, is currently under investigation. Although a greater reduction in ischemic events has been observed with dual antiplatelet therapy and the use of newer oral antiplatelet agents, there is also a progressive increase in the risk of major bleeding. Treatment decisions should be based on current practice guidelines, as well as individualized patient risk and benefit analyses.

CONCLUSION

Knowledge of the benefits and bleeding risks associated with oral antiplatelet agents, as well as guideline recommendations, can help health care providers make informed decisions regarding the most appropriate therapy for patients after ACS and PCI.

Long-Term Aspirin and Clopidogrel Response Evaluated by Light Transmission Aggregometry, VerifyNow, and Thrombelastography in Patients Undergoing Percutaneous Coronary Intervention

Background: A reduced response to aspirin and clopidogrel predicts ischemic events, but reliable tests are needed to identify low responders. We compared 3 platelet-function tests during long-term dual treatment with aspirin and clopidogrel.

Methods: Patients who underwent a percutaneous coronary intervention and were receiving a combination of 325 mg/day aspirin and 75 mg/day clopidogrel were followed for 1 year. Blood was sampled 5 times during this period for 3 tests: light transmission aggregometry (LTA) assay, with 5.0 μmol/L ADP or 1.0 mmol/L arachidonic acid (AA) used as an agonist; VerifyNow™ assay, with the P2Y12 or aspirin cartridge (Accumetrics); and thrombelastography (TEG), stimulated by 2.0 μmol/L ADP or 1.0 mmol/L AA.

Results: Twenty-six of 33 patients completed all scheduled visits. A low response to clopidogrel was found in a few patients at variable frequencies and at different visits, depending on the method and criteria used. We found a moderate correlation between the LTA (ADP) and VerifyNow (P2Y12 cartridge) results, but the TEG (ADP) results correlated poorly with the LTA and VerifyNow results. A low response to aspirin was found with the VerifyNow (aspirin cartridge) and TEG (AA) methods on 6 and 2 occasions, respectively, but not with the LTA (AA) method, except for 1 occasion caused by probable noncompliance.

Conclusions: Detecting a low response to clopidogrel depends largely on the method used. Which method best predicts ischemic events remains uncertain. A low response to aspirin is rare with AA-dependent methods used at the chosen cutoffs. In some patients, the response to clopidogrel or aspirin may be classified differently at different times, even with the same method.

Clinical implications of clopidogrel non-response in cardiovascular patients: a systematic review and meta-analysis

BSUMMARY BACKGROUND: Previous studies have shown an important risk of cardiovascular events in patients with clopidogrel biological non-response, and data have shown considerable, unexplored heterogeneity.

OBJECTIVES

To evaluate the magnitude of cardiovascular risk associated with clopidogrel non-response and to explore heterogeneity.

METHODS

This was a systematic review and meta-analysis of prospective studies of patients treated with clopidogrel for symptomatic atherothrombosis, evaluated by light transmission aggregometry with ADP and monitored prospectively for clinical ischemic events.

RESULTS

Fifteen studies were included, totaling 3960 patients, of whom 25% were considered to be clopidogrel non-responders. The global relative risk (RR) for recurrent ischemic events in clopidogrel non-responders was 3.5 [95% confidence interval (CI) 2.4-5.2, P < 0.0001]. The results of the different studies were heterogeneous (Cochran P = 0.01 and I(2) = 52%). The most recent studies yielded lower RRs [global RR = 2.9 (95% CI 2.3-3.8) after 2007, and global RR = 6.6 (95% CI 3.7-11.9) before 2007, P = 0.01]. Heterogeneity was present in the group of studies in which more than 10% of patients took glycoprotein (GP)IIb-IIIa inhibitors [Cochran P = 0.003 and I(2) = 70%; RR = 3.8 (95% CI 2.9-5.1)] and was absent in the other studies [Cochran P = 0.88 and I(2) = 0; RR = 2.5 (95% CI 1.7-3.6)]. The RR was significantly higher in studies using higher ADP maximal aggregation cut-offs (> 65%) for clopidogrel non-response than in studies using lower cut-offs [RR = 5.8 (95% CI 3.2-10.3) and RR = 2.9 (95% CI 2.2-3.7), respectively, P = 0.03].

