Frequency of nonresponse antiplatelet activity of clopidogrel during pretreatment for cardiac catheterization

Thrombosis after stent implantation: how much of a problem is there?

Drug-eluting stents reduce the occurrence of in-stent restenosis and the need for subsequent target vessel revascularization when compared with bare-metal stents. However, drug-eluting stents may be associated with delayed (or absent) endothelialization, localized hypersensitivity reactions and late stent thrombosis. Stent thrombosis is a catastrophic event, resulting in life-threatening complications. Although larger, adequately powered, randomized trials are needed to fully assess the net clinical effects of drug-eluting stents compared with bare-metal stents, the evidence, thus far, appears to suggest that the net clinical benefit of drug-eluting stents may outweigh their risks. Premature discontinuation of antiplatelet therapy is the most important predictor of stent thrombosis; therefore, patients who are candidates for implantation of drug-eluting stents should be screened for their ability to receive and tolerate uninterrupted antiplatelet therapy longer than is necessary with bare-metal stents.

Monitoring antiplatelet therapy with point-of-care platelet function assays: a review of the evidence

Multiple studies have demonstrated that subgroups of patients receiving combination therapy with aspirin and clopidogrel fail to produce the anticipated antiplatelet effect, and various terms such as ‘aspirin resistance’, ‘clopidogrel resistance’, ‘heightened post-treatment platelet reactivity’ and ‘residual platelet reactivity’ have been introduced in the medical literature. Light transmittance aggregometry is generally considered to be the gold standard for determining platelet function, but its relevance to in vivo platelet function is questionable and the logistical demands of the method make it impossible to use in daily practice. The introduction of several point-of-care platelet function assays may be the key to the widespread clinical use of platelet function testing and may identify patients who are at risk for the occurrence of adverse cardiac events. In the present paper, we discuss the current commercially available methods of assaying platelet function, including their advantages and limitations and whether they have been shown to correlate with clinical outcomes.

Common polymorphisms of CYP2C19 and CYP2C9 affect the pharmacokinetic and pharmacodynamic response to clopidogrel but not prasugrel

BACKGROUND

Thienopyridines are metabolized to active metabolites that irreversibly inhibit the platelet P2Y(12) adenosine diphosphate receptor. The pharmacodynamic response to clopidogrel is more variable than the response to prasugrel, but the reasons for variation in response to clopidogrel are not well characterized.

OBJECTIVE

To determine the relationship between genetic variation in cytochrome P450 (CYP) isoenzymes and the pharmacokinetic/pharmacodynamic response to prasugrel and clopidogrel.

METHODS

Genotyping was performed for CYP1A2, CYP2B6, CYP2C19, CYP2C9, CYP3A4 and CYP3A5 on samples from healthy subjects participating in studies evaluating pharmacokinetic and pharmacodynamic responses to prasugrel (60 mg, n = 71) or clopidogrel (300 mg, n = 74).

RESULTS

In subjects receiving clopidogrel, the presence of the CYP2C19*2 loss of function variant was significantly associated with lower exposure to clopidogrel active metabolite, as measured by the area under the concentration curve (AUC(0-24); P = 0.004) and maximal plasma concentration (C(max); P = 0.020), lower inhibition of platelet aggregation at 4 h (P = 0.003) and poor-responder status (P = 0.030). Similarly, CYP2C9 loss of function variants were significantly associated with lower AUC(0-24) (P = 0.043), lower C(max) (P = 0.006), lower IPA (P = 0.046) and poor-responder status (P = 0.024). For prasugrel, there was no relationship observed between CYP2C19 or CYP2C9 loss of function genotypes and exposure to the active metabolite of prasugrel or pharmacodynamic response.

CONCLUSIONS

The common loss of function polymorphisms of CYP2C19 and CYP2C9 are associated with decreased exposure to the active metabolite of clopidogrel but not prasugrel. Decreased exposure to its active metabolite is associated with a diminished pharmacodynamic response to clopidogrel.

Progress in clinical neurosciences: The 'antiplatelet' agents and the role of the endothelium

The antiplatelet drugs, commonly used in the prevention and treatment of cerebrovascular disease, possess a number of effects that are independent of direct antiplatelet actions. Beneficial and detrimental effects both occur. The endothelium is an important mediator of these non-antiplatelet effects. We performed a literature search to locate articles related to acetylsalicylic acid (aspirin), clopidogrel, ticlopidine, and dipyridamole and the interactions of these medications with the endothelium. The role of each of the above medications is explored in relation to vasodilation, inflammation, oxidation, platelet-leukocyte interactions, and thrombogenic tendency via platelet-vessel wall interactions.

