Unmet needs in oral antiplatelet therapy with ADP receptor blocking agents

Antiplatelet agents like aspirin and clopidogrel are treatment cornerstones for acute coronary syndromes (ACS). Drawbacks of dual therapy with these agents include slow onset and offset of effect and wide response variability. Clopidogrel may provide little benefit if administered too close to percutaneous coronary intervention (PCI) and increase major bleeding risk if given too close to coronary artery bypass grafting (CABG) or other surgery. It may not provide sufficient antiplatelet coverage prior to CABG if stopped too long before intervention and leave patients without antiplatelet coverage due to hyporesponsiveness. Prasugrel has made steps towards addressing these limitations by exhibiting more efficient metabolism, more rapid onset of effect, and greater and more consistent platelet inhibition than clopidogrel. The TRITON-TIMI38 trial in ACS patients undergoing PCI showed prasugrel produced greater ischemic event protection than clopidogrel but significantly increased major bleeding risk. AZD6140, the first reversible oral P2Y₁₂ inhibitor, provides more rapid onset of effect and greater and more consistent platelet inhibition than clopidogrel. In DISPERSE2, a phase II trial in ACS patients, AZD6140 did not increase bleeding risk, reduced bleeding risk among CABG patients, and produced numerical reductions in myocardial infarction risk. AZD6140 is being compared with clopidogrel in PLATO, a phase III trial in approximately 18000 ACS patients.     

The future of platelet function testing to guide therapy in clopidogrel low and enhanced responders

Dual oral antiplatelet therapy with aspirin and clopidogrel is the therapy of choice in patients with acute coronary syndromes and in patients undergoing coronary stent placement to lower the risk of thrombotic events. Responsiveness to aspirin and especially to clopidogrel is not uniform and is subject to considerable interindividual variability. Furthermore, there is a broad consensus that clopidogrel low response or so-called high on-treatment platelet reactivity is linked to the occurrence of ischemic events. On the other hand, evidence is accumulating that enhanced clopidogrel responders are at increased risk of bleeding. Newer antiplatelet drugs, such as prasugrel and ticagrelor, are more potent and produce more consistent inhibition of platelet aggregation via the P2Y(12) ADP platelet receptor. A variety of methods of platelet function testing are available for evaluating platelet inhibition in percutaneous coronary intervention-treated patients in order to help determine the individual risk for ischemic and bleeding complications. Although not yet routinely undertaken, platelet function testing offers the potential to tailor antiplatelet therapy for individual patients. Whether alteration of therapy based on platelet function testing improves patients' outcomes remains unclear and is currently under investigation. This article reviews the impact of antiplatelet drug responsiveness on clinical outcomes with a focus on P2Y(12) receptor inhibition as well as on current and future concepts for personalized antiplatelet strategies.     

ST-elevation myocardial infarction: the role of adjunctive antiplatelet therapy

Antiplatelet therapy to reduce the risks of recurrent myocardial infarction and restenosis after primary percutaneous coronary intervention is critically important to optimize the early treatment of ST-segment elevation myocardial infarction (STEMI). Traditionally, acetylsalicylic acid (ASA; aspirin) has been recommended for patients with suspected STEMI, but this agent targets only one of several pathways of platelet aggregation. Antiplatelet agents with different inhibitory mechanisms may act synergistically with ASA. Glycoprotein IIb/IIIa inhibitors are generally not used with fibrinolytic agents in acute STEMI management; indeed, glycoprotein IIb/IIIa inhibitors plus bolus fibrinolytics increase the risk of intracranial hemorrhage. Aggressive antiplatelet therapy with clopidogrel reduces mortality in STEMI patients and offers significant clinical benefits, without an associated increase in major bleeding events. Recent trials support the development of an early and aggressive approach to more complete platelet inhibition using clopidogrel, in combination with ASA, for patients with STEMI.     

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.     

