Aspirin and Clopidogrel Resistance in Coronary Artery Disease

Dual antiplatelet therapy has been proven effective to reduce recurrent cardiovascular eventin patients with coronary artery disease and recommended as standard therapy for acute coronary syndrome and patients who underwent percutaneous coronary intervention. The adverse clinical occurrence in patients who taking aspirin and clopidogrel associates with antiplatelet non responsiveness, in addition to repetitive bleeding incident in such a way that platelet reactivity and genetic polymorphisms investigation rises intense interest. Resistance to antiplatelet or antiplatelet non responsiveness means a phenomenon in which antiplatelet drug fails to deliver pharmacological target and it is determined by platelet function measurement. Recent laboratory methods have been developed to diagnose antiplatelet resistance, but none of them was considered as standard tool since its wide inter-individual variability and poor correlation between them. The mechanism of antiplatelet resistance is not fully understood, multifactorial, involving pharmaco dynamic and pharmacokinetic of the drugs. This review is aimed to comprehend the antiplatelet resistance mechanism and provide crucial information on managing patients who take dual antiplatelet treatment with adverse clinical events.


INTRODUCTION
Antiplatelet drugs are widely used to reduce thrombosis process in atherosclerosis patients.
There are three kinds of antiplatelet drug which have been proven their benefit to coronary artery disease (CAD), i.e. (i)cyclooxygenase (COX)-1 inhibitor such as aspirin, (ii) adenosine 5'-diphosphate (ADP) antagonist receptor such as thienopyridine (ticlopidin, clopidogrel, prasugrel), and (iii) GPIIb/IIIa antagonist such as abciximab, eptifibatide,and tirofiban.Aspirin and thienopyridinework selectively on inhibiting platelet activation and subsequent aggregation; aspirin in thromboxan A2 production through irreversibly COX-1 inhibition, while thienopyridinedisturbed ADP pathway through thrombocyte ADP receptor blockade.The prevention of platelet's clumping result in reduction of atherothrombotic event patient with CAD. 1,2,3 n a d d i t i o n t o t h e i r a d v a n t a g e s , researchers reported significant antiplatelet d e b i l i t i e s s u c h a s l o w r e s p o n s i v e n e s s (resistance), lack of platelet inhibition ability, variation of individual response, and lengthen time to recovery.Residual platelet reactivity is correlated with increased risk of major cardiovascular events. 4Chirumamilla et al.
(2012) investigated the relationship of platelet reactivity and atherosclerotic burden in patients treated with percutaneous coronary intervention (PCI).That study suggested that an increased platelet reactivity on antiplatelet treatment was correlated with higher burden of CAD and plaque calcification. 5Antiplatelet resistance is also associated with repetitive bleeding events in CAD patients. 6

H E I N C I D E N C E O F A N T I P L AT E L E T RESISTANCE
The    laboratory resistance which aspirin does not block properly with in vitro platelet reactivity.
Pharmacokinetics resistance was a condition caused by limited supply of active agents on plasma that caused low therapeutics response (such as low dose or absorption change) that improved significantly by adding aspirin in vitro dosage. 12 the clinical practice, nonadherence is probably the most frequent cause of aspirin nonresponsiveness.The lack of aspirin effect is due to the patient not taking the drug regularly or prematurely discontinue it.Aspirin discontinuation may not improve efficacy, but may raise bleeding risk and correlated with threefold increase in adverse cardiovascular events. 12,13Enteric coated aspirin is considered as a culprit of lower effectivity especially on obese patients because its tendency of low bioavaibility and bad absorption due to high-pH condition in the small intestine. 12,13,14,15e reducing bioavailibility of aspirin by gastrointestinal mucosal esterases may also occur when it takes together with proton pump inhibitor. 12Competition with other NSAIDs connected with Ser 530 enzim COX-1, in advanced causing irreversible acethylation and enzyme inactivation..In pharmacodynamics perspective, production of residual thromboxan by COX-  4). 11,13The lack of clopidogrelbio availibility  7 commonly due to non adherence patients, underdosing, and lowering prodrug intestinal absorption.Clopidogrel may interact with lipophilic statin, calcium-channel blocker, and omeprazole result in changes in hepatic cytochrome P450 isoenzymes activity and responsible on interfering clopidogrel action. 13teraction with benzodiazepine and selective serotonin reuptake inhibitor (SSRI) are also able to decrease clopidogrel biovailability. 179 Different platelet response to ADP which not increased by clopidogrel administration is also accounted as individual variability. 20