Aspirin and clopidogrel resistance: consideration and management

The efficacy with aspirin and clopidogrel treatment has been demonstrated in various clinical trials. Laboratory evaluation of platelet response in recent studies revealed that a distinctive response variability and nonresponsiveness/resistance in selected patients were associated with these antiplatelet agents. Moreover, some studies have correlated this nonresponsiveness/resistance phenomenon to the occurrence of thrombotic events. At this time there are no uniformly established methods to quantify exvivo platelet reactivity after clopidogrel and aspirin treatment of the extent of platelet inhibition by clopidogrel and aspirin. Therefore, specific treatment recommendations for patients exhibiting high platelet reactivity or poor platelet inhibition during clopidogrel or aspirin therapy are not established. A higher aspirin dose and strict compliance to therapy may overcome the occurrence of "aspirin resistance" in selected patients. A higher clopidogrel dose may be considered in patients exhibiting clopidogrel nonresponsiveness.

Thrombelastography: current clinical applications and its potential role in interventional cardiology

Thrombelastography is a bedside blood test used to assess patients' haemostatic status. It has a well-established role in hepatobiliary and cardiac surgery and is also used in obstetrics and trauma medicine to assess coagulation and identify the causes of post-operative bleeding. It is not routinely used in the diagnosis or treatment of thrombosis although recently it has been shown to predict thrombotic events post-operatively and after percutaneous intervention (PCI). In cardiovascular medicine the importance of the platelet in the pathophysiology of vascular events is increasingly apparent. As a result antiplatelet therapy is a cornerstone of the treatment for coronary disease, particularly in the setting of acute coronary syndromes. The increasing utilization of stents, particularly drug-eluting devices, in PCI has also necessitated widespread use of antiplatelet agents to minimize the risk of stent thrombosis. A quick, accurate and reliable test to measure the effect of platelet inhibition by antiplatelet agents on clotting in an individual patient would be of profound clinical value. The results from such a test could provide prognostic information, allow treatment with antiplatelet agents to be tailored to the individual and identify resistance to one or more of these agents. Optimization and tailoring of anti-platelet therapy in patients with cardiovascular disease, particularly those undergoing PCI, using such a test may reduce morbidity and mortality from thrombotic and haemorrhagic complications. Current methods of assessing platelet activity measure platelet count and function in isolation. Optical aggregation is the most widely used method for assessing platelet function but it is relatively time consuming, measures platelet function in isolation rather than in the context of clot formation and is not a bedside test. By contrast the modified thrombelastograph platelet mapping kit marketed by Haemoscope can be used to assess the effects of antiplatelet agents on ex vivo blood clotting, thus giving a measurement more relevant to in vivo responses. This represents a potentially powerful tool to assess response of individual patients to antiplatelet therapy, particularly in the context of PCI.

Platelet Inhibitors and Monitoring Platelet Function: Implications for Bleeding

Cardiovascular disease is prevalent in our medical and surgical. Patients who have atherosclerotic heart disease suffer from endothelial disorders that predispose them to plaque and thrombus formation in diseased arteries. As our knowledge of platelet physiology improves, we can understand the contribution of platelet activation to arterial disease and we can specifically inhibit that activation with platelet-inhibitory drugs. The recent increase in the number of coronary interventional procedures performed has spawned the increasing use of antiplatelet medication as prophylaxis against thrombus formation in the instrumented artery.

A comparison of prasugrel and clopidogrel loading doses on platelet function: magnitude of platelet inhibition is related to active metabolite formation

BACKGROUND

The aim of this study was to compare rate of onset, magnitude, and consistency of platelet inhibition after administration of prasugrel or clopidogrel and to relate platelet inhibition to systemic exposure to each active metabolite. Thienopyridines are prodrugs, metabolized in vivo to active metabolites that inhibit the platelet P2Y12 adenosine diphosphate (ADP) receptor.

METHODS

This was an open-label, 2-way, crossover study that randomized healthy subjects (n = 68) to an oral loading dose (LD) of prasugrel 60 mg or clopidogrel 300 mg. Platelet aggregation response to 5 and 20 micromol/L of ADP was measured by turbidometric aggregometry. Plasma concentrations of the active metabolites of prasugrel and clopidogrel were quantified by liquid chromatography with tandem mass spectrometry detection methods.

RESULTS

Inhibition of platelet aggregation (IPA) after prasugrel was significantly higher (P < .01) than that after clopidogrel from 15 minutes through 24 hours (5 micromol/L ADP) and from 30 minutes through 24 hours (20 micromol/L ADP). For 20 micromol/L ADP, the median time to reach > or = 20% IPA was 30 minutes for prasugrel and 1.5 hours for clopidogrel (P < .001). The maximum IPA was 84.1% +/- 9.5% with prasugrel versus 48.9% +/- 27.0% with clopidogrel for 5 micromol/L ADP and 78.8% +/- 9.2% versus 35.0% +/- 24.5%, respectively, for 20 micromol/L ADP (P < .001). Response to prasugrel was more consistent compared to clopidogrel (P < .01). The lower IPA response to clopidogrel was associated with lower plasma concentrations of its active metabolite (P < .001).

CONCLUSIONS

Prasugrel 60 mg LD results in more rapid, potent, and consistent inhibition of platelet function than clopidogrel 300 mg LD. Lower IPA responses to clopidogrel were associated with lower concentrations of its active metabolite.

