Platelet Aggregation According to Body Mass Index in Patients Undergoing Coronary Stenting: Should Clopidogrel Loading-Dose Be

ABSTRACT: Background. A 300 mg clopidogrel loading-dose (LD) is widely used as an adjunct antithrombotic treatment to reduce the risk of thrombotic events early after coronary stenting (CS). Antithrombotic drugs commonly used during percutaneous coronary interventions, such as heparin and platelet glycoprotein IIb/IIIa inhibitors, but not clopidogrel LD, are weight-adjusted, and few data are available on which is the most effective clopidogrel LD regimen. The aim of this study was to assess whether body mass index (BMI) influenced platelet response to clopidogrel LD in patients undergoing CS. Methods. Adenosine diphosphate (ADP)-induced platelet aggregation (PA) was assessed by light transmittance aggregometry in 48 patients on aspirin treatment undergoing CS receiving a 300 mg clopidogrel LD at intervention time. PA was assessed at baseline and up to 24 hours after intervention. Patients were divided into 2 groups according to BMI: overweight (BMI >= 25 kg/m2; 29 patients) and normal weight (BMI 2; 19 patients). PA was significantly higher in overweight than in normal weight patients at baseline (60.1 ± 18.6% versus 47.6 ± 13.5%; p = 0.01), at 24 hours (42.3 ± 18.4% versus 38.5 ± 18.3%; p = 0.02) and during the overall study time (p = 0.025). Percentage of inhibition of PA 24 hours following clopidogrel LD was suboptimal ( J INVAS CARDIOL 2004;16:169–174 Key words: clopidogrel, coronary stenting, thrombosis Although subacute stent thrombosis following coronary stenting (CS) has been significantly reduced with the use of combined antiplatelet treatment (aspirin and thienopyridine),¹ stent thrombosis still occurs in 1–2% of patients early after intervention.² Since thrombotic complications following CS commonly occur within the first 24–48 hours,^1,2 clopidogrel is generally given as the 300 mg loading-dose (LD) to obtain the maximum grade of platelet inhibition within the minimum time frame; in fact, without a LD, at least 4–5 days of treatment (75 mg/day) are required to achieve the same antiplatelet effect.³ Clopidogrel has a more favorable efficacy and safety profile and a more accelerated antiplatelet activity after first administration than ticlopidine.^4,5 Pre-treatment with a 300 mg LD has been safely used in several randomized studies.^6–8 However, LD regimens higher than the standard 300 mg have more enhanced and accelerated antiplatelet effects in the early hours following CS,^9–11 and a 600 mg LD regimen has shown to be safe and leads to a more favorable clinical outcome than conventional therapy with ticlopidine.¹² Body weight may account for individual variability in kinetic profiles of pharmacodynamic response to several drugs, including antithrombotic drugs such as heparin and platelet glycoprotein (GP) IIb/IIIa inhibitors;¹³ therefore, front-loading with these drugs is commonly weight-adjusted.¹⁴ However, this is currently not recommended for clopidogrel. Importantly, drug metabolism may be altered in overweight individuals, especially when administered as LDs, frequently leading to inadequate therapeutic levels. Therefore, dose-finding studies are generally performed to identify the correct LD in overweight individuals¹⁵ in order to guarantee an immediate adequate pharmacological effect in this group of patients. This is of crucial importance in particular during CS to reduce the early risk of thrombotic events. Body mass index (BMI) is a useful, reliable and simply calculated parameter to identify overweight individuals¹⁶ and the aim of this study was to assess the influence of BMI on platelet response to a standard 300 mg clopidogrel LD in patients undergoing CS. Methods Patient population. Forty-eight consecutive patients undergoing successful CS referred to our interventional cardiology unit were included. Patients were divided into 2 groups according to BMI: overweight (BMI >= 25 kg/m²) and normal weight (BMI 2). This is in accordance with the definitions of BMI for adults, regardless of age or sex, proposed by the World Health Organization.