Platelet thrombin receptor antagonism and atherothrombosis

Development of virtual platelets implementing the functions of three platelet membrane proteins with different adhesive characteristics

AIM

Computer simulation is a new method for understanding biological phenomena. In this report, we developed a simple platelet simulator representing platelet adhesion under blood flow conditions.

METHODS

We generated virtual platelets based on the functions of three key adhesive proteins: glycoprotein (GP) Ibα, GPIIb/IIIa and collagen receptors. The adhesive force between GPIbα and von Willebrand factor (VWF) was set to increase in association with increments in the fluid shear stress. GPIIb/IIIa acquires an adhesive force to bind with ligands only when platelets are activated following multiple GPIbα stimulation by VWF or collagen receptors.

RESULTS

Upon perfusion over the area of virtual endothelial injury, the virtual platelets adhered and became activated to form platelet thrombi. A total of 286/mm(2) of activated platelets was found to have accumulated downstream of the flow obstacle within 30 seconds, with 59/mm(2) platelets adhering upstream. The results obtained with the virtual model were consistent with those for real platelets in human blood in the presence of similarly shaped flow obstacles.

CONCLUSIONS

Our computer platelet simulator, which employs the functions of three key platelet membrane proteins, shows similar findings for adhesion in the presence and absence of blood flow obstacles.

The characteristics of thrombin in osteoarthritic pathogenesis and treatment

Osteoarthritis (OA) is a mechanical abnormality associated with degradation of joints. It is characterized by chronic, progressive degeneration of articular cartilage, abnormalities of bone, and synovial change. The most common symptom of OA is local inflammation resulting from exogenous stress or endogenous abnormal cytokines. Additionally, OA is associated with local and/or systemic activation of coagulation and anticoagulation pathways. Thrombin plays an important role in the stimulation of fibrin deposition and the proinflammatory processes in OA. Thrombin mediates hemostatic and inflammatory responses and guides the immune response to tissue damage. Thrombin activates intracellular signaling pathways by interacting with transmembrane domain G protein coupled receptors (GPCRs), known as protease-activated receptors (PARs). In pathogenic mechanisms, PARs have been implicated in the development of acute and chronic inflammatory responses in OA. Therefore, discovery of thrombin signaling pathways would help us to understand the mechanism of OA pathogenesis and lead us to develop therapeutic drugs in the future.

Disabled-2 is required for efficient hemostasis and platelet activation by thrombin in mice

OBJECTIVE

The essential role of platelet activation in hemostasis and thrombotic diseases focuses attention on unveiling the underlying intracellular signals of platelet activation. Disabled-2 (Dab2) has been implicated in platelet aggregation and in the control of clotting responses. However, there is not yet any in vivo study to provide direct evidence for the role of Dab2 in hemostasis and platelet activation.

APPROACH AND RESULTS

Megakaryocyte lineage-restricted Dab2 knockout (Dab2(-/-)) mice were generated to delineate in vivo functions of Dab2 in platelets. Dab2(-/-) mice appeared normal in size with prolonged bleeding time and impaired thrombus formation. Although normal in platelet production and granule biogenesis, Dab2(-/-) platelets elicited a selective defect in platelet aggregation and spreading on fibrinogen in response to low concentrations of thrombin, but not other soluble agonists. Investigation of the role of Dab2 in thrombin signaling revealed that Dab2 has no effect on the expression of thrombin receptors and the outside-in signaling. Dab2(-/-) platelets stimulated by low concentrations of thrombin were normal in Gαq-mediated calcium mobilization and protein kinase C activation, but were defective in Gα₁₂/₁₃-mediated RhoA-ROCKII activation. The attenuated Gα₁₂/₁₃ signaling led to impaired ADP release, Akt-mammalian target of rapamycin and integrin αIIbβ3 activation, fibrinogen binding, and clot retraction. The defective responses of Dab2(-/-) platelets to low concentrations of thrombin stimulation may contribute to the impaired hemostasis and thrombosis of Dab2(-/-) mice.

CONCLUSIONS

This study sheds new insight in platelet biology and represents the first report demonstrating that Dab2 is a key regulator of hemostasis and thrombosis by functional interplay with Gα₁₂/₁₃-mediated thrombin signaling.

Platelets and their chemokines in atherosclerosis-clinical applications

The concept of platelets as important players in the process of atherogenesis has become increasingly accepted due to accumulating experimental and clinical evidence. Despite the progress in understanding the molecular details of atherosclerosis, particularly by using animal models, the inflammatory and thrombotic roles of activated platelet s especially in the human system remain difficult to dissect, as often only the complications of atherosclerosis, i.e., stroke and myocardial infarction are definable but not the plaque burden. Platelet indices including platelet count and mean platelet volume (MPV) and soluble mediators released by activated platelets are associated with atherosclerosis. The chemokine CXCL4 has multiple atherogenic activities, e.g., altering the differentiation of T cells and macrophages by inhibiting neutrophil and monocyte apoptosis and by increasing the uptake of oxLDL and synergizing with CCL5. CCL5 is released and deposited on endothelium by activated platelets thereby triggering atherogenic monocyte recruitment, which can be attenuated by blocking the corresponding chemokine receptor CCR5. Atheroprotective and plaque stabilizing properties are attributed to CXCL12, which plays an important role in regenerative processes by attracting progenitor cells. Its release from luminal attached platelets accelerates endothelial healing after injury. Platelet surface molecules GPIIb/IIIa, GP1bα, P-selectin, JAM-A and the CD40/CD40L dyade are crucially involved in the interaction with endothelial cells, leukocytes and matrix molecules affecting atherogenesis. Beyond the effects on the arterial inflammatory infiltrate, platelets affect cholesterol metabolism by binding, modifying and endocytosing LDL particles via their scavenger receptors and contribute to the formation of lipid laden macrophages. Current medical therapies for the prevention of atherosclerotic therapies enable the elucidation of mechanisms linking platelets to inflammation and atherosclerosis.

Updates in antiplatelet agents used in cardiovascular diseases

BACKGROUND

Antiplatelet therapy is a cornerstone in coronary artery disease management. However, patients with acute coronary syndrome still remain at risk of recurrent cardiovascular events despite the advance of medical therapy.

OBJECTIVE

This article provides a review of antiplatelet agents used in cardiovascular diseases and focus on updates in the past 5 years.

METHOD

Peer-reviewed clinical trials and relevant treatment guidelines were identified from MEDLINE and Current Content database (from 1966 to April 15, 2013) using search terms aspirin, clopidogrel, prasugrel, ticagrelor, glycoprotein IIb/IIIa inhibitors, antiplatelet agents, coronary artery disease, acute coronary syndrome, pharmacology, pharmacokinetics, and pharmacodynamics. Citations from the available articles were also reviewed for additional references.