Aspirin and clopidogrel resistance

Aspirin and clopidogrel provide significant clinical benefit in patients with cardiovascular disease. However, given the complexity of platelet activation, it is not surprising that aspirin or clopidogrel prevent a small proportion of cardiovascular events. Of late, the terms aspirin and clopidogrel "resistance" have entered the physicians' lexicon, and infer a lack of therapeutic response and a single underlying mechanism, which is misleading. The incidence of "resistance" detected in studies varies with the definition applied and assay used to measure response. Rather than true resistance, however, there is a variable response that reflects the unique pharmacology and pharmacokinetics of each drug, the clinical significance of which remains to be established. True "aspirin resistance" implies that cyclooxygenase-1 is less sensitive to inactivation by aspirin. Despite 95% inhibition of serum thromboxane B(2) by aspirin, residual platelet aggregation is detected in some cases, the clinical significance of which is unknown. Heritable factors directly and indirectly related to platelet cyclooxygenase may influence aspirin response. In contrast to aspirin, the response to clopidogrel is highly variable and reflects the bioavailability of the active metabolite and not "resistance" of the receptor to inhibition.

Measuring antiplatelet drug effects in the laboratory

This review discusses the advantages and disadvantages of currently available tests for the monitoring of antiplatelet therapy (especially aspirin and clopidogrel). Many tests of platelet function are now available for clinical use, and some of these tests have been shown to predict clinical outcomes after antiplatelet therapy. However, in most of these studies, the number of major adverse clinical events was low. No published studies address the clinical effectiveness of altering therapy based on the results of monitoring antiplatelet therapy. Therefore, the correct treatment, if any, of "resistance" to antiplatelet therapy is unknown and, other than in research trials, monitoring of antiplatelet therapy in patients is not generally recommended. A clinically meaningful definition of "resistance" to antiplatelet drugs needs to be developed, based on data linking drug-dependent laboratory tests to clinical outcomes in patients.

Evidence that pre-existent variability in platelet response to ADP accounts for 'clopidogrel resistance'

BACKGROUND

Clopidogrel is a widely used antithrombotic agent that inhibits the platelet P2Y(12) adenosine diphosphate (ADP) receptor. There is increasing interest in 'clopidogrel resistance'.

OBJECTIVES

To determine whether 'clopidogrel resistance' is accounted for by a pre-existent variability in platelet response to ADP.

METHODS

Platelet response to 20 microm ADP was analyzed by four independent whole blood flow cytometric assays: platelet surface activated GPIIb-IIIa, platelet surface P-selectin, monocyte-platelet aggregates and neutrophil-platelet aggregates. In 25 consecutive, non-aspirin-treated healthy subjects, we studied platelet response before and after clopidogrel administration. In addition, we studied the platelet response in 613 consecutive aspirinated patients with or without coronary artery disease (CAD, as determined by angiography) who had or had not been treated with clopidogrel. In these patients, we tested for homogeneity of variance across all durations of clopidogrel exposure and severity of CAD by estimating the 'goodness of fit' of two independent models.

RESULTS

In the healthy subjects, pre-clopidogrel response to ADP predicted post-clopidogrel response to ADP. In the patients, clopidogrel, as expected, inhibited the platelet response to ADP. However, irrespective of the duration of clopidogrel administration, the severity of CAD, and the dose of aspirin, clopidogrel did not increase the variance in the platelet response to ADP in any of the four assays of platelet response.

CONCLUSIONS

These studies provide evidence that 'clopidogrel resistance' is accounted for by a pre-existent variability in platelet response to ADP and this variability is not increased by clopidogrel administration.

Brief review: Coronary drug-eluting stents and anesthesia

PURPOSE

Anesthesiologists managing patients with drug-eluting stents (DES) face the challenge of balancing the risks of bleeding vs perioperative stent thrombosis (ST). This article reviews DES and the influence of antiplatelet medications related to their use. A perioperative management algorithm is suggested. Novel P2Y12 antagonists currently under investigation, including cangrelor and prasugrel are considered, as well as their potential role in modification of perioperative cardiovascular risks and management of patients with DES.

SOURCE

A PubMed search of the relevant literature over the period 1985-2005 was undertaken using the terms "drug-eluting stent", "coronary artery stent", "bare metal stent", "antiplatelet medication", "aspirin", "clopidogrel."

PRINCIPAL FINDINGS

Delayed re-endothelialization may render both sirolimus-eluting and paclitaxel-eluting stents susceptible to thrombosis for a longer duration than bare metal stents. Stent thrombosis may be associated with resistance to antiplatelet medication. In patients with a DES, a preoperative cardiology consultation is essential. Elective surgery should be postponed if the duration between DES placement and noncardiac surgery is less than six months. For semi-emergent procedures, both aspirin and clopidogrel should be continued during surgery unless clearly contraindicated by the nature of the surgery. If the risk of bleeding is high, then modification of antiplatelet medications should be considered on a case-by-case basis.

CONCLUSION

A profound increase in the number of patients with DES requires anesthesiologists to be familiar with their associated antiplatelet medications, and strategies for risk modification of ST and possible hemorrhagic complications in the perioperative setting.