L A B O R AT O R Y M E A S U R E M E N T O F ANTIPLATELET RESISTANCE
Guidelines of myocardial revascularization (2014) published by ESC states that platelet function test or genetics is considered on high risk conditions such as stent thrombosis history, incompliance, resistance suspicion, and high risk bleeding (class recommendation IIb), though routine examination before and after stent implantation is not recommended. 21 vivo (bleeding time) and in vitro (light transmission aggregometry/LTA, PFA-100 system, ultegrarapidplatelet function assay-ASA) platelet function are not always reflecting drug ability to reach its pharmacology target. 22Various methods is developed to obtain the most appropriate measurement of platelet function in clinical practice. 23The former research, Aspirin Induced Platelet Effect (ASPECT) research on stable coronary artery disease, showed ALR decrement as NSAID, has its benefit on reducing resistance. 12gher platelet exchange increase the dose needed for aspirin, as American Diabetes Association recommended.Dose 75-162 mg/ 24 hour on diabetic patient with cardiovascular risk increased (as primary prevention) such as male > 50 yo, female > 60 yo with minimal 1 risk factor of atherosclerotics, or secondary prevention for those already diagnosed with atherosclerotic vascular disease. 24The proposed algorithm on handling aspirin resistance is shown on figure 4. Other CLR patients given prasugrel has not shown residual properties yet.This algorithm implementation decreasing the CLR prevalence by 86.6%. 27he former research, Aspirin Induced Platelet Effect (ASPECT) research on stable coronary artery disease, showed ALR decrement by increasing aspirin up to 325 mg/24 hour. 28Practice guideline for PCI from ACC/AHA/SCAI (2005) states that patients in whom subacute thrombosis may be fatal (such as unprotected left main, bifurcating left main, or last patent coronary vessels), platelet aggregation assessment may be considered and if it reveales inhibition of platelet agregation<50%, clopidogrel dosage should be increased up to 150 mg/24 hour (class IIb, LoE : C). 29 Aspirin resistance might be suppressed by minimalizing thromboxan production and activity or any other platelet activation pathway.Stop smoking, body weight reduction and routine physical training are mentioned to increase platelet function.Other clinical condition as hyperlipidemia, diabetes mellitus, hypertension, cardiac failure, and inflammation disturbances also disrupt aspirin activity.Incompliance, avoidance of agents interfere with aspirin absorption as proton pump inhibitor and enteric coated aspirin, agent decrease pharmacokinetics as NSAID, has its benefit on reducing resistance. 12igher platelet exchange increase the dose needed for aspirin, as American Diabetes Association recommended.Dose 75-162 mg/ 24 hour on diabetic patient with cardiovascular risk increased (as primary prevention) such as male > 50 yo, female > 60 yo with minimal 1 risk factor of atherosclerotics, or secondary prevention for those already diagnosed with atherosclerotic vascular disease. 24The proposed algorithm on handling aspirin resistance is shown on figure 4.

