Ideal P2Y12 Inhibitor in Acute Coronary Syndrome: A Review and Current Status

Dual antiplatelet therapy (DAPT) has remained the cornerstone for management of acute coronary syndrome (ACS) over the years. Clopidogrel has been the quintessential P2Y12 receptor (platelet receptor for Adenosine 5′ diphosphate) inhibitor for the past two decades. With the demonstration of unequivocal superior efficacy of prasugrel/ticagrelor over clopidogrel, guidelines now recommend these agents in priority over clopidogrel in current management of ACS. Cangrelor has revived the interest in injectable antiplatelet therapy too. Albeit the increased efficacy of these newer agents comes at the cost of increased bleeding and this becomes more of a concern when combined with aspirin. Which P2Y12i is superior over another has been intensely debated over last few years after the ISAR-REACT 5 study with inconclusive data. Three novel antiplatelet agents are already in the pipeline for ACS with all of them succeeding in phase II studies. The search for an ideal antiplatelet remains a need of the hour for optimal reduction of ischemic events in ACS.

Mimetics of the Arg-Gly-Asp Sequence: Synthesis and Studies of the Antiaggregative Properties

Abstract—

It is known that the Arg-Gly-Asp sequence in the fibrinogen molecule is key in binding to the receptors on the surface of platelets. We searched for the compounds which were able to inhibit the binding and synthesized the following analogs of this sequence: 2-acetoxybenzoyl-Arg-βAla-Asp, 4-piperidinecarbonyl-βAla-Asp, and 4-aminobezoyl-βAla-Asp. These compounds were shown to inhibit the platelets aggregation in a different degree. The 2-acetoxybenzoyl-Arg-βAla-Asp analog demonstrated the highest inhibitory activity. A decrease in the expression of the CD62p and CD63 markers on platelets was also found after the action of the Arg-Gly-Asp analogs, confirming the ability of these compounds to block the fibrinogen binding sites for the GP IIb/IIIa glycoprotein receptors.

Genetic determinants of platelet response to clopidogrel

Antiplatelet agents are the mainstay treatment in the prevention and management of atherothrombotic complications. However, a substantial interpatient variability in response to clopidogrel has been reported. Furthermore, patients with coronary artery disease and lesser platelet inhibition in response to clopidogrel are at increased risk for cardiovascular events. Clopidogrel after absorption requires two-step oxidation by the hepatic cytochrome P450 to generate its active metabolite. Polymorphisms of genes encoding the cytochrome enzymes and P-glycoprotein involved in clopidogrel absorption are regarded as major determinants of the interindividual variability in the clopidogrel-induced platelet inhibition. In our review we discuss the prevalence and clinical significance of various alleles of the genes: CYP2C19 and ABCB1 in the setting of coronary artery disease. Allele CYP2C19*2 is associated with excess of ischaemic events including myocardial infarction and stent thrombosis. On the other hand, CYP2C19*17 allele poses a serious threat of bleeding. Data concerning the prognostic value of genetic variant 3435C→T of ABCB1 remain inconclusive.

Emerging oral antiplatelet therapies for acute coronary syndromes

Emergency department physicians, along with hospitalists and interventional cardiologists, provide first-line care for patients experiencing symptoms potentially associated with acute coronary syndromes (ACS). Because these health care providers encounter and manage patients with varying degrees of risk, a clear understanding of the modes of action, benefits, and limitations of various therapeutic options is crucial for achieving optimal outcomes in the acute-care setting. Oral antiplatelet therapy has a major role in the acute care of patients with suspected ACS due to the critical role of platelets in the pathophysiology of disease. The current standard-of-care oral antiplatelet therapy for ACS is aspirin in combination with a P2Y12 adenosine diphosphate (ADP) receptor antagonist, most commonly clopidogrel. Aspirin and P2Y12 antagonists have both demonstrated efficacy in reducing morbidity and mortality in patients with ACS, but are also associated with increased bleeding risk compared with controls. Additionally, despite dual oral antiplatelet therapy, patients remain at substantial residual risk for ischemic events due to thrombotic episodes driven by platelet activation pathways that are not inhibited by these agents, including the protease-activated receptor (PAR)-1 platelet activation pathway, stimulated by thrombin. Novel oral antiplatelet agents in advanced clinical development include a direct and more readily reversible P2Y12 antagonist, ticagrelor, as well as a new class of PAR-1 antagonists, which includes vorapaxar and atopaxar. Ticagrelor has shown a significant ischemic benefit and an increase in non-surgical bleeding over clopidogrel in the large phase 3 Platelet Inhibition and Patient Outcomes trial. Results of phase 2 trials with PAR-1 antagonists suggest that these agents may provide incremental reduction in ischemic events without a bleeding liability. This hypothesis is being evaluated in 2 large ongoing phase 3 trials with vorapaxar, including the Thrombin Receptor Antagonist for Clinical Event Reduction in Acute Coronary Syndrome (TRA*CER) trial in patients with non-ST-segment elevation ACS.

