Platelet P2 receptors: old and new targets for antithrombotic drugs

G protein-coupled receptors (GPCRs): advances in structures, mechanisms, and drug discovery

G protein-coupled receptors (GPCRs), the largest family of human membrane proteins and an important class of drug targets, play a role in maintaining numerous physiological processes. Agonist or antagonist, orthosteric effects or allosteric effects, and biased signaling or balanced signaling, characterize the complexity of GPCR dynamic features. In this study, we first review the structural advancements, activation mechanisms, and functional diversity of GPCRs. We then focus on GPCR drug discovery by revealing the detailed drug-target interactions and the underlying mechanisms of orthosteric drugs approved by the US Food and Drug Administration in the past five years. Particularly, an up-to-date analysis is performed on available GPCR structures complexed with synthetic small-molecule allosteric modulators to elucidate key receptor-ligand interactions and allosteric mechanisms. Finally, we highlight how the widespread GPCR-druggable allosteric sites can guide structure- or mechanism-based drug design and propose prospects of designing bitopic ligands for the future therapeutic potential of targeting this receptor family.

Antiplatelet Therapy in Coronary Artery Disease: Now and Then

Cardiovascular disease, particularly coronary artery disease (CAD), remains the leading cause of mortality and morbidity in industrialized countries. Platelet activation and aggregation at the site of endothelial injury play a key role in the processes ultimately resulting in thrombus formation with vessel occlusion and subsequent end-organ damage. Consequently, antiplatelet therapy has become a mainstay in the pharmacological treatment of CAD. Several drug classes have been developed over the last decades and a broad armamentarium of antiplatelet agents is currently available. This review portrays the evolution of antiplatelet therapy, and provides an overview on previous and current antiplatelet drugs and strategies.

Rho GTPase Signaling in Platelet Regulation and Implication for Antiplatelet Therapies

Platelets play a vital role in regulating hemostasis and thrombosis. Rho GTPases are well known as molecular switches that control various cellular functions via a balanced GTP-binding/GTP-hydrolysis cycle and signaling cascade through downstream effectors. In platelets, Rho GTPases function as critical regulators by mediating signal transduction that drives platelet activation and aggregation. Mostly by gene targeting and pharmacological inhibition approaches, Rho GTPase family members RhoA, Rac1, and Cdc42 have been shown to be indispensable in regulating the actin cytoskeleton dynamics in platelets, affecting platelet shape change, spreading, secretion, and aggregation, leading to thrombus formation. Additionally, studies of Rho GTPase function using platelets as a non-transformed model due to their anucleated nature have revealed valuable information on cell signaling principles. This review provides an updated summary of recent advances in Rho GTPase signaling in platelet regulation. We also highlight pharmacological approaches that effectively inhibited platelet activation to explore their possible development into future antiplatelet therapies.

Potential role of IGF-1/GLP-1 signaling activation in intracerebral hemorrhage

IGF-1 and GLP-1 receptors are essential in all tissues, facilitating defense by upregulating anabolic processes. They are abundantly distributed throughout the central nervous system, promoting neuronal proliferation, survival, and differentiation. IGF-1/GLP-1 is a growth factor that stimulates neurons' development, reorganization, myelination, and survival. In primary and secondary brain injury, the IGF-1/GLP-1 receptors are impaired, resulting in further neuro complications such as cerebral tissue degradation, neuroinflammation, oxidative stress, and atrophy. Intracerebral hemorrhage (ICH) is a severe condition caused by a stroke for which there is currently no effective treatment. While some pre-clinical studies and medications are being developed as symptomatic therapies in clinical trials, there are specific pharmacological implications for improving post-operative conditions in patients with intensive treatment. Identifying the underlying molecular process and recognizing the worsening situation can assist researchers in developing effective therapeutic solutions to prevent post-hemorrhagic symptoms and the associated neural dysfunctions. As a result, in the current review, we have addressed the manifestations of the disease that are aggravated by the downregulation of IGF-1 and GLP-1 receptors, which can lead to ICH or other neurodegenerative disorders. Our review summarizes that IGF-1/GLP-1 activators may be useful for treating ICH and its related neurodegeneration.