The impact and management of acquired platelet dysfunction

Platelet function can be abnormally increased, as in association with acute vascular events, or defective, as in a variety of clinical settings. Acquired platelet dysfunction may occur at any age and range in severity from mild to life-threatening haemorrhages. Diagnostic work-up of platelet disorders requires meticulous evaluation of medical history, specifically of any drugs interfering with platelet function, careful clinical examination and a staged laboratory protocol to assess the underlying platelet defect(s). To identify hyperactive platelets ex vivo, costly procedures may be required using flow cytometry and distict epitope-specific monoclonal antibodies. Currently, this approach can be recommended for research purposes only. Drugs represent the most common cause of platelet dysfunction in our overmedicated society. While aspirin, clopigogrel (more recently also prasugrel) and integrin αIIbβ3 (GPIIb-IIIa) receptor antagonists (abciximab, eptifibatide and tirofiban) are well-known prototypes of antiplatelet drugs, other widely used agents (e.g. nonsteroidal anti-inflammatory drugs, antibiotics, serotonin reuptake inhibitors and volume expanders) can also impair platelet function and thus cause or aggravate hemorrhages. Identification of individual patients with pre-existing hemostatic defects remains crucial (i) to prevent bleeding complications, (ii) to manage symptoms adequately, (iii) to minimize the risk from invasive procedures, and (iv) to avoid unnecassary exposure to blood products. Screening for platelet dysfunction can be performed by point-of-care testing followed by platelet aggregometry in response to various agonists. While mild bleeding episodes due to antiplatelet therapy can be managed by withdrawal of the drug(s), severe hemorrhages may require immediate platelet transfusions. Apart from that, the prohemostatic armamentarium is limited to desmopressin, antifibrinolytic agents, and recombinant factor VIIa.     

Qualitative platelet defect and thrombohaemorrhagic complications in a patient with polycythaemia vera. Case 10

We describe a 67-year-old patient with polycythaemia vera and pathological functional platelet studies. He not only suffered a transient ischaemic attack despite taking of antiplatelet agents, but also showed bleeding diathesis with cerebral bleeding and spontaneous suffusions. Platelet function studies and clinical findings improved after phlebotomy and cytoreductive treatment with hydroxyurea. Thrombosis and haemorrhage account predominantly for morbidity and mortality in patients with polycythaemia vera. The pathophysiological mechanisms to explain thrombosis and bleeding in patients with myeloproliferative disorders including polycythaemia vera were intensively studied. However, up to now no clear correlation of laboratory findings in relation to clinical history of thrombosis and bleeding was demonstrated. In this report the most important pathophysiological mechanisms and therapy with antiplatelet agents are discussed.     

Rapidly regulating platelet activity in vivo with an antidote controlled platelet inhibitor

Millions of individuals are prescribed platelet inhibitors, such as aspirin and clopidogrel, to reduce their risk of thrombosis-related clinical events. Unfortunately many platelet inhibitors are contraindicated in surgical settings because of their inherent bleeding risk complicating the treatment of patients who require surgery. We describe the development of a potent antiplatelet agent, an RNA aptamer-termed Ch-9.14-T10 that binds von Willebrand factor (VWF) with high affinity and inhibits thrombosis in a murine carotid artery damage model. As expected, when this potent antiplatelet agent is administered, it greatly increases bleeding from animals that are surgically challenged. To improve this antiplatelet agent's safety profile, we describe the generation of antidotes that can rapidly reverse the activity of Ch-9.14-T10 and limit blood loss from surgically challenged animals. Our work represents the first antidote controllable antiplatelet agent, which could conceivably lead to improved medical management of patients requiring antiplatelet medication who also need surgery.     

Current Strategies for the Management of Bleeding Associated with Direct Oral Anticoagulants and a Review of Investigational Reversal Agents

