The contemporary use of antiplatelet therapy in interventional cardiology

The role of platelets in atherothrombosis

The deleterious consequences of atherothrombosis are initiated when the fibrous cap that covers a mature plaque situated in the medium- to large-diameter vessels of the coronary, cerebral, and peripheral arterial trees either ruptures or the lesion surface erodes, thereby exposing the highly prothrombotic contents to the luminal microenvironment⁽²⁾. The process of atherothrombosis can be summarized in five phases (Fig. 24.1) although such a simplistic plan belies the highly complex nature of the interaction between atherogenesis, inflammation, oxidative stress, and plaque rupture. Indeed not all plaques rupture or erode and not all plaque rupture results in ischaemic sequelae. Intense research has elucidated a cycle of repeated rupture, thrombosis, and healing with continual renovation of the fibrous plaque surface akin to normal haemostasis⁽³⁾. The central catalyst that binds these processes together, however, is the adhesion, activation, and aggregation of platelets. Plaque rupture only leads to clinical events if it leads to the intiation and propagation of thrombus. This might depend on the degree of rupture and exposure of intraplaque contents (minor erosion versus complete de-capping); the size of the plaque, so that a small amount of thrombus from a large plaque will cause temporary or permanent occlusion if the remaining non-plaque lumen is narrow; or the thrombogenicity of the patient’s blood—a minor plaque and its disruption may cause complete vessel occlusion if there is high platelet reactivity; for example in those with hypercholesterolaemia, obesity, or in smokers.

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Fig. 24.1

Five phases of atherothrombosis⁽¹⁾. ADP, adenosine diphosphate; NO, nitric oxide; PGI₂, prostacyclin; TXA₂, thromboxane A₂; VWF, von Willebrand factor. Adapted with permission from Michelson AD (ed). Platelets, 2^nd edn. Elsevier Inc/Academic Press, © 2007.

Platelet adhesion

Upon being exposed to a site of vascular injury, platelets attach to the exposed subendothelium via a glycoprotein (GP) Ib/V/IX receptor complex along with several other collagen receptors expressed on their cell surface⁽⁴⁾. The primary catalyst for this interaction is collagen, stabilized by von Willebrand factor (vWF); along with fibronectin, laminin, and thrombospondin found in the extracellular matrix of the subendothelium⁽¹⁾. The adherent platelets undergo a conformational change from smooth disc to spiky sphere allowing them to roll across the subendothelial surface which, in turn, stimulates a further change in morphology to a hemisphere via an internal signalling network. This shape transition firmly anchors the platelet to the vessel wall by increasing the surface area for contact, allowing it to overcome the high shear forces of blood flowing through the lumen. Subsequent signalling cascades flatten the platelet further thus promoting irreversible adhesion to the injured vessel wall (Fig. 24.2).

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Fig. 24.2

Conformational platelet change from disc to spiky sphere to hemisphere, ultimately resulting in a flattened cell to allow maximum surface area for aggregation to other platelets. Adapted with permission from Michelson AD (ed). Platelets, 2^nd edn. Elsevier Inc/Academic Press, © 2007.

Platelet activation and the coagulation cascade

Following platelet adhesion to the extracellular matrix, the vessel wall repair process is activated through a combination of circulating paracrine and autocrine mediators, primarily thromboxane A₂ (TXA₂), adenosine diphosphate (ADP), thrombin, and adrenaline which propagate the haemostatic response by binding to adherent platelets and synergistically inducing their activation and recruiting further circulating platelets to the growing intraluminal plug. Within the platelet there is degranulation of storage vesicles which releases ADP and serotonin; further synthesis of TXA₂ and an increase in the cell-surface expression of the GP α_IIbβ₃ (IIb/IIIa) receptor—the purpose being to prevent major external haemorrhage through the development of a haemostatic plug. It is when this plug starts to form within a vessel, which is still intact (i.e. there is no leak to the outside) but injured through either spontaneous or iatrogenic plaque damage, that they then become problematic since they can potentially lead to life-threatening vessel occlusion.

It is not only platelet activation and the platelet plug that is important. The coagulation cascade is stimulated by exposure of negatively-charged elements within the phospholipid membrane on the platelet’s surface. This ultimately results in the generation of insoluble fibrin which acts to form stabilizing cross linkages between adjacent platelets. The combination of activated platelets and coagulation cofactors along with their associated enzymes act to generate increasing amounts of thrombin which itself is a potent platelet activator⁽¹⁾. Platelets also impede fibrinolysis by secretion of factors such as plasminogen activation inhibitor 1⁽⁵⁾.

Platelet aggregation

The final stage in the formation of a persistent platelet-rich intraluminal thrombus is mediated primarily through upregulation of the GP IIb/IIIa receptor on the platelet surface and its interaction with various adhesion proteins such as fibrinogen, vWF, fibronectin, and vitronectin. Fibrinogen acts to stabilize the forming thrombus by bridging adjacent GP IIb/IIIa integrins between platelets through its thrombin-mediated conversion to fibrin (Fig. 24.3). This process is further augmented by platelet-derived bioactive tissue factor which is the primary initiator of the coagulation cascade and stimulates the conversion of prothrombin to thrombin and fibrinogen to fibrin. In effect, several positive secondary feedback loops work in unison and serve to recruit more platelets to the growing thrombus which is then strengthened by fibrin cross-linking, thus promoting further propagation of the clot^(1,4).

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Fig. 24.3

Activation of platelet GPIIb/IIIa receptors followed by cross-linking to adjacent platelets mediated by fibrinogen in the final common pathway of platelet aggregation. Reproduced from Moliterno DJ, Topol EJ. Conjunctive use of platelet glycoprotein IIb/IIIa antagonists and thrombolytic therapy for acute myocardial infarction. Thromb Haemost 1997; 78:214–19 with permission.