Targeting Platelet in Atherosclerosis Plaque Formation: Current Knowledge and Future Perspectives

Besides their role in hemostasis and thrombosis, it has become increasingly clear that platelets are also involved in many other pathological processes of the vascular system, such as atherosclerotic plaque formation. Atherosclerosis is a chronic vascular inflammatory disease, which preferentially develops at sites under disturbed blood flow with low speeds and chaotic directions. Hyperglycemia, hyperlipidemia, and hypertension are all risk factors for atherosclerosis. When the vascular microenvironment changes, platelets can respond quickly to interact with endothelial cells and leukocytes, participating in atherosclerosis. This review discusses the important roles of platelets in the plaque formation under pro-atherogenic factors. Specifically, we discussed the platelet behaviors under disturbed flow, hyperglycemia, and hyperlipidemia conditions. We also summarized the molecular mechanisms involved in vascular inflammation during atherogenesis based on platelet receptors and secretion of inflammatory factors. Finally, we highlighted the studies of platelet migration in atherogenesis. In general, we elaborated an atherogenic role of platelets and the aspects that should be further studied in the future.

Molecular Modeling Applied to the Discovery of New Lead Compounds for P2 Receptors Based on Natural Sources

P2 receptors are a family of transmembrane receptors activated by nucleotides and nucleosides. Two classes have been described in mammals, P2X and P2Y, which are implicated in various diseases. Currently, only P2Y12 has medicines approved for clinical use as antiplatelet agents and natural products have emerged as a source of new drugs with action on P2 receptors due to the diversity of chemical structures. In drug discovery, in silico virtual screening (VS) techniques have become popular because they have numerous advantages, which include the evaluation of thousands of molecules against a target, usually proteins, faster and cheaper than classical high throughput screening (HTS). The number of studies using VS techniques has been growing in recent years and has led to the discovery of new molecules of natural origin with action on different P2X and P2Y receptors. Using different algorithms it is possible to obtain information on absorption, distribution, metabolism, toxicity, as well as predictions on biological activity and the lead-likeness of the selected hits. Selected biomolecules may then be tested by molecular dynamics and, if necessary, rationally designed or modified to improve their interaction for the target. The algorithms of these in silico tools are being improved to permit the precision development of new drugs and, in the future, this process will take the front of drug development against some central nervous system (CNS) disorders. Therefore, this review discusses the methodologies of in silico tools concerning P2 receptors, as well as future perspectives and discoveries, such as the employment of artificial intelligence in drug discovery.

How Fungal Glycans Modulate Platelet Activation via Toll-Like Receptors Contributing to the Escape of Candida albicans from the Immune Response

Platelets are essential for vascular repair and for the maintenance of blood homeostasis. They contribute to the immune defence of the host against many infections caused by bacteria, viruses and fungi. Following infection, platelet function is modified, and these cells form aggregates with microorganisms leading, to a decrease in the level of circulating platelets. During candidaemia, mannans, β-glucans and chitin, exposed on the cell wall of Candida albicans, an opportunistic pathogenic yeast of humans, play an important role in modulation of the host response. These fungal polysaccharides are released into the circulation during infection and their detection allows the early diagnosis of invasive fungal infections. However, their role in the modulation of the immune response and, in particular, that of platelets, is not well understood. The structure and solubility of glycans play an important role in the orientation of the immune response of the host. This short review focuses on the effect of fungal β-glucans and chitin on platelet activation and how these glycans modulate platelet activity via Toll-like receptors, contributing to the escape of C. albicans from the immune response.

Regulation of Platelet Activation and Coagulation and Its Role in Vascular Injury and Arterial Thrombosis

Hemostasis requires tightly regulated interaction of the coagulation system, platelets, blood cells, and vessel wall components at a site of vascular injury. Dysregulation of this response may result in excessive bleeding if the response is impaired, and pathologic thrombosis with vessel occlusion and tissue ischemia if the response is robust. Studies have elucidated the major molecular signaling pathways responsible for platelet activation and aggregation. Antithrombotic agents targeting these pathways are in clinical use. This review summarizes research examining mechanisms by which these multiple platelet signaling pathways are integrated at a site of vascular injury to produce an optimal hemostatic response.