BackgroundThe management of life-threatening bleeding in patients who are receiving direct oral anticoagulants (DOACs) is a serious medical concern.ObjectiveThis review provides a concise, balanced overview of the current and future approaches for reversing the anticoagulation effects of DOACs, particularly factor Xa (FXa) inhibitors.DiscussionThe anticoagulant activity of the direct thrombin inhibitor dabigatran can be reversed by idarucizumab, but until recently, options for the management of major bleeding in patients who were receiving FXa inhibitors were limited to nonspecific strategies, including supplementation of clotting factors with prothrombin complex concentrates (PCCs) or activated PCCs for attenuating anticoagulation effects. They appear as a treatment option in many hospital guidelines despite the lack of approval by the U.S. Food and Drug Administration and the lack of rigorous medical evidence supporting their use in this setting. The development of specific reversal agents may provide improved strategies for the management of bleeding. Andexanet alfa is a modified FXa molecule approved in the United States to reverse the anticoagulant effects of FXa inhibitors (rivaroxaban and apixaban) in patients with life-threatening or uncontrolled bleeding. Ciraparantag is a small-molecule inhibitor of multiple anticoagulants that has been investigated in healthy subjects.ConclusionThe current guidelines for management of DOAC-associated bleeding are being updated to reflect that the reversal agent for rivaroxaban and apixaban is now available. For other FXa inhibitors, in the absence of a reversal agent, nonspecific strategies that include PCCs are recommended. The population of patients anticoagulated with DOACs is growing, and we hope that specific reversal agents will improve the approach to management of major bleeding in this population.     

抗血小板治疗实验室监测之我见

抗血小板药物在各种动脉血栓性疾病防治中具有重要地位。虽然目前抗血小板药物多采用固定剂量给药,但不同患者对抗血小板治疗的反应性存在明显差异。治疗后的血小板高反应性或低反应性可能与血栓事件或出血事件风险相关。基于血小板功能检测的个体化抗血小板治疗方案可能有助于预防血栓或出血不良事件的发生,但目前仍缺乏上述治疗策略能够最终改善患者预后的确切临床证据。迄今为止,对于接受抗血小板治疗的患者是否应常规进行实验室监测仍存在诸多争议。本文对抗血小板治疗反应多样性的成因及血小板功能检测是否可用于指导个体化抗血小板治疗进行讨论。     

New Antiplatelet Agents and the Role of Platelet Function Testing in Acute Coronary Syndromes

Background

Dual antiplatelet therapy is a guideline mandated for patients with acute coronary syndromes (ACS). Despite its use, thrombotic events continue to occur both early and late. Platelet function testing has been used to define the in vitro effects of new antiplatelet agents, and it has been suggested that it be used to choose therapy. The role of platelet function testing, particularly with newer antiplatelet agents, remains unclear.

Objective

We review the rationale for platelet function testing and its application in monitoring patients on antiplatelet therapy. We also review recent clinical trials of newer antiplatelet agents. On the basis of this review, we reach conclusions on the current role of antiplatelet function testing in monitoring modern antiplatelet therapy and the role of the new antiplatelet agents in the treatment of ACS.

Methods

We reviewed recent publications on platelet function testing and clinical trials of newer antiplatelet therapies compared with clopidogrel.

Results

Platelet function testing is complex, but there is now a bedside test, VerifyNow. High platelet reactivity has been associated with worse cardiovascular outcomes in patients undergoing percutaneous coronary intervention. Recent clinical trials have not found any advantage in outcomes in patients who have their therapy adjusted by monitoring their platelet function. Newer agents, prasugrel, ticagrelor, and cangrelor, produce more rapid, complete, less variable effects on platelet function than clopidogrel. Prasugrel was found to improve outcomes compared with clopidogrel in patients with ACS undergoing percutaneous intervention. Ticagrelor is beneficial in all patients with ACS and reduces cardiovascular mortality compared with clopidogrel. Cangrelor improves outcomes in patients undergoing stenting. Recent studies to assess the role of platelet function monitoring of the effects of clopidogrel and modifying treatments have not been successful.

Conclusion

Recent clinical trials have indicated that newer antiplatelet agents have advantages over clopidogrel in the treatment of ACS. Platelet function testing gives us a guide to the timing, efficacy, and variability of therapy and can correlate with poor patient outcomes; however, the use of antiplatelet function testing to tailor therapy does not seem appropriate.     