RESULTS

In unstable angina and non-ST-segment elevation myocardial infarction (MI), dual antiplatelet therapy (aspirin and clopidogrel) demonstrated a reduction in death from cardiovascular causes, nonfatal MI, or stroke (relative risk 0.80; 95% confidence interval [CI], 0.72-0.90). In ST-segment elevation MI, dual antiplatelet therapy reduced the rate of occluded infarct-related artery/death or recurrent MI (95% CI, 24%-47%). Newer agents such as prasugrel, when compared to clopidogrel, reduced death from vascular causes, MI, or stroke in patients undergoing percutaneous coronary intervention (PCI; hazard ratio [HR], 0.81; 95% CI 0.73-0.90) but not in those receiving medical management only. When compared to clopidogrel, ticagrelor reduces death from vascular causes, MI, or stroke (HR: 0.84; 95% CI, 0.77-0.92) in patients undergoing PCI or receiving medical management only. Both the agents, however, increase the risk of bleeding in certain patient population.

CONCLUSIONS

In the last 5 years, newer antiplatelet agents, including prasugrel and ticagrelor, have been demonstrated to reduce recurrent cardiovascular events compared to standard therapy and, however, also caused increase bleeding in selected patient populations. Newer agents including shorter acting P2Y12 inhibitor or antiplatelets that target other receptors are being evaluated to improve/maintain therapeutic efficacy yet minimize the risk of bleeding.

Efficacy versus safety: the dilemma of using novel platelet inhibitors for the treatment of patients with ischemic stroke and coronary artery disease

Coronary and cerebrovascular atherothrombosis are the leading cause of mortality and morbidity worldwide. Novel antiplatelet agents have been established for the management of patients with clinically evident coronary atherothrombosis and are increasingly used in these patients. These agents, however, have shown limited efficacy in the prevention of cerebrovascular events and potential harm in patients with history of stroke or transient ischemic attack. Herein, the efficacy and safety of two established antiplatelet agents in patients with stroke – aspirin and clopidogrel – are reviewed with a focus on the use and challenges related to novel antiplatelet agents – prasugrel, ticagrelor, and vorapaxar – in patients at risk for and with a history of stroke or transient ischemic attack.

Unmet needs in the management of acute myocardial infarction: role of novel protease-activated receptor-1 antagonist vorapaxar

Platelet activation with subsequent aggregation is a complex process leading to thrombus formation, which remains a key component for atherothrombotic manifestations, in particular myocardial infarction. Therefore, antiplatelet therapies are pivotal for the treatment of these patients. Current oral antiplatelet therapies used for secondary prevention of ischemic recurrences include aspirin and adenosine diphosphate P2Y12 platelet-receptor antagonists. However, despite these therapies, patients who have experienced a myocardial infarction remain at risk for ischemic recurrences. Therefore, more aggressive secondary prevention measures have been an area of research, including identifying additional targets modulating platelet-activation and -aggregation processes. Among these, thrombin-mediated platelet activation via protease-activated receptors (PARs) has been subject to extensive clinical investigation. Several PAR-1 receptor antagonists have been developed. However, vorapaxar is the only one that has completed large-scale clinical investigation. The present manuscript will provide an overview on the role of thrombin-mediated signaling, the impact of PAR-1 blockade with vorapaxar on ischemic and bleeding outcomes, and the potential role for vorapaxar in clinical practice.

New antithrombotics for secondary prevention of acute coronary syndrome

Patients with acute coronary syndrome (ACS) usually receive acetylsalicylic acid plus an adenosine diphosphate (ADP) receptor inhibitor to reduce the long‐term risk of recurrent events. However, patients receiving standard antiplatelet prophylaxis still face a substantial risk of recurrent events. Strategies involving 3 antithrombotic agents with different modes of action have now been tested. In Thrombin Receptor Antagonists for Clinical Event Reduction (TRA‐CER), compared with standard care alone, bleeding complications including intracranial hemorrhage (ICH) were increased with the addition of vorapaxar, without efficacy benefit. In Trial to Assess the Effects of SCH 530348 in Preventing Heart Attack and Stroke in Patients With Atherosclerosis (TRA 2°P–TIMI 50), the addition of vorapaxar reduced recurrent events compared with standard care in stable patients with prior myocardial infarction. This study was terminated early in patients with prior stroke owing to excess ICH, though an increased risk of ICH or fatal bleeding was not detected in patients with prior myocardial infarction. The Apixaban for Prevention of Acute Ischemic and Safety Events 2 (APPRAISE‐2) trial of standard‐dose apixaban added to standard care in patients with ACS was also stopped early owing to excess serious bleeding. However, in Rivaroxaban in Combination With Aspirin Alone or With Aspirin and a Thienopyridine in Patients With Acute Coronary Syndromes (ATLAS ACS 2 TIMI 51), fatal bleeding or fatal ICH did not increase with low‐dose rivaroxaban added to low‐dose acetylsalicylic acid‐based standard care compared with standard care alone. In that trial, a significant reduction of recurrent vascular events was shown with 3 antithrombotic regimens compared with standard care. Therefore, depending on drug dose and patient population, further reductions in recurrent vascular events after ACS may be possible in future clinical practice, with a favorable benefit‐risk profile.

Cyclophilin A is an important mediator of platelet function by regulating integrin αIIbβ3 bidirectional signalling

Cyclophilin A (CyPA) is an important mediator in cardiovascular diseases. It possesses peptidyl-prolyl cis-trans isomerase activity (PPIase) and chaperone functions, which regulate protein folding, intracellular trafficking and reactive oxygen species (ROS) production. Platelet glycoprotein receptor αIIbβ3 integrin activation is the common pathway for platelet activation. It was our objective to understand the mechanism by which CyPA-regulates αIIbβ3 activation in platelets. Mice deficient for CyPA (CyPA-/-) had prolonged tail bleeding time compared to wild-type (WT) controls despite equivalent platelet numbers. In vitro studies revealed that CyPA-/- platelets exhibited dramatically decreased thrombin-induced platelet aggregation. In vivo, formation of occlusive thrombi following FeCl3 injury was also significantly impaired in CyPA-/- mice compared with WT-controls. Furthermore, CyPA deficiency inhibited flow-induced thrombus formation in vitro. Flow cytometry demonstrated that thrombin-induced ROS production and αIIbβ3 activation were reduced in CyPA-/- platelets. Coimmunoprecipitation studies showed ROS-dependent increased association of CyPA and αIIbβ3. This association was dependent upon the PPIase activity of CyPA. Significantly, fibrinogen-platelet binding, platelet spreading and cytoskeleton reorganisation were also altered in CyPA-/- platelets. Moreover, CyPA deficiency prevented thrombin-induced αIIbβ3 and cytoskeleton association. In conclusion, CyPA is an important mediator in platelet function by regulation of αIIbβ3 bidirectionalsignalling through increased ROS production and facilitating interaction between αIIbβ3 and the cell cytoskeleton.