Thrombin receptors and their antagonists: an update on the patent literature

IMPORTANCE OF THE FIELD

Thrombin plays a central role in cardiovascular inflammation. Most of the cellular responses to thrombin are mediated by cell surface protease-activated receptors (PARs). Several preclinical studies indicate that PARs are potential targets for treating cardiovascular diseases such as thrombosis, atherosclerosis and restenosis. Among PARs, PAR-1 has emerged as an important therapeutic target.

AREAS COVERED IN THIS REVIEW

This review covers recent advances in the development of thrombin receptors antagonists. It is focused on the search for PAR-1 antagonists as this is at the moment the most promising and attractive target. However, some early promising studies on PAR-3 and -4 antagonists are also reported.

WHAT THE READER WILL GAIN

The review has been written in order to give to the reader hints and references that cover, in our opinion, the most interesting and/or promising approaches in this research field.

TAKE HOME MESSAGE

Research on PAR-1 antagonists has finally led to good clinical candidates such as SCH-530348 (Schering-Plough) and E-5555 (Eisai Co.). Clinical trials clearly demonstrate that development of PAR1 antagonists is not only possible but most likely will lead to development of antiplatelet drugs as well as of drugs useful for the treatment of inflammatory, proliferative and neurodegenerative diseases.

Vascular PAR-1: activity and antagonism

Despite major advances in antiplatelet therapies, recurrent cardiovascular events remain high after acute coronary syndrome. Furthermore, incremental benefits achieved in the reduction of atherothrombotic events have almost always been at the expense of hemorrhagic side effects. Thrombin is the most potent platelet activating factor known and it makes important interactions with the endothelium and vascular smooth muscle with proinflammatory, proatherogenic effects. Distinct from its activity within the coagulation cascade, thrombin mediates these effects via protease-activated receptor type 1 (PAR-1) in man. This review discusses the role of PAR-1 in the vasculature and the development of novel PAR-1 antagonists. These drugs may provide important antiatherothrombotic effects without attendant bleeding complications and could represent a major breakthrough for the treatment of cardiovascular diseases.

Secondary prevention strategies for coronary heart disease

Patients with established coronary heart disease (CHD) have a high risk of subsequent cardiovascular events, including myocardial infarction (MI), stroke, and death from cardiovascular disease. Adherence to evidence-based secondary prevention therapies for CHD has improved in recent years but still remains suboptimal. Mortality from CHD in the United States (US) has decreased substantially in recent decades. The decline in US deaths from CHD from 1980 through 2000 has been attributed to reductions in major risk factors and utilization of evidence-based medical therapies. It has been estimated that optimization of secondary prevention strategies could save as many as 80,000 more lives per year in the US. The American College of Cardiology (ACC) and American Heart Association (AHA) updated its guidelines for secondary prevention for patients with atherosclerotic vascular disease in 2006. The guidelines emphasize evidence-based developments in the field of CHD secondary prevention and also reinforce the need to implement these recommendations in actual clinical practice through programs such as the ACC's Guidelines Applied to Practice and the AHA's Get With The Guidelines. This review will discuss the epidemiology and risk assessment of CHD, current pharmacologic and nonpharmacologic strategies available for the secondary prevention of CHD, and summarize the guidelines and evidence that support these treatment options. There will be an emphasis on antiplatelet therapy given the important role of thrombosis in clinical cardiovascular events.

