Scientific and therapeutic insights into the role of the platelet P2Y12 receptor in thrombosis

Correlation P2Y12 Genetic Polymorphism As Risk Factor of Clopidogrel Resistance in Indonesian Stroke Patients

Background

Stroke is one of the highest causes of disability and mortality in several countries worldwide. Secondary prevention is important in the management of stroke. Clopidogrel is widely used in Asia as secondary prevention for ischemic stroke, even though several studies in Western show limited data related to clopidogrel resistance in Asia. This study aims to determine the correlation between P2Y12 genetic polymorphism and clopidogrel resistance in Indonesia.

Methods

This study was conducted on one-year duration, the subjects were chosen through the consecutive sampling method, all subjects were examined for genetics and resistance to clopidogrel. The data were analyzed through statistical analysis, a bivariate analysis was conducted to determine the correlation between several variables and the resistance variable. This study employed resistance diagnostic methods with VerifyNow. Polymorphism of receptor P2Y12 was tested with the Polymerase Chain Reaction method (PCR) and analysis of restriction fragment length polymorphism (RFLP). The genes tested in this study were G52T and C34T.

Results

The number of participants in this study was 112. Examination of gene P2Y12 showed that the majority was homozygote, wild-type C34T allele (67%), and G52T (66.1%). There was no significant correlation between clopidogrel resistance and gene G52T and C34T of P2Y12 (p > 0.05). Hb levels significantly correlated with P2Y12 G52T (p = 0.024). Meanwhile, Fatty Liver significantly correlated with P2Y12 C34T (p = 0.037).

Conclusion

Indonesia showed a low clopidogrel resistance rate and a very low C34T and G52T allele P2Y12 gene mutation, meaning that Indonesia had low mutations in the P2Y12. This is the cause of clopidogrel resistance in this study only 15%. Therefore, in a region with less clopidogrel resistance, examination of the P2Y12 gene would not give significant results.

The choline transporter Slc44a2 controls platelet activation and thrombosis by regulating mitochondrial function

Genetic factors contribute to the risk of thrombotic diseases. Recent genome wide association studies have identified genetic loci including SLC44A2 which may regulate thrombosis. Here we show that Slc44a2 controls platelet activation and thrombosis by regulating mitochondrial energetics. We find that Slc44a2 null mice (Slc44a2(KO)) have increased bleeding times and delayed thrombosis compared to wild-type (Slc44a2(WT)) controls. Platelets from Slc44a2(KO) mice have impaired activation in response to thrombin. We discover that Slc44a2 mediates choline transport into mitochondria, where choline metabolism leads to an increase in mitochondrial oxygen consumption and ATP production. Platelets lacking Slc44a2 contain less ATP at rest, release less ATP when activated, and have an activation defect that can be rescued by exogenous ADP. Taken together, our data suggest that mitochondria require choline for maximum function, demonstrate the importance of mitochondrial metabolism to platelet activation, and reveal a mechanism by which Slc44a2 influences thrombosis.

Genetic association studies have identified loci including the choline transporter SLC44A2 as a potential regulator of thrombosis. Here the authors report that loss of SLC44A2 impairs platelet activation and thrombosis in mice via a reduction of mitochondrial ATP production.

Impaired hemostatic activity of healthy transfused platelets in inherited and acquired platelet disorders: Mechanisms and implications

Platelet transfusions can fail to prevent bleeding in patients with inherited platelet function disorders (IPDs), such as Glanzmann's thrombasthenia (GT; integrin αIIbβ3 dysfunction), Bernard-Soulier syndrome [BSS; glycoprotein (GP) Ib/V/IX dysfunction], and the more recently identified nonsyndromic RASGRP2 variants. Here, we used IPD mouse models and real-time imaging of hemostatic plug formation to investigate whether dysfunctional platelets impair the hemostatic function of healthy donor [wild-type (WT)] platelets. In Rasgrp2-/- mice or mice with platelet-specific deficiency in the integrin adaptor protein TALIN1 ("GT-like"), WT platelet transfusion was ineffective unless the ratio between mutant and WT platelets was ~2:1. In contrast, thrombocytopenic mice or mice lacking the extracellular domain of GPIbα ("BSS-like") required very few transfused WT platelets to normalize hemostasis. Both Rasgrp2-/- and GT-like, but not BSS-like, platelets effectively localized to the injury site. Mechanistic studies identified at least two mechanisms of interference by dysfunctional platelets in IPDs: (i) delayed adhesion of WT donor platelets due to reduced access to GPIbα ligands exposed at sites of vascular injury and (ii) impaired consolidation of the hemostatic plug. We also investigated the hemostatic activity of transfused platelets in the setting of dual antiplatelet therapy (DAPT), an acquired platelet function disorder (APD). "DAPT" platelets did not prolong the time to initial hemostasis, but plugs were unstable and frequent rebleeding was observed. Thus, we propose that the endogenous platelet count and the ratio of transfused versus endogenous platelets should be considered when treating select IPD and APD patients with platelet transfusions.

A complementary role for tetraspanin superfamily member TSSC6 and ADP purinergic P2Y12 receptor in platelets

Tumor-suppressing subchromosomal transferable fragment cDNA 6 (TSSC6) expression is restricted to hematopoietic organs and tissues where it plays a role in hematopoietic-cell function. The ADP purinergic receptor P2Y12 is mainly expressed by platelets with important clinical significance as a target for several clinically approved antithrombotic agents. We have previously shown a physical association between P2Y12 and TSSC6 in platelets. Hence our aim was to investigate whether this physical association is translated to functional effects. To investigate this possibility, we used wild-type or TSSC6 knockout (KO) mice treated with either PBS or 50mg/kg clopidogrel. TSSC6 KO mice treated with clopidogrel exhibited synergy in delayed kinetics of clot retraction, reduced collagen-mediated platelet aggregation, and platelet spreading on fibrinogen. Platelets derived from TSSC6 mice with P2Y12 blockade form smaller thrombi when perfused over a collagen matrix under arterial flow. Clopidogrel treated TSSC6 KO arterioles showed smaller and less stable thrombi with increased tendency to embolise in vivo. These studies demonstrate a complementary role between TSSC6 and P2Y12 receptor in platelets in regulating 'outside in' integrin αIIbβ3 signalling thrombus growth and stability.