The extent to which a thrombus grows largely depends on four factors:

• ◆ The degree of plaque rupture or erosion

• ◆ The degree of luminal stenosis

• ◆ The physicochemical properties of the surface exposed to the circulation, and

• ◆ Site of the lesion relative to blood flow⁽⁶⁾.

Platelets cluster to the greatest extent at the lesion apex and continued narrowing of the artery and slower blood flow stimulates a larger, more platelet-rich clot to form which may eventually occlude the vessel to such a degree that a clinical syndrome is manifest in the absence of appropriate APT or mechanical intervention.

The role of inflammation in the formation of intraluminal thrombus has often been underestimated. It is now widely recognized that inflammation and atherothrombosis are intimately linked when once it was thought the two were situated at opposite ends of a continuum. A detailed exploration of this topic, however, is beyond the scope of this review. Suffice to say activated platelets secrete several inflammatory mediators such as: CD40 ligand, P-selectin, cyclooxygenase (COX), interleukin 1β, platelet factor 4, and platelet-derived growth factor amongst others which serve to alter the chemotactic and adhesive properties of endothelial cells^(1,4). There is now extensive research being conducted on the use of inflammatory markers as predictors of CV risk in addition to being potential targets for antiplatelet drugs⁽²⁾. It is likely that detection of the severity and degree of inflammation may become part of the clinical assessment in patients with ACS and may even have a role as a predictor for outcome. Management of those with high levels of inflammatory response such that the natural history is altered is more problematic.

Current targets for APT include the agonists TXA₂ and ADP via inhibition of COX and the P2Y₁₂ receptor respectively along with direct blockade of the GP IIb/IIIa receptor which, as previously mentioned, promotes the final common platelet aggregatory pathway. Drugs that attenuate the pro-aggregatory action of thrombin through inhibition of G-protein-linked protease-activated receptors (PARs) present on the platelet surface are also under investigation (Fig. 24.4).

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Fig. 24.4

Targets for platelet inhibition. Adapted with permission from ‘Platelets, Second Edition’ Alan D Michelson MD (eds), Elsevier Inc/Academic Press, © 2007.

Ticlopidine: a first-generation thienopyridine

Ticlopidine, the first thienopyridine to be developed, given as a twice-daily dose of 250mg has been shown to be just as effective as aspirin and significantly superior to placebo for the secondary prevention of ischaemic events in patients at high atherothrombotic risk⁽¹¹⁾. There is, however, a clear rationale for combining aspirin with a thienopyridine since they work via complementary mechanisms and have the potential to provide synergistic inhibition of the platelet aggregatory pathway; hence the evolution of DAPT.

The combination of aspirin plus oral anticoagulation (OAC), in the form of a coumadin derivative, was the first antithrombotic strategy used to prevent vaso-occlusive complications after percutaneous transluminal coronary angioplasty (PTCA) with stenting. This was until four landmark trials were published in the late 1990s: ISAR⁽¹²⁾, STARS⁽¹³⁾, FANTASTIC⁽¹⁴⁾, and MATTIS⁽¹⁵⁾. Together these studies clearly demonstrated the superiority of DAPT over OAC and aspirin in the prevention of ST and other MACE following PCI, and further significantly reduced the risk of bleeding complications. All four trials used a combination of ticlopidine and aspirin and compared them to OAC plus aspirin. Although patients were randomized to either treatment arm, each study had an open-label design. Different patient populations were studied, with variation in disease severity. Differences in peri-procedural anticoagulation and definitions of safety endpoints were also potential confounders. Despite this, all four studies came to the same conclusion: DAPT was superior to OAC plus aspirin in terms of reducing the incidence of adverse events and the rate of haemorrhagic/vascular complications following stent deployment. A subsequent meta-analysis of these trials confirmed the net clinical benefit of DAPT over OAC and aspirin⁽¹⁶⁾. This advantage was essentially driven by significant reductions in non-fatal MI and the need for repeat target vessel revascularization (TVR). Interestingly there was no significant difference in mortality between the two treatment regimens.

The association of ticlopidine with hypercholesterolaemia and, more seriously, adverse haematological effects such as neutropenia, thrombocytopenia, aplastic anaemia, and thrombotic thrombocytopenic purpura (TTP) along with its relative expense means that it has been largely superseded by clopidogrel. Its use is now reserved primarily for those who are allergic to clopidogrel.

Clopidogrel: a second-generation thienopyridine

Clopidogrel is now well established as the current thienopyridine of choice, used in combination with low-dose aspirin, as DAPT to prevent the deleterious effects of ST in the coronary stenting era and for the secondary prevention of ischaemic vascular sequelae in high-risk individuals following an atherothrombotic event such as NSTEMI. It is a pro-drug which requires two cytochrome P450-dependent oxidative steps during its metabolism by the liver to produce its active moiety. Like ticlopidine it irreversibly binds to the P2Y₁₂ receptor and therefore inhibits ADP-induced platelet aggregation and the clustering of platelet-monocyte pairings therefore also interfering with the inflammatory response which, as previously indicated, is intimately linked with the atherothrombotic cascade that follows plaque rupture/erosion.

The superior tolerability of clopidogrel when compared to ticlopidine was first established by the CLASSICS trial⁽¹⁷⁾. A cohort of 1020 patients were randomized to a regimen of either: aspirin 325mg once a day plus ticlopidine 250mg twice daily; or aspirin plus clopidogrel 75mg once daily; or aspirin plus a 300-mg loading dose (LD) of clopidogrel followed by a maintenance dose (MD) of 75mg per day after coronary stenting. The study demonstrated an essentially equivalent efficacy of clopidogrel, with or without an LD, compared to ticlopidine in terms of MACE rates between the three groups (0.9% ticlopidine, 1.5% clopidogrel 75mg per day and 1.2% clopidogrel 300mg loading followed by 75mg daily) but importantly demonstrated a much improved safety profile for clopidogrel. Consequently, due to its adverse side-effect profile and slower onset of action, ticlopidine use fell dramatically and was replaced by the much safer, equally effective and, when a loading dose was given, significantly faster-acting clopidogrel.