Biology of Platelet Purinergic Receptors and Implications for Platelet Heterogeneity

Platelets are small anucleated cells present only in mammals. Platelets mediate intravascular hemostatic balance, prevent interstitial bleeding, and have a major role in thrombosis. Activation of platelet purinergic receptors is instrumental in initiation of hemostasis and formation of the hemostatic plug, although this activation process becomes problematic in pathological settings of thrombosis. This review briefly outlines the roles and function of currently known platelet purinergic receptors (P1 and P2) in the setting of hemostasis and thrombosis. Additionally, we discuss recent novel studies on purinergic receptor distribution according to heterogeneous platelet size, and the possible implication of this distribution on hemostatic function.

Cangrelor: A New Route for P2Y12 Inhibition

Antiplatelet therapy with a P2Y12 inhibitor is a key component of treatment for patients with acute coronary syndromes undergoing percutaneous coronary intervention. Before the development of cangrelor (Kengreal, The Medicines Company, Parsippany, NJ), only oral P2Y12 inhibitors were available. Cangrelor is a reversible P2Y12 inhibitor that is administered as an intravenous infusion, and its quick onset and offset make it an appealing option for antiplatelet therapy, particularly for patients who are unable to take oral medications. Although cangrelor struggled to show benefit in early trials, the positive results of the CHAMPION PHOENIX trial led to its approval for use as an adjunct to percutaneous coronary intervention to reduce the risk of periprocedural myocardial infarction, repeat coronary revascularization, and stent thrombosis in patients who have not been treated with another P2Y12 inhibitor and are not being given a glycoprotein IIb/IIIa inhibitor. Cangrelor has also been evaluated as an option for bridging therapy in patients who must discontinue their oral P2Y12 inhibitor before coronary artery bypass grafting. This review of cangrelor will discuss its mechanism of action, its pharmacodynamics and pharmacokinetics, the clinical trial experience, and its potential place in therapy.

Antithrombotic activity of Vitis labrusca extract on rat platelet aggregation

Vitis labrusca is a grapevine that has antioxidant, neuroprotective, hepatoprotective, and anticarcinogenic activity. However, the antithrombotic effect of Vitis labrusca leaves on platelets is yet to be ascertained. We investigated the inhibitory effect of V. labrusca leaf extract (VLE) on platelet aggregation in vitro and ex vivo. The thromboxane B2 (TXB2) and serotonin concentrations were measured by ELISA. The flavonoids content was measured by ultraperformance liquid chromatography (UPLC). The antithrombotic activity of VLE was evaluated using various agonists in vitro. VLE strongly inhibited adenosine diphosphate (ADP)-induced platelet aggregation. In rats, VLE treatment (100 mg/kg) reduced ADP-stimulated platelet aggregation, without affecting tail bleeding and coagulation time. Moreover, VLE significantly suppressed TXB2 and serotonin secretion. UPLC analysis indicated that VLE contains quercetin, isorhamnetin, and rutin. Our results indicate that VLE possesses antiplatelet activity via the suppression of TXB2 and serotonin, without affecting bleeding. Further, we identified the flavonoids present in VLE. Thus, VLE may be a potential agent for the prevention of cardiovascular diseases.

BPTU, an allosteric antagonist of P2Y1 receptor, blocks nerve mediated inhibitory neuromuscular responses in the gastrointestinal tract of rodents

P2Y1 receptors mediate nerve mediated purinergic inhibitory junction potentials (IJP) and relaxations in the gastrointestinal (GI) tract in a wide range of species including rodents and humans. A new P2Y1 antagonist, with a non-nucleotide structure, BPTU, has recently been described using X-ray crystallography as the first allosteric G-protein-coupled receptor antagonist located entirely outside of the helical bundle. In this study, we tested its effect on purinergic responses in the gastrointestinal tract of rodents using electrophysiological and myographic techniques. BPTU concentration dependently inhibited purinergic inhibitory junction potentials and inhibition of spontaneous motility induced by electrical field stimulation in the colon of rats (EC50 = 0.3 μM) and mice (EC50 = 0.06 μM). Mechanical inhibitory responses were also concentration-dependently blocked in the stomach of both species. Compared to MRS2500, BPTU displays a lower potency. In the rat colon nicotine induced relaxation was also blocked by BPTU. BPTU also blocked the cessation of spontaneous contractility elicited by ADPβS and the P2Y1 agonist MRS2365. We conclude that BPTU is a novel antagonist with different structural and functional properties than nucleotidic antagonists that is able to block the P2Y1 receptor located at the neuromuscular junction of the GI tract.