¹⁶ All patients were on treatment with aspirin (100–250 mg/day) for at least 7 days and received a 300 mg clopidogrel LD at intervention time. All patients maintained clopidogrel treatment (75 mg/day) for 1 month after intervention. During intervention, unfractionated heparin (100 IU/kg) was administered and activated clotting time values were maintained above 250 seconds. Exclusion criteria were: 1) use of glycoprotein (GP) IIb/IIIa blockers before, during or after intervention; 2) the use of thienopyridines or any other antithrombotic drug in the previous 10 days; and/or 3) aspirin intolerance/allergy. Only commercially available bare-metal stents were used. All patients were followed for a 6-month period and cardiovascular events [cardiac death, myocardial infarction (MI), worsening of angina requiring new hospital admission and/or need for new coronary revascularization] were recorded. MI was defined as a 2x increase in the upper normal value of serum creatine kinase (normal range, 0–170 U/L). This study was approved by the Ethics Committee of our hospital and all patients gave their informed written consent. Assessment of platelet aggregation. Platelet aggregation (PA) induced by 6 µM adenosine diphosphate (ADP; Chrono-Log Corporation, Havertown, Pennsylvania) as agonist was assessed at baseline (with patients only on aspirin and before administration of unfractionated heparin and clopidogrel LD), and at 10 minutes, 4 hours and 24 hours following intervention and clopidogrel LD. Blood samples were drawn from a 6 French arterial sheet for baseline, 10-minute and 4-hour samples and from an antecubital vein using a 21-gauge needle for the 24-hour sample. All blood samples were collected in tubes containing 3.8% trisodium citrate and PA was assessed with platelet-rich plasma (PRP) by the turbidimetric method17 in a 2-channel aggregometer (Chrono-Log 490 Model, Chrono-Log Corporation). PRP was obtained as a supernatant after centrifugation of citrated blood at 160 g for 10 minutes. The isolated PRP was kept at 37 ºC for 20 minutes before use. Platelet-poor-plasma (PPP) was obtained by a second centrifugation of the blood fraction at 2,500 g for 10 minutes. Light transmission was adjusted to 0% with PRP and to 100% for PPP for each measurement. Curves were recorded for 5 minutes and analyzed according to international standards.¹⁷ PA was determined as the maximal percent change in light transmittance from baseline PPP as reference. Percentage (%) of inhibition of PA 24 hours following 300 mg clopidogrel LD was determined according to the following formula: % inhibition = [(Ao - At)/Ao] x 100 where At equals percent aggregation at any time point after treatment and Ao equals percent aggregation at baseline. Currently, there is no fixed value to define the degree of inhibition of antiplatelet response in patients with coronary artery disease on chronic aspirin treatment given a 300 mg clopidogrel LD. Recent reports have used an empiric value of 18 while values between 40% and 60% are equivalent of an adequate response (responders)¹⁹ and values between 10% and 40% identify individuals with a suboptimal response (or semi-responders). Accordingly, in this study a normal, or adequate, inhibitory response to a 300 mg LD was considered to be achieved when inhibition of ADP-induced PA assessed 24 hours after clopidogrel LD administration was >= 40% and considered suboptimal when Statistical analysis. Continuous variables are expressed as means ± standard deviation. Categorical variables are expressed as frequencies and percentages. Chi-square test was used to compare non-continuous variables, and the Student’s t-test was used for continuous variables. Multivariate analysis of variance (MANOVA) was performed to assess differences between groups during the study time course. Platelet response to 300 mg clopidogrel LD was assessed by: 1) evaluating the degree of PA observed 24 hours after treatment; and 2) by evaluating the degree of inhibition of PA 24 hours after treatment (defined as adequate or inadequate if above or below 40%, respectively). A multiple linear regression analysis was carried out to assess determinants of PA 24 hours after clopidogrel LD. Univariate analysis was carried out to assess determinants of inadequate platelet response to clopidogrel LD. As confounders, we analyzed BMI >= 25 kg/m², common coronary risk factors (hypertension, hyper-cholesterolemia, diabetes, smoking habit), baseline PA and clinical presentation (acute coronary syndrome or chronic stable angina). Baseline PA was not considered for univariate analysis since it is intrinsic in the definition of degree of PA inhibition and therefore led to colinearity. A p-value = 25 kg/m² (p = 0.02) and baseline PA (p = 0.045) were independently associated with a higher degree of PA 24 hours after clopidogrel LD. When considering BMI as a continuous variable, there was a positive correlation with baseline PA (p = 0.02; r = 0.35) and a positive trend with PA at 24 hours (p = 0.07; r = 0.3). Percentage of inhibition of PA 24 hours following clopidogrel LD was suboptimal (= 25 kg/m² was associated