Subsequent trials then proceeded to prove the benefits of clopidogrel use in the secondary prevention of atherothrombotic vascular disease (Table 24.1).

The CAPRIE trial compared clopidogrel to aspirin in 19 185 individuals who had recently suffered an MI, ischaemic stroke, or had established peripheral arterial disease. Clopidogrel use inferred a modest 0.51% absolute risk reduction in the composite endpoint of MI, ischaemic stroke, or vascular death but effectively the safety and tolerability of aspirin and clopidogrel were similar^(18). These results, therefore, did not effect a change in clinical practice but certainly revealed that clopidogrel was an appropriate alternative to aspirin, for instance, in those with aspirin allergy.

The landmark CURE study was the first to demonstrate the superiority of DAPT when compared to a single antiplatelet agent acting alone⁽¹⁹⁾. A total of 12 562 patients presenting within 24h of symptoms indicating UA/NSTEMI were randomized to receive clopidogrel (300mg LD followed by 75mg/day MD) or placebo in addition to aspirin (75–325mg/day MD) for 3–12 months. The primary endpoint; a composite of CV death, non-fatal MI, or stroke, occurred in 9.3% of individuals taking DAPT and 11.4% taking aspirin plus placebo, giving rise to a 2.1% absolute risk reduction overall. Of note the anti-ischaemic benefits of clopidogrel were apparent within the first 24h of drug administration, suggesting the LD was rapidly effective. They were also observed across a variety of subgroups and maintained throughout the 12-month treatment period. Conversely, however, DAPT was associated with a significant 1.0 and 2.7% increase in major and minor bleeding respectively, giving an early indication that with effective inhibition of the platelet aggregatory pathway comes the danger of excess bleeding, the need for transfusion, and vascular access complications. Surprisingly, however, a post hoc analysis of the CURE trial results revealed the observed increase in major bleeding events due to DAPT was primarily caused by increasing aspirin dose as opposed to the addition of clopidogrel (Fig. 24.6).

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Fig. 24.6

Post hoc analysis of the CURE Trial demonstrating an association between increasing aspirin dose, rather than clopidogrel, with the increase in major bleeding complications. Adapted with permission from: Bhatt DL. Aspirin resistance: more than just a laboratory curiosity. J Am Coll Cardiol 2004; 43:1127–9. Copyright © 2004 American College of Cardiology Foundation Published by Elsevier Inc.

The role of clopidogrel in PCI for UA/NSTEMI patients was established following a subgroup analysis of some 2658 patients in the CURE study who subsequently proceeded to in-hospital coronary stenting as a consequence of refractory ischaemia or MACE: the PCI-CURE study⁽²⁰⁾. Within the cohort 1313 patients were randomized to a 300-mg LD of clopidogrel followed by 75mg/day MD and 1345 randomized to placebo. PCI was conducted within a median 10-day pre-treatment period. Those patients receiving stents were then given open-label thienopyridine therapy (clopidogrel or ticlopidine) for 2–4 weeks after which time they were switched back to their pre-PCI treatment regimen. Again, the early beneficial effect of thienopyridine therapy was demonstrated in the clopidogrel arm in which significantly fewer individuals had an MI or refractory ischaemia pre-PCI when compared to placebo. At 30 days and a mean of 8 months post PCI the primary combined endpoint of CV death, MI, or urgent TVR was significantly lower in clopidogrel patients compared with the placebo arm. Since the vast majority of patients received open-label thienopyridine treatment for 4 weeks after stent deployment it follows that the period of clopidogrel treatment pre-PCI was pivotal in reducing MACE rates in the short- and long-term post procedure. The benefits were spread throughout all clinical and demographic subgroups and were maintained regardless of the timing of PCI. As with the CURE study there were more minor bleeding events in the clopidogrel arm but no significant increase in major bleeding.

The benefit of pre-PCI thienopyridine treatment was further substantiated by the CREDO study which randomized 2116 ACS patients to a 300-mg LD of clopidogrel or placebo 3–24h before planned PCI; all patients received aspirin⁽²¹⁾. Following PCI patients were administered a 28-day regimen of clopidogrel 75mg/day and were then switched back to the treatments arms to which they had been randomized pre-procedure. Similar to PCI-CURE there was a 26.9% relative risk reduction of death, MI, or stroke in those on clopidogrel after 1-year follow-up. However, there was no significant difference in the rate of death, MI, or TVR at 28 days. On closer post hoc inspection of the statistical data it appeared that the benefits in the short term were only gained in those pre-treated with clopidogrel ≥6h before PCI. Furthermore a significant (statistically just) improvement in short-term outcomes was only achieved if clopidogrel was administered at least 12h and ideally almost 24h before coronary stenting.

The role of clopidogrel in preventing adverse sequelae following STEMI was elucidated from the COMMIT/CCS-2 and CLARITY-TIMI 28 trials^(22,23). The COMMIT trial randomized 45 852 patients presenting within 24h of symptom onset with AMI (i.e. ST-segment elevation or new left bundle branch block) to clopidogrel 75mg/day during hospitalization (mean of 16 days) or placebo plus standard aspirin therapy with or without fibrinolysis. Those proceeding to primary PCI or at high risk of bleeding were excluded. There was a significant 9.0% relative risk reduction in the primary composite endpoint of death, reinfarction or stroke at hospital discharge; this was not accompanied by an increased risk of major bleeding or haemorrhagic stroke.