Current options in platelet function testing

The variable response to antiplatelet therapy has led to the use of platelet function tests to monitor the effects of antiplatelet drugs in cardiovascular diseases. The goal is to guide antiplatelet therapy to the optimal dose for the prevention or treatment of thrombosis while minimizing hemorrhagic side effects. The bleeding time is no longer recommended for use because of its nonspecificity and lack of clinical correlations. The current de facto "gold standard" test of platelet function is turbidometric platelet aggregometry. Although this method has been successful in measuring the aggregation of platelets in a glycoprotein (GP) IIb/IIIa (integrin alpha(IIb)beta(3))-dependent manner, it has several limitations, including poor reproducibility, high sample volume, requirement for sample preparation, length of assay time, requirement for a skilled technician, and cost. Therefore, new options for platelet function testing have been developed to address these disadvantages and to meet the need for point-of-care testing that can be performed at or near a patient's bedside without requiring a high degree of technical expertise. The new tests include VerifyNow (Accumetrics, San Diego, CA); Plateletworks (Helena Laboratories, Beaumont, TX); Thrombelastograph PlateletMapping System (Haemoscope Corporation, Niles, IL); Impact cone and plate(let) analyzer (DiaMed, Cressier, Switzerland); and Platelet Function Analyzer 100 (PFA-100; Dade Behring, Newark, DE). In patients treated with antiplatelet drugs, the degree of platelet inhibition, as determined by several of these new platelet function assays, has been shown to predict major adverse cardiac events.

Clopidogrel resistance?

Clopidogrel is an effective inhibitor of platelet activation and aggregation due to its selective and irreversible blockade of the P2Y(12) receptor. Combination antiplatelet therapy with clopidogrel and aspirin is an important strategy for patients with acute coronary syndromes and those undergoing percutaneous interventions. Despite significant benefits demonstrated with combination antiplatelet treatment in large clinical trials, the occurrence of adverse ischemic events, including stent thrombosis, remains a serious clinical problem. Recent studies have demonstrated distinct response variability and nonresponsiveness to clopidogrel therapy based on ex vivo platelet function measurements. Small scale investigations have suggested that nonresponsiveness may be associated with a heightened risk for adverse clinical events. The above findings have stimulated a close examination of clopidogrel metabolism.

Antiplatelet Drug Resistance and Drug-Drug Interactions: Role of Cytochrome P450 3A4

Antiplatelet therapy provided pivotal advances in the treatment of cardiovascular disease. Aspirin and thienopyridine, clopidogrel, is currently the treatment of choice in acute coronary syndromes and the prevention of thrombosis after coronary stent implantation. Despite the efficacy of this dual antiplatelet therapy in reduction of adverse coronary events in patients with acute coronary syndromes, complications persist in a subgroup of these patients. Emerging causes of aspirin and clopidogrel resistance may translate to increase risk for recurrent myocardial infarction, stroke, or cardiac related mortality. However, the mechanism of antiplatelet drug resistance remains incompletely characterized, and a sensitive and specific assay of aspirin and clopidogrel effect that reliably predicts treatment failure has not emerged. To date, evidence supporting antiplatelet drug resistance are pharmacokinetic response variability, drug-drug interaction through competitive inhibition a specific enzymatic pathway, genetic variability, and variability in the induction of enzymatic pathway in metabolic activation of prodrugs, like clopidogrel. Further investigation or guidelines are needed to optimize antiplatelet treatment strategies to identify and treat patients resistant to aspirin and/or clopidogrel.

Variability of response to clopidogrel: possible mechanisms and clinical implications

Clopidogrel has been shown to inhibit adenosine diphosphate-induced platelet aggregation and has been demonstrated to be effective in reducing the risk of arterial thrombotic events in several large clinical studies. However, the clinical benefit could be attenuated by the variability of response to the antiplatelet effects of clopidogrel in as many as 30% of patients. Multiple mechanisms likely contribute to clopidogrel variability of response, including inappropriate dosing or underdosing of clopidogrel, drug-drug interactions, and genetic polymorphisms. The best laboratory procedure to screen for possible clopidogrel variability of response remains to be determined.

Antiplatelet resistance with aspirin and clopidogrel: is it real and does it matter?

Platelets play a pivotal role in the pathophysiology of ischemic complications of atherosclerotic cardiovascular disease. Aspirin and clopidogrel are oral antiplatelet drugs that have been shown to reduce adverse clinical events across the wide spectrum of patients with atherothrombotic disease. However, recurrent ischemic events still occur in a significant proportion of patients despite treatment with these antiplatelet drugs. The concept of antiplatelet resistance therefore emerges. Although uniform definitions and standardized assays are not yet available, numerous studies have documented the interindividual variability in platelet responsiveness to oral antiplatelet drugs. Evidence is also accumulating to demonstrate that hyporesponsiveness to antiplatelet drugs in the laboratory (ie, resistance) is associated with adverse clinical events in different patient populations. Clinical application of antiplatelet resistance will require proof from prospective randomized trials that modifications of antiplatelet therapy based on tests of antiplatelet responsiveness will improve the outcomes of patients with antiplatelet resistance.

