Emerging Therapies for Acute Coronary Syndromes

In the majority of cases acute coronary syndromes (ACS) are caused by activation and aggregation of platelets and subsequent thrombus formation leading to a decrease in coronary artery blood flow. Recent focus on the treatment of ACS has centered on reducing the response of platelets to vascular injury as well as inhibiting fibrin deposition. Novel therapies include more effective P2Y12 receptor blockers thereby reducing inter-individual variability, targeting the platelet thrombin receptor (protease activated receptor 1) as well as directly inhibiting factor Xa or thrombin activity. In this review we discuss the clinical data evaluating the effectiveness of these various new ACS treatment options.


INTRODUCTION
Over the past two decades improvements in detection and management have reduced the incidence, the morbidity and the mortality of acute coronary syndromes (ACS).Yet there are more than 1 million events yearly in the United States alone and ACS remains a leading cause of death in patients over 65 years of age (Yeh et al., 2010). The term ACS denotes unstable angina, non-ST-elevation myocardial infarction and ST-elevation myocardial infarction and ischemic sudden death with the exact diagnosis dependent on the extent and severity of myocardial damage. Necropsy studies have shown that 65% of sudden deaths resulting from ACS occur from thin-cap fibroatheromas; lesions characterized as possessing a thin fibrous cap (<65 μm) overlying a necrotic core. The remaining events are precipitated by superficial erosion of the fibrous cap (Virmani et al., 2000). These processes lead to platelet activation and aggregation, the formation of fibrin rich thrombi, and the partial or total occlusion of the coronary artery resulting in an ACS. With plaque rupture or erosion there is release of tissue factor and exposure of subendothelial collagen and von Willebrand factor (vWF). Circulating platelets adhere to the exposed collagen and vWF and undergo cytoskeletal rearrangement, generation of cyclooxygenase I (COX-I) dependent thromboxane A 2 (TXA 2 ), and release the contents of stored granules including adenosine diphosphate (ADP). Interaction of TXA 2 with the TXA 2 receptor and ADP with its receptor (P2Y12) initiates positive feedback activating additional platelets in a regional manner and promoting the expression Abbreviations: ACS, acute coronary syndrome; AT-III, anti-thrombin-III; CABG, coronary artery bypass grafting; COX-I, cyclooxygenase I; GpIIb-IIIa, glycoprotein IIb-IIIa; MI, myocardial infarction; NNT, number needed to treat; NS, nonsignificant; NSTEMI, non-ST-elevation myocardial infarction; PCI, percutaneous coronary intervention; STEMI, ST-elevation myocardial infarction; TVR, target vessel revascularization; TXA 2 , thromboxane A 2 ; UA, unstable angina; vWF, von Willebrand factor. of the GpIIb-IIIa receptors. Coincident activation of factor X via tissue factor results in thrombin generation, which elicits additional platelet degranulation and TXA 2 release (Chackalamannil and Xia, 2006) and catalyzing the formation of fibrin, which serves as the cross-link between GpIIb-IIIa receptors on neighboring platelets (Figure 1).
Contemporary medical therapies are designed to promote plaque stability, reduce platelet activation and aggregation, prevent thrombus formation, and minimize myocardial oxygen demand. New pharmacotherapies for ACS reflect efforts to improve efficacy and minimize complications of anti-platelet and anti-thrombotic therapies by increasing target specificity and reducing interindividual variation in therapeutic response. In this review we will describe recent advances in pharmacological therapy for ACS with particular attention to platelet aggregation and thrombus formation. Agents targeting glycoprotein IIb/IIIa (Gp IIb/IIIa) receptors will not be discussed.

