Platelet resistance is best defined as a lack of the desired pharmacologic effect

2Chapter 1 Anti-Platelet Resistance Oral Antiplatelet Therapy Resistance: Definition, diagnosis, and clinical implications Saurabh Gupta, MD Peter J. Casterella, MD Executive Summary Platelet resistance is best defined as a lack of the desired pharmacologic effect of an antiplatelet medication when it is taken as directed. Antiplatelet therapy resistance is common, with approximately half of all patients having resistance to either aspirin, clopidogrel, or both. It also confers an increased risk of adverse outcomes associated with acute coronary syndromes and percutaneous coronary interventions. Optimal management for patients with platelet resistance has not yet been established. Standardized methods of platelet function testing and interpretation, as well as integration of point-of-service testing, may assist in adjusting antiplatelet therapy to improve outcomes in patients with platelet resistance. Likewise, newer pharmacologic agents may demonstrate better efficacy and a lower incidence of resistance compared to the current standard of care of aspirin and clopidogrel. 1

SIS 2007 Yearbook Introduction Antiplatelet therapy with aspirin (ASA) 75-325 mg/ day offers protection against myocardial infarction (MI), stroke, and death. The mechanism of aspirin s antiplatelet action was first described in 1971 by British pharmacologist John Vane. In platelets, ASA irreversibly inhibits the enzyme cyclooxygenase-1 (COX-1), which in turns inhibits production of thromboxane A2, a facilitator of platelet aggregation (Figure 1). Significant benefit is evident among patients with acute coronary syndromes, such as unstable angina or acute MI, patients undergoing percutaneous coronary and peripheral intervention, and those with chronic ischemic vascular disease, or cerebrovascular disease. With regard to safety, efficacy, and cost effectiveness, ASA has demonstrated the best risk-benefit ratio of any available therapy for acute MI. Clopidogrel and ticlopidine are both thienopyridine agents with similar therapeutic effects, although clopidogrel has a better safety and side effect profile. Clopidogrel is a prodrug that undergoes activation by the cytochrome P450 3A4 system to become an irreversible inhibitor of the platelet P2Y12 receptor by blocking the binding of adenosine 5 -diphosphate (ADP). Blocking ADP leads to direct inhibition of the binding of fibrinogen to the glycoprotein IIb/ IIIa complex, thereby inhibiting platelet aggregation. Clopidogrel has demonstrated significant benefit across a wide spectrum of coronary artery disease clinical scenarios including ST elevation myocardial infarction (STEMI), non-stemi, and routine percutaneous coronary intervention (PCI). Ticlopidine, as mentioned, is similarly effective but associated with frequent side effects including rash and gastrointestinal upset, as well as the rare but potentially life-threatening occurrence of neutropenia. Still, it remains an alternative when clopidogrel therapy is not tolerated. Parenteral glycoprotein (GP) IIb/IIIa antagonists inhibit the final common pathway of platelet aggregation, the crossbridging of platelets secondary to Clopidogrel bisulfate, Ticlopidine HCI GP/IIb/IIIa (fibrinogen Receptor, IV agents) aspirin COX ADP ADP Activation Didyridamole Phosphodiesterase Collagen Thrombin TXA ADP = adenosine diphosphate; TXA2 = thromboxane A; COX = cyclooxygenase. Schafer AI. Am J Med, 1996, 101:199-209. fibrinogen binding to the activated GP IIb/IIIa receptor, resulting in near complete inhibition of platelet aggregation. Platelet Resistance: Definition, Incidence, Prevalence, and Outcomes Definition of Platelet Resistance Platelet resistance is best defined as a lack of the desired pharmacologic effect of an antiplatelet medication when it is taken as directed. However, the definition is complicated by the absence of standardized methods for platelet function assessment and the use of multiple assays and agonists. The occurrence of an ischemic clinical event while on antiplatelet therapy, although reflective of treatment failure, does not necessarily indicate that resistance is present. There are two methods of platelet aggregation testing. The first, light transmission platelet aggregometry (LTA), has been the gold standard of platelet function analysis for decades. However, this technology is tedious, time consuming, and not available in all hospitals. Furthermore, results of LTA are not immediately available, and thus cannot be used to guide acute therapies in the catheterization laboratory setting. The second, point-of-care