Reversal of thienopyridine-induced platelet dysfunction following desmopressin administration

Adenosine diphosphate (ADP)-receptor antagonists are widely used for thrombus prevention, although reversing their platelet dysfunction is difficult. This study evaluated the ability of desmopressin to reverse clopidogrel-induced platelet dysfunction. Sprague-Dawley rats received either clopidogrel (30 mg/kg) or placebo, followed 4 h later by saline or desmopressin (0.15, 0.3, or 0.6 μg/kg). Bleeding times and platelet aggregation studies were subsequently performed. A bleeding time >25 min was considered "prolonged." The median bleeding time for clopidogrel-exposed rats was 21 min, vs. 6 min for controls (p < 0.01). Progressively higher doses of 1-deamino-8-D-arginine vasopressin (DDAVP) were associated with a reduced number of rats with prolonged bleeding time (p = 0.001). Higher doses of DDAVP were also associated with a reduction in the median (IQR) bleeding time; 29 (13.5-30) min in rats receiving clopidogrel without DDAVP vs. 19 (12-28) min in rats receiving clopidogrel and 0.6 μg/kg DDAVP. The step-wise dosing of DDAVP resulted in a 54 % reduction in meeting the endpoint of prolonged bleeding time (OR 0.46; p = 0.025; 95 % CI 0.23-0.91). Platelet aggregation was observed in all control rats, but only some of those clopidogrel-treated rats who received 0.6 μg/kg DDAVP. In this model of an ADP-receptor antagonist, DDAVP results in partial reversal of clopidogrel-induced platelet dysfunction.

Basics of Antithrombotic Therapy for Cardiovascular Disease: Pharmacologic Targets of Platelet Inhibitors and Anticoagulants

Arterial thrombus formation is the common pathophysiologic process of cardiovascular disease manifestations, requiring interplay between platelets and coagulation factors. Current platelet inhibitors block the formation of thromboxane A₂ and interfer with adenosine diphosphate stimulation mediated by the P2Y₁₂ receptor. Novel antiplatelet agents blocking these and other pathways are under clinical development. Thrombin represents a bridge between platelets and coagulation. Indirect and direct thrombin inhibitors are pivotal in clinical settings. Other key coagulation factors include factors IX and X which are therapeutic targets of current and novel anticoagulants. This article reviews the pathophysiology of arterial thrombosis and current and novel antiplatelet and anticoagulant agents.

Highly electronegative LDL from patients with ST-elevation myocardial infarction triggers platelet activation and aggregation

Platelet activation and aggregation underlie acute thrombosis that leads to ST-elevation myocardial infarction (STEMI). L5-highly electronegative low-density lipoprotein (LDL)-is significantly elevated in patients with STEMI. Thus, we examined the role of L5 in thrombogenesis. Plasma LDL from patients with STEMI (n = 30) was chromatographically resolved into 5 subfractions (L1-L5) with increasing electronegativity. In vitro, L5 enhanced adenosine diphosphate-stimulated platelet aggregation twofold more than did L1 and induced platelet-endothelial cell (EC) adhesion. L5 also increased P-selectin expression and glycoprotein (GP)IIb/IIIa activation and decreased cyclic adenosine monophosphate levels (n = 6, P < .01) in platelets. In vivo, injection of L5 (5 mg/kg) into C57BL/6 mice twice weekly for 6 weeks shortened tail bleeding time by 43% (n = 3; P < .01 vs L1-injected mice) and increased P-selectin expression and GPIIb/IIIa activation in platelets. Pharmacologic blockade experiments revealed that L5 signals through platelet-activating factor receptor and lectin-like oxidized LDL receptor-1 to attenuate Akt activation and trigger granule release and GPIIb/IIIa activation via protein kinase C-α. L5 but not L1 induced tissue factor and P-selectin expression in human aortic ECs (P < .01), thereby triggering platelet activation and aggregation with activated ECs. These findings indicate that elevated plasma levels of L5 may promote thrombosis that leads to STEMI.

High-dose atorvastatin on the pharmacodynamic effects of double-dose clopidogrel in patients undergoing percutaneous coronary interventions: The ACHIDO (Atorvastatin and Clopidogrel HIgh DOse in stable patients with residual high platelet activity) study

OBJECTIVES

The goal of this study was to investigate the impact of high-dose atorvastatin on the pharmacodynamic (PD) effects of double-dose clopidogrel in statin-naive patients with stable coronary artery disease (CAD) and high-on-treatment platelet reactivity (HTPR) while on standard-dose clopidogrel before percutaneous coronary intervention (PCI).

BACKGROUND

Patients with HTPR are at increased risk of adverse cardiovascular events after PCI. High-dose statins improve prognosis in high-risk patients by lipid- and nonlipid-related mechanisms, including antithrombotic effects.

METHODS

The ACHIDO (Atorvastatin and Clopidogrel HIgh DOse in stable patients with residual high platelet activity) study was a randomized PD study of high-dose (80 mg) atorvastatin in addition to double-dose (150 mg) clopidogrel (atorvastatin group, n = 38) versus double-dose clopidogrel alone (control group, n = 38) in patients with HTPR. HTPR was defined as P2Y(12) reaction units (PRU) ≥235 by the VerifyNow P2Y12 assay. Platelet reactivity was evaluated immediately before PCI and at 10 and 30 days.

RESULTS

Patients randomized to atorvastatin had lower PRU values (188 ± 48 vs. 223 ± 53 PRU, p < 0.01; primary endpoint) and HTPR rates (16% vs. 42%, p < 0.01) at 30 days than patients in the control group. Statin treatment (odds ratio [OR]: 3.8, p = 0.011), baseline PRU <298 (OR: 10.7, p = 0.0001), noncarrier status of CYP2C19*2 loss-of-function allele (OR: 2.9, p = 0.043), and age (OR: 0.94, p = 0.032) were variables significantly associated with optimal PD response (PRU <235) at 30 days. No correlations were found between PRU and lipid fractions.