The discovery and development of prasugrel

INTRODUCTION

Prasugrel (CS-747, LY640315) is a third-generation thienopyridine, which gained approval by the FDA in 2009 for its use in patients with acute coronary syndrome undergoing percutaneous coronary intervention.

AREAS COVERED

This article focuses on the preclinical profile of prasugrel. Using published preclinical and clinical studies, the authors summarize the pharmacokinetics, pharmacodynamics, and pharmacogenomics of prasugrel and their distinguishing features in efficacy and safety.

EXPERT OPINION

Prasugrel has a more rapid, more potent antiplatelet effect with less interindividual response variability when compared to clopidogrel. Those therapeutic advantages are attributed to features of its chemical structure that favor the metabolic conversion of prasugrel to its active metabolite. However, the increased risk of bleeding has been associated with a greater antiplatelet effect and dosing profile; this is especially the case in those patients who are at a higher risk of bleeding complications. It is therefore important for an optimal dosing strategy of prasugrel to be identified to provide a formulation that has the best balance for efficacy and safety.

Delineation of ligand binding and receptor signaling activities of purified P2Y receptors reconstituted with heterotrimeric G proteins

P2Y receptors are G protein coupled receptors that respond to extracellular nucleotides to promote a multitude of signaling events. Our laboratory has purified several P2Y receptors with the goal of providing molecular insight into their: (1) ligand binding properties, (2) G protein signaling selectivities, and (3) regulation by RGS proteins and other signaling cohorts. The human P2Y₁ receptor and the human P2Y₁₂ receptor, both of which are intimately involved in ADP-mediated platelet aggregation, were purified to near homogeneity and studied in detail. After high-level expression from recombinant baculovirus infection of Sf9 insect cells, approximately 50% of the receptors were successfully extracted with digitonin. Purification of nearly homogeneous epitope-tagged P2Y receptor was achieved using metal-affinity chromatography followed by other traditional chromatographic steps. Yields of purified P2Y receptors range from 10 to 100 μg/l of infected cells. Once purified, the receptors were reconstituted in model lipid vesicles along with their cognate G proteins to assess receptor function. Agonist-promoted increases in steady-state GTPase assays demonstrated the functional activity of the reconstituted purified receptor. We have utilized this reconstitution system to assess the action of various nucleotide agonists and antagonists, the relative G protein selectivity, and the influence of other proteins, such as phospholipase C, on P2Y receptor-promoted signaling. Furthermore, we have identified the RGS expression profile of platelets and have begun to assess the action of these RGS proteins in a reconstituted P2Y receptor/G protein platelet model.

Long-term (trophic) purinergic signalling: purinoceptors control cell proliferation, differentiation and death

The purinergic signalling system, which uses purines and pyrimidines as chemical transmitters, and purinoceptors as effectors, is deeply rooted in evolution and development and is a pivotal factor in cell communication. The ATP and its derivatives function as a 'danger signal' in the most primitive forms of life. Purinoceptors are extraordinarily widely distributed in all cell types and tissues and they are involved in the regulation of an even more extraordinary number of biological processes. In addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion, there is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body. In this article, we focus on the latter in the immune/defence system, in stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption, as well as in cancer.

Peripheral arterial disease in diabetes: is there a role for genetics?

Atherosclerotic occlusion of vessels outside of the heart is commonly referred to as peripheral arterial disease (PAD). The lower extremity is the most common site of PAD and its development is associated with the same risk factors involved in general atherosclerosis. However, there is emerging evidence that other risk factors may play a key role in the development of PAD. Over the past decade polymorphism in a number of genes has been shown to contribute to the risk of developing PAD. These genes can be classified into proartherosclerosis or proatherothrombosis based on the known gene function. Moreover, they can be categorized as "novel" polymorphism when the function of the genes is not known or when the specific gene within an associated genetic locus is not known. It is intriguing that not only are gene polymorphisms associated with PAD being identified, but more recently studies are now finding gene polymorphisms that may be important in development of this syndrome only in the contest of certain environmental factors such as diabetes. Currently how these gene-environment interactions contribute to the pathogenesis of PAD is poorly understood but will likely play a critical role in future understanding of this complex disease.

Ticagrelor effectively and reversibly blocks murine platelet P2Y12-mediated thrombosis and demonstrates a requirement for sustained P2Y12 inhibition to prevent subsequent neointima

OBJECTIVE

Our goal was to study the effects of ticagrelor on murine platelet function and thrombosis and characterize the time course of P2Y(12) inhibition required to inhibit neointima formation following vascular injury.

METHODS AND RESULTS

Mice were treated with ticagrelor or vehicle. Platelet aggregation and P-selectin expression were assessed over time, and thrombus formation was assessed in laser-injured cremasteric arterioles of P2Y(12)+/+ and P2Y(12)-/- mice. Neointima formation in FeCl(3)-injured carotid artery was assessed in C57BL/6 mice treated with different regimens of ticagrelor. Ticagrelor inhibited platelet aggregation and P-selectin expression in a dose-dependent, reversible manner. Ticagrelor inhibited thrombus formation to the same extent as seen in P2Y(12)-/- mice. Neointima formation was markedly reduced in mice treated with ticagrelor before and 4 hours after injury (neointima area: control, 39 921±22 749 μm(2), versus ticagrelor, 3705±2600 μm(2); P<0.01), whereas administration of ticagrelor either before injury only or from 4 hours postinjury was ineffective.

