Oral administration of the thrombin receptor antagonist E5555 (atopaxar) attenuates intimal thickening following balloon injury in rats

Protease-Activated Receptors (PARs): Biology and Therapeutic Potential in Perioperative Stroke

Perioperative stroke is a devastating complication that occurs during surgery or within 30 days following the surgical procedure. Its prevalence ranges from 0.08 to 10% although it is most likely an underestimation, as sedatives and narcotics can substantially mask symptomatology and clinical presentation. Understanding the underlying pathophysiology and identifying potential therapeutic targets are of paramount importance. Protease-activated receptors (PARs), a unique family of G-protein-coupled receptors, are widely expressed throughout the human body and play essential roles in various physiological and pathological processes. This review elucidates the biology and significance of PARs, outlining their diverse functions in health and disease, and their intricate involvement in cerebrovascular (patho)physiology and neuroprotection. PARs exhibit a dual role in cerebral ischemia, which underscores their potential as therapeutic targets to mitigate the devastating effects of stroke in surgical patients.

Dual blockade of protease-activated receptor 1 and 2 additively ameliorates diabetic kidney disease

Protease-activated receptors (PARs) are coagulation protease targets, and they increase expression of inflammatory cytokines and chemokines in various diseases. Of all PARs, previous reports have shown that PAR1 or PAR2 inhibition is protective against diabetic glomerular injury. However, how PAR1 and PAR2 cooperatively contribute to diabetic kidney disease (DKD) pathogenesis and whether dual blockade of PARs is more effective in DKD remain elusive. To address this issue, male type I diabetic Akita mice heterozygous for endothelial nitric oxide synthase were used as a model of DKD. Mice (4 mo old) were divided into four treatment groups and administered vehicle, PAR1 antagonist (E5555, 60 mg·kg^-1·day^-1), PAR2 antagonist (FSLLRY, 3 mg·kg^-1·day^-1), or E5555 + FSLLRY for 4 wk. The results showed that the urinary albumin creatinine ratio was significantly reduced when both PAR1 and PAR2 were blocked with E5555 + FSLLRY compared with the vehicle-treated group. Dual blockade of PAR1 and PAR2 by E5555 + FSLLRY additively ameliorated histological injury, including mesangial expansion, glomerular macrophage infiltration, and collagen type IV deposition. Marked reduction of inflammation- and fibrosis-related gene expression in the kidney was also observed. In vitro, PAR1 and PAR2 agonists additively increased mRNA expression of macrophage chemoattractant protein 1 or plasminogen activator inhibitor-1 in human endothelial cells. Changes induced by the PAR1 agonist were blocked by a NF-κB inhibitor, whereas those of the PAR2 agonist were blocked by MAPK and/or NF-κB inhibitors. These findings suggest that PAR1 and PAR2 additively contribute to DKD pathogenesis and that dual blockade of both could be a novel therapeutic option for treatment of patients with DKD.

Vorapaxar expands antiplatelet options

Vorapaxar is the first substance of a new class of antiplatelet drugs that has been tested in large clinical trials. The protease-activated receptor 1 (PAR-1) antagonist inhibits thrombin-induced platelet activation to prevent atherothrombosis. In the phase 3 trials TRACER (acute coronary syndrome) and TRA 2P-TIMI 50 (stable atherosclerosis) reducing ischemic events with vorapaxar came at the cost of bleeding.

TRACER compared vorapaxar to placebo in 12 944 patients who had non-ST-segment elevation acute coronary syndromes on top of contemporary treatment including dual antiplatelet therapy (aspirin and clopidogrel). Vorapaxar reduced ischemic events non-significantly, but increased bleeding significantly, therefore not justifying triple antiplatelet therapy in this setting. Follow-up was stopped early because of bleeding. TRA 2P-TIMI 50 examined 26 449 patients who had a history of myocardial infarction, ischemic stroke, or peripheral arterial disease. Vorapaxar reduced ischemic events and increased bleeding both significantly. Recruitment of patients with prior stroke was stopped early. Net clinical outcome and subgroup analyses suggested that vorapaxar could be beneficial for patients with prior myocardial infarction – but no history of stroke.

Recent Progress in Research on the Pathogenesis of Pulmonary Thromboembolism: An Old Story with New Perspectives

Pulmonary thromboembolism (PTE) is part of a larger clinicopathological entity, venous thromboembolism. It is also a complex, multifactorial disorder divided into four major disease processes including venous thrombosis, thrombus in transit, acute pulmonary embolism, and pulmonary circulation reconstruction. Even when treated, some patients develop chronic thromboembolic pulmonary hypertension. PTE is also a common fatal type of pulmonary vascular disease worldwide, but earlier studies primarily focused on the pathological changes in the blood component of the disease. With contemporary advances in molecular and cellular biology, people are becoming increasingly aware of coagulation pathways, the function of vascular smooth muscle cells, microparticles, and the inflammatory pathways that play key roles in PTE. Combined hypoxia and immune research has revealed that PTE should be regarded as a class of complex diseases caused by multiple factors involving the vascular microenvironment and vascular cell dysfunction.