CONCLUSIONS

High-dose atorvastatin significantly improved the PD effects of double-dose clopidogrel in our stable CAD patients with HTPR undergoing PCI (Atorvastatin and Clopidogrel HIgh DOse in stable patients with residual high platelet activity [ACHIDO]; NCT01335048).

Antiplatelet and anticoagulant therapy for atherothrombotic disease: the role of current and emerging agents

Coronary atherothrombotic disease, including chronic stable angina and acute coronary syndromes (ACS), is associated with significant global burden. The acute clinical manifestations of atherothrombotic disease are mediated by occlusive arterial thrombi that impair tissue perfusion and are composed of a core of aggregated platelets, generated by platelet activation, and a superimposed fibrin mesh produced by the coagulation cascade. Long-term antithrombotic therapies, namely oral antiplatelet agents and anticoagulants, have demonstrated variable clinical effects. Aspirin and P2Y12 adenosine diphosphate (ADP) receptor antagonists have been shown to reduce the risk for thrombosis and ischaemic events by blocking the thromboxane (Tx) A2 and platelet P2Y12 activation pathways, respectively, whereas the benefits of oral anticoagulants have not been consistently documented. However, even in the presence of aspirin and a P2Y12 receptor antagonist, the risk for ischaemic events remains substantial because platelet activation continues via pathways independent of TxA2 and ADP, most notably the protease-activated receptor (PAR)-1 platelet activation pathway stimulated by thrombin. Emerging antithrombotic therapies include those targeting the platelet, such as the new P2Y12 antagonists and a novel class of oral PAR-1 antagonists, and those inhibiting the coagulation cascade, such as the new direct factor Xa antagonists, the direct thrombin inhibitors, and a novel class of factor IX inhibitors. The role of emerging antiplatelet agents and anticoagulants in the long-term management of patients with atherothrombotic disease will be determined by the balance of efficacy and safety in large ongoing clinical trials.

Synthetic isoxazole as antiplatelet agent

Abstract Nine synthetic isoxazoles were evaluated as antiplatelet agents and studied the possible mechanism of more active compound. The initial screening was evaluating all compounds against platelet aggregation assays. The most active compound was isoxazole 8 showing an inhibition of platelet aggregation around 70%. In subsequent experiments, ADP and collagen were used as agonists to explore the possible inhibitory mechanisms of isoxazole 8 in platelet aggregation and secretion. We reported the effect of isoxazole 8 for reducing the expression of inflammatory markers, such as soluble CD40 ligand (sCD40L) and soluble P-selectin (sP-selectin), on activated platelets. Of this form, an inhibition of sCD40L and sP-selectin can prevent the onset of an atherosclerotic lesion.

Antiplatelet therapy: new pharmacological agents and changing paradigms

Recurrent atherothrombotic events in patients with acute coronary syndromes (ACS) and/or those undergoing percutaneous coronary intervention (PCI) are essentially platelet-driven processes, underscoring the need for effective pharmacological platelet inhibition. Dual antiplatelet therapy with aspirin and clopidogrel has been, for over a decade, the mainstay of antiplatelet management in ACS/PCI. However, atherothrombotic events continue to occur in a relevant proportion of subjects despite the benefit of this combination, which has led to the clinical development of newer and more potent antiplatelet drugs. Two of these, prasugrel and ticagrelor, have been recently approved for clinical use. The scope of this manuscript is to provide an up-to-date overview on new antiplatelet drugs in the setting of ACS and PCI, including the most recent advances on newly approved agents as well as on emerging compounds in clinical development.

A critical role of thrombin/PAR-1 in ADP-induced platelet secretion and the second wave of aggregation

BACKGROUND

The stable or second wave of platelet aggregation often observed in ADP-stimulated platelet-rich plasma (PRP) with an artificially lowered extracellular calcium level has been attributed to enhanced thromboxane A2 (TXA2 ) generation and inhibition of ectonucleotidase activity. However, the role of thrombin in ADP-induced platelet secretion and the second wave of aggregation is unknown.

OBJECTIVES AND METHODS

We employed aggregometry, flow cytometry, immunoblotting and ELISA to determine whether and how thrombin participates in ADP-induced platelet secretion and the second wave of aggregation.

RESULTS

ADP induces a phosphoinositide 3-kinase (PI3K) pathway-dependent thrombin generation, presumably resulting from the cleavage of αII b β3 -associated prothrombin. Generated thrombin subsequently activates protease-activated receptor-1 (PAR-1) and mediates dense granule secretion and the second wave of platelet aggregation in ADP-stimulated citrated PRP. Thus, ADP-induced dense granule secretion and the second wave of platelet aggregation in PRP were similarly and non-additively blocked by thrombin inhibitor hirudin, PAR-1 antagonist SCH-79797 or PI3K inhibitor wortmannin. Moreover, ADP stimulation caused the dissociation of prothrombin from αII b β3 and an increased plasma thrombin level; both were prevented by wortmannin. Furthermore, the wortmannin-inhibited second wave of platelet aggregation by ADP was restored by a subaggregation concentration of PAR-1 activating peptide SFLLRN. Blocking TXA2 production with indomethacin or restoring extracellular calcium to physiological concentration did not influence this thrombin/PAR-1 dependence.

CONCLUSIONS

A PI3K-dependent thrombin generation and the resultant PAR-1 activation serve as an indispensable mechanism to relay the platelet activation process induced by ADP.

Thrombin receptor antagonism in antiplatelet therapy

Activated platelets play a crucial role in the pathogenesis of atherothrombotic disease and its complications. Even under treatment of antiplatelet drugs, such as acetylsalicylic acid and P2Y12 antagonists, morbidity and mortality rates of thromboembolic complications remain high. Hence, the therapeutic inhibition of protease-activated receptor (PAR)-1, which is activated by thrombin, is a novel promising approach in antiplatelet therapy. Recent data suggest that PAR-1 is mainly involved in pathological thrombus formation, but not in physiological hemostasis. Therefore, PAR-1 inhibition offers the possibility to reduce atherothrombotic events without increasing bleeding risk. So far, two emerging PAR-1 antagonists have been tested in clinical trials: vorapaxar (SCH530349; Merck & Co., Whitehouse Station, NJ, USA) and atopaxar (E5555; Eisai, Tokyo, Japan). Although in TRA-CER vorapaxar showed an unfavorable profile for patients with acute coronary syndrome in addition to standard therapy, it revealed promising results for patients with prior myocardial infarction in TRA 2P-TIMI50. Depending on the status of clinical approval, vorapaxar might be an option for patients with peripheral arterial disease to reduce limb ischemia. The second PAR-I antagonist, atopaxar, tended towards reducing major cardiovascular adverse events in acute coronary syndrome patients in a phase II trial. However, although statistically not significant, bleeding events were numerically increased in atopaxar-treated patients compared with placebo. Furthermore, liver enzymes were elevated and the relative corrected QT interval was prolonged in atopaxar-treated patients. Currently, the development of atopaxar by Eisai is discontinued. The future of this novel class of antithrombotic drugs will depend on the identification of patient groups in which the risk–benefit ratio is favorable.