CONCLUSIONS

Ticagrelor effectively and reversibly inhibits P2Y(12)-mediated platelet function and thrombosis in mice. P2Y(12) inhibition is required both at the time of and after injury to effectively inhibit neointima formation. Additional studies are warranted to evaluate the role of P2Y(12) inhibition in preventing restenosis.

Roles and interactions among protease-activated receptors and P2ry12 in hemostasis and thrombosis

Toward understanding their redundancies and interactions in hemostasis and thrombosis, we examined the roles of thrombin receptors (protease-activated receptors, PARs) and the ADP receptor P2RY12 (purinergic receptor P2Y G protein-coupled 12) in human and mouse platelets ex vivo and in mouse models. Par3(-/-) and Par4(+/-) mouse platelets showed partially decreased responses to thrombin, resembling those in PAR1 antagonist-treated human platelets. P2ry12(+/-) mouse platelets showed partially decreased responses to ADP, resembling those in clopidogrel-treated human platelets. Par3(-/-) mice showed nearly complete protection against carotid artery thrombosis caused by low FeCl(3) injury. Par4(+/-) and P2ry12(+/-) mice showed partial protection. Increasing FeCl(3) injury abolished such protection; combining partial attenuation of thrombin and ADP signaling, as in Par3(-/-):P2ry12(+/-) mice, restored it. Par4(-/-) mice, which lack platelet thrombin responses, showed still better protection. Our data suggest that (i) the level of thrombin driving platelet activation and carotid thrombosis was low at low levels of arterial injury and increased along with the contribution of thrombin-independent pathways of platelet activation with increasing levels of injury; (ii) although P2ry12 acts downstream of PARs to amplify platelet responses to thrombin ex vivo, P2ry12 functioned in thrombin/PAR-independent pathways in our in vivo models; and (iii) P2ry12 signaling was more important than PAR signaling in hemostasis models; the converse was noted for arterial thrombosis models. These results make predictions being tested by ongoing human trials and suggest hypotheses for new antithrombotic strategies.

Elinogrel: pharmacological principles, preclinical and early phase clinical testing

Antiplatelet drug therapy represents the cornerstone of treatment for cardiovascular atherothrombotic disease processes. Dual antiplatelet therapy with aspirin and oral ADP-receptor antagonists such as clopidogrel are currently the standard care for prevention of ischemic events in patients with acute coronary syndrome and who are undergoing percutaneous coronary intervention. However, despite the clinical benefit associated with clopidogrel therapy, this drug has several limitations, including a broad interindividual response variability, drug–drug interactions, slow onset of action and irreversible platelet inhibition, emphasizing the need for novel P2Y12-receptor antagonists. Elinogrel (PRT060128) is a reversible, potent and competitive inhibitor of the P2Y12 receptor with a fast onset and offset of action that can be administered by both oral and intravenous routes and rapidly achieves near complete platelet inhibition. Preclinical and early-phase clinical testing have shown promising results with this novel compound, which awaits further testing in outcome-driven clinical trials. This article provides an overview of the current level of knowledge regarding elinogrel, focusing on its pharmacologic properties and preclinical and early-phase clinical development.

Antiplatelet effects of piplartine, an alkamide isolated from Piper tuberculatum: possible involvement of cyclooxygenase blockade and antioxidant activity

Objectives

Piplartine (piperlongumine; 5,6-dihydro-1-[1-oxo-3-(3,4,5-trimethoxyphenyl]-2(1H) pyridinone) is an alkaloid amide isolated from Piper species (Piperaceae). It has been reported to show multiple pharmacological activities in vitro and in vivo.

Methods

We evaluated the in-vitro antiplatelet effect of piplartine isolated from the roots of P. tuberculatum, on human platelet aggregation induced in platelet-rich plasma by the agonists collagen, adenosine 5′-diphosphate (ADP), arachidonic acid (AA) and thrombin.

Key findings

Piplartine (100μg/ml) caused a 30% inhibition in platelet aggregation when collagen was the agonist. At 200 μg/ml, piplartine significantly inhibited the aggregation induced by arachidonic acid (100%), collagen (59%) or ADP (52%) but not that induced by thrombin. The highest concentration of piplartine (300 μg/ml) inhibited thrombin- (37%), ADP- (71%) and collagen- (98%) induced aggregation. The inhibitory effect of piplartine on ADP-induced platelet aggregation was not modified by pretreatment with pentoxifylline (a phosphodiesterase inhibitor), l-arginine (a substrate for nitric oxide synthase) or ticlopidine (a P2Y12 purinoceptor antagonist). However, aspirin, a well-known inhibitor of cyclooxygenase, greatly increased the inhibitory effect of piplartine on arachidonic-acid-induced platelet aggregation.

Conclusions

The mechanism underlying the piplartine antiplatelet action is not totally clarified. It could be related to the inhibition of cyclooxgenase activity and a decrease in thromboxane A2 formation, similar to that occurring with aspirin. This and other possible mechanisms require further study.

