Antiplatelet agents

Cardiovascular diseases are the leading cause of morbidity in western countries, accounting for an annual worldwide mortality of 17 million, with an annual cost of around half a trillion dollars in the US alone.¹ Platelets play a major role in arterial thrombosis, which is the final event complicating cardiovascular diseases as well as peripheral vascular diseases, and antiplatelet therapy improves survival of patients with these disorders.² The multiple pathways of platelet activation limit the effect of specific receptor/pathway inhibitors, resulting in limited clinical efficacy. In contrast, inhibitors of the final common pathway of platelet activation, binding of the major integrin GPIIbIIIa to its ligands (fibrinogen, von Willebran factor and others), have the potential of complete platelet inhibition, which is independent of the activation pathway. Aspirin (acetyl salicylic acid) is the most commonly used drug in western countries and is the drug of choice for cardiovascular diseases due to its good cost effectiveness profile.³ Although already used in the ancient world as an anti-inflammatory agent, aspirin was adopted for antithrombotic therapy only in the 1960s.⁴ Nowadays, 1 out of every 5 US citizens is taking aspirin on a daily basis, mostly for cardiovascular disease prevention. Aspirin improves clinical outcome in all cardiovascular syndromes, including acute events, primary and secondary prevention, with an overall reduction of serious vascular events by about one quarter.⁵ In stable coronary heart disease it was found that a low dose of aspirin (50–100 mg/d) is as effective as the higher does of 300 mg/d but is associated with a lower rate of major bleeding.⁶ However, in patients with a higher risk of cardiovascular events, aspirin alone is not sufficiently effective. Thus, combining aspirin with clopidogrel (thienopyridine derivative) is the treatment of choice and is also cost effective in patients presenting with acute coronary syndrome (ACS).⁷ The introduction of percutaneous intervention (PCI) technology in cardiology was associated with the need to protect the treated vessels against early and late thrombosis as well as against re-occlusion.⁸ Again, the combination of aspirin with P2Y12 receptor blockers (first ticlopidine and later clopidogrel) has proved to be effective in preventing some of these complications.⁹ This dual antiplatelet treatment should be extended for a longer period particularly in patients in whom a drug-eluting stent (DES) is placed, where stent thrombosis may occur late after implantation. A more aggressive antiplatelet approach was the development of the intravenous platelet integrin GPIIbIIIa inhibitors, which improved outcome of patients undergoing PCI.^10,–13 These potent platelet inhibitors are currently used during PCI procedures. Oral versions of these drugs were developed later but were found to be either ineffective or not safe for secondary prevention application.⁹ The beneficial effect of antiplatelet drugs is associated with an increased risk of bleeding. This is true even with the currently used low dose aspirin; thus, the application of these drugs is limited in some patients and often requires gastric protection medications.¹⁴ In a recent evaluation by the Cochrane group it was found that combination therapy of aspirin and clopidogrel is beneficial only in patients with ACS and for those undergoing PCI, whereas in those with a high risk or an established disease without the characteristics of an acute syndrome, bleeding complications may outweigh the protective effect.¹⁵ In addition, some rare cases of thrombotic thrombocytopenic purpura (TTP) were reported in patients receiving clopidogrel, a complication that requires an urgent intervention.¹⁶ Selected recommendations for antiplatelet therapy are summarized in Table 1 .