ASPIRIN AND P2Y12 INHIBITORS
Aspirin irreversibly blocks COX-I, inhibiting generation of TXA 2 and consequent platelet activation. In an early study assessing the effect of aspirin in ACS, 1266 patients with unstable angina were randomized to 324 mg aspirin or placebo. Use of aspirin resulted in a 51% reduction of either death or MI at 12 weeks (p = 0.0002); without an increase in bleeding with sustained benefit at 1 year follow up (−43%, p = 0.008; Lewis et al., 1983). Subsequent smaller studies corroborated this benefit, and a meta-analysis of 15 randomized trials including 135,000 patients identified a reduction in the combined incidence of MI, stroke, or vascular death in ACS patients treated with aspirin (−30 ± 4%, p < 0.0001). Subsequently, virtually every trial assessing ACS pharmacotherapy has been performed on the background of aspirin therapy (Lewis et al., 1983;ISIS Collaborative Group, 1988;Antiplatelet Trialists' Collaboration, 1994;Antithrombotic Trialists' Collaboration, 2002). www.frontiersin.org The term "aspirin resistance" initially denoted patients who suffered a vascular event while on aspirin therapy, yet specifically refers to the inability of aspirin to sufficiently suppress TXA 2 production or platelet aggregation in vitro (Hennekens et al., 2004). The determination of aspirin resistance is complicated by the variable prevalence (6-26%) among studies depending on the laboratory assay employed (Gum et al., 2001;Gasparyan et al., 2008). Small prospective observational studies have identified increases in the risk of MI (OR 2.0, CI 1.2-3.4, p = 0.006), cardiovascular death (OR 3.5, CI 1.7-7.4, p < 0.001), and composite death, MI or CVA (OR 3.12, CI 1.1-8.9, p = 0.03) in patients with high platelet reactivity (HPR) on aspirin therapy (Eikelboom et al., 2002;Gum et al., 2003;Gasparyan et al., 2008). As a result, the concept of dual anti-platelet therapy (DAPT) emerged in order to reduce the rate of vascular events among those with HPR on aspirin therapy, and specifically to reduce the rate of reinfarction during and after percutaneous coronary intervention (PCI).
Clopidogrel is a thienopyridine pro-drug (Table 1; Figure 2) that requires cytochrome 2C19 (CYP2C19) biotransformation to the active metabolite, an irreversible P2Y12 receptor inhibitor. In a large prospective secondary prevention trial (n = 19,185), clopidogrel alone compared to aspirin did not significantly reduce recurrent vascular events (stroke, MI, or vascular death) among those with a history of myocardial infarction (−3.7%, p = 0.66), and was associated with similar rates of bleeding (9.27 vs. 9.28%; CAPRIE Steering Committee, 1996). However, when administered together, clopidogrel and aspirin provide more complete and uniform suppression of platelet activation via dual pathways for the lifespan of the platelet (average of 5-7 days; Moshfegh et al., 2000). The CURE trial randomized 12,562 patients with UA or NSTEMI to aspirin or DAPT with aspirin and clopidogrel. Patients randomized to DAPT had a reduced incidence of composite cardiovascular death, non-fatal MI, or stroke which was evident at 24 h (HR 0.66, CI 0.51-0.86, p < 0.05) and persisted for at least 12 months (HR 0.80, CI 0.72-0.90, p < 0.001). The event rate was driven primarily by a reduction in non-fatal MI. Not unexpectedly, the combination of clopidogrel and aspirin was associated with increased major bleeding rates (HR 1.38, CI 1.12-1.67, p = 0.001), although there was no difference in life threatening bleeding. Among the subgroup managed with a planned invasive strategy, the addition of clopidogrel was also associated with a significant reduction in composite cardiovascular death, non-fatal MI, or urgent target vessel revascularization at 30 days (HR 0.70, CI 0.50-0.97, p = 0.03), with no difference in major bleeding (HR 1.13, CI 0.61-2.1, p = 0.69; Mehta et al., 2001;Yusuf et al., 2001;Wright et al., 2011).