A randomized comparison of high clopidogrel loading doses in patients with non-ST-segment elevation acute coronary syndromes: the ALBION (Assessment of the Best Loading Dose of Clopidogrel to Blunt Platelet Activation, Inflammation and Ongoing Necrosis) trial

OBJECTIVES

We sought to compare the antiplatelet effects of three clopidogrel loading doses (LDs).

BACKGROUND

Administration of a 300-mg clopidogrel LD is beneficial in situations requiring rapid platelet inhibition. Whether higher LDs can provide further benefits remains unknown.

METHODS

Patients (n = 103) with non-ST-segment elevation acute coronary syndromes were randomized to receive a 300-mg, 600-mg, or 900-mg clopidogrel LD, given on top of other standard therapy (including acetylsalicylic acid). The main outcome measure was inhibition of adenosine diphosphate-induced inhibition of platelet aggregation (IPA); inhibition of platelet activation, inflammatory markers, troponin I release, and major adverse cardiac events also were evaluated; all measures were blindly evaluated.

RESULTS

Compared with the 300-mg LD, greater doses were associated with significantly greater platelet inhibition, with dose-effect relationships observed for onset of action, maximal plateau, 24-h areas under the curves of IPA, and rates of low IPA (<10% at 6 h), using 20 micromol/l major adverse cardiac events. A significant dose-response was also observed for the vasodilator-stimulated phosphoprotein index, a measure of P2Y(12) receptor inhibition. Similar but nonsignificant trends were observed for troponin release and major adverse cardiac events. Bleeding rates were similar in each group.

CONCLUSIONS

In low-to-moderate risk patients with non-ST-elevation acute coronary syndromes, clopidogrel LDs >300 mg provide a faster onset of action, a higher IPA plateau, and greater reductions in platelet activation during the first 24 h. A 900-mg LD may induce a greater antiplatelet effect than 600 mg, when compared with the standard 300-mg regimen. These findings require further clinical confirmation.

Current strategies with eptifibatide and other antiplatelet agents in percutaneous coronary intervention and acute coronary syndromes

Antiplatelet and anticoagulation therapies are essential for the prevention of thromboembolic-induced myocardial ischaemia in non-ST-elevation acute coronary syndromes and the ischaemic complications of percutaneous coronary intervention. Although heparin, direct thrombin inhibitors and oral platelet activation inhibitors provide substantial benefit, only glycoprotein (GP) IIb/IIIa inhibitors block the final common pathway leading to platelet aggregation, and the American College of Cardiology/American Heart Association guidelines recommend GP IIb/IIIa inhibitors as an integral component of care in these patients. Abciximab, eptifibatide and tirofiban all act through the GP IIb/IIIa receptor; however, variations in clinical outcomes among patients receiving these agents may be related to their structural and pharmacological differences, as well as to patient demographics. Data indicate that eptifibatide, at the current recommended dosing schedule, achieves the highest level of consistent platelet inhibition compared with current doses of abciximab and tirofiban.

Clopidogrel use in coronary artery disease

Platelet adhesion, activation and aggregation are central to the pathophysiology of the acute coronary syndromes. Clopidogrel, an oral thienopyridine derivative, is a platelet adenosine diphosphate receptor antagonist capable of inhibiting platelet activation. During the last decade, the utility of clopidogrel has been evaluated in several common clinical scenarios in a large number of patients. The benefits of clopidogrel in patients with stable coronary artery disease undergoing elective percutaneous coronary interventions and in patients presenting with acute coronary syndromes are now well established. This review outlines the pharmacology of clopidogrel, highlights the results of clopidogrel trials in the setting of acute coronary syndromes, and presents areas of uncertainty and potential future work.

A Prospective, Randomized Platelet-Function Study of Heparinized Oxygenators and Cardiotomy Suction

OBJECTIVE

The purpose of this study was to determine if substitution of a heparin-coated oxygenator and salvaged autologous blood for cardiotomy suction would improve platelet function.

DESIGN

A prospective, randomized trial.

SETTING

A large academic medical center.

PARTICIPANTS

Sixty adult patients undergoing coronary artery bypass graft surgery with cardiopulmonary bypass (CPB).

INTERVENTIONS

Patients were randomized into 1 of 4 groups in a 2 x 2 factorial design by oxygenator (heparinized v nonheparinized) and blood salvage during CPB (cardiotomy suction v salvaged autologous blood).

MEASUREMENTS AND MAIN RESULTS

Primary outcome measures were platelet function, glass-bead retention, platelet dense-body adenosine triphosphate secretion, platelet-rich plasma (PRP) aggregometry, Plateletworks platelet-function analyzer (Helena Laboratories Corp, Allen Park, MI), and platelet count. Secondary outcome measures were chest-tube drainage and allogeneic blood transfusion requirements. All platelet-function tests except thrombin-receptor activator peptide-induced PRP aggregometry showed a reduction in platelet function during and immediately after CPB (all p < 0.05). The only statistically significant difference in platelet-function tests between the groups was the glass-bead assay at 5 minutes before discontinuation of CPB (p < 0.05). This difference resolved 10 minutes after protamine administration. There were no differences between the groups in the amount of blood transfused, chest-tube drainage, and routine laboratory test results.