platelet function assays, have become available more recently and are analogous to an activated clotting time (ACT) in that results are immediately available to guide therapy. Light transmission aggregometry and point-ofcare platelet function assays typically express results as percent aggregation. The assessment of platelet function after administration of platelet inhibitors may be expressed as residual platelet aggregation, or as level of platelet inhibition, which is the inverse of residual platelet aggregation. The most widely accepted definition of ASA resistance is 70% residual platelet aggregation with 10 μmol ADP stimulation or 20% residual platelet aggregation with 0.5mg/ml arachadonic acid stimulation (1). With clopidogrel, a less than 10% change in 5 μmol ADP-mediated platelet aggregation compared to baseline defines the presence of clopidogrel resistance (2). Platelet resistance does not appear to be an all or none phenomenon. Some patients demonstrate nearly complete resistance to aspirin, clopidogrel, or both agents, while others have a partial, but incomplete response of their platelets to these agents. While the optimal dose of aspirin has not been established, a consistent dose of clopidogrel is utilized for routine clinical use. It is unclear whether one size fits all dosing of clopidogrel is appropriate, and it has been postulated 2

Chapter 1 Anti-Platelet Resistance that some individuals may require a higher maintenance dose of clopidogrel to achieve the desired platelet inhibitory effect. Aspirin Resistance: Incidence and Outcomes The incidence of aspirin resistance varies from 5-60% of patients with cardiovascular disease depending upon the definition and assay used, as well as the population studied (1). The etiology of aspirin resistance is poorly understood and likely involves a combination of clinical, cellular, and genetic factors (Figure 2) (3). Ischemic stroke patients with aspirin resistance have an increased risk of future cardiovascular events, including stroke, MI, or vascular death (4). In a study of 151 patients undergoing non-urgent PCI, the incidence of aspirin resistance was 19.2%. Despite pretreatment with clopidogrel, those patients who were aspirin-resistant had a significantly higher rate of CK-MB and troponin I elevation post-procedure than those who were aspirin-sensitive (51.7% vs. 24.6%, p=0.006) (5). Interestingly, there was a significantly higher incidence of women in the aspirin-resistant group (44.8%) compared with the aspirin-sensitive group (19.7%) (p=0.007). CLINICAL FACTORS Non-absorption Interaction with NSAIDs Acute coronary syndrome Congestive heart failure Hyperglycemia Catecholamine surge CELLULAR FACTORS Insufficient suppression of COX-1 Over-expression of COX-2 mrna Erythrocyte induced platelet activation Increased norepinephrine Generation of 8-iso-PGF 2α Resolvins GENETIC FACTORS COX-1 GP IIb/IIIa receptor polymorphism Collagen receptor polymorphism vwf receptor polymorphism P2Y1 single nucleotide polymorphism ASPIRIN RESISTANCE Figure 2 Factors Contributing to Aspirin Resistance* *adapted from Bhatt DL. J Am Coll Cardiol 2004;43:1127-1129 Clopidogrel Resistance: Prevalence and Outcomes The prevalence of clopidogrel resistance varies from 4-30% of patients with coronary artery disease (CAD) depending upon the clinical scenario, test assay, and clopidogrel dose (2,6,7). A number of extrinsic and intrinsic factors have been hypothesized as operative in the etiology of clopidogrel resistance (Figure 3) (7). Marked inter-individual variability in the platelet inhibitory response from clopidogrel has been demonstrated (Figure 4) (8). Among patients undergoing PCI, those with high pretreatment platelet reactivity are the most likely to demonstrate post-treatment resistance to clopidogrel (2). The early risk of PCI is related to the level of platelet aggregation during the procedure. Baseline platelet aggregation was measured in 802 consecutive patients receiving a 600 mg loading dose of clopidogrel before elective coronary stenting. Nearly 40% of patients demonstrated suboptimal platelet inhibition. By quartiles of increasing platelet aggregation, the 30-day incidence of death, MI, and target lesion revascularization was 0.5% in the first quartile, 0.5% in the second, 3.1% in the third, and 3.5% in the fourth (p = 0.034) (9). Time from clopidogrel loading has a strong effect on the change in platelet aggregation, but even patients on chronic clopidogrel therapy who exhibit high ADP-induced platelet aggregation are at increased risk for post-procedural ischemic events (10). Clopidogrel resistance has been