Drug-like Antagonists of P2Y Receptors-From Lead Identification to Drug Development

P2Y receptors are expressed in virtually all cells and tissue types and mediate an astonishing array of biological functions, including platelet aggregation, smooth muscle cell proliferation, and immune regulation. The P2Y receptors belong to the G protein-coupled receptor superfamily and are composed of eight members encoded by distinct genes that can be subdivided into two groups on the basis of their coupling to specific G-proteins. Extensive research has been undertaken to find modulators of P2Y receptors, although to date only a limited number of small-molecule P2Y receptor antagonists have been approved by drug/medicines agencies. This Perspective reviews the known P2Y receptor antagonists, highlighting oral drug-like receptor antagonists, and considers future opportunities for the development of small molecules for clinical evaluation.

Protease receptor antagonism to target blood platelet therapies

Platelet activation and thrombus formation play a central role in ischemic vascular disease. Thrombin, an especially potent physiologic agonist mediating in vivo activation of platelets, acts via a unique family of G-protein-coupled receptors called protease-activated receptors (PARs) with a broad tissue expression. This review focuses on current antiplatelet therapies as well as innovative approaches to targeting PARs in patients with atherothrombotic vascular disease.

Synthesis and Anti-Platelet Activity of Thiosulfonate Derivatives Containing a Quinone Moiety

Thiosulfonate derivatives based on quinones were synthesized for studying “structure-activity relationship” compounds with an acylated and a free amino-group. Anti-platelet activity of the synthesized compounds was determined and the influence of substituents on the activity of the derivatives was assessed.

Chloroform fraction of Euphorbia maculata has antiplatelet activity via suppressing thromboxane B2 formation

Euphorbia maculata (EM) is a traditionally used antidiarrheal, antibacterial, antifungal and antioxidant agent. However, the effects of EM on platelet activity remain to be elucidated. Therefore, the present study investigated the antiplatelet effect of various EM extract fractions on platelet aggregation in rats. The antiplatelet activity of the EM fractions on collagen or adenosine diphosphate (ADP)‑induced platelet aggregation was evaluated in vitro and ex vivo. Thromboxane B2 (TXB2) formation, rat‑tail bleeding time and coagulation time were also measured. Among the fractions, the chloroform fraction of EM (CFEM) significantly inhibited ADP‑induced platelet aggregation in vitro. Furthermore, oral administration of 50 mg/kg CFEM to rats significantly reduced ADP‑induced platelet aggregation without increasing the tail bleeding time or coagulation time. In addition, EM significantly inhibited the level of TXB2 formation in a dose‑dependent manner. These results suggest that CFEM exhibits antiplatelet activity, without causing bleeding, via the suppression of TXB2 formation. CFEM may be a type of food which has the potential for preventing cardiovascular disease.

4-Benzothiazole-7-hydroxyindolinyl diaryl ureas are potent P2Y1 antagonists with favorable pharmacokinetics: low clearance and small volume of distribution

Current antithrombotic discovery efforts target compounds that are highly efficacious in thrombus reduction with less bleeding liability than the standard of care. Preclinical data suggest that P2Y1 antagonists may have lower bleeding liabilities than P2Y12 antagonists while providing similar antithrombotic efficacy. This article describes our continuous SAR efforts in a series of 7-hydroxyindolinyl diaryl ureas. When dosed orally, 4-trifluoromethyl-7-hydroxy-3,3-dimethylindolinyl analogue 4 was highly efficacious in a model of arterial thrombosis in rats with limited bleeding. The chemically labile CF3 group in 4 was then transformed to various groups via a novel one-step synthesis, yielding a series of potent P2Y1 antagonists. Among them, the 4-benzothiazole-substituted indolines had desirable PK properties in rats, specifically, low clearance and small volume of distribution. In addition, compound 40 had high i.v. exposure and modest bioavailability, giving it the best overall profile.

