Normalization of platelet reactivity in clopidogrel-treated subjects

BACKGROUND: Aspirin (ASA) + clopidogrel are commonly used in acute coronary syndrome (ACS), but persistent antiplatelet effects may complicate surgery. METHODS AND RESULTS: To study the possibility of normalizing platelet reactivity after ASA + clopidogrel treatment, 11 healthy subjects received a 325-mg ASA + clopidogrel loading dose (300 or 600 mg dependent on study arm), followed by 81 mg of ASA + 75 mg of clopidogrel daily for 2 days. Platelet reactivity was assessed by light transmittance aggregometry (LTA) [challenged by adenosine diphosphate (ADP), arachidonic acid (AA), collagen, and thrombin receptor activating peptide (TRAP)] and flow cytometry for platelet activation by GPIIb/IIIa receptor exposure pretreatment, 4 and 72 h postload. To normalize platelet reactivity, increasing amounts of pooled platelets from five untreated volunteers [volunteers (V)-platelet-rich plasma (PRP)] were added ex vivo to the subject's PRP (S-PRP). At both 4 and 72 h, 40% and 50% V-PRP were needed to overcome platelet disaggregation in the 300 or 600 mg arms, respectively, after ADP challenge; an additional 10% V-PRP fully normalized aggregation. Recovery of function was linear with each incremental increase of V-PRP. ADP-induced GPIIb/IIIa activation showed the same pattern as LTA (r = 0.74). Forty percent V-PRP was required to normalize platelet function to AA, collagen, and TRAP. CONCLUSION: Our results suggest that the pre-operative transfusion of 10 platelet concentrate units (the equivalent of 40% V-PRP) after a 300-mg clopidogrel loading or 12.5 units (50% V-PRP) after a 600 mg loading may adequately reverse clopidogrel-induced platelet disaggregation to facilitate postoperative hemostasis. An additional 2.5 units fully normalized platelet function. The potential clinical implications of our observations could include shorter hospitalizations and reduced bleeding complications. But these observations should be fully explored in an in vivo clinical setting with clopidogrel-treated patients before and after surgery.     

High on‐aspirin platelet reactivity as measured with aggregation‐based,cyclooxygenase‐1 inhibition sensitive platelet function tests is associated with the occurrence of atherothrombotic events

Summary. Background: High on‐aspirin platelet reactivity (HAPR) is associated with atherothrombotic events following percutaneous coronary intervention (PCI). The aim of the present study was to identify the platelet function test sensitive for platelet cyclooxygenase‐1 inhibition that best predicts atherothrombotic events. Methods and results: Nine hundred and fifty‐one consecutive patients on dual antiplatelet therapy undergoing elective PCI were enrolled. On‐aspirin platelet reactivity was measured in parallel by arachidonic acid (AA)‐induced light transmittance aggregometry (AA‐induced LTA), the VerifyNow® Aspirin Assay (VerifyNow® Aspirin Assay), the arachidonic acid prestimulated IMPACT‐R (IMPACT‐R AA) and the PFA‐100 collagen/epinephrine cartridge (PFA COL/EPI). Cut‐offs for HAPR were established by receiver‐operator characteristic curve analysis. At 1‐year follow‐up, the composite of all‐cause death, non‐fatal acute myocardial infarction, stent thrombosis and ischemic stroke occurred more frequently in patients with HAPR when assessed by LTA [10.1% vs. 6.0%, P = 0.020 (n = 925)] and VerifyNow^® [13.3% vs. 5.9%, P = 0.015 (n = 422)]. The VerifyNow^® ASA assay (AUC = 0.78) and, to a lesser extent, AA‐induced LTA (AUC = 0.73) added significantly to a model consisting of clinical and procedural risk factors in predicting atherothrombotic events. In contrast, the IMPACT‐R (n = 791) and the PFA Collagen/Epinephrine (n = 719) were unable to discriminate between patients with and without primary endpoint at 1‐year follow‐up. None of the platelet function tests was able to identify patients at risk for bleeding. Conclusions: AA‐induced LTA and the VerifyNow^® ASA test were able to identify aspirin‐treated patients undergoing PCI with stenting at risk for atherothrombotic events. The VerifyNow^® Aspirin Assay had the highest predictive accuracy. None of the tests was able to identify patients at higher risk of bleeding.     