Despite these population level benefits, there is considerable inter-individual variability in response to clopidogrel that at least partially result from heterogeneity in CYP2C19 . The most common CYP2C19 variant is CYP2C19(2,  which has an allelic frequency between 25 and 55% depending on the ethnic population. Individuals with at least one copy of CYP2C19(2 have reduced plasma concentrations of the active clopidogrel metabolite and have reduced in vitro platelet responsiveness to clopidogrel (Kim et al., 2008;Kubica et al., 2011). Moreover, the CYP2C19(2 variant has been associated with significant increases in the risk of vascular events in a number of prospective studies and sub-studies of large ACS trials (reviewed elsewhere; Angiolillo et al., 2007;Kubica et al., 2011). The concept of "tailored" anti-platelet therapy has emerged to describe an approach of providing stronger platelet inhibition to those patients with a lower risk of bleeding, in the early phases of ACS when ischemic complications are the highest, or in patients with residual HRP on DAPT (Wiviott et al., 2007;Antman et al., 2008). The latter have been identified as a high risk subset, with as much as a 6.7-fold increase in the 30-day risk of composite death, myocardial infarction, or revascularization in those undergoing PCI (Hochholzer et al., 2006). In patients with HPR, clopidogrel dose escalation can incrementally reduce platelet activity and decrease the incidence of HPR from 37 to 14% (p = 0.002; Gladding et al., 2008). However, whether HPR should dictate subsequent therapy is unclear. The GRAVITAS trial randomized patients that had undergone PCI with subsequent identification of HPR to placebo or an additional loading dose of clopidogrel (600 mg) and increased maintenance therapy (150 mg daily). There was no difference in the composite MI, cardiovascular death, or stent thrombosis rate at 6 months (HR 1.01, CI 0.58-1.76), despite a dose-associated reduction in HPR in those randomized to higher-dose clopidogrel (38 vs. 60%, p < 0.001; Price et al., 2011).
In order to overcome the impact of CYP2C19 heterogeneity on platelet responsiveness novel ADP receptor antagonists have been developed. Prasugrel is a thienopyridine structurally similar to clopidogrel (Table 1; Figure 2) but less dependent on CYP2C19 biotransformation, and provides faster and more pronounced platelet inhibition (Table 1; Mega et al., 2009b). In TRITON-TIMI 38, patients (n = 13,608) with ACS and planned PCI were randomized to prasugrel (60 mg loading dose followed by 10 mg daily) or clopidogrel (300 mg loading dose followed by 75 mg daily) for a median of 14.5 months. Prasugrel significantly reduced the incidence of non-fatal MI, (HR 0.76, CI 0.67-0.85, p < 0.001) driving a significant reduction in the composite cardiovascular outcome that also included cardiovascular death and stroke (HR 0.81, CI 0.73-0.90, p < 0.001). This benefit was associated with a significant increase in the risk of bleeding, including that classified as life threatening (HR 1.5, CI 1.1-2.1, p = 0.01) and fatal (HR 4.2, CI 1.6-11.1, p = 0.002; Wiviott et al., 2007).
TRIGGER-PCI, designed to evaluate the efficacy of prasugrel in patients undergoing PCI with HPR on clopidogrel therapy, was stopped after a preliminary analysis revealed low event rates and an unlikely benefit of prasugrel. The ongoing TRILOGY-ACS trial is evaluating prasugrel in patients with ACS undergoing medical management with HPR on clopidogrel therapy (Chin et al., 2010).