CLARITY-TIMI 28 studied the addition of clopidogrel (300-mg LD plus 75mg/day MD) to a standard regimen of aspirin at the time of fibrinolysis compared with placebo in 3491 individuals presenting within 12h of acute STEMI. Clopidogrel therapy resulted in a significant absolute reduction of 6.7% in the primary efficacy endpoint (a composite of occluded infarct-related artery at angiography – Thrombolysis In Myocardial Infarction [TIMI] grade 0/1, death or recurrent MI before angiography) and a 2.5% absolute reduction in the composite endpoint of CV death, recurrent MI or urgent TVR. This was, in some ways, an odd study since the primary endpoint was measured at 8 days and was based on angiographic data. Therefore the intervention undertaken as a result of the angiography would have influenced the clinical endpoints at 1 month, which were not impressive. Although benefits were achieved with no significant increase in major bleeding including intracranial haemorrhage, clopidogrel in the absence of PCI for STEMI has not really taken off. Thus despite these two trials, if a patient receives thrombolysis there appears to be little conviction amongst clinicians that post lytic clopidogrel is mandated. However, if patients go for PCI whether it be for failed thrombolysis (i.e. rescue PCI) or post successful lysis as a means of risk stratification, as is becoming standard practice, then clopidogrel should be used.

Like the CURE study, a subgroup analysis of those 1863 patients who proceeded to PCI after fibrinolysis in the CLARITY-TIMI 28 cohort was also performed: the PCI-CLARITY trial⁽²⁴⁾. Patients were randomized to receive a 300-mg LD followed by a 75mg/day MD of clopidogrel or placebo. The median time from drug initiation to PCI was 3 days. At 30-day follow-up patients pre-treated with clopidogrel (since most patients who received coronary stenting were given open-label thienopyridine) had a 46% reduction in the rate of CV death, recurrent MI, or stroke. The benefit of clopidogrel pre-treatment remained consistent regardless of the fibrinolytic agent used, whether a GP IIb/IIIa inhibitor was added or whether PCI was emergent or planned.

It is important to note that large-scale randomized controlled trials of clopidogrel versus placebo in the setting of primary PCI have not been conducted. It is accepted, however, that primary PCI is the treatment of choice if STEMI patients present within 90 minutes of symptom onset to an interventional centre. The combined evidence from PCI-CURE, CREDO, and PCI-CLARITY has been implemented to justify the use of DAPT before, during, or after primary PCI to positively impact on ischaemic events.

Resistance to dual antiplatelet therapy

Theoretically, inhibition of the two main amplification pathways of platelet aggregation should be superior to inhibition of either pathway alone. Despite appropriate DAPT, however, a significant minority of patients continue to suffer MACE following ACS and coronary stenting. Late ST, especially in the era of DES, can occur in 1–2% of individuals following PCI and is reported to progress at 0.3–0.6% each year. In real terms this can account for 30 000 MACE per annum worldwide. Death, MI, or stroke occur in 8.5%, and 21.4% require revascularization a year after the index PCI procedure⁽²¹⁾. This has lead to the postulation that those individuals suffering further events may represent a cohort of patients who have less than adequate response to aspirin and clopidogrel.

The term ‘resistance’ is controversial in this context since it has been used to encompass both the failure of an agent to prevent the clinical condition for which it is intended and also failure to achieve its full pharmacokinetic and/or pharmacodynamic effect. Since the pathophysiology of atherothrombosis is complex, comprising thrombosis, inflammation, innate vascular biology, and changing haemodynamics, no single type of agent can be expected to abolish ischaemic events completely. Furthermore, a patient may have the appropriate platelet response to a given therapy but have recurrent events mediated by non-platelet factors. For these reasons it would seem reasonable to categorize patients suffering recurrent events on therapy as having treatment failure, while limiting the term resistance to those for whom the antiplatelet drug does not achieve its pharmacological effect which is inhibition of platelet aggregation (IPA). Resistance in its broadest sense can be referred to as the continued occurrence of ischaemic events despite adequate duration of APT, dosing, and compliance alongside optimal stent deployment.

Aspirin resistance

The concept of aspirin resistance is made more problematic since there is a lack of definitive evidence to suggest that altering therapy in response to discovering this phenomenon actually improves clinical outcomes. Indeed, although aspirin strongly inhibits the production of TXA₂, it may still fail to inhibit platelet aggregation since there are a number of other potent agonists that will continue to activate platelets. Furthermore no standard has been set or widely accepted for determining biochemical aspirin resistance; as such an estimated incidence of between 5–45% has been postulated depending on the specificity of the assay and the definition of resistance used along with the population under study. At present, AA-stimulated platelet aggregation assessed by light transmittance aggregometry (LTA) or platelet mapping by thromboelastography are the most commonly used laboratory assays, but these putative tests may not be specific or sensitive enough to fully establish so called ‘aspirin resistance’.

The cause of aspirin resistance, if it truly exists, remains contentious and is most likely multi-factorial (Fig. 24.7). Ultimately, however, given the issues listed here it is perhaps more appropriate to refer to the occurrence of adverse ischaemic events despite aspirin therapy as ‘treatment failure’ rather than hyporesponsiveness or resistance. There is also no evidence to suggest that increasing the dose of aspirin will overcome treatment failure. Indeed doses of 75–150mg per day may be more effective at preventing MACE compared to doses up to 10 times as high⁽²⁵⁾. Additionally increasing doses of aspirin have consistently been shown to increase bleeding complications.

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Fig. 24.7

Potential causes of aspirin resistance. Adapted with permission from Bhatt DL. Aspirin resistance: more than just a laboratory curiosity. J Am Coll Cardiol 2004; 43:1127–9.