Clopidogrel resistance

Clopidogrel resistance has been used as one of the terms employed in the literature to describe different degrees of ex vivo low platelet inhibition after clopidogrel administration. In addition to the diverse nomenclature, the characterisation of clopidogrel resistance has also been problematic because different authors have given different definitions. The mechanisms responsible for this decreased platelet response are not yet clearly defined, some hypotheses have been put forward but not yet demonstrated. Although there have been no large prospective studies demonstrating that the degree of platelet inhibition is directly related to clinical outcomes, several recent studies and reports have shown an association between less platelet inhibition and more adverse events after percutaneous coronary interventions with clopidogrel therapy suggesting that clopidogrel resistance may be a marker for increased risk of recurrent cardiovascular events. Larger scale investigations are needed to support these findings.

Platelet activation triggered by Chlamydia pneumoniae is antagonized by 12-lipoxygenase inhibitors but not cyclooxygenase inhibitors

Chlamydia pneumoniae is a respiratory pathogen that has been linked to cardiovascular disease. We have recently shown that C. pneumoniae activates platelets, leading to oxidation of low-density lipoproteins. The aim of the present study was to evaluate the inhibitory effects of different pharmacological agents on platelet aggregation and secretion induced by C. pneumoniae. Platelet interaction with C. pneumoniae was studied by analyzing platelet aggregation and ATP-secretion with Lumi-aggregometry. Platelet aggregation and ATP-secretion induced by C. pneumoniae was markedly inhibited by the NO-donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP), an effect that was counteracted by the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). Pre-treatment of platelets with the 12-lipoxygenase (12-LOX) inhibitors cinnamyl-3,4-dihydroxy-alpha-cyanocinnamate (CDC) and 5,6,7-trikydroxyflavone (baicalein) completely blocked the activation, whereas the cyclooxygenase (COX) inhibitors 2-acetyloxybenzoic acid (aspirin) and (8E)-8-[hydroxy-(pyridin-2-ylamino)methylidene]-9-methyl-10,10-dioxo-10$l;(6)thia-9-azabicyclo[4.4.0]deca-1,3,5-trien-7-one (piroxicam) had no inhibitory effects. Opposite to C. pneumoniae-induced activation, platelets stimulated by collagen were inhibited by the COX-inhibitors but were unaffected by the 12-LOX-inhibitors. The platelet activating factor (PAF) antagonist Ginkgolide B blocked the C. pneumoniae-induced platelet activation, whereas the responses to collagen were unaffected. Furthermore, the P2Y1 and P2Y12 purinergic receptor antagonists 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS2179) and N(6)-(2-methyl-thioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (cangrelor) inhibited the aggregation and secretion caused by C. pneumoniae. It is well-known that the efficacy of COX inhibitors in the prevention and treatment of cardiovascular disease varies between different patients, and that patients with low responses to aspirin have a higher risk to encounter cardiovascular events. The findings in this study showing that platelets stimulated by C. pneumoniae are unaffected by COX inhibitors but sensitive to 12-LOX inhibitors, may thus be of importance in future management of atherosclerosis and thrombosis.

Clopidogrel resistance: role of body mass and concomitant medications

INTRODUCTION

Platelets have a central role in the development of arterial thrombosis and subsequent cardiovascular events. An appreciation of this has made antiplatelet therapy the cornerstone of cardiovascular disease management. Recent studies have described the phenomenon of clopidogrel resistance but the possible mechanisms are still unclear.

PATIENTS AND METHODS

The aim of this study was to compare the characteristics (risk profile, previous diseases, medications, hemorheological variables and plasma von Willebrand factor and soluble P-selectin levels) of patients in whom clopidogrel provided effective platelet inhibition with those in whom clopidogrel was not effective in providing platelet inhibition. 157 patients with chronic cardio- and cerebrovascular diseases (83 males, mean age 61+/-11 yrs, 74 females, 63+/-13 yrs) taking 75 mg clopidogrel daily (not combined with aspirin) were included in the study.

RESULTS

Compared with clopidogrel-resistant patients (35 patients (22%), patients who demonstrated effective clopidogrel inhibition had a significantly lower BMI (26.1 vs. 28.8 kg/m2, p<0.05). Patients with ineffective platelet aggregation were significantly more likely to be taking benzodiazepines (25% vs. 10%) and selective serotonin reuptake inhibitors (28% vs. 12%) (p<0.05). After an adjustment to the risk factors and medications BMI (OR 2.62; 95% CI: 1.71 to 3.6; p<0.01), benzodiazepines (OR 5.83; 95% CI: 2.53 to 7.1; p<0.05) and SSRIs (OR 5.22; 95% CI: 2.46 to 6.83; p<0.05) remained independently associated with CLP resistance. There was no significant difference in the rheological parameters and in the plasma levels of adhesive molecules between the two examined groups.

CONCLUSION

The background of ineffective clopidogrel medication is complex. Drug interactions may play a role on clopidogrel bioavailability, on the other hand, the significant difference in BMI between the two examined groups suggests that clopidogrel therapy should be weight-adjusted.

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

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

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Plasma ectonucleotidases prevent desensitization of purinergic receptors in stored platelets: importance for platelet activity during thrombus formation

BACKGROUND

Platelets (PLTs) contain purinergic receptors for ATP (P2X1) and ADP (P2Y1 and P2Y12) that rapidly desensitize upon stimulation with these nucleotides. In vivo, this is antagonized by ectonucleotidases on the surface of endothelial cells and white blood cells (WBCs). The receptor desensitization of ATP- and ADP-induced responses of PLTs stored in plasma without WBCs was investigated.

STUDY DESIGN AND METHODS

ATP- and ADP-induced PLT shape change (shear-induced) aggregation and Ca2+ signaling were measured in the presence or absence of plasma. Degradation of nucleotides in plasma was quantified by high-performance liquid chromatography.