Two novel anti-platelet agents have recently entered clinical trials for ACS: atopaxar and vorapaxar. Both are synthetic antagonists of the platelet thrombin receptor (protease activated receptor 1), and have the potential advantage of impeding platelet aggregation during rapid thrombin formation such as ACS (Morrow et al., 2009). In phase II trials with atopaxar, Japanese patients with ACS (n = 241) were randomized to 50-200 mg daily or placebo in addition to standard therapy (>96% aspirin, >89% clopidogrel) for 12 weeks. The bleeding rate was similar among all doses of atopaxar and placebo (5.0 vs. 6.6%, NS). However, there was an increased frequency of liver enzyme elevation among patients randomized to the 100 (6.2%) and 200 mg (14.8%) doses (Goto et al., 2010b). In phase II trial of vorapaxar, patients (n = 1030) with stable CAD undergoing non-urgent PCI receiving vorapaxar (10-40 mg load, with 0.5-2.5 mg daily) for 60 days had bleeding rates that were comparable to placebo (41% grouped vorapaxar vs. 35% placebo, NS). Moreover, those randomized to vorapaxar had non-significant reductions composite death or myocardial infarction (5% grouped vorapaxar vs. 7% placebo, NS; Becker et al., 2009). Phase III trials including more than 40,000 patients with ACS (TRACER) or a history of vascular disease (TrA2P-TIMI 50) are underway (Morrow et al., 2009;Van De Werf, 2010).

ANTI-THROMBOTIC AGENTS HEPARINS
The heparins complex with anti-thrombin-III (AT-III) and accelerate AT-III mediated inhibition of factor Xa and thrombin (Figure 1). Unfractionated heparin (UFH) represents the cornerstone of anti-thrombotic therapy in ACS, but has a number of limitations. UFH poorly inhibits clot-associated thrombin activity resulting in persistent proaggregant and procoagulant thombin activity at the site of arterial thrombosis. Additional concerns with UFH include heparin-induced thrombocytopenia and thrombosis (HITT), the necessity of frequent monitoring, and platelet-activating effects at therapeutic doses (Xiao and Theroux, 1998).
Low molecular weight heparin (LMWH) provides greater factor Xa inhibition, a reduced incidence of HITT, and more predictable bioavailability and elimination characteristics than UFH. A number of trials comparing enoxaparin to UFH demonstrated comparable or superior efficacy with regard to ischemic ACS endpoints yet small increases in bleeding with enoxaparin, particularly when invasive strategies were employed (Antman et al., 1999Goodman et al., 2000;Wallentin et al., 2003;Ferguson et al., 2004). A meta-analysis encompassing 12 randomized trials (n = 49,088) comparing enoxaparin to UFH in ACS concluded that there were significant reductions in the incidence of MI (HR 0.75, CI 0.65-0.86, p < 0.001) at 30 days, with an increase in major bleeding (HR 1.25, CI 1.04-1.50, p = 0.02). The net clinical benefit endpoint (death, MI, or major bleeding), approached significance at 30 days (HR 0.90, CI 0.76-1.00, p = 0.051; Murphy et al., 2007). Other LMWHs have also been evaluated in ACS but are in less frequent clinical use due to inferior clinical results (tinzaparin, dalteparin) or increased bleeding (nadroparin; Klein et al., 1997;Michalis et al., 2003;Katsouras et al., 2005; Table 2). As LMWHs are cleared renally and given that monitoring tests are less reliably available, UFH is preferable in those with advanced chronic kidney disease.

DIRECT FACTOR XA INHIBITORS
Direct factor Xa inhibitors block the coagulation cascade at an earlier step and are able to inhibit free and clot-associated factor Xa. Currently there is one direct factor Xa inhibitor FDA approved for use in ACS (fondaparinux) with three others in preclinical trials (apixaban, rivaroxaban, otamixaban). Fondaparinux is a synthetic pentasaccharide that mimics the AT-III binding portion of heparin and enhancing the anti-factor Xa activity >300-fold (Table 3; Figure 3). It is administered parenterally, eliminated   renally, with little inter-individual variability and no need for routine monitoring. The utility of fondaparinux in ACS was first assessed in the dose finding PENTALYSE trial. STEMI patients (n = 333) undergoing thrombolysis were randomized to 48-72 h of UFH or one of five fondaparinux doses for 5-7 days. Patients receiving fondaparinux had lower 30 day revascularization rates (39 vs. 51%, p = 0.054) with a similar major bleeding risk (7.1 vs. 7.1%; Coussement et al., 2001). In the PENTUA trial patients with UA or NSTEMI were randomized to one of four doses of fondaparinux or enoxaparin for a period for 3-7 days after identification of ACS. Fondaparinux at the lowest dose (2.5 mg daily) provided a non-significant reduction in composite death, MI, or recurrent ischemia at 9 days (27.9 vs. 35.7% in the enoxaparin group, NS) with no major bleeding and comparable rates of minor bleeding (3.9 vs. 4.8%, NS; Simoons et al., 2004).