Biological effects of aspirin and clopidogrel in a randomized cross-over study in 96 healthy volunteers

BACKGROUND: Some data suggest that biological 'resistance' to aspirin or clopidogrel may influence clinical outcome. OBJECTIVE: The aim of this study was to evaluate the relationship between aspirin and clopidogrel responsiveness in healthy subjects. METHODS: Ninety-six healthy subjects were randomly assigned to receive a 1-week course of aspirin 100 mg day(-1) followed by a 1-week course of clopidogrel (300 mg on day 1, then 75 mg day(-1)), or the reverse sequence, separated by a 2-week wash-out period. The drug effects were assessed by means of serum TxB2 assay, platelet aggregation tests, and the PFA -100 and Ultegra RPFA -Verify Now methods. RESULTS: Only one subject had true aspirin resistance, defined as a serum TxB2 level > 80 pg microL(-1) at the end of aspirin administration and confirmed by platelet incubation with aspirin. PFA-100 values were normal in 29% of the subjects after aspirin intake, despite a drastic reduction in TxB2 production; these subjects were considered to have aspirin pseudo-resistance. Clopidogrel responsiveness was not related to aspirin pseudo-resistance. Selected polymorphisms of platelet receptor genes were not associated with either aspirin or clopidogrel responsiveness. CONCLUSIONS: In healthy subjects, true aspirin resistance is rare and aspirin pseudo-resistance is not related to clopidogrel responsiveness.     

The Residual Platelet Aggregation after Deployment of Intracoronary Stent (PREDICT) score.

BACKGROUND: Recent studies suggest a high interindividual variability of response to clopidogrel associated with adverse cardiovascular outcome. Different clinical factors are considered to influence a persistent residual platelet aggregation (RPA) despite conventional antiplatelet therapy. OBJECTIVES: To investigate clinical factors that affect RPA after 600-mg clopidogrel loading in a large unselected cohort of patients with symptomatic CAD. METHODS: The study population included a consecutive cohort of 1,092 patients treated with coronary stenting for stable angina and acute coronary syndromes (ACS). Residual platelet activity was assessed by ADP (20 micromol L(-1))-induced platelet aggregation >or= 6 h after LD. Eleven clinical factors were included in the primary analysis. RESULTS: In multivariate regression analysis increased RPA was significantly influenced by ACS, reduced LV-function, diabetes mellitus, renal failure (creatinine > 1.5 mg dL(-1)), and age > 65 years. In a factor-weighed model the risk for high RPA increased with higher score levels (OR for patients with a score of 1-3, 1.21, 95% CI 0.7-2.1; score 4-6, OR 2.0, 95% CI 1.17-3.5; P = 0.01; score 7-9, OR 3.3, 95% CI 1.8-6.0). During a 30-day follow-up the incidence of major adverse events was higher in patients with RPA in the upper tertile (4.8% vs. 2.5% in the 2nd and 1.5% in the 1st tertile; P < 0.05). CONCLUSIONS: The PREDICT score provides a good tool to estimate residual platelet activity after clopidogrel LD by easily available patient details. Additionally, we demonstrate its association with short-term outcome. Thus, patients with a high score may benefit from intensified antiplatelet therapy by improved platelet inhibition and risk reduction for thromboischemic events.     

口服精氨酸抑制血小板钙波动机制研究

目的　观察口服精氨酸对正常人血小板钙波动的影响并探讨其作用机制。方法　选择10名至少 2周内未服用任何药物 ,血小板聚集功能正常的健康男性志愿者 10例。年龄 ( 2 6 7± 3 6)岁 ,体重 ( 67 9± 5 0 )kg。志愿者口服L 精氨酸 4d ,每日 2 0g,分 3次服用。用激光扫描共聚焦显微镜观察口服精氨酸前后单个血小板钙波动变化。结果　服用精氨酸后 ,血清精氨酸浓度 [( 80 90±3 0 17) μmol/L和 ( 2 0 0 .19± 4 2 .99) μmol/L ,P <0 0 1],血小板胞浆内环磷酸鸟苷浓度 [( 1 91±0 2 0 )pmol/ 10 9血小板和 ( 2 .14± 0 .3 4)pmol/ 10 9血小板 ,P <0 0 5 ]比服用前升高 ;二磷酸腺苷诱导的血小板钙波动峰值降低。结论　口服精氨酸可以通过激活血小板的左旋精氨酸 /一氧化氮通路 ,促进一氧化氮和环磷酸鸟苷的合成使二磷酸腺苷诱导的血小板钙波动峰值降低。     

Clinical predictors of dual aspirin and clopidogrel poor responsiveness in stable cardiovascular patients from the ADRIE study