CONCLUSIONS

The risk of ischemic events associated with clopidogrel non-response is now more precisely defined. The risk is heterogeneous across studies, possibly because of an interaction with GPIIb-IIIa inhibitors and the use of different cut-offs to identify non-responders.

Response to ticagrelor in clopidogrel nonresponders and responders and effect of switching therapies: the RESPOND study

BACKGROUND

The antiplatelet effects of the Platelet Inhibition and Patient Outcomes (PLATO) trial dose of ticagrelor in patients nonresponsive to clopidogrel and after they switch agents are unknown.

METHODS AND RESULTS

Patients with stable coronary artery disease on aspirin therapy received a 300-mg clopidogrel load; nonresponders were identified by light transmittance aggregometry. In a 2-way crossover design, nonresponders (n=41) and responders (n=57) randomly received clopidogrel (600 mg/75 mg once daily) or ticagrelor (180 mg/90 mg twice daily) for 14 days during period 1. In period 2, all nonresponders switched treatment; half of the responders continued the same treatment, whereas the others switched treatment. Inhibition of platelet aggregation was higher in nonresponders treated with ticagrelor compared with clopidogrel (P<0.05). Treatment with ticagrelor among nonresponders resulted in a >10%, >30%, and >50% decrease in platelet aggregation from baseline in 100%, 75%, and 13% of patients, respectively. Platelet aggregation fell from 59+/-9% to 35+/-11% in patients switched from clopidogrel to ticagrelor and increased from 36+/-14% to 56+/-9% in patients switched from ticagrelor to clopidogrel (P<0.0001 for both). Platelet reactivity was below the cut points previously associated with ischemic risk measured by light transmittance aggregometry, VerifyNow P2Y(12) assay, and vasodilator-stimulated phosphoprotein phosphorylation in 98% to 100% of patients after ticagrelor therapy versus 44% to 76% of patients after clopidogrel therapy.

CONCLUSIONS

Ticagrelor therapy overcomes nonresponsiveness to clopidogrel, and its antiplatelet effect is the same in responders and nonresponders. Nearly all clopidogrel nonresponders and responders treated with ticagrelor will have platelet reactivity below the cut points associated with ischemic risk. Clinical Trial Registration- http://www.clinicaltrials.gov. Unique Identifier: NCT00642811.

Time does not heal every wound: coronary stent thrombosis of a bare-metal stent more than one decade after its implantation

In the present report, we describe a unique case of very late stent thrombosis with a bare-metal stent that occurred more than a decade after stent implantation. Potential explanations for the late stent thrombosis are non-compliance to aspirin, late acquired malapposition of the stent, progression of atherosclerosis or in-stent restenosis. In our patient, none of these explanations seems to have played a role. Although the occurrence of (very) late stent thrombosis is not uncommon with drug-eluting stent (DES), it is rather unusual with bare-metal stent (BMS). Nevertheless, cardiologists should be aware of the potential complication of late stent thrombosis, even with bare-metal stents.

Variability of residual platelet function despite clopidogrel treatment in patients with peripheral arterial occlusive disease