In the largest ACS trial to date (OASIS-5), over 20,000 patients with UA or NSTEMI were randomized to fondaparinux (2.5 mg daily) or enoxaparin (1 mg/kg twice daily) for a mean of 6 days. There was no difference in the incidence of composite death, nonfatal MI, or refractory ischemia (HR 1.01, CI 0.9-1.13) between fondaparinux and enoxaparin. However, major bleeding was www.frontiersin.org significantly decreased with fondaparinux at 9 days (HR 0.52, CI 0.44-0.61, p < 0.001) and all cause mortality was lower at 30 (HR 0.83, CI 0.71-0.91, p = 0.02) and 180 days (HR 0.89, CI 0.80-1.0, p = 0.05; Yusuf et al., 2006a). OASIS-6 was designed to determine whether fondaparinux was superior to UFH in STEMI patients who underwent mechanical revascularization or pharmacological thrombolysis (Yusuf et al., 2006b). Patients were randomized to fondaparinux (2.5 mg daily for 8 days or until discharge) or UFH (for 24-48 h); those in which UFH was deemed unnecessary (primarily following streptokinase administration) were randomized to fondaparinux or placebo. Fondaparinux was associated with a significant reduction in the incidence of the death or reinfarction at 30 days (HR 0.86, CI 0.77-0.96, p = 0.008); an effect that persisted to 6 months. There was no difference in bleeding rates.
The reductions in the composite cardiovascular outcomes with fondaparinux in the OASIS trials were restricted to those who underwent conservative management or thrombolysis, while the benefit of fondaparinux was not apparent in patients who underwent primary PCI. Furthermore, fondaparinux was associated with increased catheter related thrombi, prompting the subsequent use of procedural intravenous fondaparinux. In the event patients managed under a conservative strategy proceed to PCI on fondaparinux, current guidelines recommend a transition to an agent with anti-thrombin activity.
Otamixaban is a synthetic parenteral factor Xa inhibitor with a rapid onset of action and biliary clearance (Table 3; Figure 3). Phase II trials in patients undergoing planned PCI and those with high risk ACS are promising. Compared to UFH with eptifibatide, otamixaban has been associated with reductions in the rate of death or myocardial infarction at 7 days (HR 0.52, CI 0.28-0.98 with 0.105 mg/kg/h) with comparable rates of bleeding (3.1 vs. 2.7%; Cohen et al., 2007;Sabatine et al., 2009). The ongoing TAO trial is powered for ischemic endpoints and is evaluating otamixaban vs. UFH and eptifibatide in patients with ACS undergoing a planned invasive strategy.
Two orally administered synthetic factor Xa inhibitors are presently in phase III trials: rivaroxaban and apixaban (Table 3; Figure 3; Rothberg et al., 2005). Rivaroxaban is an orally administered factor Xa inhibitor, with high bioavailability and a rapid onset of action. ATLAS TIMI 46 was a phase II trial in which patients admitted with ACS and subsequently stabilized were randomized to placebo or escalating doses of rivaroxaban. Compared to placebo, rivaroxaban was associated with a non-significant decrease in the primary efficacy endpoint (death, MI, stroke, recurrent ischemia requiring revascularization) at the lowest dose (5 mg twice daily; HR 0.60, CI 0.29-1.25) with dose-dependent increases in bleeding (HR 1.71, CI 0.76-3.85 with 5 mg twice daily; Mega et al., 2009a). A phase III trial of 2.5 and 5 mg twice daily rivaroxaban is presently underway (ATLAS 2 TIMI 51).