CONCLUSIONS

The authors concluded that the effects of these changes to the CPB circuit were small and inconsequential in this cohort of patients.

Drug Insight: clopidogrel nonresponsiveness

Platelet reactivity to agonists and subsequent activation are important factors that affect the development of atherothrombosis and resultant ischemic events. Pharmacologic intervention with clopidogrel and aspirin during acute coronary syndromes and percutaneous coronary intervention is considered the gold standard for attenuating platelet activation and aggregation. Despite significant benefits reported with dual antiplatelet treatment in major clinical trials, the occurrence of adverse ischemic events, including stent thrombosis, remains a serious clinical problem. Nonresponsiveness, also called resistance, to current clopidogrel regimens might play a part in the occurrence of ischemic events. Various mechanisms have been implicated in nonresponsiveness to clopidogrel, including variability in intestinal absorption and hepatic conversion to the active metabolite, drug-drug interactions and receptor polymorphisms. Increased loading and maintenance doses and the use of new and more-potent P2Y12-receptor blockers might overcome the phenomenon of clopidogrel nonresponsiveness. The aim of this article is to provide a comprehensive and current review of clopidogrel response variability and nonresponsiveness.

Measurement of patients' bivalirudin plasma levels by a thrombelastograph ecarin clotting time assay: a comparison to a standard activated clotting time

Standard activated clotting time (ACT) tests have a poor correlation to bivalirudin levels, leading to uncertainty regarding adequate anticoagulation in percutaneous coronary intervention patients. We tested a Thrombelastograph (TEG) ecarin clotting time (ECT) assay for sensitivity to bivalirudin using blood from 80 patients undergoing interventional cardiology procedures with bivalirudin anticoagulation. This was compared to a standard Hemochron ACT assay using diatomaceous earth. With the TEG assay, the direct thrombin activator, ecarin, was used to initiate coagulation and measured as the reaction time. Plasma samples were evaluated for bivalirudin by a chromogenic assay at an independent hematological laboratory. Linear regression of the standard ACT versus bivalirudin level gave an r = 0.306 whereas the TEG ECT gave a much higher r2 = 0.746 (both P < 0.0001). The TEG ECT should prove more useful than the standard ACT for monitoring bivalirudin anticoagulation across the clinically therapeutic range.

Oral antiplatelet therapy and percutaneous coronary intervention

The benefit of oral antiplatelet therapy following percutaneous coronary intervention (PCI) with intracoronary stent implantation is well established. Combined aspirin with clopidogrel or ticlopidine therapy is superior to aspirin alone in reducing thrombotic events after stent placement. Clopidogrel is the drug of choice, given that its efficacy is comparable to ticlopidine and it has a superior safety profile. Despite dual antiplatelet therapy, patients remain at risk of recurrent vascular events. Optimal timing, duration and dosage of antiplatelet therapy remain controversial. Recent evidence suggests additional benefit with clopidogrel pretreatment, high clopidogrel loading dose and long-term dual antiplatelet therapy post-PCI in high-risk patients.

Clopidogrel resistance: fact and fiction

The antiplatelet agent clopidogrel in combination with aspirin has been shown to reduce thrombotic events in patients with acute coronary syndromes and/or who are undergoing percutaneous coronary intervention. However, a large interindividual response variability to clopidogrel has been described. The reported rates of inadequate clopidogrel response vary considerably depending on the definition and methodologies used to measure the inhibition of platelet function. Recently, several (small) studies have demonstrated the clinical relevance of an inadequate response to clopidogrel. Moreover, several factors have been associated with a high interindividual variability in response to clopidogrel. These are: dosing, impaired intestinal absorption, cytochrome P450 3A4 and 3A5 activity, drug–drug interactions, polymorphisms of the receptors involved in the process of arterial thrombosis and hemostasis, and the method of measurement of platelet function. Future research for the evaluation of clopidogrel resistance should be based on the assessment of selective P2Y12 receptor inhibition (e.g., the vasodilator-stimulated phosphoprotein-assay or the measurement of stabilization of platelet aggregates) with quick and simple tests. Only then can we reveal the true prevalence and impact of clopidogrel resistance.

High post-treatment platelet reactivity identified low-responders to dual antiplatelet therapy at increased risk of recurrent cardiovascular events after stenting for acute coronary syndrome

BACKGROUND AND OBJECTIVES

Low response to antiplatelet therapy may be a risk factor for the development of ischemic complications in patients with non-ST segment elevation acute coronary syndrome (NSTE ACS) undergoing coronary stenting.