Platelet function profiles in patients with diabetes mellitus

Patients with diabetes mellitus (DM) are at high risk for several cardiovascular disorders such as coronary heart disease, stroke, peripheral arterial disease, and congestive heart failure. DM has reached epidemic proportions and its strong association with coronary artery disease is responsible for increased cardiovascular morbidity and mortality. DM patients are characterized by platelet hyperreactivity, which contribute to the enhanced atherothrombotic risk of these subjects. Several mechanisms are involved in the hyperreactive platelet phenotype characterizing DM patients. Furthermore, a large proportion of DM patients show inadequate response to standard antiplatelet treatments and high rate of adverse recurrent cardiovascular events despite compliance with standard antiplatelet treatment regimens. Therefore, new antiplatelet treatment regimens are warranted in DM patients to reduce their atherothrombotic risk. The present manuscript provides an overview on the current status of knowledge on platelet function profiles in patients with DM and therapeutic considerations.

Risk of Intracranial Hemorrhage With Protease-Activated Receptor-1 Antagonists

Background and Purpose—

Recent clinical trial data suggest that protease-activated receptor-1 (PAR-1) antagonists may increase the risk of intracranial hemorrhage. Our objective was to investigate the qualitative and quantitative risks of intracranial hemorrhage in patients receiving PAR-1 antagonist therapy.

Methods—

Pubmed, EMBASE, Cochrane Central Register of Controlled Trials, and Clinicaltrials.gov from 1966 to May 2012, were searched to identify relevant studies. We included randomized controlled trials that included a comparison of PAR-1 antagonist with placebo and in which the total number of patients and intracranial hemorrhage events were reported separately for active treatment and control groups. Summary incidence rates, relative risks, and 95% confidence intervals (CIs) were calculated using random-effects models. Between-study heterogeneity was assessed using the I 2 statistic.

Results—

In 9 PAR-1 antagonist trials with 42000 patients with a history of thrombotic vascular disease or acute coronary syndrome, PAR-1 antagonist treatment was associated with increased risk of intracranial hemorrhage (0.59% vs 0.30%; relative risk, 1.98; 95% CI, 1.46–2.68; P <0.00001; number needed to harm, 345). There was no heterogeneity across trials ( P =0.84; I 2 =0%), PAR-1 antagonist agent ( P =0.52), treatment duration ( P =0.38), or trial-qualifying event ( P =0.59). Risk of death from any cause or a cardiovascular cause did not differ between active treatment and control groups.

Conclusion—

In a pooled analysis of data from 9 trials, PAR-1 antagonist therapy was associated with an increased risk for intracranial hemorrhage.

Platelet function in health and disease: from molecular mechanisms, redox considerations to novel therapeutic opportunities

Increased oxidative stress appears to be of fundamental importance in the pathogenesis and development of several disease processes. Indeed, it is well known that reactive oxygen species (ROS) exert critical regulatory functions within the vascular wall, and it is, therefore, plausible that platelets represent a relevant target for their action. Platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, ROS may participate in the regulation of platelet activation. It is already established that enhanced ROS release from the vascular wall can indirectly affect platelet activity by scavenging nitric oxide (NO), thereby decreasing the antiplatelet properties of endothelium. On the other hand, recent data suggest that platelets themselves generate ROS, which may evoke pro-thrombotic responses, triggering many biological processes participating in atherosclerosis initiation, progression, and complication. That oxidative stress may alter platelet function is conceivable when considering that antioxidants play a role in the prevention of cardiovascular disease, although the precise mechanism accounting for changes attributable to antioxidants in atherosclerosis remains unknown. It is possible that the effects of antioxidants may be a consequence of their enhancing or promoting the antiplatelet effects of NO derived from both endothelial cells and platelets. This review focuses on current knowledge regarding ROS-dependent regulation of platelet function in health and disease, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Residual platelet activation through protease-activated receptors (PAR)-1 and -4 in patients on P2Y12 inhibitors

BACKGROUND

Dual antiplatelet therapy with aspirin and thienopyridines has improved outcomes of patients after coronary stent implantation. However, current knowledge suggests that thrombin generation is not affected by inhibition of the P2Y12 receptor, and therefore, platelet activation may still occur.

METHODS

The response to agonists specific for protease-activated receptors (PAR)-1 and -4 was tested by multiple electrode impedance aggregometry in 82 patients on stable doses of clopidogrel or prasugrel, and in 55 healthy controls.

RESULTS

Based on the consensus cut-off value for adenosine diphosphate (ADP) responsiveness, only one out of 19 patients on prasugrel, but 22 out of 63 patients on clopidogrel had high on-treatment residual platelet reactivity in response to exogenous ADP (p=0.01). Among the patients with adequate ADP P2Y12 receptor inhibition (n=59), we still observed 32 patients (54.2%) with normal response to the PAR-1 activator SFLLRN (26 patients on clopidogrel, 81.2%; 6 patients on prasugrel, 18.8%), and 37 patients (63.8%) with a normal response to the PAR-4 activator AYPGKF (29 patients on clopidogrel, 78.4%; 8 patients on prasugrel, 21.6%). The degree of PAR-agonists inducible platelet activation was directly correlated with the activation induced by ADP (r>0.5 and p<0.001 for both agonists). Moreover, SFLLRN and AYPGKF inducible platelet reactivities were strongly correlated (r=0.75, p<0.001).

CONCLUSION

PAR responsiveness is preserved in the majority of patients with adequate clopidogrel-mediated inhibition of the platelet P2Y12 receptor, and still in about 20% of those with adequate inhibition by prasugrel.

Safety and efficacy of protease-activated receptor-1 antagonists in patients with coronary artery disease: a meta-analysis of randomized clinical trials

BACKGROUND

Thrombin receptor antagonists blocking protease-activated receptor-1 (PAR-1) on platelets represent a new class of oral antiplatelet agents for patients with atherothrombotic disease manifestations.

OBJECTIVES

We investigated the safety and efficacy of PAR-1 antagonists in patients with coronary artery disease (CAD).