Clopidogrel resistance

In contrast to the phenomenon of aspirin resistance there is evidence to suggest that clopidogrel resistance is a definite pathopharmacotherapeutic entity that can lead to adverse clinical outcomes. There is wide interindividual variation in clopidogrel response. Currently, no laboratory definition of true clopidogrel resistance has been universally accepted. It is agreed that clopidogrel resistance occurs when the drug is unable to achieve adequate IPA. Since there are a number of different assays that can be used to assess this phenomenon it follows that there are a number of empiric cut-off values that have been adopted to suggest non-responsiveness. None of these tests have been fully standardized to measure clopidogrel responsiveness, leading to significant interlaboratory variation in results. Prevalence figures for non-responders to clopidogrel range from 4–30% 24h after drug administration depending on the technique used to measure platelet aggregation and the presence of factors contributing to greater baseline platelet reactivity.

Optical LTA is considered the historical gold standard for assessing platelet function. It uses spectrophotometric measurement of platelet aggregation in platelet-rich plasma in response to ADP as an agonist. The procedure carries with it a number of drawbacks: it is labour-intensive, requires expert personnel, has time-consuming centrifugation steps which may injure platelets, tests in an artificial milieu devoid of red and white blood cells, and can be subject to several interlaboratory differences.

Impedance aggregometry, on the other hand, uses electrical resistance between two electrodes immersed in whole blood to measure platelet aggregation. It offers improved sensitivity over LTA since it requires no centrifugation step and therefore no platelet ‘injury’ occurs; inclusion of giant platelets for functional assessment; and a shorter time to perform the test under more physiological conditions. The Multiplate^® analyzer, for instance, allows measurement of platelet function in whole blood by using impedance aggregometry and can be used for a variety of applications such as monitoring of APT and assessment of perioperative platelet function disorders in a near-patient environment (Fig. 24.8).

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Fig. 24.8

The Multiplate^® analyzer can be used to monitor response to antiplatelet therapy using whole blood impedance aggregometry.

Since clopidogrel is specific for P2Y₁₂, the P2Y₁ receptor can still be activated and contribute to platelet aggregation. This can be a confounding factor when using LTA or impedance aggregometry. The vasodilator-stimulated phosphoprotein phosphorylation (VASP-P) assay is more specific to the P2Y₁₂ pathway. Levels of VASP-P and dephosphorylation reflect P2Y₁₂ inhibition and activation respectively and can be measured quite easily by quantitative flow cytometry. This is fast-becoming the only standardized P2Y₁₂-specific assay available. Furthermore, point-of-care assays for platelet aggregation have a promising role in guiding future APT. The VerifyNow^® P2Y₁₂ system uses a similar principle to that employed by the VASP-P assay by using a combination of ADP and PGE₁ as agonists in order to specifically evaluate inhibition of the P2Y₁₂ pathway (Fig. 24.9). PGE₁ increases VASP-P by stimulation of adenylate cyclase. Binding of ADP to P2Y₁₂ leads to inhibition of adenylate cyclase thereby reducing PGE₁-induced VASP-P levels. If, however, P2Y₁₂ receptors are successfully blocked by clopidogrel, addition of ADP will have no theoretical effect on PGE₁-stimulated VASP-P levels. The VerifyNow^® P2Y₁₂ system has recently been approved by the Food and Drug Administration in the United States but remains primarily a research tool and is yet to be used widely in the clinical forum.

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Fig. 24.9

The Point-of-Care VerifyNow^® P2Y₁₂ Assay.

Several studies have established clopidogrel resistance as an emerging clinical entity and proposed a link with adverse clinical outcomes. What are common to all of them, however, are relatively small sample sizes and short follow-up periods. They have not been sufficiently powered to detect a causal association between clopidogrel resistance and MACE. One of the largest studies found a normal distribution of clopidogrel responsiveness amongst 544 individuals consisting of healthy volunteers, patients post coronary stenting, those with heart failure, and after stroke⁽²⁶⁾. The authors subsequently categorized hyporesponders, hyperresponders, and the remaining individuals as standard responders using the mean IPA achieved as a reference point. Another prospectively studied 60 patients who underwent primary PCI following presentation with STEMI to determine whether variability in response to clopidogrel affected clinical outcomes. Patients were stratified into four quartiles according to the percentage reduction of ADP-induced platelet aggregation. Forty per cent of patients in the first quartile, who were considered non-responders, sustained a MACE during 6-month follow-up compared to one patient in the second quartile and none in the third and fourth⁽²⁷⁾. The largest study thus far looked at platelet reactivity following clopidogrel administration in 804 patients who had received DES during PCI. They found that patients with over 70% post-clopidogrel platelet aggregation in vitro had a nearly fourfold increase in definite or probable ST as compared with clopidogrel responders⁽²⁸⁾. More recently Price and colleagues measured post-clopidogrel treatment platelet reactivity with the VerifyNow^® P2Y₁₂ assay in 380 patients undergoing PCI with sirolimus-eluting stents to determine whether there was an optimal cut-off value for platelet reactivity in predicting 6-month MACE rates. They found that those patients with high post-treatment platelet reactivity had significantly higher rates of CV deaths, ST and overall MACE⁽²⁹⁾.

Several mechanisms of clopidogrel resistance have been postulated and encompass genetic, cellular, and clinical factors either acting alone or in conjunction (Table 24.2). Clinical factors can range from poor drug compliance to suboptimal stent deployment and increased baseline platelet reactivity due to increased body mass index, diabetes mellitus, and insulin resistance. Again a common thread of small sample sizes, empirical cut-off points for responsiveness, lack of robust clinical outcome data, differing methods of assessing platelet aggregation, and contradictory findings exists. No one mechanism has been accepted as the true cause of clopidogrel resistance. Particular interest has, however, focussed recently on several functional polymorphisms found in genes encoding cytochrome P450 isoenzymes involved in the hepatic biotransformation of clopidogrel to its active form. Abnormal function variants of the CYP2C9 and CYP2C19 genotypes have been associated with a decreased pharmacodynamic response to clopidogrel resulting in less exposure to the active metabolite⁽³⁰⁾. The relationship between these genotypes and their corresponding pharmacodynamic effect must now be confirmed in larger cohorts and a link to clinical outcomes established.