2-Amino-1,3,4-thiadiazoles in the 7-hydroxy-N-neopentyl spiropiperidine indolinyl series as potent P2Y1 receptor antagonists

Blockade of the P2Y1 receptor is important to the treatment of thrombosis with potentially improved safety margins compared with P2Y12 receptor antagonists. Investigation of a series of urea surrogates of the diaryl urea lead 3 led to the discovery of 2-amino-1,3,4-thiadiazoles in the 7-hydroxy-N-neopentyl spiropiperidine indolinyl series as potent P2Y1 receptor antagonists, among which compound 5a was the most potent and the first non-urea analog with platelet aggregation (PA) IC50 less than 0.5 μM with 10 μM ADP. Several 2-amino-1,3,4-thiadiazole analogs such as 5b and 5f had a more favorable pharmacokinetic profile, such as higher Ctrough, lower Cl, smaller Vdss, and similar bioavailability compared with 3.

Identification of 1-{2-[4-chloro-1'-(2,2-dimethylpropyl)-7-hydroxy-1,2-dihydrospiro[indole-3,4'-piperidine]-1-yl]phenyl}-3-{5-chloro-[1,3]thiazolo[5,4-b]pyridin-2-yl}urea, a potent, efficacious and orally bioavailable P2Y(1) antagonist as an antiplatelet agent

Spiropiperidine indoline-substituted diaryl ureas had been identified as antagonists of the P2Y1 receptor. Enhancements in potency were realized through the introduction of a 7-hydroxyl substitution on the spiropiperidinylindoline chemotype. SAR studies were conducted to improve PK and potency, resulting in the identification of compound 3e, a potent, orally bioavailable P2Y1 antagonist with a suitable PK profile in preclinical species. Compound 3e demonstrated a robust antithrombotic effect in vivo and improved bleeding risk profile compared to the P2Y12 antagonist clopidogrel in rat efficacy/bleeding models.

A phase 1 study of prasugrel in patients with sickle cell disease: pharmacokinetics and effects on ex vivo platelet reactivity

AIMS

Prasugrel is a novel thienopyridine P2Y12 adenosine diphosphate (ADP) receptor antagonist that inhibits ADP-mediated platelet activation and aggregation. Accordingly, it may be useful in reducing platelet-related ischaemia in sickle cell disease (SCD). Exposure to prasugrel's active metabolite (Pras-AM) and its antiplatelet activity in SCD have not been investigated.

METHODS

Thirteen adult patients with SCD and an equal number of matched healthy control subjects were studied before and after 12 days of 5.0 or 7.5 mg day(-1) prasugrel treatment. Platelet reactivity was assessed by light transmission aggregometry (LTA), impedance aggregometry (MEA), VerifyNow® P2Y12, vasodilator-stimulated phosphoprotein (VASP) phosphorylation and Plateletworks. Exposure to Pras-AM was also assessed.

RESULTS

At baseline, patients with SCD showed increased platelet reactivity vs. healthy control subjects with VerifyNow (408 vs. 323 P2Y12 reaction units (PRU), respectively, P = 0.003) and MEA (106 vs. 77 area under the aggregation curve (AU.min), P = 0.002); lower platelet reactivity index with VASP flow cytometry (59 vs. 79% platelet reactivity index (PRI), P = 0.018); and no significant differences with LTA, VASP enzyme-linked immunosorbent assay or Plateletworks. Relative to baseline, prasugrel significantly reduced platelet reactivity by all assays in both populations (all P < 0.05). Prasugrel was well tolerated, with no bleeding-related events in patients with SCD. The mean concentration-time profiles of Pras-AM were comparable between healthy subjects and patients with SCD following a single 10 mg prasugrel dose and following the 12th dose of 7.5 or 5 mg prasugrel.

CONCLUSIONS

Results demonstrate that in response to prasugrel, patients with SCD and healthy subjects have similar degrees of platelet inhibition and exposure to Pras-AM, and provide a basis for further study of prasugrel in patients with SCD.

Conformationally constrained ortho-anilino diaryl ureas: discovery of 1-(2-(1'-neopentylspiro[indoline-3,4'-piperidine]-1-yl)phenyl)-3-(4-(trifluoromethoxy)phenyl)urea, a potent, selective, and bioavailable P2Y1 antagonist

Preclinical antithrombotic efficacy and bleeding models have demonstrated that P2Y1 antagonists are efficacious as antiplatelet agents and may offer a safety advantage over P2Y12 antagonists in terms of reduced bleeding liabilities. In this article, we describe the structural modification of the tert-butyl phenoxy portion of lead compound 1 and the subsequent discovery of a novel series of conformationally constrained ortho-anilino diaryl ureas. In particular, spiropiperidine indoline-substituted diaryl ureas are described as potent, orally bioavailable small-molecule P2Y1 antagonists with improved activity in functional assays and improved oral bioavailability in rats. Homology modeling and rat PK/PD studies on benchmark compound 3l will also be presented. Compound 3l was our first P2Y1 antagonist to demonstrate a robust oral antithrombotic effect with mild bleeding liability in the rat thrombosis and hemostasis models.