associated with adverse outcomes in a variety of clinical scenarios. Up to 25% of STEMI patients undergoing primary PCI are resistant to clopidogrel and have an increased risk of adverse cardiovascular events including stent thrombosis (11). A recent study evaluated responsiveness to clopidogrel in 804 consecutive patients receiving sirolimus- Extrinsic Factors Dose Drug-Drug Interactions (i.e. Lipitor) CYP3A4 Activity Variable Pro-Drug absorption Variable active metabolite clearance Intrinsic Factors P2Y12 receptor variability in numbers or affinity for drug Increased intrinsic ADP levels Up-regulation of other platelet activation pathways Figure 3 Potential Mechanisms of Clopidogrel Resistance* *adapted from Nguyen TA et al. J Am Coll Cardiol 2005;45:1157-1164 3

SIS 2007 Yearbook Figure 4 Distribution of 5μmol of adenosine diphosphate (ADP)-induced residual platelet aggregation in 544 patients after receiving clopidogrel therapy* *from Serebruany VL et al. J Am Coll Cardiol 2005;45:246-251 or paclitaxel-eluting stents. All patients received aspirin (325 mg) and a loading dose of 600 mg of clopidogrel followed by a maintenance dose of 75 mg daily. 13% of patients were not responsive to clopidogrel. At 6 months, the incidence of stent thrombosis was 8.6% in nonresponders and 2.3% in responders (p<0.001). By multivariate analysis, nonresponsiveness to clopidogrel was a strong independent predictor of stent thrombosis (hazard ratio [HR] 3.08, 95% confidence interval [CI] 1.32 to 7.16, p=0.009) (12) Patients with unstable angina undergoing PCI demonstrate lower inhibition of platelet aggregation following clopidogrel loading compared to patients with stable angina (13). These findings suggest that patients presenting with acute coronary syndromes (ACS) are a subgroup of patients with an increased likelihood of clopidogrel resistance. Treatment Modification in Patients with Platelet Resistance Aspirin Resistance The optimal treatment of ASA resistance remains unclear. It has been theorized that an increased aspirin dose may overcome resistance in some patients, especially those who are partial responders. However, while some small in-vitro studies show that increasing the dose of aspirin may overcome biological resistance, several large trials have demonstrated no significant difference in clinical outcomes with varying doses of ASA (14). There is also in-vitro evidence suggesting that aspirin-resistant patients may benefit from clopidogrel due to their increased platelet sensitivity to ADP (7,15). Similarly, it has been proposed that the use of a loading dose of clopidogrel may prevent adverse events in aspirin-resistant patients undergoing PCI. However, Chen and colleagues demonstrated that aspirin-resistant patients pre-treated with 300 mg of clopidogrel prior to PCI still had a greater than two-fold increase in the incidence of CK-MB elevation following PCI (5). Additionally, Lev and colleagues reported that 47% of aspirin-resistant individuals demonstrated concurrent clopidogrel resistance as well (16). Thus, it seems unlikely that the use of clopidogrel in aspirin-resistant patients will be a suitable therapeutic option. 4

Chapter 1 Anti-Platelet Resistance Clopidogrel Resistance As with aspirin resistance, the optimal therapeutic adjustment in the setting of clopidogrel resistance has yet to be determined. Gurbel and colleagues demonstrated that following PCI, a 300 mg loading dose of clopidogrel, followed by 75 mg daily maintenance dose is associated with an incidence of resistance of 63% at 2 hours, 31% at 2 and 5 days, and 15% at 30 days. The declining incidence of resistance over time suggests that a higher loading and/or maintenance dose might overcome resistance in some individuals (2). In a subsequent study, Gurbel and colleagues demonstrated superior levels of platelet inhibition in the first 24 hours after PCI with a 600 mg vs. 300 mg loading dose of clopidogrel (17). However, further increases in clopidogrel loading have not been shown to be beneficial. Price and colleagues reported that a 900 mg dose of clopidogrel provided no benefit in terms of magnitude or time to maximal platelet inhibition compared with a 600 mg dose, but both are superior to a 300 mg dose (18). Use of a Glycoprotein IIb/IIIa Antagonist In the setting of PCI, a potential way to combat both aspirin and clopidogrel resistance is the use of a parenteral glycoprotein (GP) IIb/IIIa antagonist. Prior clinical studies of the GP IIb/IIIa inhibitor class showed significant reductions in major adverse cardiac events, leading to the widespread use of these agents with PCI in the late 1990s and early 