METHODS

We prospectively studied the platelet response to both clopidogrel and aspirin in 106 NSTE ACS consecutive patients undergoing percutaneous coronary intervention (PCI) with stenting. A single post-treatment blood sample was obtained just before PCI and analyzed by platelet aggregometry using both ADP and arachidonic acid (AA) as agonists to explore the responses to clopidogrel and aspirin, respectively. Patients were divided into quartiles according to the ADP or AA induced maximal intensity of platelet aggregation. Patients of the highest quartile (quartile 4) were defined as the 'low-responders'.

RESULTS

Twelve recurrent cardiovascular (CV) events occurred during the 1-month follow-up. Clinical outcome was significantly associated with platelet response to clopidogrel [Quartile 4 vs. 1, 2, 3: OR (95% CI) 22.4 (4.6-109)]. Low platelet response to aspirin was significantly correlated with clopidogrel low response (P = 0.003) but contributed less to CV events [OR (95%CI): 5.76 (1.54-35.61)].

CONCLUSIONS

A post-treatment ADP-induced platelet aggregation performed just before PCI identifies low responders to dual antiplatelet therapy with an increased risk of recurrent CV events.

Resistance to antiplatelet drugs: current status and future research

Platelet reactivity and activation are important factors during the development of atherothrombotic processes and subsequent ischaemic complications. Pharmacological agents that suppress platelet function are proved to be the most efficient in the prevention and treatment of thrombotic complications. As the activation of platelets during thrombus generation involves many complex and redundant pathways, simultaneous use of different antiplatelet drugs that are directed against different targets have been effective in reducing adverse clinical events. The main antiplatelet drugs are aspirin (which inhibits thromboxane synthesis), thienopyridines (which block P2Y12 receptors) and glycoprotein IIb/IIIa antagonists (which block glycoprotein IIb/IIIa receptors). In recent years, resistance or nonresponsiveness to antiplatelet therapy has been reported and, more importantly, are linked to the occurrence of adverse cardiovascular events. New treatment strategies to overcome nonresponsiveness are being sought. A focus on the development of simple, reproducible and user friendly point-of-care methods to determine aspirin/clopidogrel responsiveness should be undertaken to assist clinicians in tailoring antiplatelet therapy to the individual patient.

Aprotinin Reduces Bleeding and Blood Product Use in Patients Treated With Clopidogrel Before Coronary Artery Bypass Grafting

BACKGROUND

An increased proportion of patients undergoing urgent coronary artery bypass graft surgery (CABG) is being treated with clopidogrel, an irreversible platelet inhibitor. Clopidogrel in combination with aspirin is known to augment bleeding, transfusion requirements, and reoperation rates after CABG. Aprotinin, a protease inhibitor, is approved for use in cardiac surgery to reduce bleeding. The aim of this study was to investigate whether or not intraoperative use of aprotinin decreases bleeding and number of transfusions after CABG in patients treated with clopidogrel less than 5 days before surgery.

METHODS

We retrospectively reviewed the medical records of all consecutive patients, with preoperative clopidogrel exposure less than 5 days before surgery, who underwent urgent CABG at our institution during 1 year (n = 33). Eighteen patients received a full-dose aprotinin regime intraoperatively whereas 15 patients not receiving aprotinin served as a control group.

RESULTS

The two groups were comparable with respect to baseline characteristics and operative data. Mean postoperative bleeding was 710 mL (95% confidence interval [CI]: 560 to 860) in the aprotinin group versus 1,210 mL (95% CI: 860 to 1550) in the control group (p = 0.004). The aprotinin group received fewer transfusions of packed red blood cells (0.9 U, 95% CI: 0.1 to 1.7, versus 2.7 U, 95% CI: 1.4 to 4.1; p = 0.01), platelets (0.1 U, 95% CI: 0 to 0.3, versus 0.6 U, 0.2 to 0.9; p = 0.02), and fewer blood product units (1.1 U, 95% CI: 0.1 to 2.0, versus 3.7 U, 95% CI: 2.1 to 5.4; p = 0.002). There were 3 reoperations for bleeding, all in the control group (p = 0.05).

CONCLUSIONS

Aprotinin reduces bleeding, transfusion requirements of packed red blood cells, platelets, and total blood units in patients on clopidogrel undergoing urgent CABG.

[Variable extent of platelet responsiveness to clopidogrel inhibition: "clopidogrel resistance"?]

During percutaneous coronary angioplasty, platelet inhibition by clopidogrel and aspirin has drastically decreased the risk of thrombotic occlusion of the stented vessels. However, despite the widespread use of these drugs, the incidence of acute or subacute stent thrombosis remains elevated, concerning 1 to 2% of the treated patients. Considerable differences in the responsiveness to clopidogrel could be observed, suggesting a possible underlying biological resistance. "Clopidogrel resistance" has recently been associated to an increased risk of thrombotic events following coronary angioplasty. Variations in enteric absorption, biotransformation in the liver by the CYP3A4, changes in the ADP receptor P2Y12, abnomalies of intraplatelet signal transduction, extent of platelet activation, class angina, diabetes mellitus may account for the considerable interindividual response variability widely reported. In this view, laboratory tests evaluating "clopidogrel resistance" might be useful tools for the identification and follow-up of patients at higher thrombotic risk. Indeed, in these patients, further platelet inhibition can be achieved by higher doses of clopidogrel.