Monitoring of antiplatelet therapy. Current limitations, challenges, and perspectives

Screening of platelet function can be performed by point-of-care testing followed by platelet aggregometry in response to agonists such as collagen, adenosine diphosphate, epinephrine, and arachidonic acid. Despite in use for decades, this technique is not well standardized. Monitoring of antiplatelet therapy is increasingly applied in patients at high risk for re-thrombosis or bleeding. To assess pharmacological inhibition of platelet function, agonist-induced platelet aggregation, thromboxane B2 (TxB2) and vasodilator-stimulated protein phosphorylation (VASP) are being measured. While serum TxB2 levels of < 2 ng/ml reflect aspirin-induced inhibition of cyclo-oxygenase-1 activity with high sensitivity, VASP exhibits a wide variability upon treatment with clopidogrel or prasugrel. Multiple studies reveal an association between high residual platelet reactivity and adverse cardiovascular events in patients on antiplatelet therapy. However, despite the plethora of platelet function assays currently under investigation, their use in daily practice cannot be recommended. This is due to several reasons: (i) there is no consensus on the method and a respective cut-off value associated with clinical adverse outcome, and (ii) data demonstrating any benefit of tailored antiplatelet therapy and its monitoring (based on assessment of platelet functions) are still limited. Thus, appropriate identification of 'resistant' or 'poor responders' to antiplatelet agents remains challenging in clinical practice.     

Aggressive chronic platelet inhibition with prasugrel and increased cancer risks: revising oral antiplatelet regimens?

The TRITON-TIMI 38 was a head-to-head trial to assess the efficacy and safety of the experimental antiplatelet agent prasugrel vs. standard care with clopidogrel on top of aspirin. Besides some ischemic protection at expense of overwhelming bleeding disadvantage, prasugrel treated patients experienced three times higher rate of colonic neoplasms then after clopidogrel, and this difference was significant. Importantly, known gastrointestinal bleeding preceded the diagnosis of colonic neoplasms only in half of the patients. Three potential mechanisms responsible for such harmful association are reviewed, namely: (i) direct hazard of the experimental drug on cancer occurrence and progression; (ii) indirect modulation of tumor growth; and (iii) enhanced metastatic dissemination due to instability of platelet-tumor cell aggregates, or/and inability to keep the disease locally due by much more potent long-term platelet inhibition should be considered. Significant excess of cancer after prasugrel is alarming, and can be reasonably explained, with critical clinical implications not only for prasugrel further development, but also for existing and future chronic antiplatelet strategies. If the hypothesis that oral aggressive platelet inhibition cause higher cancer risks will turn out to be true, then intensity of platelet inhibition, and especially duration of chronic antiplatelet therapy should be reconsidered. More delicate platelet inhibition, and shorter exposure to oral antiplatelet agents will prevail.     

Dyspnoea after antiplatelet agents: the AZD6140 controversy

Recent randomised studies suggest that experimental oral reversible platelet P2Y12 receptor inhibitor, AZD6140, causes dyspnoea. This also raises similar concerns about the parent compound, and another adenosine triphosphate (ATP) analogue (AR-69931MX or cangrelor), which is currently in Phase 3 trial in patients undergoing coronary interventions. We analysed package inserts, and available clinical trials safety data for antiplatelet agents with regard to the incidence of dyspnoea. We found that dyspnoea is a very rare complication of the presently approved platelet inhibitors, mostly caused by underlying disease, rather than antiplatelet therapy per se. The main reasons for respiratory distress after oral (AZD6140), and intravenous (cangrelor) agents may be the development of mild asymptomatic thrombotic thrombocytopenic purpura, fluid retention and dyspnoea because of the reversible nature of these drugs. Also, these agents are ATP analogues, which rapidly metabolise to adenosine, a well-known bronchoprovocator causing dyspnoea as well. In summary, dyspnoea is seldom considered, there are no treatment algorithms when it does occur, plausible mechanisms exist and despite these plausible mechanisms, the true cause of dyspnoea in these exposed individuals is unknown. Additional pulmonary function testing, immunological investigations and platelet receptor studies are urgently needed to determine the cause of dyspnoea after AZD6140, and to point out how such serious adverse reactions can be prevented, or at least minimised, raising potential concerns about this drug.     