PATIENTS/METHODS

Randomized, placebo-controlled trials of the PAR-1 antagonists atopaxar or vorapaxar in CAD patients were identified. The primary safety endpoint was the composite of Thrombolysis In Myocardial Infarction (TIMI) clinically significant bleeding. The primary efficacy endpoint was the composite of death, myocardial infarction (MI) or stroke.

RESULTS

A total of 41 647 patients from eight trials were included. PAR-1 antagonists were associated with higher risks of TIMI clinically significant (odds ratio [OR] 1.48, 95% confidence interval [CI] 1.39-1.57, P < 0.001), major (OR 1.46, 95% CI 1.28-1.67, P < 0.001) and minor (OR 1.67, 95% CI 1.40-2.00, P < 0.001) bleeding than placebo in the fixed-effects model. PAR-1 antagonists reduced the composite of death, MI or stroke as compared with placebo (OR 0.87, 95% CI 0.81-0.92, P < 0.001), driven by a lower risk of MI (OR 0.85, 95% CI 0.78-0.92, P < 0.001). Conversely, PAR-1 antagonists and placebo did not differ in terms of risk of death (OR 0.99, 95% CI 0.90-1.09, P = 0.81) or stroke (OR 0.96, 95% CI 0.84-1.10, P = 0.59).

CONCLUSIONS

PAR-1 antagonists decrease ischemic events in patients with CAD as compared with placebo, mainly driven by a reduction in MI, at the cost of an increased risk of clinically significant bleeding.

Hydrogen sulfide-releasing aspirin derivative ACS14 exerts strong antithrombotic effects in vitro and in vivo

OBJECTIVE

Hydrogen sulfide (H(2)S)-releasing NSAIDs exert potent anti-inflammatory effects beyond classical cyclooxygenase inhibition. Here, we compared the platelet inhibitory effects of the H(2)S-releasing aspirin derivative ACS14 with its mother compound aspirin to analyze additional effects on platelets.

METHODS AND RESULTS

In platelets of mice fed with ACS14 for 6 days (50 mg/kg per day), not only arachidonic acid-induced platelet aggregation but also ADP-dependent aggregation was decreased, an effect that was not observed with an equimolar dose of aspirin (23 mg/kg per day). ACS14 led to a significantly longer arterial occlusion time after light-dye-induced endothelial injury as well as decreased thrombus formation after ferric chloride-induced injury in the carotid artery. Bleeding time was not prolonged compared with animals treated with equimolar doses of aspirin. In vitro, in human whole blood, ACS14 (25-500 µmol/L) inhibited arachidonic acid-induced platelet aggregation, but compared with aspirin additionally reduced thrombin receptor-activating peptide-, ADP-, and collagen-dependent aggregation. In washed human platelets, ACS14 (500 µmol/L) attenuated αIIbβ3 integrin activation and fibrinogen binding and increased intracellular cAMP levels and cAMP-dependent vasodilator-stimulated phosphoprotein (VASP) phosphorylation.

CONCLUSIONS

The H(2)S-releasing aspirin derivative ACS14 exerts strong antiaggregatory effects by impairing the activation of the fibrinogen receptor by mechanisms involving increased intracellular cyclic nucleotides. These additional antithrombotic properties result in a more efficient inhibition of thrombus formation in vivo as achieved with aspirin alone.

Atopaxar. A novel player in antiplatelet therapy?

Atopaxar, also known as E 5555 is a novel reversible protease-activated receptor-1 (PAR-1) thrombin receptor antagonist. To date, Atopaxar has been investigated in phase II trials with focus on safety and tolerability in patients with acute coronary syndromes or stable coronary artery disease on top of standard antiplatelet therapy. Atopaxar was generally well tolerated, however a rise in liver enzymes and prolongation of the QTcF interval were observed. The data suggest, that atopaxar administration may promote some minor bleeding complications, but does not seem to significantly increase the risk of major bleeding. Although not powered for efficacy, the currently available data suggest potential benefits in patients at high risk for recurrent ischemic events on top of standard antiplatelet therapy. In conclusion, more studies (e.g. phase III) are needed to evaluate efficacy and safety of atopaxar.

Prasugrel versus tirofiban bolus with or without short post-bolus infusion with or without concomitant prasugrel administration in patients with myocardial infarction undergoing coronary stenting: the FABOLUS PRO (Facilitation through Aggrastat By drOpping or shortening Infusion Line in patients with ST-segment elevation myocardial infarction compared to or on top of PRasugrel given at loading dOse) trial

OBJECTIVES

The authors sought to compare the effect on inhibition of platelet aggregation (IPA) of prasugrel therapy versus tirofiban bolus with or without a post-bolus short drug infusion in ST-segment elevation myocardial infarction (STEMI) patients.

BACKGROUND

The degree and rapidity of IPA after prasugrel alone with or without concomitant glycoprotein IIb/IIIa inhibition in STEMI patients is unknown.

METHODS

A total of 100 STEMI patients randomly received prasugrel 60 mg versus 25 μg/kg tirofiban bolus with or without post-bolus 2-h infusion of tirofiban, with or without concomitant prasugrel. IPA at light transmission aggregometry was performed throughout 24 h. The primary endpoint was IPA stimulated with 20 μmol/l adenosine diphosphate (ADP) at 30 min.

RESULTS

At 30 min, patients in the prasugrel group showed a significantly lower IPA to 20 μmol/l ADP stimulation as compared with tirofiban-treated patients (36 ± 35 vs. 87 ± 31, p < 0.0001). Similarly, patients taking prasugrel showed a suboptimal degree of platelet inhibition for at least 2 h compared with tirofiban patients. Post-bolus tirofiban infusion was necessary to maintain a high level of IPA beyond 1 h after bolus administration if concomitant clopidogrel was given, whereas the bolus-only tirofiban and concomitant prasugrel led to the higher and more consistent IPA levels after both ADP and thrombin receptor-activating peptide stimuli than either therapy alone.

CONCLUSIONS

Our study shows that prasugrel administration leads to a suboptimal IPA for at least 2 h in STEMI patients. Yet, prasugrel, given in association with a bolus only of glycoprotein IIb/IIIa inhibitor, obviates the need of post-bolus infusion and almost completely abolishes residual variability of IPA after treatment. (Facilitation through Aggrastat By drOpping or shortening Infusion Line in patients with ST-segment elevation myocardial infarction compared to or on top of PRasugrel given at loading dOse [The FABOLUS PRO trial]; NCT01336348).