Optimizing the dose of clopidogrel

The potential clinical validity of clopidogrel resistance has led to several studies designed to elucidate the optimal loading and maintenance doses for clopidogrel that could overcome hyporesponsiveness and yet remain safe in terms of bleeding rates. The ARMYDA-2 study indicated the benefit of pre-treatment with a 600-mg LD when compared with 300mg in reducing peri-procedural MI in patients with stable angina or NSTEMI undergoing planned PCI, without any increase in bleeding hazards⁽³¹⁾.

The ALBION and ISAR-CHOICE studies looked at increasing the LD further to 900mg^(32,33). A higher LD did display a greater degree of platelet inhibition; more rapid onset of action (approximately 2h although maximal IPA may not be achieved until 3–4h); and a lower percentage of non-responders compared to the standard 300-mg strategy but the differences observed between the 600mg and 900mg regimens were less remarkable and not significant. These studies confirm the dose-dependent inhibitory effects of clopidogrel but also show there is a threshold for the platelet inhibitory function that can be achieved which may be linked to saturation of intestinal absorption as opposed to a limit on hepatic metabolism.

To establish further the mechanistic aspects of clopidogrel resistance Bonello and colleagues undertook a prospective, randomized, multicentre study of 162 patients due to undergo coronary stenting. Clopidogrel resistance was defined as a VASP-P index of >50% after a 600-mg LD. Patients who were non-responsive to clopidogrel were randomized to a control group or to a VASP-guided group; the latter received additional boluses of clopidogrel to reduce the VASP-P index to below 50% as measured by the VerifyNow^® P2Y₁₂ assay. Although the study sample was small there was a significantly lower MACE rate in the VASP-guided group suggesting that adjusting the LD according to clopidogrel response was appropriate, safe (i.e. there was no difference in bleeding between the two groups) and could potentially improve clinical outcome⁽³⁴⁾.

The standard 75mg/day MD of clopidogrel requires 3–7 days to achieve maximal IPA. The ISAR-CHOICE-2 study revealed the advantage of a 150-mg clopidogrel MD over standard dosing in patients 1 month after low-risk PCI⁽³⁵⁾. Additionally the OPTIMUS study showed that a 150-mg MD of clopidogrel resulted in pronounced platelet inhibition of numerous platelet function measures compared to the standard 75mg daily in patients with diabetes mellitus; although a significant number of patients continued to display high post-treatment platelet reactivity⁽³⁶⁾. More definitive data on the optimal dosing of clopidogrel has come from the CURRENT-OASIS 7 trial, the results of which were first reported at the European Society of Cardiology (ESC) Annual Congress in September 2009. Over 25 000 patients presenting with ST and non-ST-elevation ACS intended for early (≤72h) revascularization were enrolled into a 2 × 2 factorial trial and randomized to either 600-mg LD followed by 150mg/day for 1 week then 75mg/day MD of clopidogrel or a 300-mg LD followed by 75mg/day MD⁽³⁷⁾. Individuals were also randomized to receive either high-dose (300–325mg) or low-dose (75–100mg) aspirin in an open-label manner.

Of the 17 232 patients who did receive early intervention the primary composite endpoint of CV death, MI, and stroke at 30 days occurred in 4.5% of those on the ‘standard’ clopidogrel regimen and 3.9% on the ‘augmented’ regimen (p = 0.036); the benefit primarily driven by a reduction in MI. Of the 7855 patients who did not proceed to PCI, through no significant CAD identified on angiography or those scheduled for CABG, there was no significant difference in the primary endpoint. Overall the rate of ST was significantly higher in the standard group compared to the augmented clopidogrel therapy cohort (1.2 vs. 0.7%; p = 0.001). There was no significant difference in the primary endpoint between a high- or low-dose aspirin strategy although, conversely, there was also no difference in bleeding indices between the two strategies either.

Data from CURRENT-OASIS 7 suggests that those patients scheduled to undergo PCI following an ACS should receive a 600-mg LD of clopidogrel which is already advocated in Europe and is current practice. A MD of 150mg for 1 week following PCI would be simple enough to institute thereafter and then patients could return to the 75-mg MD that we are all familiar with. Whether this practice will become commonplace in the ‘real world’ is yet to be seen. In the meantime current European guidelines on the use of DAPT in patients undergoing PCI are listed in Table 24.3 and indicate 600-mg preloading and 75mg/day maintenance doses.

Dual antiplatelet therapy for patients on long-term oral anticoagulation requiring PCI

By far the most common indication for OAC is atrial fibrillation (AF). It affects 5% of those aged over 65 years and almost 10% of people above the age of 80. This is thought to double over the next two generations. Since AF commonly coexists with vascular disease it follows that the incidence of patients in AF on warfarin therapy presenting with an ACS or requiring scheduled PCI is set to rise exponentially. To date, the decision to continue or withdraw OAC following PCI has largely been empiric and generally based on the ‘guesstimated’ risk by the physician of the thromboembolic complications balanced against the putative risk of bleeding. General consensus might suggest that there are very few instances in which OAC can be stopped safely and a complete switch to DAPT be made after coronary stenting: low thromboembolic-risk AF and low risk venous thromboembolism being the only likely scenarios.