Vein graft failure: from pathophysiology to clinical outcomes

Occlusive arterial disease is a leading cause of morbidity and mortality worldwide. Aside from balloon angioplasty, bypass graft surgery is the most commonly performed revascularization technique for occlusive arterial disease. Coronary artery bypass graft surgery is performed in patients with left main coronary artery disease and three-vessel coronary disease, whereas peripheral artery bypass graft surgery is used to treat patients with late-stage peripheral artery occlusive disease. The great saphenous veins are commonly used conduits for surgical revascularization; however, they are associated with a high failure rate. Therefore, preservation of vein graft patency is essential for long-term surgical success. With the exception of 'no-touch' techniques and lipid-lowering and antiplatelet (aspirin) therapy, no intervention has hitherto unequivocally proven to be clinically effective in preventing vein graft failure. In this Review, we describe both preclinical and clinical studies evaluating the pathophysiology underlying vein graft failure, and the latest therapeutic options to improve patency for both coronary and peripheral grafts.

Scoparone interferes with STAT3-induced proliferation of vascular smooth muscle cells

Scoparone, which is a major constituent of Artemisia capillaries, has been identified as an anticoagulant, hypolipidemic, vasorelaxant, anti-oxidant and anti-inflammatory drug, and it is used for the traditional treatment of neonatal jaundice. Therefore, we hypothesized that scoparone could suppress the proliferation of VSMCs by interfering with STAT3 signaling. We found that the proliferation of these cells was significantly attenuated by scoparone in a dose-dependent manner. Scoparone markedly reduced the serum-stimulated accumulation of cells in the S phase and concomitantly increased the proportion of cells in the G0/G1 phase, which was consistent with the reduced expression of cyclin D₁, phosphorylated Rb and survivin in the VSMCs. Cell adhesion markers, such as MCP-1 and ICAM-1, were significantly reduced by scoparone. Interestingly, this compound attenuated the increase in cyclin D promoter activity by inhibiting the activities of both the WT and active forms of STAT3. Similarly, the expression of a cell proliferation marker induced by PDGF was decreased by scoparone with no change in the phosphorylation of JAK2 or Src. On the basis of the immunofluorescence staining results, STAT3 proteins phosphorylated by PDGF were predominantly localized to the nucleus and were markedly reduced in the scoparone-treated cells. In summary, scoparone blocks the accumulation of STAT3 transported from the cytosol to the nucleus, leading to the suppression of VSMC proliferation through G1 phase arrest and the inhibition of Rb phosphorylation. This activity occurs independent of the form of STAT3 and upstream of kinases, such as Jak and Src, which are correlated with abnormal vascular remodeling due to the presence of an excess of growth factors following vascular injury. These data provide convincing evidence that scoparone may be a new preventative agent for the treatment of cardiovascular diseases.

Thrombin Receptor Protease-Activated Receptor 4 Is a Key Regulator of Exaggerated Intimal Thickening in Diabetes Mellitus

Background—

Diabetes mellitus predisposes to thrombotic and proliferative vascular remodeling, to which thrombin contributes via activation of protease-activated receptor (PAR) 1. However, the use of PAR-1 inhibitors to suppress remodeling may be limited by severe bleeding. We recently reported upregulation of an additional thrombin receptor, PAR-4, in human vascular smooth muscle cells exposed to high glucose and have now examined PAR-4 as a novel mediator linking hyperglycemia, hypercoagulation, and vascular remodeling in diabetes mellitus.

Methods and Results—

PAR-4 expression was increased in carotid atherectomies and saphenous vein specimens from diabetic versus nondiabetic patients and in aorta and carotid arteries from streptozotocin-diabetic versus nondiabetic C57BL/6 mice. Vascular PAR-1 mRNA was not increased in diabetic mice. Ligated carotid arteries from diabetic mice developed more extensive neointimal hyperplasia and showed greater proliferation than arteries from nondiabetic mice. The augmented remodeling response was absent in diabetic mice deficient in PAR-4. At the cellular level, PAR-4 expression was controlled via the mRNA stabilizing actions of human antigen R, which accounted for the stimulatory actions of high glucose, angiotensin II, and H 2 O 2 on PAR-4 expression, whereas cicaprost via protein kinase A activation counteracted this effect.

Conclusions—

PAR-4 appears to play a hitherto unsuspected role in diabetic vasculopathy. The development of PAR-4 inhibitors might serve to limit mainly proliferative processes in restenosis-prone diabetic patients, particularly those patients in whom severe bleeding attributed to selective PAR-1 blockade or complete thrombin inhibition must be avoided or those who do not require anticoagulation.