Plant extracts inhibit ADP-induced platelet activation in humans: their potential therapeutic role as ADP antagonists

Adenosine diphosphate (ADP) plays a pivotal role in platelet activation. Platelet hyperactivity is associated with vascular disease and also has a key role in haemostasis and thrombosis. ADP activates platelets through three purinoceptor subtypes, the G(q)-coupled P2Y(1) receptor, G(i)-coupled P2Y(12) receptor and P2X(1) ligand-gated cation channel. Platelet ADP purinergic receptors are therefore suitable targets for antiplatelet drugs. Thienopyridines such as clopidogrel and ticlopidine, as well as other ADP receptor antagonists like prasugrel, ticagrelor, cangrelor and elinogrel have demonstrated clinical benefits via the inhibition of the selective purinergic ADP receptor, P2Y(12). However, they still have limitations in their mode of action and efficacy, like increased risk of bleeding. Thus, the ongoing pursuit to develop newer and more effective antiplatelet agents continues. There is a growing interest in the purinergic antiplatelet properties exhibited by plant extracts. This article considers the following: pomolic acid isolated from Licania pittieri, brazilin isolated from the heartwood of Caesalpinia sappan L, phylligenin isolated from the twigs of Muraltia vulpina, bark oil of Gonystylus velutinus, seed and bark extracts from Aesculus hippocastanum L. and red wine phenolics and catechins isolated from green tea. Moreover, the method used to investigate platelet purinergic receptors should be considered, since using a more sensitive, high-resolution platelet sizer can sometimes detect platelet variations when the light transmission method was not able to do so. The exact mechanisms by which these plant extracts work need further investigation. They all however inhibit ADP-induced activation in human platelets. This could explain, at least in part, the protective effect of plant extracts as antiplatelet agents.

Blockade of the purinergic P2Y12 receptor greatly increases the platelet inhibitory actions of nitric oxide

Circulating platelets are constantly exposed to nitric oxide (NO) released from the vascular endothelium. This NO acts to reduce platelet reactivity, and in so doing blunts platelet aggregation and thrombus formation. For successful hemostasis, platelet activation and aggregation must occur at sites of vascular injury despite the constant presence of NO. As platelets aggregate, they release secondary mediators that drive further aggregation. Particularly significant among these secondary mediators is ADP, which, acting through platelet P2Y12 receptors, strongly amplifies aggregation. Platelet P2Y12 receptors are the targets of very widely used antithrombotic drugs such as clopidogrel, prasugrel, and ticagrelor. Here we show that blockade of platelet P2Y12 receptors dramatically enhances the antiplatelet potency of NO, causing a 1,000- to 100,000-fold increase in inhibitory activity against platelet aggregation and release reactions in response to activation of receptors for either thrombin or collagen. This powerful synergism is explained by blockade of a P2Y12 receptor-dependent, NO/cGMP-insensitive phosphatidylinositol 3-kinase pathway of platelet activation. These studies demonstrate that activation of the platelet ADP receptor, P2Y12, severely blunts the inhibitory effects of NO. The powerful antithrombotic effects of P2Y12 receptor blockers may, in part, be mediated by profound potentiation of the effects of endogenous NO.

Purinergic signaling in inflammatory cells: P2 receptor expression, functional effects, and modulation of inflammatory responses

Extracellular ATP and related nucleotides promote a wide range of pathophysiological responses via activation of cell surface purinergic P2 receptors. Almost every cell type expresses P2 receptors and/or exhibit regulated release of ATP. In this review, we focus on the purinergic receptor distribution in inflammatory cells and their implication in diverse immune responses by providing an overview of the current knowledge in the literature related to purinergic signaling in neutrophils, macrophages, dendritic cells, lymphocytes, eosinophils, and mast cells. The pathophysiological role of purinergic signaling in these cells include among others calcium mobilization, actin polymerization, chemotaxis, release of mediators, cell maturation, cytotoxicity, and cell death. We finally discuss the therapeutic potential of P2 receptor subtype selective drugs in inflammatory conditions.