There is marked heterogeneity in the use of antithrombotic regimens in warfarin-eligible patients post coronary stenting. A survey of 24 internationally-renowned interventional centres revealed that just over half used standardized protocols leaving the remainder to operator-directed decisions based on the balance between thromboembolic versus haemorrhagic risk⁽⁴⁰⁾. The study did reveal triple therapy (DAPT plus OAC) to be the most prescribed regimen in the majority (83%) of centres.

Current joint European and US guidelines for the management of AF recommend triple therapy for a ‘brief period’ following PCI. This should then be followed by a maintenance regimen of OAC and clopidogrel for 3–12 months, depending on the stent platform used, after which clopidogrel is stopped and warfarin therapy continued indefinitely. Owing to a lack of robust randomized controlled trial (RCT) data these recommendations can only be assigned a level of evidence C and grade IIB (i.e. benefits ≥ risks)⁽⁴¹⁾.

To date, the majority of published data on triple therapy post PCI has taken the form of small, observational, single-centre retrospective studies or registry analysis where safety endpoints have predominated over clinical efficacy measures. What is clearly needed, therefore, is a large multicentre RCT that is sufficiently powered for clinical outcomes to determine which antithrombotic strategy provides net clinical benefit for this expanding cohort of patients on chronic OAC undergoing PCI. The ‘Finding Appropriate anti-Coagulation strategies to improve Outcomes after coRonary Stenting: The FACTORS Trial, is being planned in the UK.

Prasugrel: a third-generation thienopyridine

With the emergence of clopidogrel resistance as a probable clinical entity and the increasingly interventional treatment of ACS including STEMI, it has become apparent that there is a need for more potent antiplatelet agents which display less interindividual variability, have a faster onset of action and a good tolerability profile. Prasugrel, like clopidogrel, is a thienopyridine pro-drug which causes irreversible inhibition of the P2Y₁₂ receptor; has time- and dose-dependent antiplatelet effects, and requires hepatic biotransformation to form the active metabolite from the parent molecule in vivo (see Fig. 24.5). Unlike clopidogrel it requires only one oxidative step to form its active moiety which is, therefore, generated much faster, more efficiently, and in much higher concentration, giving rise to approximately 10 times more antiplatelet potency and a more consistent IPA despite both metabolites displaying similar antiplatelet activity in vitro. Indeed, in patients with stable atherosclerosis a 60-mg LD of prasugrel has been shown to achieve a more rapid and greater degree of IPA within 30min than that achieved by a 600-mg LD of clopidogrel over 24h⁽⁴²⁾. In addition the metabolism of prasugrel is less likely to be affected by polymorphisms in CYP2C19 P450 iso-enzymes.

The JUMBO-TIMI 26 trial compared three different dose regimens of prasugrel against the standard dosing of clopidogrel in 904 patients due to undergo PCI. There was a comparable safety profile in terms of bleeding between all four groups and a trend, albeit not statistically significant, towards benefit in favour of prasugrel for the secondary composite endpoint of MACE at 30 days⁽⁴³⁾. The PRINCIPLE-TIMI 44 two-phase crossover trial compared prasugrel to high-dose clopidogrel in 201 patients undergoing planned PCI⁽⁴⁴⁾. Patients were initially randomized to a 60-mg LD of prasugrel or 600-mg LD of clopidogrel with a primary endpoint of ADP-induced platelet aggregation, as measured by optical LTA, the VASP-P index, and VerifyNow^® P2Y₁₂ assays, at 6h. In the second phase of the trial, post-PCI patients entered a 28-day crossover comparison in which they were randomized to 14 days of prasugrel at 10mg/day followed by 14 days of clopidogrel at 150mg/day or vice versa. In both of the loading and maintenance phases, prasugrel was shown to cause significantly greater IPA compared to a high-dose clopidogrel strategy.

These Phase II clinical trials lead to the Phase III TRITON-TIMI 38, a large randomized multicentre head-to-head study of prasugrel (60-mg LD followed by 10mg/day MD) versus clopidogrel (300-mg LD followed by 75mg/day MD) in 13608 patients presenting with the entire spectrum of moderate- to high-risk ACS due to undergo primary or delayed PCI⁽⁴⁵⁾. Initial results were promising: the primary efficacy endpoint of CV death, non-fatal MI, and stroke occurred in 12.1% of clopidogrel patients and 9.9% receiving prasugrel (p <0.001). A significant difference between the two strategies had already emerged by day 3, presumably secondary to prasugrel’s more potent and rapid onset of action and persisted throughout the entire 15-month follow-up period. There were also significant reductions in the secondary endpoints of ST (1.1% versus 2.4%; p <0.001) and urgent TVR (2.5% versus 3.7%; p <0.001) in favour of prasugrel. On more detailed analysis, however, benefits in the primary composite endpoint were driven predominantly by a reduced rate of non-fatal MI (7.3% versus 9.5%; p <0.001); there was indeed no significant difference in the rate of CV death, non-fatal stroke, or all-cause mortality. The other major issue with this trial was that it was US-based and patients were randomized once the angiogram had been done—in the UK and Europe as has been clearly outlined earlier, pre-loading with clopidogrel (600mg >24h) is thought, albeit on non-prospective comparative data, to be the standard of care. It is unclear whether prasugrel would have demonstrated the same advantages if it had been compared with patients pre-loaded with clopidogrel. However the reduction in opportunity for pre-loading in the time-dependant STEMI patient has prompted the National Institute of Health and Clinical Excellence (NICE) to support its use in this group of patients.

With greater platelet inhibition there is the inevitable hazard of more bleeding events (Table 24.4). Significantly more major bleeding (2.4% versus 1.8%; p = 0.03); life-threatening bleeding (1.4% versus 0.9%; p=0.01); and fatal bleeding (0.4% versus 0.1%; p = 0.002) was seen in the prasugrel arm of the trial. Post hoc analysis identified the following subgroups particularly at risk:

• ◆ Patients with known cerebrovascular disease

• ◆ Those aged ≥ 75 years, and

• ◆ Body weight <60kg.

