Antithrombotic Properties of the Thromboxane A2/Prostaglandin H2 Receptor Antagonist S18886 on Prevention of Platelet-Dependent Cyclic Flow Reductions in Dogs

Crosstalk between Platelet and Bacteria: A Therapeutic Prospect

Platelets are typically recognized for their roles in the maintenance of hemostasis and vascular wall repair to reduce blood loss. Beyond hemostasis, platelets also play a critical role in pathophysiological conditions like atherosclerosis, stroke, thrombosis, and infections. During infection, platelets interact directly and indirectly with bacteria through a wide range of cellular and molecular mechanisms. Platelet surface receptors such as GPIbα, FcγRIIA, GPIIbIIIa, and TLRs, etc. facilitate direct interaction with bacterial cells. Besides, the indirect interaction between platelet and bacteria involves host plasma proteins such as von Willebrand Factor (vWF), fibronectin, IgG, and fibrinogen. Bacterial cells induce platelet activation, aggregation, and thrombus formation in the microvasculature. The activated platelets induce the Neutrophil Extracellular Traps (NETs) formation, which further contribute to thrombosis. Thus, platelets are extensively anticipated as vital immune modulator cells during infection, which may further lead to cardiovascular complications. In this review, we cover the interaction mechanisms between platelets and bacteria that may lead to the development of thrombotic disorders. Platelet receptors and other host molecules involved in such interactions can be used to develop new therapeutic strategies to combat against infection-induced cardiovascular complications. In addition, we highlight other receptor and enzyme targets that may further reduce infection-induced platelet activation and various pathological conditions.

Perspective: Tyrosine phosphatases as novel targets for antiplatelet therapy

Arterial thrombosis is the primary cause of most cases of myocardial infarction and stroke, the leading causes of death in the developed world. Platelets, highly specialized cells of the circulatory system, are key contributors to thrombotic events. Antiplatelet drugs, which prevent platelets from aggregating, have been very effective in reducing the mortality and morbidity of these conditions. However, approved antiplatelet therapies have adverse side effects, most notably the increased risk of bleeding. Moreover, there remains a considerable incidence of arterial thrombosis in a subset of patients receiving currently available drugs. Thus, there is a pressing medical need for novel antiplatelet agents with a more favorable safety profile and less patient resistance. The discovery of novel antiplatelet targets is the matter of intense ongoing research. Recent findings demonstrate the potential of targeting key signaling molecules, including kinases and phosphatases, to prevent platelet activation and aggregation. Here, we offer perspectives to targeting members of the protein tyrosine phosphatase (PTP) superfamily, a major class of enzymes in signal transduction. We give an overview of previously identified PTPs in platelet signaling, and discuss their potential as antiplatelet drug targets. We also introduce VHR (DUSP3), a PTP that we recently identified as a major player in platelet biology and thrombosis. We review our data on genetic deletion as well as pharmacological inhibition of VHR, providing proof-of-principle for a novel and potentially safer VHR-based antiplatelet therapy.

Novel antiplatelet drugs in clinical development

The clinical value of antiplatelet compounds strongly depends on the benefit-risk balance between their anti-thrombotic effects and the bleeding risk they incur. This ratio is especially important in the treatment of cerebro-vascular disease. Several novel compounds in clinical development hold promise to improve this benefit-risk ratio.

Daily administration of the TP receptor antagonist terutroban improved endothelial function in high-cardiovascular-risk patients with atherosclerosis

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Terutroban is a selective TP receptor antagonist, i.e. a specific antagonist of the thromboxane A(2) and prostaglandin endoperoxide receptors, shown to improve endothelial function after a single administration in patients with coronary artery disease.

WHAT THIS STUDY ADDS

• This randomized, double-blind, placebo-controlled trial demonstrates that repeated-dose terutroban for 15 days improves endothelial function and inhibits thromboxane A(2) -induced platelet aggregation in high-cardiovascular-risk patients taking 300 mg of aspirin per day. Terutroban may prove useful for preventing cardiovascular events in such patients.

AIMS

The specific TP receptor antagonist terutroban improves endothelial function after a single dose in patients with coronary artery disease. Our aim was to evaluate the effects and dose dependency of repeated-dose terutroban on endothelial function and platelet aggregation in high-cardiovascular-risk patients with carotid atherosclerosis.

METHODS

We randomly allocated 48 patients taking 300 mg aspirin per day to placebo or to one of three terutroban dosages (2.5, 5 or 10 mg) for 15 days in a double-blind study. Flow-mediated vasodilatation was evaluated before and 2 h after the first oral dose on day 0 and 2 h after the last oral dose on day 14.

RESULTS

On day 0 and day 14, all three terutroban dosages improved flow-mediated vasodilatation and abolished platelet aggregation induced by the TP receptor agonist U46619, without changing the aggregation response to ADP or collagen.

CONCLUSION

Terutroban, by chronically improving endothelium-dependent vasodilatation and inhibiting platelet aggregation, may prove useful for preventing cardiovascular events in high-risk patients.

The thromboxane/endoperoxide receptor (TP): the common villain

The stimulation of thromboxane/endoperoxide receptors (TP) elicits diverse physiological/pathophysiological reactions, including platelet aggregation and contraction of vascular smooth muscle. Furthermore, the activation of endothelial TP promotes the expression of adhesion molecules and favors adhesion and infiltration of monocytes/macrophages. In various cardiovascular diseases, endothelial dysfunction is predominantly the result of the release of endothelium-derived contracting factors that counteract the vasodilator effect of nitric oxide produced by the endothelial nitric oxide synthase. Endothelium-dependent contractions involve the activation of cyclooxygenases, the production of reactive oxygen species along with that of endothelium-derived contracting factors, which diffuse toward the vascular smooth muscle cells and activate their TP. TP antagonists curtail the endothelial dysfunction in diseases such as hypertension and diabetes, are potent antithrombotic agents, and reduce vascular inflammation. Therefore, TP antagonists, because of this triple activity, may have a unique potential for the treatment of cardiovascular disorders.