New antiplatelet agents

PURPOSE OF REVIEW

Cardiovascular events are still a major concern despite dual antiplatelet treatment with aspirin and clopidogrel. Moreover, recent developments that improved platelet inhibition were associated with increased bleeding complications, suggesting that personal dose adjustment might be needed. This review is dedicated to new developments in antiplatelet agents other than ADP P2Y12 receptor blockers, which are discussed in a separate article of this issue.

RECENT FINDINGS

The thrombin receptor protease activable receptor-1 antagonist SCH530348 and the thromboxane receptor antagonist terutroban are two promising agents that are now undergoing phase III clinical trials. Cilostazol, a phosphodiesterase III inhibitor, is currently tested in phase IV trials for several new indications. Despite previous disappointing results with oral GPIIbIIIa antagonists, new compounds with unique features are under development. In addition, targeting intracellular signaling pathways is a new area of research, with several attractive targets.

SUMMARY

Recent development of new antiplatelet agents, including inhibitors of thrombin receptor, thromboxane receptor, phosphodiesterase, glycoprotein VI, GPIIbIIIa and signaling pathways, may lead to a more effective and yet safe antiplatelet therapy with an improved clinical outcome.

Effect of pharmaceutical interventions targeting thromboxane receptors and thromboxane synthase in cardiovascular and renal diseases

The present review focuses on the roles of thromboxane A2 (TxA2) in arterial thrombosis, atherogenesis, vascular stent-related ischemic events and renal proteinuria. Particular emphasis is laid on therapeutic interventions targeting the TxA2 (TP) receptors and TxA2 synthase (TS), including dual TP-receptor antagonists and TS inhibitors. Their significant inhibitory efficacies on arterial thrombogenesis, atherogenesis, restenosis after stent placement, vasoconstriction and proteinuria indicate novel and improved treatments for cardiovascular and selected renal diseases. New therapeutic interventions of the TxA2 pathway may also be beneficial for patients with poor biological antiplatelet drug response, for example, to aspirin and/or clopidogrel. These new TP/TS agents offer novel improved treatments to efficiently and simultaneously interfere with thrombogenesis and atherogenesis, and to enlarge the existing panel of platelet inhibitors for efficient prophylaxis and treatment of arterial thrombosis and renal proteinuria.

Protease-activated receptors in cardiovascular health and diseases

The platelet, once thought to be solely involved in clot formation, is now known to be a key mediator in various other processes such as inflammation, thrombosis, and atherosclerosis. Therefore, antiplatelet agents have become paramount in the prevention and management of various cardiovascular diseases. However, the currently most widely used antiplatelet drugs, aspirin and clopidogrel, have been shown to reduce the risk of serious vascular events only by approximately one quarter. Similarly, oral glycoprotein IIb/IIIa antagonists have been associated with excess mortality, thus restricting the use of parental glycoprotein IIb/IIIa antagonists to the treatment of acute clinical conditions. Thus, for the prevention of cardiovascular diseases, there is still a clinical need for antiplatelet drugs with higher antithrombotic efficacy but with safety profiles that allow for a preventive long-term administration. Thrombin signaling through the protease-activated receptors (PARs) has been shown to influence a wide range of physiologic and pathologic responses in cardiovascular systems. Thus, interference with PARs appears to be a promising strategy to develop new antiplatelet agents with higher efficacy. This review focuses on the cardiovascular actions of PARs that play a role in normal cardiovascular physiology and that are likely to contribute to cardiovascular diseases.

P2Y1 receptor antagonists as novel antithrombotic agents

The P2Y(1) and P2Y(12) purinergic receptors are responsible for mediating adenosine diphosphate (ADP) dependent platelet aggregation. Evidence from P2Y(1) knockout studies as well as from nucleotide-based small molecule P2Y(1) antagonists has suggested that the antagonism of this receptor may offer a novel and effective method for the treatment of thrombotic disorders. Herein, we report the identification and optimization of a series of non-nucleotide P2Y(1) antagonists that are potent and orally bioavailable.