Discovery of diarylurea P2Y(1) antagonists with improved aqueous solubility

Preclinical data suggests that P2Y1 antagonists, such as diarylurea compound 1, may provide antithrombotic efficacy similar to P2Y12 antagonists and may have the potential of providing reduced bleeding liabilities. This manuscript describes a series of diarylureas bearing solublizing amine side chains as potent P2Y1 antagonists. Among them, compounds 2l and 3h had improved aqueous solubility and maintained antiplatelet activity compared with compound 1. Compound 2l was moderately efficacious in both rat and rabbit thrombosis models and had a moderate prolongation of bleeding time in rats similar to that of compound 1.

Effects of P2Y(1) receptor antagonism on the reactivity of platelets from patients with stable coronary artery disease using aspirin and clopidogrel

BACKGROUND AND PURPOSE

P2Y(1) is a purine receptor that triggers platelet aggregation. Its inhibition was studied in patients with stable coronary artery disease (CAD) receiving standard anti-platelet therapy.

EXPERIMENTAL APPROACH

Blood samples from 10 patients on aspirin therapy (ASA, 80 mg·day(-1) ) were withdrawn before and 24 h after the administration of 450 mg clopidogrel (ASA/C) and were anti-coagulated with citrate or hirudin/PPACK in the presence or absence of the P2Y(1 ) inhibitor MRS2179 (M, 100 µM). Platelet responses to ADP (2.5 µM) and TRAP (2.5 µM), and collagen-induced thrombosis under flow conditions were analysed.

KEY RESULTS

Compared with ASA, ASA + M strongly inhibited ADP-induced peak platelet aggregation (88%), late aggregation (84%), P-selectin expression (85%) and α(IIb) β(3) activation (62%) (28%, 65%, 70% and 51% inhibition, respectively, for ASA/C vs. ASA). ASA + M also inhibited platelet/monocyte and platelet/neutrophil conjugate formation by 69% and 71% (57% and 59% for ASA/C vs. ASA). In TRAP-activated blood, ASA + M unexpectedly inhibited α(IIb) b(3) activation by 30%. In blood perfused in collagen-coated glass capillaries (shear rate of 1500 s(-1) ), ASA/C prevented thrombus growth beyond 5 min in relation to thrombus fragments embolization. ASA + M with or without clopidogrel completely prevented thrombus formation. Finally, ex vivo addition of MRS2179 and ASA to the blood of healthy donors markedly blocked thrombus formation on collagen in flow conditions, in contrast to ASA plus the P2Y(12) inhibitor 2-MeSAMP.

CONCLUSIONS AND IMPLICATIONS

Through particularly efficient complementarities with ASA to inhibit platelet activation and thrombus formation, the inhibition of P2Y(1) in the blood of patients with CAD appears to play a more important role than previously anticipated.

Anti-platelet therapy: cyclo-oxygenase inhibition and the use of aspirin with particular regard to dual anti-platelet therapy

Aspirin and P2Y(12) antagonists are commonly used anti-platelet agents. Aspirin produces its effects through inhibition of thromboxane A(2) (TXA(2)) production, while P2Y(12) antagonists attenuate the secondary responses to ADP released by activated platelets. The anti-platelet effects of aspirin and a P2Y(12) antagonist are often considered to be separately additive. However, there is evidence of an overlap in effects, in that a high level of P2Y(12) receptor inhibition can blunt TXA(2) receptor signalling in platelets and reduce platelet production of TXA(2). Against this background, the addition of aspirin, particularly at higher doses, could cause significant reductions in the production of prostanoids in other tissues, e.g. prostaglandin I(2) from the blood vessel wall. This review summarizes the data from clinical studies in which dose-dependent effects of aspirin on prostanoid production have been evaluated by both plasma and urinary measures. It also addresses the biology underlying the cardiovascular effects of aspirin and its influences upon prostanoid production throughout the body. The review then considers whether, in the presence of newer, more refined P2Y(12) receptor antagonists, aspirin may offer less benefit than might have been predicted from earlier clinical trials using more variable P2Y(12) antagonists. The possibility is reflected upon, that when combined with a high level of P2Y(12) blockade the net effect of higher doses of aspirin could be removal of anti-thrombotic and vasodilating prostanoids and so a lessening of the anti-thrombotic effectiveness of the treatment.