2000s. GP IIb/IIIa use lessened with more recent studies such as REPLACE 2 (19) and ISAR REACT (20), which demonstrated that not all PCI patients require aggressive antiplatelet therapy. However, the ISAR REACT 2 trial showed that ACS patients benefit from the addition of a GP IIb/IIIa antagonist to a baseline therapy of aspirin, clopidogrel 600 mg loading dose, and intravenous heparin when undergoing PCI (21). This finding of a benefit with GP IIb/IIIa antagonists in higher risk ACS patients may be associated with the higher incidence of platelet resistance demonstrated in ACS patients. It seems plausible that a bona fide indication for the use of GP IIb/IIIa inhibitors may be the presence of platelet resistance, especially in the clinical scenario of ACS. The RESISTOR (Research Evaluation to Study Individuals who Show Thromboxane or P2Y12 Receptor Resistance) trial is currently evaluating the role of GP IIb/IIIa inhibitors in PCI in the setting of platelet resistance. In this study, elective PCI patients who demonstrate platelet resistance on pre-procedure platelet function testing are randomized to GP IIb/IIIa inhibitor therapy with eptifibatide vs. placebo. The primary endpoint is myonecrosis as assessed by cardiac biomarker elevations. Future Directions in the Management of Platelet Resistance Effectively dealing with and resolving the clinical challenges posed by platelet resistance hinges upon several critical issues including: 1. The need for standardized, convenient, and easily interpretable point-of-care platelet function assays. 2. Development of standardized terminology and nomenclature to define platelet resistance and to characterize platelet function response to inhibitory therapies. 3. Development of new pharmacologic therapeutic agents to address the shortcomings of aspirin and clopidogrel. Increased use of point-of-care platelet function assays may lead to better therapy. However, to integrate the results of such assays into patient management, it will be necessary to standardize features such as agonist type and strength used to test specific anti-platelet agents, in addition to nomenclature of reporting test results. The ideal platelet function test will use whole blood (no requirement for specimen preparation), will accurately assess platelet responsiveness even in the setting of baseline therapy with antiplatelet medications, and will provide results in 10 minutes or less. Measurement of responsiveness to antiplatelet therapy creates the potential for individualized therapy. Eventually, prospective, randomized clinical trials will determine if monitoring of platelet function response to therapy and subsequent adjustment of antiplatelet therapy translates into improved clinical outcomes. Newer pharmacologic agents may demonstrate better efficacy and a lower incidence of resistance compared to the current standard of care of aspirin and clopidogrel. Prasugrel is a third generation thienopyridine with greater potency, more rapid onset, more consistent platelet inhibition, and a lower incidence of resistance compared to clopidogrel (22). Prasugrel is currently under evaluation in the TRITON TIMI 38 study, which is comparing it to clopidogrel in the setting of PCI. Other novel agents such as cangrelor and AZD6140 are rapid, reversible P2Y12 inhibitors that are direct inhibitors as opposed to prodrugs. 5

SIS 2007 Yearbook Conclusion O ral antiplatelet therapy resistance is common and conveys an increased risk of adverse outcomes with ACS and PCI. Patients with platelet resistance may require higher doses of oral antiplatelet therapies or more potent antiplatelet therapies (i.e. GP IIb/IIIa antagonists) to optimize outcomes. Standardized methods of platelet function testing and interpretation, along with integration of point-of-service testing may assist in individualizing antiplatelet therapy to improve outcomes in patients with platelet resistance. The advancement of new agents with a more reliable pharmacologic effect and reduced incidence of resistance may further assist in combating the deleterious effects of antiplatelet therapy resistance. References 1. Gum PA, Kottke-Marchant K, Poggio ED, Gurm H, Welsh PA, Brooks L, et al. Profile and prevalence of aspirin resistance in patients with cardiovascular disease. Am J Cardiol 2001;88:230-5. 2. Gurbel PA, Bliden KP, Hiatt BL, O Connor CM. Clopidogrel for coronary stenting: Response variability, drug resistance, and the effect of pretreatment platelet reactivity. Circulation 2003;107:2908-2913. 3. Bhatt DL. Aspirin resistance: More than just a laboratory curiosity. J Am Coll Cardiol 2004;43:1127-1129. 