[Clopidogrel resistance: myth or reality].

The efficacy of clopidogrel, as an established anti-platelet agent for the acute coronary syndrome treatment and for the thrombotic complications prevention after percutaneous coronary angioplasty and coronary artery stenting has been supported by the evidence of several major randomized clinical trials. However, some patients treated with clopidogrel have a minor, but still a risk of coronary artery thrombosis and sudden death. Moreover, clopidogrel was not effective in patients after ischemic stroke, as well as it was not effective for the primary prevention of vascular events. Several authors proposed the theory of " clopidogrel-resistance " . However, this theory is based on a limited number of laboratory findings, and was not supported by the evidence of clinical studies. The phenomena of " clopidogrel-resistance " could be masked by the low patient compliance, while the rate of such patients could exceed 30% after one year of treatment. The offered methods for overcoming clopidogrel-resistance include doubling or tripling of the loading dose (600-900 mg vs. 300mg) or administration one or two more potent antiplatelet agents. However, such approach could not only increase the risk of major and fatal bleedings, but could have a potential to reduce patients compliance, and consequently increase the risk of thrombosis. Only multicenter randomized study with hard outcome or ideally survival endpoint, supported by comprehensive serial platelet assessment, strict compliance rules including measurement of clopidogrel metabolite(s) will determine whether " clopidogrel resistance " is a real danger (as suggested by the platelet biomarkers), or an artificial tool (as suggested by the randomized clinical evidence) introduced to help novel antiplatelet agents to gain the vascular market share.

Platelet inhibition beyond conventional antiplatelet agents: expanding role of angiotensin receptor blockers, statins and selective serotonin reuptake inhibitors.

Aspirin, dipyridamole, cilostazol, thienopyridines and glycoprotein IIb/IIIa inhibitors represent the classical examples of the established antiplatelet agents commonly used for the secondary prevention in patients after vascular events. Obviously, the era of expanding antiplatelet regimens and indications may require new agents as the substitutes, or additions to the available strategies. However, recent results of the majority of antiplatelet trials strongly suggest boarder line advantages in clinical outcomes, and higher associated bleeding risks with the novel antiplatelet agents or/and regimens. Moreover, unexpected failures, such as lack of efficacy of clopidogrel and aspirin combination for ischaemic stroke prevention (MATCH), or use of the same antiplatelet regimen for the primary vascular prevention (CHARISMA) raise legitimate concerns that the concept 'the more the better' may not be valid. Broad use of statins, angiotensin receptor blockers and selective serotonin reuptake inhibitors may be in part responsible for the lack of impressive results with the antiplatelet therapy because each of these drug classes per se inhibits platelets. In this review, we discuss the available evidence and potential clinical significance of these findings.

Rebound platelet activation after termination of prasugrel and aspirin therapy due to confirmed non-compliance in patient enrolled in the JUMBO Trial.

Therapy with aspirin and/or adenosine diphosphate (ADP) receptor blockers is associated with better outcomes via inhibition of platelet activity, and subsequent reduction of ischemic vascular events. Non-compliance (NC) is a well-recognised hazard limiting the clinical utility of antiplatelet agents, and, probably worsening outcomes. However, comprehensive platelet characteristics of a confirmed NC patient after acute vascular event have never been reported within a major randomised trial with ADP-receptor antagonists. A 48-year-old male patient, well-educated, was among patients enrolled in the platelet sub-study for the JUMBO trial. He received 325 mg of aspirin daily for 9 months, presented with unstable angina for urgent coronary intervention, and was successfully reperfused with two intracoronary stents. The patient was randomised to a 60 mg prasugrel loading dose, and 10 mg of prasugrel daily for 30 days. Platelets were assessed at baseline, 4 and 24 h, and at 30 days after acute coronary event utilising ADP-, and collagen-induced conventional aggregometry, rapid cartridge-based analyser and flow cytometry. Loading with prasugrel resulted in significant inhibition of platelet activity during and after stenting. However, after assessing platelet biomarkers at 30 days, voluntary withdrawal from the antiplatelet agents was suspected. Based on the platelet activity characteristics, NC was later confirmed, and the patient admitted that he stopped taking both prasugrel and aspirin shortly after discharge due to minor bleeding episodes after shaving. Major platelet activity biomarkers of the index NC patient were compared with those from compliant prasugrel-, clopidogrel-treated patients, and healthy controls. The platelet tests uniformly revealed rebound activation by all platelet measures (at least twofold increase) while being especially high for ADP-, and collagen-induced aggregation, platelet/endothelial cell adhesion molecule-1 (PECAM-1), glycoprotein (GP)Ib, GPIIb/IIIa activity, P-selectin, protease activated receptor (PAR)-1 thrombin receptor (activated and intact epitopes), and thrombospondin expression. The clinical benefits of antiplatelet agents are not only denied in NC outpatients, but may put them at additional risk for worsened vascular outcomes due to the rebound platelet activation. Proclaimed 'resistance' to antiplatelet agents may at least in part be a result of NC, especially in the chronic uncontrolled setting. Enforcing compliance will improve outcomes in the clinical trials, and save lives of patients really receiving antiplatelet therapy.