Phase II trials with apixaban have yielded similar results . The phase III APRAISE-2 trial, patients with ACS (n = 7832) were randomized to placebo or 5 mg twice daily of apixaban in addition to standard DAPT. At a mean follow up of 241 days, apixaban therapy was not associated with a reduction in composite cardiovascular death, MI, or stroke (HR 0.97, CI 0.8-1.1, p = 0.51) while resulting in increased major bleeding (HR 2.59, CI 1.5-4.5, p = 0.001; Alexander et al., 2011).

DIRECT THROMBIN INHIBITORS
Unlike the heparins, direct thrombin inhibitors are independent of AT-III, reduce the formation of fibrin monomers and abrogate the direct platelet-activating effects of thrombin on platelets. Direct thrombin inhibitors are also active on clot-bound thrombin, providing a regional specificity that heparins lack. Bivalirudin is approved for ACS, while dabigatran is currently in clinical trials.
Finally, the HORIZONS-AMI trial was designed to specifically address whether or not bivalirudin offered a superior risk-benefit profile in patients with STEMI undergoing planned PCI. Patients with STEMI (n = 3602) within the preceding 12 h were randomized to heparin and GpIIb-IIIa or bivalirudin monotherapy (0.75 mg/kg bolus, then 1.75 mg/kg/h), with antithrombin therapies generally discontinued at PCI completion. Bivalirudin was associated with a reduced incidence of the composite outcome (death, reinfarction, revascularization, stroke, or major bleeding) at 30 days, driven primarily by decreased rates of major bleeding (HR 0.60, CI 0.46-0.77, p < 0.001), and death. All cause mortality at 30 days was significantly lower in the bivalirudin group (HR 0.66, CI 0.44-1.0, p = 0.047), an effect maintained at 3 year follow up. The bleeding and survival benefits of bivalirudin were evident in those that had received heparin therapy prior to randomization, and at all stages of renal disease -subgroups in where LMWH have proven less safe (Ferguson et al., 2004;Cohen et al., 2006), or are unadvised due to primary renal elimination and concern for bleeding risk (Robson, 2000;Sun et al., 2007). Whether or not early initiation of bivalirudin provides incremental benefit is the focus of the ongoing EUROMAX trial, in which patients with STEMI and

GRAVITAS
Patients that underwent DES implantation with HPR on clopidogrel therapy were randomized to standard (75 mg daily) or high dose clopidogrel (150 mg daily). Primary endpoint was CV death, NFMI, definite, or probable stent thrombosis High dose clopidogrel did not reduce composite primary outcome (Price et al., 2011) PRASUGREL TRITON-TIMI 38 sub-studies Prasugrel conferred a reduction in primary endpoint in patients not undergoing stent implantation (HR 0.82, CI 0.53-1.25, p = 0.27), with an increase in non-CABG related major bleeding (0 vs. 5 events; Pride et al., 2009) Prasugrel reduced incidence of primary endpoint in patients in patients that did (HR 0.76, CI 0.64-0.90) and did not (HR 0.78, CI 0.63-0.97) receive GpIIb-IIIa inhibitor therapy (O'donoghue et al., 2009) Prasugrel resulted in greater platelet inhibition than clopidogrel in patients admitted with ACS both at 1-2 h and at 30 days (Michelson et al., 2009) Prasugrel reduced incidence of primary endpoint at 30 days (HR 0.68, CI 0.54-0.87) and 15 months (HR 0.79, CI 0.65-0.97) in patients with STEMI; increased major post-CABG bleeding (HR 6.53, CI 1.78-23.94;Montalescot et al., 2009) SWAP trial Phase II trial in patients after ACS on clopidogrel 75 daily and then randomized to continued clopidogrel, prasugrel 10 mg daily, or prasugrel 60 mg once then 10 mg daily Compared to maintenance clopidogrel, prasugrel therapy