Figure 2 .
Figure 2. Pharmacokinetics and pharmacodynacis which influenced drug clinical outcome.This figure is modified from Rocca and Petrucci (2012) 10.
Hulot et al (2006) conducted pharmacogenetic study to figure out the mechanism of clopidogrel hyporesponsiveness which related to genetic variation.They concluded that CYP2C19*2 lossof-function allele was correlated with poor antiplatelet responsiveness of clopidogrel in young healthy male volunteers.The capability of clopidogrel to impede ADP-induced platelet aggregation demonstrates a wide individual variabiality.
due to LTAhas a low reproducibility, interindividual variability on ADP induced platelets aggregation baseline, and the logistical condition make a difficulty to use in daily practice.24Ultegra Rapid Platelet Function Assay (RPFA)-Verify Now ® is a simple, rapid (less than 5 minutes), user-friendly, high reproducibility, and point-of-care assessment which use AA or propylgallate to measure aspirin effects and ADP to find out the effect of P2Y 12 inhibitor.24Platelet function is assessed from the ability of activated platelet to bind fibrinogen which is detected by turbidimetric-based optical.Using light source, blood specimens is added to mixing chamber (cartridge) which contains fibrinogen-coated beads and a specific agonist.Clotting of functional platelets will raise the light transmission and the amount of fibrinogen binding by the activated platelets is determined.Three different assays are available to verify the inhibition of platelet function by anti-platelet drugs (GPIIb/IIIa inhibitor, aspirin, and clopidogrel).The result of aspirin and P2Y12 test is reported as Aspirin Reaction Unit (ARU) and P2Y12 Reaction Unit (PRU), respectively. 23,25The cutt-off value of aspirin and clopidogrel resistance are ≥ 550 ARU and ≥ 240 PRU. 6M A N A G E M E N T O F A N T I P L AT E L E T RESISTANCE Experts recommend three strategies to overcome antiplatelet resistance: increasing the dosage, adding other agent (such as gylcoprotein IIb/IIIa inhibitor, cilostazol), or substituting with more potent agent (such as prasugrel, ticagrelor). 6Alegria-Barrero et al (2010) convey an algorithm on overcoming antiplatelet resistance in patient undergo PCI (figure 3).After PCI procedure, patient receives dual antiplatelet drug (aspirin 100 mg and clopidogrel 75 mg).When aspirin or clopidogrel resistance is indicated, increase aspirin dosage up to 325 mg or clopidogrel 150 mg, respectively.It may also be switched with trifusal for aspirin resistance or prasugrel/ticagrelor/prasugrel for clopidogrel resistance. 26Neubaueret al. (2011) investigate 504 patients following PCI in single center study whoreceived 500 mg aspirin (loading dose), continued with 100 mg per 24 hour and 600 mg clopidogrel (loading dose), continued with 75 mg per 24 hour.All participants undergo whole blood aggregometry examination > 48 hours (no more than 72 hour) after stent implantation.The researcher applies the tailored algorithm on managing antiplatelet resistance which called "The Bochum clopidogrel and aspirin plan (BOCLA-Plan)".Study result identified 30,8% clopidogrel low-responders (CLR) and 19,4% aspirin low-responders (ALR).Aspirin dosage adjustment gradually at 300 mg/day, then 500 mg/day, resulted on 5,4% residual ALR.This means aspirin maintenance dose modification is adequately success for ALR.Meanwhile, 69%CLR patients are successfully treated with increased dosage of clopidogrel (150 mg/24 hour).The remaining 12.7% CLR who is contraindicated with prasugrel or inavailability of prasugrel, show adequate response with ticlopidine.Other CLR patients given prasugrel has not shown residual properties yet.This algorithm implementation decreasing the CLR prevalence by 86.6%.27

Figure 3 .
Figure 3. Algorithm of antiplatelet resistance mangement.Reprinted with permission from Alegria-Barrero (2010)26 Resistance to antiplatelet means a phenomenon in which antiplatelet drug fail to deliver pharmacological target and it is determined by platelet function measurement.Antiplatelet resistance is associated with recurrent adverse cardiovascular events, higher burden of CAD, plaque calcification, and repetitive bleedingoccurences.Studies develop various laboratory methods to recognize antiplatelet resistance, but none of them is considered as standard tool since its wide inter-individual variability and poor correlation between them.The published guidelines does not recommend to check the platelet function test routinely in periprocedural of PCI, unless the patient on high risk situation in Acta Cardiologia Indonesiana (Vol. 3 No. 1): …-… contraindicated with prasugrel or inavailability of prasugrel, show adequate response with ticlopidine.

Figure 4 . 1 )
Figure 4. Proposed algorithm of managing aspirin resistance on coronary artery disease.

Figure 4 .
Figure 4. Proposed algorithm of managing aspirin resistance on coronary artery disease.

Table 1 .
7revalence of aspirin resistance based on laboratory assay7 7hrombotic events.The action's mechanism of aspirin and clopidogrel are shown on figure 1. 8 DEFINITION OF ANTIPLATELET RESISTANCE European Society of Cardiology (ESC) Working Group on Thrombosis states that definition of antiplatelet resistance remains unclear since the standardized platelet function test has not been established to define whether it is responders or non-responders (resistance).The terminology of antiplatelet resistance consists of two features: clinical and laboratory.A patient who receives antiplatelet drug routinely but still experiences in cardiovascular event implied as clinical resistance.Source of table: Saraf et al. (2009)7

Table 2 .
7revalence of clopidogrel resistance based on laboratory assay7

Table 3 .
7nderlying mechanisms of antiplatelet resistance7 • High incidencee of shear stress, collagen, thrombin and other platelet activation pathway.