Available data regarding efficacy and safety of antiplatelet therapy are based entirely on the clinical outcome. Although platelet aggregation testing was developed in parallel to the introduction of aspirin as an antiplatelet drug,⁴ none of the major studies that established the role of aspirin and clopidogrel in cardiovascular diseases was based on the laboratory response of the individual patient to the specific drug. However, evidence of variable laboratory response to aspirin can be found since the 1970s.¹⁷ One reason for the lack of laboratory response of patients treated with antiplatelet drugs is the labor-intensive nature of the original platelet aggregation test, which did not allow massive screening and testing. Recently several point-of-care (POC) methods have been developed, allowing more accessible testing of the effect of these drugs.^18,–21 These developments led to several studies where patients under either aspirin or clopidogrel were tested for the laboratory response to these drugs by different methods.^22,23 These early reports have raised the question of whether pharmacological or functional tests are better markers for monitoring the effect of antiplatelet agents. The mechanism of the variable response to aspirin and clopidogrel was extensively studied as well. Data regarding aspirin responsiveness revealed a complete pharmacological response (inhibition of COX-1) in almost every treated patient.² However, when functional testing was applied, a variable response was observed among ACS patients, with a significantly higher rate of low laboratory response to aspirin (sometimes referred to aspirin “resistance”) among patients in the acute phase as compared with those at a later stable phase. The mechanism of this dynamic response to aspirin was found to stem from platelet activation at the acute phase, associated with residual response to arachidonic acid in spite of a complete COX-1 inhibition.²⁴ This residual response was proposed to be driven in part by an ADP effect.²⁵ Thus aspirin “resistance” seems to reflect the disease burden in the acute event rather than reflecting the genetic variability. The compliance of patients who require chronic aspirin therapy is yet another factor that is of concern. Similar studies of the response to clopidogrel have yielded different results. Although a reduced laboratory response is observed among some 20% to 30% of patients during ACS, there is still a significant range of response to the drug in stable patients as well as among healthy volunteers.²⁶ This variable response seems to reflect in part a genetic polymorphism of cytochrome p-450 where clopidogrel (as a prodrug) is converted into its active metabolite.²⁷ Other factors which may affect responsiveness to clopidogrel include use of other drugs that are metabolized by cytochrome P450 and gastrointestinal absorptions, etc. An early report by Gum et al was the first to document a correlation between aspirin “resistance” among patients with stable angina and clinical outcome.²⁸ Additional studies came to the same conclusion, demonstrating higher rates of recurrent cardiovascular events among “resistant” patients as compared with those responding to the drugs.²⁹ A major limitation of these studies was the relatively low statistical power due to low patient numbers. Recent meta analyses of several prospective studies have shown that indeed ACS patients with laboratory resistance to aspirin and clopidogrel are at a higher risk of recurrent cardiovascular events.^30,31 Thus resistance to aspirin in ACS seems to confer an odds ratio of around 3 for recurrent events, whereas in clopidogrel nonresponders, recurrent events are even more frequent. Additional support for the role of disease burden in the development of laboratory resistance to antiplatelet drugs comes from the observation of low responsiveness to these drugs among patients with diabetes mellitus. A subgroup analysis for diabetic patients in the Antithrombotic Trialists collaboration meta-analysis, showed a non-significant reduction of major cardiovascular events in diabetes mellitus (7% reduction; 5000 patients) as compared with the clear benefit of aspirin for high-risk non-diabetic patients (22% reduction).⁵ In addition, in two recent studies of patients with diabetes, aspirin given for primary prevention did not decrease the risk of cardiovascular events.^32,33 Platelet reactivity in diabetic patients still remains high under dual antiplatelet treatment with aspirin and clopidogrel.³⁴ 

In the search for improved efficacy, and in view of the observed variable responses to clopidogrel, an improved laboratory response was reported upon increasing the loading dose from 300 mg to 600 mg and increasing the maintenance dose from 75 mg/d to 150 mg/d.³⁵ Increasing the loading dose to 600 mg in laboratory-resistant patients was associated with improved clinical outcome as well.³⁶ These studies set the stage for the next step where increasing the dose, or changing the drug combination based on individual laboratory response, is evaluated for its potential beneficial effect on clinical outcome. Several such studies, currently underway, together with others will provide valuable information regarding a potential adaptation of individual dosing strategy of antiplatelet drugs.³⁷ 

Another approach taken by the industry was an effort to improve the effect of thienopyridine derivatives. The first example of such a drug is the prasugrel, which was found to be more potent than clopidogrel due to its higher rate of conversion into the active metabolite. This is due to its resistance to plasma esterase inhibition and to the fact that only single oxidative step is needed for its conversion to the active metabolite.³⁸ In addition, in contrast to clopidogrel prasugrel was found not to be affected by genetic variations in cytochrome P450.³⁹ A recent study compared the effect of prasugrel with that of clopidogrel in ACS patients.⁴⁰ In this study, when clopidogrel was applied at the original loading dose of 300 mg followed by 75 mg/d, prasugrel was found to be more effective with a significant reduction in myocardial infarctions and stent thrombosis, but with a higher rate of major bleeding events. In a subset of diabetic patients the overall benefit of this drug was more significant, with a better efficacy and yet with a lower rate of major bleeding.⁴¹ This finding is in accordance with the relatively high rate of platelet drug “resistance” observed in diabetic patients, suggesting a better effect of the more potent drug in this group as compared with non-diabetic patients with ACS. Several other new P2Y12 inhibitors are currently at different stages of clinical development, including those which reversibly inhibit the P2Y12 receptor, either by oral (Ticagrelor) or intravenous (Cangrelor) administration.^42,43 The observation of better efficacy achieved by applying a more potent P2Y12 inhibitor led to the initiation of numerous clinical trials aiming to compare clinical outcome in ACS patients under standard combination therapy of aspirin and clopidogrel, with those under adjusted higher dose of clopidogrel based on POC functional testing.³⁷ At the time of the manuscript preparation there are no official reports from these studies, some of which are expected to conclude soon. In view of the available data it seems likely that dose adjustment as well as modification of drug combinations based on functional testing may be adopted at least for several subsets of patients in the near future.