Bleeding risk and reversal strategies for old and new anticoagulants and antiplatelet agents

Summary. The most important adverse effect of antithrombotic treatment is the occurrence of bleeding. In the case of severe bleeding in a patient who uses anticoagulant agents or when a patient on anticoagulants needs to undergo an urgent invasive procedure, it may be useful to reverse anticoagulant treatment. Conventional anticoagulants such as vitamin K antagonists may be neutralized by administration of vitamin K or prothrombin complex concentrates, whereas heparin and heparin derivatives can be counteracted by protamine sulphate. The anti‐hemostatic effect of aspirin and other antiplatelet strategies can be corrected by the administration of platelet concentrate and/or desmopressin, if needed. Recently, a new generation of anticoagulants with a greater specificity towards activated coagulation factors as well as new antiplatelet agents have been introduced and these drugs show promising results in clinical studies. A limitation of these new agents may be the lack of an appropriate strategy to reverse the effect if a bleeding event occurs, although experimental studies show hopeful results for some of these agents.     

Ibrutinib‐associated bleeding: pathogenesis,management and risk reduction strategies

Ibrutinib is an irreversible inhibitor of Bruton's tyrosine kinase (Btk) that has proven to be an effective therapeutic agent for multiple B‐cell‐mediated lymphoproliferative disorders. Ibrutinib, however, carries an increased bleeding risk compared with standard chemotherapy. Bleeding events range from minor mucocutaneous bleeding to life‐threatening hemorrhage, due in large part to the effects of ibrutinib on several distinct platelet signaling pathways. There is currently a minimal amount of data to guide clinicians regarding the use of ibrutinib in patients at high risk of bleeding or on anticoagulant or antiplatelet therapy. In addition, the potential cardiovascular protective effects of ibrutinib monotherapy in patients at risk of vascular disease are unknown. Patients should be cautioned against using non‐steroidal anti‐inflammatory drugs, fish oils, vitamin E and aspirin‐containing products, and consider replacing ibrutinib with a different agent if dual antiplatelet therapy is indicated. Patients should not take vitamin K antagonists concurrently with ibrutinib; direct oral anticoagulants should be used if extended anticoagulation is strongly indicated. In this review, we describe the pathophysiology of ibrutinib‐mediated bleeding and suggest risk reduction strategies for common clinical scenarios associated with ibrutinib.     

Tirofiban preserves platelet loss during continuous renal replacement therapy in a randomised prospective open-blinded pilot study

Introduction  

Approximately one third of all patients with cardiogenic shock suffer from acute kidney injury. Percutaneous coronary intervention, intra-aortic balloon pump, and continuous renal replacement therapy (CRRT) require effective antiplatelet therapy and anticoagulation, resulting in a high risk for platelet loss and bleeding events. The reversible platelet glycoprotein IIb/IIIa receptor inhibitor tirofiban was investigated to preserve platelet number and activation in a prospective open-blinded endpoint evaluation study.     

Evaluation of the clinical utility of platelet aggregation studies in the long-term follow-up of patients with atherosclerotic vascular disease.

The present study was designed to evaluate the usefulness of laboratory monitoring of antiplatelet therapy by means of a multiparametric evaluation of in vitro platelet aggregation tests in the attempt to individually optimize a given therapeutic regimen. The presence of a condition of hyperaggregability was shown in approximately 80% of patients with different forms of atherosclerotic vascular disease not undergoing any therapeutic regimen with antiplatelet agents. Conversely, a significant decrease in platelet activity was observed in patients undergoing different therapies based on acetylsalicylic acid (ASA), ticlopidine, or indobufen. The similar antiaggregatory effect of low-dose vs. high-dose ASA therapies was also shown. Dipyridamole alone showed no antiaggregatory effect, which, in turn, was reached only by the addition of ASA. Nevertheless, the association of ASA plus dipyridamole did not show any stronger antiplatelet effect than ASA alone. The evaluation of in vitro platelet activity in a group of patients treated with picotamide failed to show any significant change in comparison with the untreated group, probably due to the short half-life of picotamide in man and/or to its capability of reversibly antagonizing the action of thromboxane at receptor level. The evaluation of a long-term follow-up of 90 patients treated with different antiplatelet agents supports the idea that a multiparametric analysis of in vitro platelet aggregation may provide valuable help in monitoring and optimizing a given therapeutic regimen.     