was associated with greater platelet inhibition as early as 2 h (60 mg load then 10 mg daily) and 6 days (10 mg daily) that persisted through 14 days (Angiolillo et al., 2010) TICAGRELOR PLATO sub-studies Ticagrelor reduced primary endpoint (HR 0.84,, no difference in major bleeding (HR 0.99, CI 0.89-1.10) in those undergoing invasive strategy  In STEMI and planned reperfusion, ticagrelor reduced the incidence of primary endpoint (HR 0.87, CI 0.75-1.01), and all cause mortality (HR 0.82, CI 0.67-1.0), with no difference in major bleeding (Steg et al., 2010a) In those with CKD ticagrelor reduced the incidence of primary endpoint (HR 0.77, CI 0.65-0.90), and all cause mortality (HR 0.72, CI 0.58-0.89;James et al., 2010)

ATLAS ACS TIMI 46
Patients with stabilized ACS (n = 3,491) randomized to rivaroxaban or placebo and treated for 6 months. Primary safety endpoint: clinically significant bleeding; Primary efficacy endpoint: death, NFMI, stroke, or recurrent ischemia At 5 mg twice daily, rivaroxaban was associated with increased bleeding (HR 1.71, CI 0.76-3.85), and a non-significant reduction in primary efficacy endpoint (HR 0.60, CI 0.29-1.25; Mega et al., 2009a) APIXABAN APPRAISE Patients with stabilized ACS (n = 1,715) randomized to one of four doses of apixaban or placebo. Primary outcome was major or clinically relevant bleeding.

SEPIA-ACS TIMI 42
Patients with NSTEMI (n = 3,241) were randomized to one of five otamixaban doses or UFH plus eptifibatide. Primary efficacy endpoint: death, MI, urgent revascularization, or bailout GpIIb-IIIa inhibitor use. Primary safety endpoint: CABG unrelated bleeding Otamixaban at an intermediate dose (0.105 mg/kg/h) was associated with reduced incidence of primary efficacy endpoint (R 0.61, CI 0.36-1.02), with a non-significant increase in bleeding (3.1 vs. 2.7%; Sabatine et al., 2009) planned PCI will be provided bivalirudin or UFH by emergency medical personnel.
Dabigatran is a synthetic, competitive, and reversible direct thrombin inhibitor with good oral bioavailability (Table 3; Figure 3). Recently approved for the prevention of stroke in patients with atrial fibrillation (Connolly et al., 2009), dabigatran is undergoing evaluation in ACS. In a phase II dose finding study, patients with ACS and at high risk for additional events were randomized to dabigatran twice daily or placebo a median of 7 days after the event and treated for 6 months. Preliminary findings demonstrated no difference in rate of composite cardiovascular death, non-fatal myocardial infarction, or stroke (3.8% on placebo, 4.6% 50 mg, 3.5% 150 mg) or the risk of bleeding (0.54% on placebo, 0.81% 50 mg, 1.73% 150 mg), although there was a Frontiers in Pharmacology | Cardiovascular and Smooth Muscle Pharmacology trend toward increased minor bleeding at every dose .

CONCLUSION
Platelet aggregation and thrombus formation comprise the intermediary events between plaque instability and MI, and pharmacological inhibition of these events has led to substantial reductions in ischemic complications and mortality. Newer agents are characterized by more predictable pharmacokinetics, reduced inter-individual response variability, and activity at the site of coronary occlusion. Recently approved agents (bivalirudin, fondaparinux, prasugrel, ticagrelor) have improved outcomes in clinical trials by more effectively balancing the anti-ischemic benefits and bleeding risks during ACS. Identifying the timing and setting of anti-platelet and anti-coagulant initiation, and the subgroups in which one agent provides a comparably favorable risk-benefit profile remains an area of active investigation ( Table 4).