Blockade of the higher affinity receptor PAR 1 is a new target for antiplatelet drugs. SCH530348 is a highly selective PAR-1 inhibitor that does not interfere with thrombin-mediated cleavage of fibrinogen. It has successfully moved into phase III clinical trials after showing a good safety profile, even when added to the combination of clopidogrel and aspirin.⁴⁴ 

Thromboxane receptor inhibitors have certain pharmacological advantages over aspirin: in addition to blocking the effect of TxA₂ on platelets, they also inhibit other thromboxane receptor ligands such as endoperoxides, prostanoids and isoprostanes. They antagonize the effects of TxA₂ on thromboxane receptors present on other cells such as monocytes and vascular cells, and preserve the beneficial COX-1 endothelial production of prostacyclin, leading to inhibition of platelet aggregation and vasodilation.⁴⁵ Multiple thromboxane receptor antagonists have been developed; however, only a few have progressed beyond phase II trials because of safety concerns.⁴³ 

As opposed to the clear benefit of intravenous administration, oral GPIIb/IIIa agents given for long-term secondary prevention in ACS and PCI failed to provide protection from recurrent ischemic events and some were paradoxically associated with an increase in adverse events and mortality.⁴⁶ The reason behind this paradoxical effect might be that binding of ligand mimetic antagonists to the receptor causes a conformational change of the receptor, which may result in outside-in signaling, platelet activation and possibly paradoxical thrombosis.⁴⁷ In addition, the conformational change might expose new platelet epitopes, resulting in antibody formation and thrombocytopenia in some patients. A novel low-molecular-weight compound with unique features was recently developed.⁴⁸ 

The compound targets the αIIb unit but not β3; when exposed to purified αIIbβ3, it selectively inhibits the αIIbβ3-receptor, without receptor priming and increased fibrinogen binding. Derivatives of this compound with possible higher affinity are now under development. Such a compound could potentially allow chronic application of GPIIbIIIa inhibitors.

Cilostazol, a phosphodiesterase III inhibitor, is approved by the US Food and Drug Administration for intermittent claudication. In addition to its antiplatelet effects, it was shown to reduce smooth muscle proliferation and intimal hyperplasia after endothelial injury. A recent meta-analysis (only 5428 patients) concluded that the drug is safe and effective in reducing the risk of restenosis and repeated revascularization after PCI. As the authors state, the currently available data should still be cautiously looked upon due to the potential bias effects of small studies.⁴⁹ 

Several stage IV studies are currently running in patients after PCI, testing the effect of this drug for the treatment and prevention of cerebral infarction, and a few studies are dedicated to diabetic patients (www.clinicaltrials.gov).

Learning more about platelet signaling pathways has opened new horizons for antiplatelet treatment. Research on G-protein–coupled receptors and their intracellular pathways has recently been summarized.⁴³ Interactions of the αIIbβ3 cytoplasmic domains with specific regulatory proteins during αIIbβ3 signaling may also provide new targets for antiplatelet drug development. In knockout mice, deletion of 3 C-terminal β3 residues (arginine-glycine-threonine [RGT]) disrupted c-Src–mediated αIIbβ3 signaling, while retaining β3 residues necessary for talin-dependent fibrinogen binding. Unlike control mice, β3 (Δ 760-762) mice were protected from carotid artery thrombosis after vessel injury with FeCl₃. However, disruption of integrin signaling at the level of the β3 cytoplasmic domain might also affect the function of αVβ3 in endothelial cells, osteoclasts, and other cells; this should be further tested in animal models in the future.⁵⁰ Again, such a development could be translated in the future to a new class of antiplatelet therapy. β-nitrostyrenes may represent a new class of tyrosine kinase inhibitors with potent antiplatelet activity. The first compound that was discovered is 3, 4-methylenedioxy-β-nitrostyrene (MNS), a tyrosine kinase inhibitor of Src and Syk that prevents protein tyrosine phosphorylation and cytoskeletal association of GPIIb/IIIa and talin. It was shown to potently inhibit GPIIb/IIIa activation and platelet aggregation caused by various agonists. New derivatives from this family are now under development.⁵¹ 

A better understanding of the individual patient response may pave the way to improve efficacy and safety of antiplatelet therapy. This goal may be achieved by either dose adjustment of the drug based on functional testing, by changing drug combinations or by the introduction of more potent and safer drugs.