RESULTS

Washed PLTs became refractory for ATP and ADP in shape change, aggregation, and Ca2+ responses during a 90-minute incubation at 37 degrees C. The PLT responses mediated by P2X1, P2Y1, and P2Y12 receptors gradually reduced or disappeared. When plasma was present, however, the PLTs persistently showed high responses to ATP and ADP. Heat treatment of plasma abolished this effect. Also under conditions of flow and high shear, PLTs in plasma kept high P2X1 activity, mediating aggregate formation. In isolated plasma, not containing WBCs, nucleotides were degraded in the order of ADP/UDP>ATP/UTP. Degradation of ATP was partly inhibited by blocking the ecto-NTPDase CD39, whereas degradation of both ATP and ADP was inhibited by blocking ectopyrophosphatase/phosphodiesterase activity. Part of the nucleotide-degrading activities appeared to be membrane-bound.

CONCLUSION

Ectonucleotidases in plasma preserve the functionality of P2X1 and P2Y receptors. Upon PLT storage, these plasma activities are essential to ensure adequate (shear-dependent) formation of aggregates and thrombi.

Continuous signaling via PI3K isoforms beta and gamma is required for platelet ADP receptor function in dynamic thrombus stabilization

Signaling from collagen and G protein-coupled receptors leads to platelet adhesion and subsequent thrombus formation. Paracrine agonists such as ADP, thromboxane, and Gas6 are required for platelet aggregate formation. We hypothesized that thrombi are intrinsically unstable structures and that their stabilization requires persistent paracrine activity and continuous signaling, maintaining integrin alpha(IIb)beta3 activation. Here, we studied the disassembly of human and murine thrombi formed on collagen under high shear conditions. Platelet aggregates rapidly disintegrated (1) in the absence of fibrinogen-containing plasma; (2) by blocking or inhibiting alpha(IIb)beta3; (3) by blocking P2Y12 receptors; (4) by suppression of phosphoinositide 3-kinase (PI3K) beta. In murine blood, absence of PI3Kgamma led to formation of unstable thrombi, leading to dissociation of multiplatelet aggregates. In addition, blocking PI3Kbeta delayed initial thrombus formation and reduced individual platelet-platelet contact. Similarly without flow, agonist-induced aggregation was reversed by late suppression of P2Y12 or PI3K isoforms, resulting in single platelets that had inactivated alpha(IIb)beta3 and no longer bound fibrinogen. Together, the data indicate that continuous outside-in signaling via P2Y12 and both PI3Kbeta and PI3Kgamma isoforms is required for perpetuated alpha(IIb)beta3 activation and maintenance of a platelet aggregate. This novel concept of intrinsic, dynamic thrombus instability gives possibilities for the use of antiplatelet therapy.

Regulation of platelet functions by P2 receptors

The main role of blood platelets is to ensure primary hemostasis, which is the maintenance of vessel integrity and cessation of bleeding upon injury. While playing a major part in acute arterial thrombosis, platelets are also involved in inflammation, atherosclerosis, and angiogenesis. ADP and ATP play a crucial role in platelet activation, and their receptors are potential targets for antithrombotic drugs. The ATP-gated cation channel P2X(1) and the two G protein-coupled ADP receptors, P2Y(1) and P2Y(12), selectively contribute to platelet aggregation and formation of a thrombus. Owing to its central role in the growth and stabilization of a thrombus, the P2Y(12) receptor is an established target of antithrombotic drugs such as clopidogrel. Studies in P2Y(1) and P2X(1) knockout mice and selective P2Y(1) and P2X(1) antagonists have shown that these receptors are also attractive targets for new antithrombotic compounds. The potential role of platelet P(2) receptors in the involvement of platelets in inflammatory processes is also discussed.

Correction of changes in ADP-induced platelet aggregation with vasopressin analogue desglycinamide-arginine-vasopressin

We studied the effect of vasopressin analogue desglycinamide-arginine-vasopressin on changes in platelet hemostasis produced by intragastric administration of Ticlid or clopidogrel (inhibitors of ADP-induced platelet aggregation). Intranasal administration of the peptide under conditions of hemorrhagic diathesis produced a hemostatic effect and normalized some parameters of blood coagulation.

The platelet P2 receptors in arterial thrombosis

ADP and ATP play a crucial role in hemostasis and thrombosis and their receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the two G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well established target of antithrombotic drugs like clopidogrel which has proved efficacious in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knock-out mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs. Since both P2X1 and P2Y1 receptor inhibition result in milder prolongation of the bleeding time as compared to P2Y12 inhibition, the idea is put forward that combination of P2 receptor antagonists could improve efficacy with diminished hemorrhagic risk. However, further studies are required to validate such a point of view.

Pharmacodynamics, pharmacokinetics, and safety of the oral reversible P2Y12 antagonist AZD6140 with aspirin in patients with atherosclerosis: a double-blind comparison to clopidogrel with aspirin

AIMS

This double-blind, parallel-group study was conducted to assess the pharmacodynamics, pharmacokinetics, and safety of AZD6140, the first oral, reversible adenosine diphosphate (ADP) receptor antagonist.

METHODS AND RESULTS

Patients (n = 200) with atherosclerosis were randomized to receive AZD6140 50, 100, or 200 mg twice daily (bid) or 400 mg daily (qd) or clopidogrel 75 mg qd for 28 days. All groups received aspirin 75-100 mg qd. AZD6140 (100 and 200 mg bid, 400 mg qd) rapidly and nearly completely inhibited ADP-induced platelet aggregation after initial dosing (day 1) and at day 28. On day 1, peak final-extent inhibition of platelet aggregation (IPA) was observed 2-4 h post-dose with AZD6140, whereas clopidogrel minimally inhibited platelet aggregation (mean percentage IPA < 20%, all time points). Four hour post-dose at steady state, the three higher doses of AZD6140 produced comparable final-extent mean percentage IPA (approximately 90-95%), which exceeded that with AZD6140 50 mg bid or clopidogrel (approximately 60%). AZD6140 was generally well tolerated. All bleeding events, except one in a patient receiving 400 mg qd, were minor and of mild-to-moderate severity.