Residual platelet function despite treatment with clopidogrel may predict an unfavourable cardiovascular outcome. The majority of studies have investigated the effects of clopidogrel administration in conjunction with aspirin in patients undergoing percutaneous coronary intervention. The primary objective of the present study was to assess the platelet response to clopidogrel in the absence of aspirin in patients with peripheral arterial occlusive disease (PAOD) and to investigate whether non-responsiveness to clopidogrel is reproducible during long-term follow-up. Fifty-four clinically stable PAOD patients on a maintenance dose of 75 mg/d clopidogrel were enrolled in this study. Platelet function was assessed at baseline and after a median follow-up of 18 months using light transmittance aggregometry (LTA) with 2 microM ADP as an agonist. HPLC-coupled mass spectrometry was used to detect clopidogrel and clopidogrel carboxylic acid, the main metabolite of clopidogrel. Residual platelet function, as defined by late aggregation values within the reference range (i.e., >43%), was observed in 35.2% of patients at baseline and 17.6% during follow-up. During the observation period, 26.5% had switched from responder to non-responder status or vice versa. Among non-responders, either clopidogrel or its metabolite was detected in 89.5% and 83.3% of patients at baseline and at follow-up, respectively. We conclude that non-responsiveness to clopidogrel as determined by ADP-induced LTA is not stable over time. This phenomenon cannot be attributed to non-compliance alone.

The significance of clopidogrel low-responsiveness on stent thrombosis and cardiac death assessed by the verifynow p(2)y(12) assay in patients with acute coronary syndrome within 6 months after drug-eluting stent implantation

Background and Objectives

Clopidogrel resistance or low-responsiveness may be associated with recurrent atherothrombotic events after drug-eluting stent (DES) implantation. We prospectively evaluated the association between clopidogrel resistance assessed by the Verifynow™ P₂Y₁₂ assay (Accumetrics, San Diego, CA, USA) and stent thrombosis (ST) or cardiac death (CD) in patients with acute coronary syndrome (ACS) within 6 months after DES implantation.

Subjects and Methods

We enrolled 237 consecutive patients (160 males, 65.2±10.3 years) with ACS who received a DES implantation. The composite endpoint was defined to CD or ST by Academic Research Consortium definitions within 6 months post-implantation. Clopidogrel resistance was defined as <20% inhibition of the P₂Y₁₂ receptor.

Results

Baseline demographic characteristics were similar between 142 normal individuals and 95 clopidogrel resistant patients. CD occurred in one case (0.7%) in the normal group and two cases (2.13%) in the resistant group (p=0.344). There was no episode of ST in the normal group and four episodes in the resistant group (4.2%, four definite ST) (p=0.035). Univariate logistic regression revealed an adjusted odds ratio (OR) for composite end point of CD or ST of 9.646 {95% confidence interval (CI) 1.139-81.679}, and multivariate logistic regression for composite end point revealed an OR of 12.074 (95% CI 1.205-120.992).

Conclusion

Clopidogrel low-responsiveness assessed by the Verifynow™ P₂Y₁₂ assay is an independent predictor of ST and composite end point of ST or CD in patients with ACS within 6 months after DES implantation.

Aspirin and clopidogrel hyporesponsiveness and nonresponsiveness in patients with coronary artery stenting

Patients undergoing coronary artery stenting receive an antiplatelet regimen to reduce the risk of antithrombotic complications. Current guidelines recommend the use of acetyl salicylic acid (aspirin) and clopidogrel as evidenced by large clinical trials. There has been a concern about variable responses of patients to aspirin and clopidogrel which may predispose them to subacute stent thrombosis or late stent thrombosis. Up to 25% of patients with acute myocardial infarction (AMI) undergoing percutaneous coronary intervention (PCI) were found to have hyporesponsiveness or resistance to clopidogrel which may predispose them to recurrent events. Dual antiplatelet regimen is a standard therapy in these patients and there is always a concern about variable responses to aspirin and clopidogrel predisposing them to acute coronary syndrome (ACS). Prevalence of this hyporesponsiveness or resistance may be due to noncompliance, genetic mutations, co-morbid situations and concomitant use of other drugs. This issue is of considerable importance in the era of coronary drug eluting stents when a long-term dual antiplatelet regimen is needed. This paper is a review for clinicians taking care of such patients with hyporesponsiveness or nonresponsiveness to dual antiplatelet regimen.