Atopaxar: a review of its mechanism of action and role in patients with coronary artery disease

Platelet activation and aggregation is a complex and key process in thrombus formation after the rupture of an atherosclerotic plaque, which can lead to an acute coronary syndrome. Aspirin, an irreversible inhibitor of thromboxane A2 synthesis, in combination with an inhibitor of P2Y12 ADP platelet receptors (clopidogrel, prasugrel or ticagrelor), represents the current standard of care of antiplatelet therapy for patients with acute coronary syndrome and in those patients undergoing percutaneous coronary intervention. Despite the benefit of these agents, the risk of thrombotic events and bleeding complications may still occur while on such antiplatelet treatment regimens, thus representing an important limitation. Thrombin is one of the most important platelet activators. The inhibition of thrombin-mediated platelet activation by means of protease-activated receptor-1 inhibitors represents an attractive therapeutic option for patients with atherothrombotic disease processes. This article provides an overview on atopaxar (E5555), an orally active protease-activated receptor-1 antagonist that has recently completed Phase II clinical investigation.

[Periinterventional antiplatelet therapy: from bench to bedside]

The recruitment and activation of blood platelets initiate ischemic events in patients with coronary artery diseases (myocardial infarction). Moreover, platelet activation and aggregation are key events triggering early and late thrombotic complications (stent thromboses) in patients successfully treated with coronary stent implantation. Hence, the inhibition of the thromboxane A2 synthesis by aspirin and the ADP-receptor pathway by clopidogrel have drastically reduced the risk of ischemic complications following coronary stenting. However, clopidogrel has several inherent limitations, including the high incidence of non- or low-responders and the increased risk of bleeding due to the prolonged persistence of its anti-platelet effect. The latter is of particular concern in patients who require non-deferrable surgery. In recent years, novel anti-platelet substances have been clinically evaluated and are now approved for use in patients. These substances offer several potential advantages over the previous strategies and could pave the way for more individualized anti-platelet regimens. In this review, we give an overview on the available clinical data evaluating the benefits of these novel anti-platelet substances.

Pharmacokinetics and pharmacodynamics of the novel PAR-1 antagonist vorapaxar in patients with end-stage renal disease

PURPOSE

To determine whether impaired renal function alters the pharmacokinetics (PK) of vorapaxar or its ability to inhibit thrombin receptor agonist peptide (TRAP)-induced platelet aggregation.

METHODS

This was an open-label study in which 8 patients with end-stage renal disease (ESRD) on hemodialysis and 7 matched (based on age, gender, weight, and height) healthy controls were administered a single 10-mg oral dose of vorapaxar. Blood samples for vorapaxar PK and pharmacodynamic analysis were collected predose and at frequent intervals up to 6 weeks postdose.

RESULTS

Mean vorapaxar bioavailability (based on area under the curve of plasma vorapaxar concentration over time) was identical in the two subject groups; the ESRD/healthy geometric mean ratio (GMR, expressed in percent) was 98. Mean maximum observed plasma concentration (77.4-98.2 ng/mL) was numerically lower in patients with ESRD compared with matched controls (GMR=76; 90% confidence interval=48 to 118). Median time of maximum observed plasma concentration was 2 h in both subject groups. The observed means for elimination half-life were 186 and 231 h in the ESRD and control groups, respectively. Inhibition of platelet aggregation was similar in the two groups. Four out of 15 (27%) subjects reported adverse events, all of which were characterized by the investigator as mild and unrelated to treatment.

CONCLUSIONS

ESRD had no clinically relevant effect on the PK profile of vorapaxar or its ability to inhibit TRAP-induced platelet aggregation.

Antiplatelet therapy: thrombin receptor antagonists

Activated platelets stimulate thrombus formation in response to rupture of an atherosclerotic plaque or endothelial cell erosion, promoting atherothrombotic disease. Multiple pathways contribute to platelet activation. Aspirin, an irreversible inhibitor of thromboxane A2 synthesis, in combination with clopidogrel, an inhibitor of P2Y(12) adenosine diphosphate platelet receptors, represent the current standard-of-care of antiplatelet therapy for patients with acute coronary syndrome and for those undergoing percutaneous coronary intervention. Although these agents have demonstrated significant clinical benefit, the increased risk of bleeding and the recurrence of thrombotic events represent substantial limitations. Thrombin is one of the most important platelet activators. The inhibition of protease-activated receptor 1 showed a good safety profile in preclinical studies. In fact, phase II studies with vorapaxar (SCH530348) and atopaxar (E5555) showed no increase of bleeding events in addition to the current standard-of-care of antiplatelet therapy. Although the results of phase III trials for both drugs are awaited, this family is a promising new addition to the current clinical practice for patients with atherothrombotic disease, not only as an alternative, but also as additional therapy.

Increased atherothrombotic burden in patients with diabetes mellitus and acute coronary syndrome: a review of antiplatelet therapy

Patients with diabetes mellitus presenting with acute coronary syndrome have a higher risk of cardiovascular complications and recurrent ischemic events when compared to nondiabetic counterparts. Different mechanisms including endothelial dysfunction, platelet hyperactivity, and abnormalities in coagulation and fibrinolysis have been implicated for this increased atherothrombotic risk. Platelets play an important role in atherogenesis and its thrombotic complications in diabetic patients with acute coronary syndrome. Hence, potent platelet inhibition is of paramount importance in order to optimise outcomes of diabetic patients with acute coronary syndrome. The aim of this paper is to provide an overview of the increased thrombotic burden in diabetes and acute coronary syndrome, the underlying pathophysiology focussing on endothelial and platelet abnormalities, currently available antiplatelet therapies, their benefits and limitations in diabetic patients, and to describe potential future therapeutic strategies to overcome these limitations.

Overcoming limitations of current antiplatelet drugs: a concerted effort for more profitable strategies of intervention

Platelets play a central role in the pathophysiology of atherothrombosis, an inappropriate platelet activation leading to acute ischemic complications (acute myocardial infarction, ischemic stroke). In view of this, platelets are a major target for pharmacotherapy. Presently, the main classes of antiplatelet agents approved for the use in such complications are aspirin and thienopyridines. Although antiplatelet treatment with these two types of drugs, alone or in combination, leads to a significant reduction of non-fatal myocardial infarction (-32%), non-fatal stroke (-25%), and of cardiovascular death (-17%), a residual risk persists. Newer antiplatelet agents have addressed some, but not all, these limitations. Vis-à-vis their net clinical benefit, the higher potency of some of them is associated with a rise in bleeding complications. Moreover, newer thienopyridines do not show advantages over and above the older ones as to reduction of stroke. A concerted effort that takes into consideration clinical, genetic, and laboratory information is increasingly recognized as a major direction to be pursued in the area. The well-established road signs of clinical epidemiology will provide major information to define newer potentially useful targets for platelet pharmacology.