CONCLUSION

AZD6140 100 and 200 mg bid were well tolerated and were superior to AZD6140 50 mg bid and clopidogrel 75 mg qd with regard to antiplatelet efficacy.

Association between platelet P2Y12 haplotype and risk of cardiovascular events in chronic coronary disease

INTRODUCTION

A positive association was recently described between P2Y12 platelet receptor H1 and H2 haplotypes and peripheral artery disease. We tested the described P2Y12 receptor haplotypes in a group of patients with coronary artery disease.

STUDY DESIGN AND METHODS

The P2Y12 platelet receptor H1 and H2 haplotypes was tested in a group of 540 patients enrolled in the Medical, Angioplasty, or Surgery Study II (MASS II), a randomized trial comparing treatments for patients with coronary artery disease (CAD) and preserved left ventricular function. After a 3-year follow-up period, the incidence of the composite end point of cardiac death, myocardial infarction, and refractory angina requiring revascularization was determined in the H1/H1, H1/H2 and H2/H2 haplotype groups. We used Student's t-test and the chi-square test to analyze the differences among groups and Kaplan-Meier method to calculate survival curves. Risk was assessed with the use of a Cox proportional-hazards model.

RESULTS

The frequency of haplotypes among studied patients were 410 (75.9%) H1/H1, 119 (22.0%) H1/H2 and 11 (2.1%) H2/H2. The baseline clinical characteristics, mean clinical follow-up time and received treatment of each genotype group were similar. We did not disclose any association between haplotype groups regarding the incidence of any of the studied cardiovascular end-points.

CONCLUSION

This is the first report studying the association of P2Y12 platelet receptor H1 and H2 haplotype and cardiovascular events. Our findings do not provide evidence for a strong association between H1/H1 and H1/H2 haplotypes and a increased risk of cardiovascular events in a population with CAD. Future works should address the role of the H2/H2 haplotype as a genetic marker for cardiovascular events.

The novel suramin analogue NF864 selectively blocks P2X1 receptors in human platelets with potency in the low nanomolar range

The role of ATP-stimulated P2X1 receptors in human platelets is still unclear. They may act alone or in synergy with other pathways, such as P2Y1 or P2Y12 receptors, to accelerate and enhance calcium mobilisation, shape change and aggregation. To date very few pharmacological means of selectively inhibiting platelet P2X1 receptors have been described, although recent work has shown that suramin is a useful lead compound for the development of high-affinity P2X1 antagonists. We therefore investigated the effects of a series of bivalent and tetravalent suramin analogues on alphabeta meATP (P2X1 receptors)-induced or ADP (P2Y1 receptors)-induced intracellular calcium increases and shape change, as well as on ADP-induced aggregation (P2Y1 & P2Y12 receptors) in human platelets. Changes in intracellular calcium were measured using standard fluorescence techniques, while shape change and aggregation were determined by turbidimetry. The novel tetravalent compound NF864 (8,8',8'',8'''-(carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino)))tetrakis-naphthalene-1,3,5-trisulfonic acid-dodecasodium salt) proved to be the most potent platelet P2X1 antagonist reported to date, blocking alphabeta meATP-induced Ca2+ increases and shape change in a concentration-dependent manner, with a pA2 of 8.17 and 8.49, respectively. The ability to inhibit the platelet P2X1 receptor displayed the following order : NF864 > NF449 > or = NF110 > NF023 = MK-HU1 = suramin. A different antagonistic profile was observed for ADP-induced Ca2+ increases, shape change and aggregation; however, overall four compounds showed sufficient ability to selectively inhibit P2X1 responses, with the order NF110 > NF449 > or = NF864 > or = MK-HU1. Therefore, these compounds should prove useful tools for investigating the functional significance of platelet P2X1 receptors in thrombosis and haemostasis, NF864 being the most promising compound.

Inhibition of platelet functions and thrombosis through selective or nonselective inhibition of the platelet P2 receptors with increasing doses of NF449 [4,4',4'',4'''-(carbonylbis(imino-5,1,3-benzenetriylbis-(carbonylimino)))tetrakis-benzene-1,3-disulfonic acid octasodium salt]

Our aim was to determine whether the newly described P2X1 antagonist NF449 [4,4',4'',4'''-(carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino)))tetrakis-benzene-1,3-disulfonic acid octasodium salt] could selectively antagonize the platelet P2X1 receptor and how it affected platelet function. NF449 inhibited alpha,beta-methyleneadenosine 5'-triphosphate-induced shape change (IC50 = 83 +/- 13 nM; n = 3) and calcium influx (pA2 = 7.2 +/- 0.1; n = 3) (pIC50 = 6.95) in washed human platelets treated with apyrase to prevent desensitization of the P2X1 receptor. NF449 also antagonized the calcium rise mediated by the P2Y1 receptor, but with lower potency (IC50 = 5.8 +/- 2.2 microM; n = 3). In contrast, it was a very weak antagonist of the P2Y12-mediated inhibition of adenylyl cyclase activity. Selective blockade of the P2X1 receptor with NF449 led to reduced collagen-induced aggregation, confirming a role of this receptor in platelet activation induced by collagen. Intravenous injection of 10 mg/kg NF449 into mice resulted in selective inhibition of the P2X1 receptor and decreased intravascular platelet aggregation in a model of systemic thromboembolism (35 +/- 4 versus 51 +/- 3%) (P = 0.0061; n = 10) but without prolongation of the bleeding time (106 +/- 16 versus 78 +/- 7 s; n = 10) (N.S.; P = 0.1209). At a higher dose (50 mg/kg), NF449 inhibited the three platelet P2 receptors. This led to a further reduction in platelet consumption compared with mice injected with saline (13 +/- 4 versus 42 +/- 3%) (P = 0.0002; n = 5). NF449 also reduced dose-dependently the size of thrombi formed after laser-induced injury of mesenteric arterioles. Overall, our results indicate that NF449 constitutes a new tool to investigate the functions of the P2X1 receptor and could be a starting compound in the search for new antithrombotic drugs targeting the platelet P2 receptors.