Mechanism of action and clinical development of platelet thrombin receptor antagonists

Atherothrombotic disease is the leading cause of death worldwide. Currently, dual antiplatelet therapy with aspirin and ADP receptor antagonists has shown improved short- and long-term clinical outcomes but is associated with increased bleeding risk, and the rates of recurrent ischemic events still remain high. Selective inhibition of the principal protease-activated receptor (PAR)-1 for thrombin, the most potent platelet activator, represents a promising novel strategy to reduce ischemic events without increasing the risk of bleeding. Two PAR-1 antagonists are currently being tested in clinical trials: SCH 530348 and E5555. Both have demonstrated an antiplatelet effect without increasing bleeding time in preclinical trials. Results of Phase II trials showed that SCH 530348, in addition to standard antiplatelet therapy, was well tolerated and not associated with increased bleeding risk. The safety and tolerability of E5555 is being evaluated in patients with coronary artery disease and non-ST-segment elevation acute coronary syndrome in four Phase II clinical trials. Two large-scale Phase III trials assessing the efficacy of SCH 530348 in addition to the standard of care are currently ongoing. This article provides an overview of the current status of knowledge on platelet thrombin receptor antagonists, focusing on pharmacologic properties and clinical development.     

Elinogrel, an orally and intravenously available ADP-receptor antagonist. How does elinogrel affect a personalized antiplatelet therapy?

The antiplatelet therapy with aspirin and the ADP-receptor blocker clopidogrel is currently the standard medication after coronary intervention or after acute coronary syndrome to prevent recurrent ischemic events and reduce mortality. However, high interindividual response variability to antiplatelet treatment is described in up to 44% of treated patients. A poor response to clopidogrel is caused by multifactorial mechanisms. Individual risk assessment including platelet function testing (PFT) can help to identify high risk patients, although recent randomized trials to investigate effects of PFT-guided therapy have failed to detect an impact on prognostic outcome. Poor response to standard antiplatelet agents can be overcome by switching to alternate substances. Elinogrel is a novel competitive, reversible ADP-receptor antagonist available in oral and intravenous formulation. Additional treatment with elinogrel showed advantages over clopidogrel, including more rapid, less variable, and more complete inhibition of platelet function without significantly increased bleeding complications. This review gives an overview over the investigational drug elinogrel for use in a personalized antiplatelet approach.     

MRS2500 [2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate], a potent, selective, and stable antagonist of the platelet P2Y1 receptor with strong antithrombotic activity in mice

The platelet P2Y(1) ADP receptor is an attractive target for new antiplatelet drugs. However, because of the lack of strong and stable antagonists, only a few studies have suggested that pharmacological inhibition of the P2Y(1) receptor could efficiently inhibit experimental thrombosis in vivo. Our aim was to determine whether the newly described potent and selective P2Y(1) receptor antagonist MRS2500 [2-iodo-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate] could inhibit platelet function ex vivo and experimental thrombosis in mice in vivo. MRS2500 was injected intravenously into mice, and its effect on ex vivo platelet aggregation and in several models of thrombosis in vivo was determined. MRS2500 displayed high potency and stable and selective P2Y(1) receptor inhibition ex vivo. Although MRS2500 injection resulted in only moderate prolongation of the bleeding time, it provided strong protection in systemic thromboembolism induced by infusion of a mixture of collagen and adrenaline. MRS2500 also potently inhibited localized arterial thrombosis in a model of laser-induced vessel wall injury with two degrees of severity. Moreover, combination of MRS2500 with clopidogrel, the irreversible inhibitor of the platelet P2Y(12) receptor for ADP, led to increased antithrombotic efficacy compared with each alone. These results add further evidence for a role of the P2Y(1) receptor in thrombosis and validate the concept that targeting the P2Y(1) receptor could be a relevant alternative or complement to current antiplatelet strategies.