[New era in antiplatelet therapy based on results from recent clinical trials]

Antiplatelet therapy has been successful in reducing mortality and morbidity in cardiovascular diseases (e.g. acute coronary syndromes). Recent advances in understanding the molecular basis of the role of platelets in atherothrombosis have enabled the development of new agents with the potential to further reduce mortality and morbidity. Some limitations associated with the use of aspirin and clopidogrel have led to potential alternatives, including more potent ADP antagonists such as prasugrel, ticagrelor, cangrelor, and thrombin receptor antagonists, which have shown additional benefit in large randomized controlled trials. These new agents open a realistic prospect of a personalized choice of the most appropriate antiplatelet therapy tailored for an individual patient and a clinical condition.

Platelets in atherosclerosis

Beyond obvious functions in haemostasis and thrombosis, platelets are considered to be essential in proinflammatory surroundings such as atherosclerosis, allergy, rheumatoid arthritis and even cancer. In atherosclerosis, platelets facilitate the recruitment of inflammatory cells towards the lesion sites and release a plethora of inflammatory mediators, thereby enriching and boosting the inflammatory milieu. Platelets do so by interacting with endothelial cells, circulating leukocytes (monocytes, neutrophils, dendritic cells, T-cells) and progenitor cells. This cross-talk enforces leukocyte activation, adhesion and transmigration. Furthermore, platelets are known to function in innate host defense through the release of antimicrobial peptides and the expression of pattern recognition receptors. In severe sepsis, platelets are able to trigger the formation of neutrophil extracellular traps (NETs), which bind and clear pathogens. The present antiplatelet therapies that target key pathways of platelet activation and aggregation therefore hold the potential to modulate platelet-derived immune functions by reducing cellular interactions of platelets with other immune components and by reducing the secretion of inflammatory proteins into the milieu. The objective of this review is to update and discuss the current perceptions of the platelet immune constituents and their prospect as therapeutic targets in an atherosclerotic setting.

Pharmacodynamics and pharmacokinetics of the novel PAR-1 antagonist vorapaxar (formerly SCH 530348) in healthy subjects

PURPOSE

The aim of our study was to evaluate the pharmacology of vorapaxar (SCH 530348), an oral PAR-1 antagonist, in healthy volunteers.

METHODS AND RESULTS

In two randomized, placebo-controlled studies, subjects received either single ascending doses of vorapaxar (0.25, 1, 5, 10, 20, or 40 mg; n = 50), multiple ascending doses of vorapaxar (1, 3, or 5 mg/day for 28 days; n = 36), a loading dose (10 or 20 mg) followed by daily maintenance doses (1 mg) for 6 days (n = 12), or placebo. Single 20- and 40-mg doses of vorapaxar completely inhibited thrombin receptor activating peptide (TRAP)-induced platelet aggregation (>80% inhibition) at 1 h and sustained this level of inhibition for ≥72 h. Multiple doses yielded complete inhibition on Day 1 (5 mg/day) and Day 7 (1 and 3 mg/day). Adverse events were generally mild, transient, and unrelated to dose.

CONCLUSION

Vorapaxar provided rapid and sustained dose-related inhibition of platelet aggregation without affecting bleeding or clotting times.

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.

Atherothrombosis and the role of antiplatelet therapy

Atherothrombosis remains a major global public health problem. Chronic atherosclerotic disease is often clinically silent and coexists across vascular beds, but when complicated by thrombosis can result in acute coronary syndrome, stroke, transient ischaemic attack and critical limb ischaemia. Platelets play a role in the development of chronic atherosclerotic disease and are a key mediator of clinical events in atherothrombosis. Numerous trials have examined the role of antiplatelet agents in primary and secondary prevention and several new antiplatelet drugs are under development. In secondary prevention, there is evidence of clear benefit of single and in some cases dual antiplatelet therapy in the prevention of recurrent cerebro-vascular complications. Dual antiplatelet therapy has emerged as the standard of care in acute coronary syndromes, with aspirin typically being used in combination with clopidogrel or one of the newer more potent antiplatelet agents. Conversely, in chronic stable coronary disease, no benefit has yet been convincingly demonstrated from dual antiplatelet therapy. In cerebro-vascular disease, aspirin monotherapy remains the cornerstone of prevention of recurrent events, with clopidogrel or the combination of aspirin and dipyridamole being only modestly more efficacious. In primary prevention, the evidence for the routine use of aspirin or any other antiplatelet agent is mixed and suggests this should only be considered on an individual basis in high-risk groups where the thrombotic risk outweighs the risk of major bleeding complications.

Impact of adjunctive cilostazol therapy on platelet function profiles in patients with and without diabetes mellitus on aspirin and clopidogrel therapy

Cilostazol is a platelet inhibitor which when added to aspirin and clopidogrel has shown to reduce the risk of recurrent ischaemic events without an increase in bleeding. These clinical benefits have shown to be more pronounced in patients with diabetes mellitus (DM). However, it remains unknown whether cilostazol exerts different pharmacodynamic effects in patients with and without DM. This was a randomised, double-blind, placebo-controlled, cross-over pharmacodynamic study comparing platelet function in patients with and without DM on aspirin and clopidogrel therapy. Patients (n=111) were randomly assigned to either cilostazol 100 mg or placebo twice daily for 14 days and afterwards crossed-over treatment for another 14 days. Platelet function was performed at baseline, 14 days post-randomisation, and 14 days post-cross-over. Functional testing to assess P2Y12 signalling included flow cytometric analysis of phosphorylation status of vasodilator-stimulated phosphoprotein measured by P2Y12 reactivity index (PRI), light transmittance aggregometry and VerifyNow. Thrombin generation processes were also studied using thrombelastography. Significantly lower PRI values were observed following treatment with cilostazol compared with placebo both in DM and non-DM groups (p < 0.0001). The absolute between-treatment differences of PRI between groups was a 35.1% lower in patients with DM (p=0.039). Similar results were obtained using all other functional measures assessing P2Y12 signalling. Thrombin generation was not affected by cilostazol. Cilostazol reduces platelet reactivity both in patients with and without DM, although these pharmacodynamic effects are enhanced in patients with DM. Despite the marked platelet inhibition, cilostazol does not alter thrombin-mediated haemostatic processes, which may explain its ischaemic benefit without the increased risk of bleeding.