The platelet P2 receptors as molecular targets for old and new antiplatelet drugs

Platelet activation by ADP and ATP plays a crucial role in haemostasis and thrombosis, and their so-called P2 receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the 2 G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. The P2Y1 receptor is responsible for ADP-induced shape change and weak and transient aggregation, while the P2Y12 receptor is responsible for the completion and amplification of the response to ADP and to all platelet agonists, including thromboxane A2 (TXA2), thrombin, and collagen. The P2X1 receptor is involved in platelet shape change and in activation by collagen under shear conditions. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well-established target of antithrombotic drugs like ticlopidine or clopidogrel, which have proved efficacy in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knockout mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs.

Interplay between P2Y(1), P2Y(12), and P2X(1) receptors in the activation of megakaryocyte cation influx currents by ADP: evidence that the primary megakaryocyte represents a fully functional model of platelet P2 receptor signaling

The difficulty of conducting electrophysiologic recordings from the platelet has restricted investigations into the role of ion channels in thrombosis and hemostasis. We now demonstrate that the well-established synergy between P2Y(1) and P2Y(12) receptors during adenosine diphosphate (ADP)-dependent activation of the platelet alpha(IIb)beta(3) integrin also exists in murine marrow megakaryocytes, further supporting the progenitor cell as a bona fide model of platelet P2 receptor signaling. In patch clamp recordings, ADP (30 microM) stimulated a transient inward current at -70 mV, which was carried by Na(+) and Ca(2+) and was amplified by phenylarsine oxide, a potentiator of certain transient receptor potential (TRP) ion channels by phosphatidylinositol 4,5-bisphosphate depletion. This initial current decayed to a sustained phase, upon which repetitive transient inward cation currents with pre-dominantly P2X(1)-like kinetics were super-imposed. Abolishing P2X(1)-receptor activity prevented most of the repetitive currents, consistent with their activation by secreted adenosine triphosphate (ATP). Recordings in P2Y(1)-receptor-deficient megakaryocytes demonstrated an essential requirement of this receptor for activation of all ADP-evoked inward currents. However, P2Y(12) receptors, through the activation of PI3-kinase, played a synergistic role in both P2Y(1) and P2X(1)-receptor-dependent currents. Thus, direct stimulation of P2Y(1) and P2Y(12) receptors, together with autocrine P2X(1) activation, is responsible for the activation of nonselective cation currents by the platelet agonist ADP.

Resistance to clopidogrel: a review of the evidence

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

Differential regulation and relocalization of the platelet P2Y receptors after activation: a way to avoid loss of hemostatic properties?

In the present study, we investigated the desensitization and trafficking of the P2Y1 and P2Y12 receptors after agonist-induced stimulation of platelets or astrocytoma cells transfected with the P2Y1 or P2Y12 receptors fused to green fluorescent protein. In platelets and in transfected cells, exposure to 10 microM ADP caused desensitization of the P2Y1 receptor-driven calcium signal, whereas the P2Y12 receptor-mediated inhibition of cAMP formation was not affected. Plasma membranes from ADP-stimulated platelets also retained P2Y12 activity. Agonist-induced P2Y1 receptor desensitization was accompanied by its internalization in platelets and transfected cells. In contrast, although a substantial fraction of P2Y12 receptors was rapidly and transiently internalized, most of the P2Y12 receptors remained at the plasma membrane. Activated P2Y1 receptors were internalized through a clathrin-dependent pathway in cells and platelets, whereas the P2Y12 receptors seemed to use a distinct, clathrin-independent pathway. Together, these data indicate that the P2Y1 and P2Y12 receptors are differentially regulated upon activation. The absence of desensitization of the Gi protein-coupled P2Y12 receptor-dependent responses could represent a mechanism to preserve the hemostatic properties of otherwise unresponsive platelets.

Flow cytometric analysis of intraplatelet VASP phosphorylation for the detection of clopidogrel resistance in patients with ischemic cardiovascular diseases

Interindividual variability of the inhibitory effect of clopidogrel on platelet functions leading to clopidogrel resistance has been described in some patients with ischemic cardiovascular disease. A reliable laboratory test is therefore needed to identify patients insufficiently protected by this antiplatelet treatment. The phosphorylation of vasodilator-stimulated phosphoprotein (VASP), an intraplatelet actin regulatory protein, is dependent on the level of activation of the platelet P2Y12 receptor, which is targeted by clopidogrel. The aim of this study was to use a flow cytometric VASP phosphorylation assay to evaluate the efficacy of clopidogrel therapy. The platelet reactivity index (PRI), expressed as a percentage, is the difference in VASP fluorescence intensity between resting (+PGE1) and activated (+ADP) platelets. In vitro, the PRI was strongly correlated with the inhibition of platelet aggregation induced by specific blockade of the P2Y12 receptor by the competitive antagonist AR-C69931MX (R = 0.72, P < 0.0001). Ex vivo, the PRI was 78.3 +/- 4.6% in 47 healthy donors, 79.0 +/- 4.1% in 34 patients not receiving clopidogrel and 61.1 +/- 17.0% in 33 patients treated with clopidogrel (P < 0.0001). In the clopidogrel group, the PRI values were widely dispersed (from 6.6 to 85.8%) and more than 30% of these patients had a PRI equivalent of values in patients not receiving clopidogrel. The flow cytometric analysis of VASP phosphorylation seems to be a suitable test to evaluate the efficacy of clopidogrel treatment. This assay demonstrated a wide interindividual variability of the inhibitory response of platelets to clopidogrel and showed that one-third of the patients treated appeared to be 'unprotected' by this therapy.