Clopidogrel, a P2Y12 receptor antagonist, potentiates the inflammatory response in a rat model of peptidoglycan polysaccharide-induced arthritis

The P2Y12 receptor plays a crucial role in the regulation of platelet activation by several agonists, which is irreversibly antagonized by the active metabolite of clopidogrel, a widely used anti-thrombotic drug. In this study, we investigated whether reduction of platelet reactivity leads to reduced inflammatory responses using a rat model of erosive arthritis. We evaluated the effect of clopidogrel on inflammation in Lewis rats in a peptidoglycan polysaccharide (PG-PS)-induced arthritis model with four groups of rats: 1) untreated, 2) clopidogrel-treated, 3) PG-PS-induced, and 4) PG-PS-induced and clopidogrel-treated. There were significant differences between the PG-PS+clopidogrel group when compared to the PG-PS group including: increased joint diameter and clinical manifestations of inflammation, elevated plasma levels of pro-inflammatory cytokines (IL-1 beta, interferon (IFN) gamma, and IL-6), an elevated neutrophil blood count and an increased circulating platelet count. Plasma levels of IL-10 were significantly lower in the PG-PS+clopidogrel group compared to the PG-PS group. Plasma levels of platelet factor 4 (PF4) were elevated in both the PG-PS and the PG-PS+clopidogrel groups, however PF4 levels showed no difference upon clopidogrel treatment, suggesting that the pro- inflammatory effect of clopidogrel may be due to its action on cells other than platelets. Histology indicated an increase in leukocyte infiltration at the inflammatory area of the joint, increased pannus formation, blood vessel proliferation, subsynovial fibrosis and cartilage erosion upon treatment with clopidogrel in PG-PS-induced arthritis animals. In summary, animals treated with clopidogrel showed a pro-inflammatory effect in the PG-PS-induced arthritis animal model, which might not be mediated by platelets. Elucidation of the mechanism of clopidogrel-induced cell responses is important to understand the role of the P2Y12 receptor in inflammation.

Upstream treatment of acute coronary syndrome in the ED

Rapid risk stratification, selection of downstream management options, and institution of initial pharmacotherapy are essential to ensure that patients admitted to the emergency department with acute coronary syndromes receive optimal care. A broad range of antiplatelet and antithrombotic medications is available that permits tailoring of initial pharmacotherapy to each patient's risk status. In the urgent setting, thienopyridines (clopidogrel and prasugrel) carry limitations including response variability and increased risk for bleeding in patients requiring subsequent coronary artery bypass graft surgery. Glycoprotein IIb-IIIa receptor inhibitors, although they are highly effective in preventing ischemic events, must be used with care to reduce bleeding risk. Bivalirudin, a relatively new direct thrombin inhibitor, represents another upstream option but is costly and does not have approval for this indication. Simplified institutional management paradigms can streamline the process of selecting appropriate pharmacotherapy and aid in care delivery that will optimize patient outcomes.

Piperazinyl-glutamate-pyrimidines as potent P2Y12 antagonists for inhibition of platelet aggregation

Piperazinyl-glutamate-pyrimidines were prepared with oxygen, nitrogen, and sulfur substitution at the 4-position of the pyrimidine leading to highly potent P2Y12 antagonists. In particular, 4-substituted piperidine-4-pyrimidines provided compounds with exceptional potency. Pharmacokinetic and physicochemical properties were fine-tuned through modifications at the 4-position of the piperidine ring leading to compounds with good human PRP potency, selectivity, clearance and oral bioavailability.

Oral antiplatelet therapy in unstable angina/non-ST-segment elevation myocardial infarction, ST-segment elevation myocardial infarction, and percutaneous coronary intervention: is it time for a guideline update?

Oral antiplatelet drugs are central in the management of acute coronary syndromes, including unstable angina/non-ST-segment elevation myocardial infarction, ST-segment elevation myocardial infarction, and percutaneous coronary intervention. However, the mechanisms that confer the benefits of these agents also increase bleeding risk. Thus, the selection of appropriate pharmacotherapy requires close attention to the delicate balance between reducing the risk of ischemic events and minimizing bleeding risk. A critical review of the 2007 guidelines in these areas and new clinical data with currently available and soon-to-be-approved antiplatelet agents are presented.