with a suboptimal inhibitory response to clopidogrel LD (Table 3). Clinical follow-up. In the initial post-stenting period (up to 48 hours post-CS), three events occurred: one cardiac death due to thrombotic occlusion and 2 non-Q wave MIs. Two of the 3 events (1 death and 1 non-Q wave MI) occurred in overweight patients who had a suboptimal response to antiplatelet treatment. At 1-month follow-up, no further events had occurred. At 6-month follow-up, a total number of 13 events had occurred. These occurred more frequently in overweight patients (10/29; 34%) than in normal weight patients (3/19; 16%), although this finding was not statistically significant (p = 0.2). Discussion The present study demonstrates that overweight patients on chronic aspirin treatment undergoing CS have a higher platelet reactivity before intervention that continues to be elevated up to 24 hours following a 300 mg clopidogrel LD when compared to normal weight patients. Furthermore, a significant number of overweight patients (more than half) had a suboptimal antiplatelet response to such treatment. A higher platelet reactivity and a lower response to antiplatelet treatment, as observed in this study, has been associated with a higher risk of adverse cardiac events,²⁰ therefore suggesting that a higher clopidogrel LD and/or an adjunctive antithrombotic treatment may be required to adequately inhibit PA early after CS in overweight patients. The thrombotic burden early after CS is increased due to the release of prothrombotic factors,²¹ such as tissue factor, adhesion molecules, inflammatory mediators, and because of the intrinsic thrombogenicity of bare metal stents.^1,2 In such a setting, adequate inhibition of platelet function is fundamental to overcome the risk of thrombotic events, which generally leads to severe complications (cardiac death, MI, need for urgent revascularization). The present study confirms data from previous studies in which platelet function was shown to be enhanced in overweight individuals.²² Platelet function results also increased in diabetic patients^23,24 and may explain the higher risk of thrombotic complications observed in these patients.^25,26 In this study, in accordance with the pathophysiology of this metabolic disorder, there was a larger number of diabetics, defined according to WHO criteria, among overweight patients.²⁷ Interestingly, a subgroup analysis identified that diabetics presented a significantly higher degree of baseline PA (63 ± 22% versus 52% ± 15%; p = 0.05) and a lower response to a 300 mg clopidogrel LD (42% ± 22% versus 30% ± 17%; p = 0.05) than non-diabetics. However, diabetes was not found to be an independent predictor of PA 24 hours after clopidogrel LD or of an inadequate platelet response. Perhaps the small sample size of this study may have led to these results. The higher degree of PA observed in overweight patients in this study may be due to the increased levels of platelet cytostolic calcium and plasma cathecolamines found in overweight individuals.^22,28 It may be argued that the increased PA observed at 24 hours and the lower response to a 300 mg clopidogrel LD in overweight patients in this study may be related to the increased PA already present at baseline. However, PA does not continue to decrease in overweight patients as it does in normal weight patients following the fourth hour (Figure 1), suggesting differences in the metabolic rate of clopidogrel among these individuals. Clopidogrel is inactive in vitro and a hepatic biotransformation by the cytochrome P450 3A4 (CYP3A4) is necessary to express the full antiaggregating activity of the drug.²⁹ In particular, the activity of CYP3A4 is reduced in overweight individuals³⁰ and this may explain the lower response to a 300 mg clopidogrel LD in these patients. Atorvastatin is also metabolized by CYP3A4 and modulates clopidogrel-induced antiplatelet effects;²⁹ however, there were no differences in treatment among groups in our study. Therefore, these pharmacological considerations represent a rationale for the use of a higher clopidogrel LD in overweight individuals. Use of higher LD has been suggested since a more homogenous antiplatelet effect and a reduced individual variability within the same time frame are obtained. In particular, platelet function studies using clopidogrel LD higher than the standard 300 mg dose demonstrated a higher degree of inhibition of platelet function^9,10 and at the same time have been shown to be safe without an increased bleeding risk.