Emerging roles for P2X1 receptors in platelet activation

The platelet surface membrane possesses three P2 receptors activated by extracellular adenosine nucleotides; one member of the ionotropic receptor family (P2X(1)) and two members of the G-protein-coupled receptor family (P2Y(1) and P2Y(12)). P2Y(1) and P2Y(12) receptors have firmly established roles in platelet activation during thrombosis and haemostasis, whereas the importance of the P2X(1) receptor has been more controversial. However, recent studies have demonstrated that P2X(1) receptors can generate significant functional platelet responses alone and in synergy with other receptor pathways. In addition, studies in transgenic animals indicate an important role for P2X(1) receptors in platelet activation, particularly under conditions of shear stress and thus during arterial thrombosis. This review discusses the background behind discovery of P2X(1) receptors in platelets and their precursor cell, the megakaryocyte, and how signalling via these ion channels may participate in platelet activation.

Multiple platelet defects identified by flow cytometry at diagnosis in acute myeloid leukaemia

Summary Previous findings of megakaryocytic hypogranulation and dysmegakaryocytopoietic features in acute myeloid leukaemia (AML) strongly indicate defects in platelet production. The bleeding tendency of these patients may result from dysregulated platelet production, resulting in thrombocytopenia as well as qualitative platelet defects. The present study examined platelet function at diagnosis in 50 AML patients by whole blood flow cytometry. Following in vitro platelet agonist stimulation, platelet activation markers were analysed and compared with 20 healthy individuals. To detect recent in vivo platelet activation, plasma soluble P-selectin (sP-selectin) was measured. Flow cytometric analysis of platelet activation markers demonstrated reduced CD62P [35.6 vs. 118.5 x 10(3) molecules of equivalent soluble fluorochrome (MESF); P < 0.0001], CD63 (11.3 vs. 50.7 x 10(3) MESF; P < 0.0001), and PAC-1 (41.5 vs. 90.5%; P = 0.0001) while reductions in CD42b were abnormal (45.6 vs. 70%; P < 0.0001). sP-selectin levels were similar in patients and healthy controls (0.04 vs. 0.27 fg/platelet; P = 0.84). The presented data indicate that AML pathogenesis may result in multiple platelet defects, involving adhesion, aggregation, and secretion and demonstrate that flow cytometry is a feasible method for platelet function analysis in patients with thrombocytopenia.

Central role of the P2Y12 receptor in platelet activation

Platelet activation occurs in response to vessel injury and is important for the arrest of bleeding. Platelet activation during disease states leads to vascular occlusion and ischemic damage. The P2Y(12) receptor, activated by ADP, plays a central role in platelet activation and is the target of P2Y(12) receptor antagonists that have proven therapeutic value.

Central role of the P2Y12 receptor in platelet activation

Platelet activation occurs in response to vessel injury and is important for the arrest of bleeding. Platelet activation during disease states leads to vascular occlusion and ischemic damage. The P2Y(12) receptor, activated by ADP, plays a central role in platelet activation and is the target of P2Y(12) receptor antagonists that have proven therapeutic value.

P2Y12 H2 haplotype is associated with peripheral arterial disease: a case-control study

BACKGROUND

We recently described a gain-of-function haplotype, called H2, of the adenosine diphosphate (ADP) receptor P2Y12 gene associated with increased ADP-induced platelet aggregation ex vivo in healthy volunteers. Because platelets play a key role in atherosclerosis and arterial thrombosis, we tested the possible link between the H2 haplotype and the risk of peripheral arterial disease (PAD) in a case-control study.

METHODS AND RESULTS

We studied 184 consecutive male patients under 70 years of age with PAD and 330 age-matched control subjects free of symptomatic PAD and with no cardiovascular history. Mean age was 57.1+/-7.2 years (cases) and 56.7+/-7.6 years (control subjects). The H2 haplotype was more frequent in patients with PAD than in control subjects (30% and 21%, respectively; OR, 1.6; CI, 1.1 to 2.5; P=0.02 in univariate analysis). This association with PAD remained significant in multivariate regression analysis (OR, 2.3; CI, 1.4 to 3.9; P=0.002) after adjustment for diabetes, smoking, hypertension, hypercholesterolemia, and other selected platelet receptor gene polymorphisms.

CONCLUSIONS

These data point to a role of the H2 haplotype in atherosclerosis and raise the possibility of relative thienopyridine resistance in carriers of the P2Y12 H2 haplotype.

Constitutive activity and inverse agonists of G protein-coupled receptors: a current perspective

Over the last decade, the ability to detect agonist-independent signal transduction by G protein-coupled receptors has in turn resulted in the detection and study of ligands able to block this activity. Such ligands are generically described as inverse agonists. Considerable attention has recently been devoted to the presence and roles of endogenous antagonist/inverse agonists and the concept that inverse agonists may have specific therapeutic benefits compared with neutral antagonists.