4. Grotemeyer KH, Scharafinski HW, Husstedt IW. Two-year follow-up of aspirin responder and aspirin non responder. A pilot-study including 180 post-stroke patients. Thrombosis Research 1993;71:397-403. 5. Chen W-H, Lee P-Y, Ng W, Tse H-F, Lau C-P. Aspirin resistance is associated with a high incidence of myonecrosis after non-urgent percutaneous coronary intervention despite clopidogrel pretreatment. J Am Coll Cardiol 2004;43:1122-1126. 6. Muller I, Besta F, Schulz C, Massberg S, Schonig A, Gawaz M. Prevalence of clopidogrel non-responders among patients with stable angina pectoris scheduled for elective coronary stent placement. Thromb Haemost 2003;89:783-7. 7. Nguyen TA, Diodati JG, Pharand C. Resistance to clopidogrel: A review of the evidence. J Am Coll Cardiol 2005;45:1157-1164. 8. Serebruany VL, Steinhubl SR, Berger PB, Malinin AI, Bhatt DL, Topol EJ. Variability in platelet responsiveness to clopidogrel among 544 individuals. J Am Coll Cardiol 2005;45:246-251. 9. Hochholzer W, Trenk D, Bestehorn H-P, Fischer B, Valina CM, Ferenc M, et al. Impact of the degree of peri-interventional platelet inhibition after loading with clopidogrel on early clinical outcome of elective coronary stent placement. J Am Coll Cardiol 2006;48:1742-1750. 10. Bliden KP, DiChiara J, Tantry US, Bassi AK, Chaganti SK, Gurbel PA. Increased risk in patients with high platelet aggregation receiving chronic clopidogrel therapy undergoing percutaneous coronary intervention: Is the current antiplatelet therapy adequate? J Am Coll Cardiol 2007;49:657-666. 11. Matetzky S, Shenkman B, Guetta V, Shechter M, Bienart R, Goldenberg I, et al. Clopidogrel resistance is associated with increased risk of recurrent atherothrombotic events in patients with acute myocardial infarction. Circulation 2004;109:3171-3175. 6

Chapter 1 Anti-Platelet Resistance 12. Buonamici P, Marcucci R, Migliorini A, Gensini GF, Santini A, Paniccia R, et al. Impact of platelet reactivity after clopidogrel administration on drug-eluting stent thrombosis. J Am Coll Cardiol 2007;49:2312-2317. 13. Soffer D, Moussa I, Harjaim KJ, Boura JA, Dixon SR, Grines CL, et al. Impact of angina class on inhibition of platelet aggregation following clopidogrel loading in patients undergoing coronary intervention: Do we need more aggressive dosing regimens in unstable angina? Catheterization and Cardiovascular Interventions 2003;59:21-25. 14. Collaborative meta-analysis of randomised trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high risk patients. BMJ 2002;324:71-86. 15. Macchi L, Christiaens L, Brabant S, Sorel N, Allal J, Mauco G, et al. Resistance to aspirin in vitro is associated with increased platelet sensitivity to adenosine diphosphate. Thromb Res 2002;107:45-9. 16. Lev EI, Patel RT, Maresh KJ, Guthikonda S, Granada J, DeLao T, et al. Aspirin and clopidogrel drug response in patients undergoing percutaneous coronary intervention: The role of dual drug resistance. J Am Coll Cardiol 2006;47:27-33. 17. Gurbel PA, Bliden KP, Zaman KA, Yoho JA, Hayes KM, Tantry US. Clopidogrel loading with eptifibatide to arrest the reactivity of platelets: Results of the clopidogrel loading with eptifibatide to arrest the reactivity of platelets (clear platelets) study. Circulation 2005;111:1153-1159. 18. Price MJ, Coleman JL, Steinhubl SR, Wong GB, Cannon CP, Teirstein PS. Onset and offset of platelet inhibition after high-dose clopidogrel loading and standard daily therapy measured by a pointof-care assay in healthy volunteers. The American Journal of Cardiology 2006;98:681-684. 19. Lincoff AM, Bittl JA, Harrington RA, Feit F, Kleiman NS, Jackman JD, et al. Bivalirudin and provisional glycoprotein iib/iiia blockade compared with heparin and planned glycoprotein iib/iiia blockade during percutaneous coronary intervention: Replace-2 randomized trial. JAMA 2003;289:853-63. 20. Kastrati A, Mehilli J, Schuhlen H, Dirschinger J, Dotzer F, ten Berg JM, et al. A clinical trial of abciximab in elective percutaneous coronary intervention after pretreatment with clopidogrel. N Engl J Med 2004;350:232-8. 21. Kastrati A, Mehilli J, Neumann FJ, Dotzer F, ten Berg J, Bollwein H, et al. Abciximab in patients with acute coronary syndromes undergoing percutaneous coronary intervention after clopidogrel pretreatment: The isar-react 2 randomized trial. JAMA 2006;295:1531-8. 22. Jernberg T, Payne CD, Winters KJ, Darstein C, Brandt JT, Jakubowski JA, et al. Prasugrel achieves greater inhibition of platelet aggregation and a lower rate of non-responders compared with clopidogrel in aspirin-treated patients with stable coronary artery disease. Eur Heart J 2006;27:1166-1173. 7