Resistance to clopidogrel: a review of the evidence

Current available data show that about 4% to 30% of patients treated with conventional doses of clopidogrel do not display adequate antiplatelet response. Clopidogrel resistance is a widely used term that remains to be clearly defined. So far, it has been used to reflect failure of clopidogrel to achieve its antiaggregatory effect. The interpatient variability in clopidogrel response is multifactorial. It can be due to extrinsic or intrinsic mechanisms. Among extrinsic mechanisms are the possibility of clopidogrel underdosing in patients undergoing stenting or with acute coronary syndrome, and drug-drug interactions involving CYP3A4. Intrinsic mechanisms include genetic polymorphisms of the P2Y(12) receptor and of the CYP3As, accrued release of adenosine diphosphate, or up-regulation of other platelet activation pathways. Presently, there is no definite demonstration of an association between low responsiveness to clopidogrel and thrombotic events. The optimal level of clopidogrel-induced platelet inhibition, which will correlate quantitatively with clopidogrel's ability to prevent atherothrombotic events is still lacking. Furthermore, because there is no single and validated platelet function assay to measure clopidogrel's antiplatelet effect, it is not justified to routinely look for clopidogrel resistance in the clinical setting. This review discusses currently available evidence surrounding the variability in the antiplatelet response to clopidogrel.

Bisphosphonates as anti-inflammatory agents in ankylosing spondylitis and spondylarthropathies

NSAIDs remain the cornerstone of the treatment of ankylosing spondylitis (AS) and spondylarthropathies (SpA), and have been successfully used for a long time in these diseases. However, some patients remain refractory or intolerant to NSAIDs and new effective treatments have recently emerged, namely TNF-alpha-blocker agents. Other therapeutic options targeting the bone, such as bisphosphonates, have also been tried in refractory AS or SpA patients. The anti-inflammatory properties of bisphosphonates give the rationale for the use of these compounds in AS and SpA, and include the inhibition of antigen presenting cells, the modulation of pro-inflammatory cytokine generation, and also a decreased bone mass in AS. Open trials using pamidronate gave favourable results, and one controlled study comparing the efficacy of pamidronate 10 versus 60mg showed that the 60mg dose was effective in AS. Further studies are required to confirm these preliminary data and to better determine the optimal regimen (dosage and rhythm) of administration.

Platelet function analysis

Since the last guidelines for BCSH platelet function testing were written in the late 1980s, many new tests have become available. Previously most platelet function tests were traditionally utilized to aid in the diagnosis and management of patients with platelet and haemostatic disorders. Most traditional tests were also largely restricted to the specialized laboratory or centre. However, nowadays there is also much renewed interest in monitoring the efficacy of anti-platelet therapy and measuring platelet hyper-function. A number of dedicated platelet function instruments have now become available that are much simpler to use and are beginning to be utilized as point of care instruments. These can now provide measurement of platelet function within whole blood without the requirement of sample processing. Some are also beginning to be incorporated into routine clinical use and can be utilized as not only as general screening tests of platelet function but to monitor anti-platelet therapy and to potentially assess both risk of bleeding and/or thrombosis. Modern flow cytometric-based platelet function analysis now also provides a wide variety of specific tests that can assess different aspects of platelet biology that are useful for diagnostic purposes. This review will highlight some of these of new tests/instruments and discuss their potential utility both within the haemostasis laboratory but also as potential point of care instruments.

A novel modification of the Thrombelastograph assay, isolating platelet function, correlates with optical platelet aggregation

Flow cytometry, singlet platelet counting, and optical aggregation have been used to monitor clopidogrel and glycoprotein IIb/IIIa (GPIIb/IIIa) platelet antagonists. Optical aggregation is considered the gold standard, but neither it nor flow cytometry is convenient in larger-scale clinical studies or point-of-care systems. Singlet platelet counting, a point-of-care assay correlated with optical platelet aggregation, only provides a measurement of platelet function at a single point in time. The Thrombelastograph is used to assay whole blood for thrombin-generated maximal clot-shear elasticity, referred to as the maximal amplitude (MA). Although platelet dysfunction, thrombocytopenia, and the in vitro effect of strong inhibitors such as IIb/IIIa antagonists can be observed, with thrombin generation milder platelet inhibitors cannot be assessed. We modified the Thromboelastograph assay, using reptilase and factor XIIIa, to form a clot, without thrombin generation, in heparinized whole blood. The resulting clot MA is dependent on added platelet agonists such as ADP or arachidonic acid, is sensitive to platelet antagonists, and provides a continuous measure of platelet function more analogous and better correlated with optical aggregation. This novel modification of the Thromboelastograph assay should prove to be a useful point-of-care whole-blood assay with which to monitor the effects of GPIIb/IIIa, ADP, and thromboxane A(2)-receptor-inhibiting drugs in patients.