Platelet inhibition with prasugrel (CS-747) compared with clopidogrel in patients undergoing coronary stenting: the subset from the JUMBO study.

BACKGROUND: Based on the preclinical and phase 1 studies, prasugrel, a novel platelet ADP P2Y12 receptor blocker, may be a more potent platelet inhibitor than clopidogrel. This study compared the antiplatelet properties of prasugrel in a small subset of patients enrolled in the JUMBO trial, and compared with historic clopidogrel treated controls. METHODS AND RESULTS: Nine patients undergoing coronary stenting were randomised to one of three arms of prasugrel (40 mg loading, and 7.5 mg maintenance, n = 1; 60/10 mg, n = 4; or 60/15 mg, n = 2), or clopidogrel (300/75 mg, n = 2). Aspirin and GP IIb/IIIa inhibitors were permitted. Platelet activity was assessed at baseline, at 4, and 24 hours, and at 30 days after stent implantation in substudy participants, and compared with 124 historic controls who received clopidogrel. Independent of the loading, or maintenance dose, patients treated with prasugrel exhibited significantly more potent platelet inhibition as determined by ADP, and collagen induced aggregation, Ultegra Analyser, and surface expression of PECAM-1, GPIIb/IIIa antigen, and activity with PAC-1 antibody, GPIb, P-selectin, CD40-ligand, GP37, and thrombospondin receptor expression when compared with those treated with clopidogrel. There were no differences between antiplatelet agents with regard to vitronectin, LAMP-1, PAR-1 (intact and cleaved epitopes) thrombin receptor expression, or formation of platelet-monocyte microparticles. Expression of GPIIb antigen, vitronectin, and LAMP-3 receptor were not affected by both agents. Two patients treated with prasugrel 10 mg/daily exhibited complete inhibition of collagen induced aggregation at 30 days. CONCLUSION: At the dosing regimens chosen in the JUMBO trial, it seems that prasugrel is a more potent antiplatelet agent than clopidogrel. Two episodes of profound platelet inhibition, which are not seen with clopidogrel, raise the possibility of higher bleeding risks especially during long term prasugrel use. Whether stronger platelet inhibition will yield better clinical outcomes and/or increased bleeding remains to be determined in an ongoing comparative phase 3 superiority trial (TRITON).

Selective serotonin reuptake inhibitors yield additional antiplatelet protection in patients with congestive heart failure treated with antecedent aspirin.

Clinical depression has been identified as an independent risk factor for increased mortality in patients with coronary artery disease. Enhanced platelet activity has been suggested as the mechanism responsible for this adverse association. Selective serotonin reuptake inhibitors (SSRIs) are known to inhibit platelets in patients undergoing coronary stenting. We sought to determine whether concomitant therapy with SSRIs would yield additional anti-platelet benefit in patients with congestive heart failure (CHF) already treated with antecedent aspirin. A total of 88 patients with left ventricular ejection fraction (LVEF) <40% or CHF symptoms in the setting of preserved systolic function and NYHA Class II-IV were analyzed. Of these, 23 patients (26%) were chronic SSRI users (SSRI+), and 65 patients were free from SSRI therapy (SSRI-). All patients received aspirin (325 mg) for at least 1 month prior to platelet studies. Platelets were assessed by aggregometry, flow cytometry and a rapid analyzer. The SSRI+ group exhibited a substantial decrease in platelet activity when compared with SSRI- patients, as manifested by a significant reduction in ADP- (P=0.001), and collagen-induced (P=0.02) aggregation, and the expression of PECAM-1 (P=0.03), GPIb (P=0.03), GP IIb/IIIa antigen (P=0.02) and GP IIb/IIIa activity with PAC-1 antibody (P=0.04) and P-selectin (P=0.02). Therapy with SSRIs also resulted in the reduced formation of platelet-leukocyte microparticles (P=0.01). Epinephrine-induced aggregation in plasma, collagen-induced whole blood aggregation, closure time and expression of vitronectin receptor, CD63, CD107a, CD107b and CD151 did not differ between groups. In patients with CHF already on aspirin, SSRI therapy was associated with further inhibition of platelet function. This observation may help to explain some of the clinical benefits associated with SSRI therapy. Further clinical trials may help to elucidate the potential outcome benefits of SSRIs in other potential thrombotic circumstances.

Paradoxical activation of major platelet receptors in the methadone-maintained patients after single pill of aspirin.