Preclinical evaluation of S18886 in an experimental model of coronary arterial thrombosis

The specific thromboxane receptor antagonist, S18886, was evaluated for prevention of coronary arterial thrombosis and myocardial ischemia-reperfusion in anesthetized canines. For the primary thrombosis study in left circumflex (LCX) coronary artery, 26 dogs were randomized to receive either vehicle (n = 7) or intravenous S18886 (0.3 mg/kg, n = 6; 1.0 mg/kg, n = 6; and 3.0 mg/kg, n = 7). The respective times to occlusion after S18886 were as follows: 56.8 +/- 9.3, 83.5 +/- 14.9, and 92.4 +/- 15.7 minutes compared to 43.3 +/- 8.2 minutes after vehicle. S18886 caused a minimal increase in tongue bleeding time and a significant decrease in ex vivo platelet aggregation to arachidonic acid or U46619. Another 37 dogs were randomized to receive placebo (n = 12), clopidogrel 1.0 mg/kg p.o. QDX3 (n = 9), clopidogrel + S18886 0.3 (n = 9) or 1.0 (n = 7) mg/kg intravenous. Clopidogrel produced a 50% reduction in adenosine diphosphate-induced platelet aggregation and a slight increase in the time to occlusion. However, clopidogrel + S18886 1.0 mg/kg prevented occlusive thrombus formation in most of the coronary vessels over 6 hours. S18886 did not alter myocardial infarct size in the ischemia-reperfusion model. In conclusion, S18886 alone caused a dose-dependent prolongation in the time to primary occlusive coronary artery thrombosis, whereas S18886 + clopidogrel displayed effective in preventing occlusive thrombus formation with only a moderate increase of tongue-bleeding time.

Experimental models of thrombosis and atherosclerosis

Atherothrombosis is a complex disease which includes two different pathologies: atherosclerosis, the process of plaque formation in the arterial wall and thrombosis, the formation of a blood clot mostly at the site of a ruptured atherosclerotic lesion. Animal models for both pathologies have been useful to understand their aetiology and their evolution and they were used to evaluate the efficacy of new treatments. Numerous models to study venous and arterial thrombosis have been described. Thus in the rat, venous thrombosis induced by lesion/stasis, e.g. in the vena cava, and arterial thrombosis by lesioning of the vessel wall are frequently used. The resulting blood clot formation is measured either directly (weight of the thrombus) or indirectly (reduction in blood flow). More complex models have been developed in large animals such as dogs and pigs in order to examine coronary thrombosis; the principle always being the arterial lesion that causes the thrombus formation. The effect of the TP-receptor antagonist terutroban (S 18886) on different thrombosis models has been evaluated and this has allowed to conclude on the powerful anti-thrombotic effects of this agent and has contributed to its progression into clinical development. In the past the most frequently used model of atherosclerosis was the hypercholesterolemic rabbit; both plaque formation and its consequences on vascular, endothelial, function have been largely studied in this model. More recently genetically engineered mouse models of atherosclerosis have been introduced and they are now largely studied to characterize the disease and to evaluate new drugs. The two models mostly used are the ApoE(-/-) and the LDL receptor(-/-) mice. Studies with terutroban have illustrated that this TP-receptor antagonist prevents lesion formation in mouse and rabbit models illustrating its interesting anti-atherosclerotic properties and demonstrating the role played by endothelial TP-receptors in atherogenesis. In conclusion, experimental models to study atherosclerosis and thrombosis have been developed and used to study the etiology and the evolution of atherothrombotic disease. They have also been of great value to predict anti-thrombotic and/or anti-atherosclerotic properties of new substances such as terutroban, that may become novel treatments for this complex cardiovascular disease.

Terutroban et récepteurs TP endothéliaux dans l’athérogenèse

Treatment of thrombotic diseases implicates the use of anti-platelet agents, anti-coagulants and pro-fibrinolytic substances. Amongst the anti-platelet drugs, aspirin occupies a unique position. As soon as it became evident that the major action of aspirin is indirect blockade, through inhibition of cyclooxygenase (COX), of the production of thromboxane A2 (TXA2), a powerful vasoconstrictor and platelet activator, research for new anti-thrombotics that interact more specifically with the production and/or the action of TXA2 was started. Terutroban (S 18886) is a selective antagonist of TP receptors, the receptors for TXA2, that are present on platelets and on vascular smooth muscle cells, but also on endothelial cells. The role played by the platelet and smooth muscle cell TP receptors in thrombotic disease is well known, and preclinical and clinical studies with terutroban have illustrated the powerful antithrombotic effects of this agent. The implication of endothelial TP receptors in the development of atherosclerotic disease has only been examined during the past five years and studies with terutroban have been crucial for understanding the role of these endothelial receptors in cardiovascular physiopathology. The goal of the present review is to discuss the arguments in favour of the hypothesis suggesting that activation of endothelial TP receptors, by causing expression of adhesion molecules, favours adhesion and infiltration of monocytes/macrophages in the arterial wall, thereby stimulating the development of atherosclerosis. The review will also highlight the important contribution of the studies performed with terutroban in this research area. The triple activity (anti-thrombotic, anti-vasoconstrictor, anti-atherosclerotic) observed with terutroban in preclinical studies, stressed by the first results in clinical development, places terutroban as an innovative drug with a unique potential for treatment of cardiovascular disorders.