¹² Preliminary data from our group comparing 300 mg versus 600 mg LD regimens have confirmed these findings; furthermore, a 600 mg LD reduced individual variability to antiplatelet treatment and was associated with a similar inhibitory effect in normal weight and overweight patients.³¹ A subanalysis of the CAPRIE study demonstrated that there is no need for weight adjustment for long-term clopidogrel treatment (75 mg/day).^32,33 However, kinetic profiles of pharmacodynamic response to LD regimens differ from that of long-term drug administration and to date no studies have assessed antiplatelet effects induced by clopidogrel LD in overweight individuals. The rationale for a 300 mg clopidogrel LD regimen in clinical practice comes from dose-finding studies performed in healthy subjects.^6,11 However, cardiovascular patients not only have a higher thrombotic burden than healthy individuals, but may have blunted antiplatelet effects to antithrombotic drugs related to intrinsic features of the atherosclerotic disease process and its complications (e.g., “primed platelets”).^34,35 Furthermore, cardiovascular patients are generally on aspirin, while healthy volunteers in dose-finding studies were not. Previous studies have demonstrated that clopidogrel pre-treatment (at least 6 hours before intervention) is associated with a better outcome.^7,8 Clopidogrel pre-treatment ensures a higher degree of platelet inhibition in the earliest hours after coronary intervention than achieved if administered at intervention time.³⁶ However, administration of clopidogrel as a LD is generally recommended only when coronary anatomy is known.³⁷ Since “real world” coronary interventions are frequently performed immediately after diagnostic angiography, this offsets the possibility of routine administration of clopidogrel before patients enter the cath lab. Therefore, there is still a need to define the optimal clopidogrel LD in this setting in order to adequately inhibit PA within the minimum time frame and the results from this study suggest the use of a higher clopidogrel LD in overweight patients to reach this goal. Larger studies with assessment of platelet function and use of different LD regimens, preferably in a randomized fashion, are warranted to define the optimal clopidogrel LD to be administered at intervention time. Study limitations. There are several limitations to this study. First, the small sample size of the study does not allow the finding of significant clinical differences. In fact, platelet function studies, such as the present one, are necessarily performed in a limited number of individuals because of the costs and around-the-clock personnel required to process blood immediately after sampling. However, since thrombotic events are associated with an increased platelet reactivity²⁰ and this leads to severe complications, the implications of our data remain clinically relevant. In the present study, PA was assessed during the first 24 hours. Although previous reports have shown inhibition of ADP-induced aggregation to be maximal at 24 hours after clopidogrel loading administration with no significant increased inhibition over the following days,³⁸ perhaps platelet response becomes progressively adequate over a longer time period in overweight patients.^32,33 However, the period of assessment of PA performed in our study is critical since the majority of thrombotic complications occur within the first hours after CS. The definition of responders/non-responders to antiplatelet treatment is arbitrary and therefore may be an arguable issue. Furthermore, platelet response varies significantly among individuals and different results may be obtained during platelet function studies depending on the agonist used and its concentration. Currently, there is no fixed cut-off value of ADP-induced PA to define an adequate antiplatelet response in patients undergoing CS on chronic aspirin treatment receiving a 300 mg clopidogrel LD. However, in the present study, ADP was used as an agonist since clopidogrel acts by blocking the ADP platelet receptor and a cut-off value of 40% of baseline aggregation was considered on the basis of previous reports.19Conclusion. In conclusion, a 300 mg clopidogrel LD does not inhibit PA in overweight patients to the same extent as normal weight patients early after CS, setting a significant number of overweight patients at an increased early thrombotic risk. These data suggest that overweight patients may need a higher LD of clopidogrel and/or an adjunctive antithrombotic treatment to adequately inhibit PA early after CS. Further studies to elucidate the optimal antithrombotic regimen in overweight patients undergoing CS should be performed to confirm this hypothesis.