BACKGROUND: Chronic drug abuse is an established cause of acute coronary events and sudden death. The association between use of narcotics and platelet abnormalities is well described. However, it is still not clear, how aspirin affects expression of major platelet receptors in chronic drug users with coronary artery disease, especially those recovering in the methadone clinic maintenance program. We sought to compare how a single pill of non-enteric coated aspirin (325-mg) affects human platelets in patients with coronary artery disease dependent on methadone use. METHODS: Data from 30 subjects were analyzed, eight of them were the chronic drug addicts, and participated in a methadone recovery program. Platelets were assessed twice at baseline (pre-aspirin), and after 3-24 hours (post-aspirin). The expression of platelet receptors was determined by using the following monoclonal antibodies: CD31 (PECAM-1), CD41 (GPIIb), CD42b (GPIb), CD51/CD61 (vitronectin receptor), CD62p (P-selectin), CD63 (LIMP or LAMP-3), CD107a (LAMP-1), CD151 (PETA-3), and PAC-1 for fibrinogen-platelet binding determination (PharMingen, San Diego, CA). Platelet-leukocyte interactions were assessed by using dual antibodies for a pan-platelet marker (CD151), together with CD14, a monocyte/macrophage marker. RESULTS: In a drug free group, digestion of a single tablet of aspirin resulted in a significantly (p<0.05) diminished expression of PECAM-1, GP IIb, fibrinogen binding with PAC-1 antibody, GP Ib, P-selectin, and CD151. In contrast, patients receiving methadone exhibited opposite trends, resultant in a paradoxical activation of major platelet receptors after digestion of aspirin. These differences reached statistical significance (p<0.05) for PECAM-1, GPIIb, and P-selectin expression. CONCLUSION: There are some fundamental differences in the responsiveness to aspirin in chronic methadone users when compared with drug-free patients. Suspecting narcotics abuse may be critical in patients with limited aspirin efficacy, or those who developed an aspirin resistance. Unexpected platelet activation may indeed represent a missing link between use of narcotics and enhanced incidence of vascular death in this high-risk population.

Uniform platelet activation exists before coronary stent implantation despite aspirin therapy.

BACKGROUND: Platelets play an important role in the natural history of coronary artery disease. Enhanced platelet aggregation and receptor expression unquestionably occur after coronary stent implantation; however, the functional characteristics of platelets before stenting have not been fully elucidated. METHODS: Platelets were assessed before intervention by platelet-rich plasma aggregation (PA) with 5 mmol adenosine diphosphate (ADP) and 1 mg/mL collagen; whole blood aggregation (WBA) by 1 mg/mL collagen; shear-induced closure time (CT); contractile force (CF); and expression of 9 surface receptors by flow cytometry in 126 patients undergoing elective coronary artery stent placement. All patients received aspirin for at least 7 days. The data were compared with those from 64 healthy volunteers. RESULTS: Each test revealed sustained platelet activation in patients undergoing coronary stenting compared with control values. These differences were significant for collagen-induced PA (P =.031); CF (P =.0001); expression of glycoprotein (GP) IIb/IIIa (P =.0001); P-selectin (P =.0008); platelet/endothelial cell adhesion molecule (PECAM)-1 (P =.0001); CD107a (P =.0001); CD107b (P =.0004); and CD63 (P =.009). CONCLUSION: Platelets are indeed activated before coronary stenting despite antecedent therapy with aspirin.

Platelet activation as a universal trigger in the pathogenesis of acute coronary events after cocaine abuse.

Numerous reports document the widespread use of cocaine in most parts of the world, which confers an increased risk of vascular morbidity and mortality. The mechanisms responsible for this association are multifactorial, but platelet activation might play a substantial role linking these events. Contradictory data exist regarding the cellular mechanisms of cocaine's effects on thrombocytes. In terms of therapeutic interventions, a possible activation of platelets would conceptually require more aggressive anti-platelet therapy with aspirin, clopidogrel or other compounds, however, no data exist to date to support this approach. Further studies elucidating these issues are warranted. This review summarizes the latest and often confusing data on the interaction between cocaine and platelets in certain in vitro, animal and clinical scenarios.

Platelet function and fibrinolytic agents: two sides of a coin?

Fibrinolytic therapy is the established treatment for the management of patients with ST elevation acute myocardial infarction (AMI). Present fibrinolytic regimens have a number of shortcomings, including the failure to produce early and sustained reperfusion, as well as failure to prevent reocclusion in at least some patients. Platelets play an important role in coronary thrombosis responsible for AMI. The effect of coronary fibrinolysis on platelets has been extensively debated in the literature with evidence of both platelet activation and inhibition. Among fibrinolytic agents, tissue plasminogen activator (t-PA) is considered to be the mainstay in the treatment of coronary artery disease. The native t-PA molecule has been modified in an attempt to achieve improved lytic characteristics with less risk of bleeding. The result is a group of mutant t-PA variants considered third-generation plasminogen activators. TNK-t-PA is one bioengineered variant of t-PA. Another third-generation plasminogen activator is reteplase (r-PA). Like TNK-t-PA, it is a variant of t-PA that has been developed to establish a more rapid, complete, and stable coronary artery patency, thus promising reduced mortality. Both r-PA and TNK-t-PA are effective when given as bolus therapy. This feature may facilitate more rapid treatment as well as decrease overall costs of treatment. New fibrinolytic regimens include potent antiplatelet agents that may improve sustained reperfusion. This review summarizes the latest and often confusing data on the interaction between fibrinolytic therapy and platelets in certain in vitro, animal and clinical scenarios.