In a pre-specified subgroup analysis the 3421 patients presenting with STEMI who received either primary or delayed PCI demonstrated a significant reduction in the primary efficacy endpoint (9.8% versus 12.3%; p = 0.02) without a significant increase in major bleeding at 15 months⁽⁴⁶⁾. Prasugrel also demonstrated greater efficacy in patients at higher risk of ST, for instance: those needing longer stents, bifurcation lesions, those with renal dysfunction, diabetes, and those presenting with STEMI; again with no significant increase in major bleeding⁽⁴⁷⁾. A further subanalysis looked at the diabetic cohort of patients who are known to have high platelet reactivity and a greater risk of being poorly responsive to clopidogrel therapy (see Table 24.2). Of the 2947 patients with diabetes the primary efficacy endpoint was reached in 12.2% of patients with prasugrel and 17.0% taking clopidogrel at 15 months (p <0.001). The occurrence of major bleeding, however, was not statistically significantly different between the groups⁽⁴⁸⁾. There was significantly less incidence of ST in the prasugrel group which has led NICE to provisionally recommend its use in those suffering ST. Individuals thought to be at high risk of suffering ST may be better served by giving them prasugrel as a primary agent as opposed to after the adverse event occurring first. The ultimate role of prasugrel remains undefined. NICE have produced its final Appraisal Consultation Document on prasugrel in August 2009 and will issue formal guidance in October 2009⁽⁴⁹⁾. Salient points that have arisen from the consultation so far are summarized in Table 24.5. In effect the NICE recommendations are for prasugrel to be used in those patients suffering STEMI, those who have evidence of previous ST and in diabetics.

Ticagrelor

Ticagrelor or AZD6140 is the first oral reversible ADP receptor antagonist which directly inhibits the P2Y₁₂ receptor but does not require hepatic metabolism for its activity. It is a non-thienopyridine which blocks platelet reactivity more consistently and completely than clopidogrel with a lower degree of inter-individual response variability. Additionally, a reversible antiplatelet effect may confer a clinical advantage for patients awaiting coronary artery bypass graft surgery since rapid recovery of platelet function can occur without having to wait for 5 days due to irreversible inhibition caused by thienopyridine administration.

The PLATO Phase III clinical trial which compared AZD6140 (180mg LD then 90-mg twice daily MD) with clopidogrel (300-mg LD plus a further bolus at time of PCI then 75mg/day MD) in 18 624 NSTEMI and STEMI ACS patients has now been completed⁽⁵³⁾. Results reported first at the ESC Annual Congress in September 2009 were encouraging. Like the landmark CURE trial, PLATO enrolled the entire continuum of ACS with or without ST-segment elevation unlike TRITON-TIMI 38 which was predominantly a PCI trial. At 12 months there was a significant difference in the primary endpoint of death from vascular causes, MI, or stroke in patients taking ticagrelor compared with clopidogrel (9.8% vs 11.7%; p <0.001) regardless of whether patients had received a higher dose of clopidogrel or whether they proceeded to PCI or not. There were also significant differences in individual secondary end-points such as death from vascular causes and MI but no difference in the rate of stroke between the two groups. Although known to have a more potent antiplatelet effect, ticagrelor did not cause a significant increase in overall major bleeding (11.6% vs 11.2%; p = 0.43) although there was a higher rate of non procedure-related major bleeding when compared to clopidogrel (4.5% vs 3.8%; p = 0.03). Interestingly, although much has been said about the reversibility of AZD6140, there was no difference in bleeding between the two drugs in those patients who proceeded to CABG surgery. Perhaps most striking, however, was the significant difference in the rate of all-cause mortality which was 4.5% with ticagrelor and 5.9% with clopidogrel (p <0.001). The authors have equated this to saving 14 lives per 1000 treated patients although it must be remembered that the study was not adequately powered to detect such a difference. All in all, ticagrelor may well represent a paradigm shift in APT but care must be taken not to forget the ramifications of compliance issues with twice-daily dosing and quality of life concerns raised by an increased incidence of dyspnoea associated with the drug. This is the first published trial for this agent—others will be needed. We will need to await the PCI subgroup data to be presented at the Transcatheter Cardiovascular Therapeutics meeting in September 2009 to allow comparisons with the Prasugrel TRITON data.

Cangrelor

Cangrelor is a selective and competitive P2Y₁₂ antagonist suitable for intravenous administration. It is an adenosine triphosphate analogue with more potent antiplatelet activity than clopidogrel. CHAMPION PCI and CHAMPION PLATFORM are two currently ongoing prospective multi-centre phase III clinical trials evaluating the efficacy of cangrelor against clopidogrel. The CHAMPION study has (in June 2009) been stopped for lack of efficacy. Details will follow in due course.

Protease-activated receptor antagonists

Thrombin-induced platelet aggregation can occur in the absence of TXA₂ and ADP thus giving an idea of how potent a platelet agonist it is. Thrombin activates platelets via two protease-activated receptors (PARs): PAR1 and PAR4; the former mediates platelet activation at low concentrations of thrombin whereas the latter requires high concentrations of thrombin for its activity (see Fig. 24.4). Currently two experimental PAR1 antagonist compounds: E555 and SCH530348 are under phase II investigation. In addition the currently ongoing TRANSCENDENCE PCI multi-centre randomized trial has been designed to evaluate the effects of SCH530348 against those of a GP IIb/IIIa inhibitor in terms of major and minor bleeding. The results should tell us more on the safety and efficacy of thrombin receptor blockade.