Thrombin receptor antagonism in antiplatelet therapy

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

Matrix metalloproteases and PAR1 activation

Cardiovascular diseases, including atherothrombosis, are the leading cause of morbidity and mortality in the United States, Europe, and the developed world. Matrix metalloproteases (MMPs) have recently emerged as important mediators of platelet and endothelial function, and atherothrombotic disease. Protease-activated receptor-1 (PAR1) is a G protein-coupled receptor that is classically activated through cleavage of the N-terminal exodomain by the serine protease thrombin. Most recently, 2 MMPs have been discovered to have agonist activity for PAR1. Unexpectedly, MMP-1 and MMP-13 cleave the N-terminal exodomain of PAR1 at noncanonical sites, which result in distinct tethered ligands that activate G-protein signaling pathways. PAR1 exhibits metalloprotease-specific signaling patterns, known as biased agonism, that produce distinct functional outputs by the cell. Here we contrast the mechanisms of canonical (thrombin) and noncanonical (MMP) PAR1 activation, the contribution of MMP-PAR1 signaling to diseases of the vasculature, and the therapeutic potential of inhibiting MMP-PAR1 signaling with MMP inhibitors, including atherothrombotic disease, in-stent restenosis, heart failure, and sepsis.

Atopaxar. A novel player in antiplatelet therapy?

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

Exogenous thrombin delivery promotes collateral capillary arterialization and tissue reperfusion in the murine spinotrapezius muscle ischemia model

OBJECTIVE

We examined the effects of exogenously delivered thrombin on cell recruitment in skeletal muscle and the formation of new collateral arterioles in the microvasculature in response to ligation-induced ischemia.

METHODS

Thrombin or vehicle was locally applied to both ligated and nonoperated Balb/c spinotrapezius muscles, which were harvested after three or seven days, imaged using confocal microscopy, and analyzed.

RESULTS

Thrombin treatment resulted in accelerated arterialization of collateral capillaries and accelerated tissue reperfusion in ischemic muscles. Uninjured muscle treated with thrombin displayed increased vascular cell adhesion molecule 1 expression on arteriole and venule endothelium, increased expression of smooth muscle α-actin on capillary-sized vessels, increased infiltration by CD11b(+) leukocytes, and mast cell infiltration and degranulation.

CONCLUSIONS

Exogenous delivery of thrombin enhances microvascular collateral development in response to ischemic insult, and accelerates tissue reperfusion. Elicited responses from multiple cell types probably contribute to these effects.

PAR-1 inhibitors: a novel class of antiplatelet agents for the treatment of patients with atherothrombosis

Stroke and myocardial infarction are leading causes of death and disability worldwide. Typically, these events are triggered by the rupture or erosion of "vulnerable" atherosclerotic plaque, a phenomenon termed atherothrombosis.Three platelet activation pathways are presumed to be particularly important in the genesis of atherothrombosis and are triggered by 1) cyclo-oxygenase (COX)-1 mediated thromboxane A2 (TXA2) synthesis and activation via the TXA2 receptor, 2) adenosine diphosphate (ADP) via the P2Y12 receptor, and 3) thrombin via the protease activated receptor (PAR)-1.Despite the efficacy of aspirin and of a growing family of P2Y12 receptor antagonists on the first 2 pathways, major cardiovascular events continue to occur in patients with coronary and cerebrovascular disease, suggesting that thrombin-mediated platelet activation may contribute to these adverse events.Recently, a novel class of antiplatelet agents able to inhibit thrombin-mediated platelet activation has been developed, PAR-1 inhibitors. In this chapter, we will discuss the rationale underlying the development of this novel class of agents focus on the two drugs in the most advanced stages of development: vorapaxar (SCH530348) and atopaxar (E5555).

Novel agents for anti-platelet therapy

Anti-platelet therapy plays an important role in the treatment of patients with thrombotic diseases. The most commonly used anti-platelet drugs, namely, aspirin, ticlopidine, and clopidogrel, are effective in the prevention and treatment of cardio-cerebrovascular diseases. Glycoprotein IIb/IIIa antagonists (e.g., abciximab, eptifibatide and tirofiban) have demonstrated good clinical benefits and safety profiles in decreasing ischemic events in acute coronary syndrome. However, adverse events related to thrombosis or bleeding have been reported in cases of therapy with glycoprotein IIb/IIIa antagonists. Cilostazol is an anti-platelet agent used in the treatment of patients with peripheral ischemia, such as intermittent claudication. Presently, platelet adenosine diphosphate P2Y(12) receptor antagonists (e.g., clopidogrel, prasugrel, cangrelor, and ticagrelor) are being used in clinical settings for their pronounced protective effects. The new protease-activated receptor antagonists, vorapaxar and atopaxar, potentially decrease the risk of ischemic events without significantly increasing the rate of bleeding. Some other new anti-platelet drugs undergoing clinical trials have also been introduced. Indeed, the number of new anti-platelet drugs is increasing. Consequently, the efficacy of these anti-platelet agents in actual patients warrants scrutiny, especially in terms of the hemorrhagic risks. Hopefully, new selective platelet inhibitors with high anti-thrombotic efficiencies and low hemorrhagic side effects can be developed.