Summary. Background: Poor response to both aspirin and clopidogrel (dual poor responsiveness [DPR]) is a major risk factor for recurrent ischemic events. Objectives: The aim of this study was to identify factors associated with DPR, defined with specific tests, and derive a predictive clinical score. Methods: We studied 771 consecutive stable cardiovascular patients treated with aspirin (n = 223), clopidogrel (n = 111), or both drugs (n = 437). Aspirin responsiveness was evaluated by serum thromboxane (Tx)B₂ assay, and clopidogrel responsiveness by calculating the platelet reactivity index (PRI) on the basis of the phosphorylation status of the vasodilator phosphoprotein. The analysis was focused on patients treated with both drugs, and on independent predictors of DPR. Results: Among patients on dual therapy, there was no relevant correlation between TxB₂ levels and PRI values (r = 0.11). Sixty‐seven patients (15.4%) had DPR. Diabetes [odds ratio (OR) 1.89, 95% confidence interval (CI) 1.06–3.39], high body weight (> 86 kg vs. < 77 kg, OR 4.74, 95% CI 2.49–9.73), low aspirin dose (75–81 mg vs. ≥ 160 mg, OR 0.12, 95% CI 0.09–0.93) and high C‐reactive protein (CRP) level (> 1.6 mg L^?1 vs. < 0.6 mg L^?1, OR 3.66, 95% CI 1.74–8.72) were independently associated with DPR, via increased TxB₂ levels, increased PRI, or both. These associations with TxB₂ and PRI were reproduced across the whole population. With use of a factor‐weighed score (c‐index = 0.74), the predicted prevalence of DPR was 57% in the highest strata of the score as compared with < 4% for the lowest strata. Conclusions: Diabetes, body weight, the aspirin dose and CRP levels are readily available independent predictors of DPR, and some are potential targets for reducing its prevalence.     

Determinants of the interindividual variability in response to antiplatelet drugs

The aim of this review article is to discuss the main determinants of the interindividual variability in response to antiplatelet agents. The main sources of pharmacokinetic and pharmacodynamic variability are reviewed, with particular emphasis on aspirin and clopidogrel. The term 'resistance' is uninformative of the mechanism(s) underlying interindividual variability in response to these antiplatelet agents, and is potentially misleading. Increased awareness of the distinct factors potentially interfering with the desired antiplatelet effects of aspirin or clopidogrel, particularly avoidable drug interactions, may ultimately result in better patient management than requesting unnecessary costly tests of platelet function. Similarly, new studies addressing the interindividual variability in response to these antiplatelet agents should rely upon mechanism-based biochemical end-points rather than platelet aggregation measurements. As with any drug used to prevent atherothrombosis, treatment 'failure' can occur with aspirin or clopidogrel perhaps not surprisingly, given the multifactorial nature of atherothrombosis. There is no scientific basis for changing antiplatelet therapy in the face of a treatment 'failure', as we cannot be sure whether a second vascular event occurring in the same patient will reflect the same pathophysiological event that led to the first. Moreover, we have no controlled evidence that changing therapy is a more effective strategy than maintaining an evidence-based therapy.     

Cyclooxygenase-1 haplotype modulates platelet response to aspirin

BACKGROUND: Aspirin (acetylsalicylic acid) irreversibly inhibits platelet cyclooxygenase (COX)-1, the enzyme that converts arachidonic acid (AA) to the potent platelet agonist thromboxane (TX) A2. Despite clear benefit from aspirin in patients with cardiovascular disease (CAD), evidence of heterogeneity in the way individuals respond has given rise to the concept of 'aspirin resistance.'AIMS: To evaluate the hypothesis that incomplete suppression of platelet COX as a consequence of variation in the COX-1 gene may affect aspirin response and thus contribute to aspirin resistance. PATIENTS AND METHODS: Aspirin response, determined by serum TXB2 levels and AA-induced platelet aggregation, was prospectively studied in patients (n = 144) with stable CAD taking aspirin (75-300 mg). Patients were genotyped for five single nucleotide polymorphisms in COX-1 [A-842G, C22T (R8W), G128A (Q41Q), C644A (G213G) and C714A (L237M)]. Haplotype frequencies and effect of haplotype on two platelet phenotypes were estimated by maximum likelihood. The four most common haplotypes were considered separately and less common haplotypes pooled. RESULTS: COX-1 haplotype was significantly associated with aspirin response determined by AA-induced platelet aggregation (P = 0.004; 4 d.f.). Serum TXB2 generation was also related to genotype (P = 0.02; 4 d.f.). CONCLUSION: Genetic variability in COX-1 appears to modulate both AA-induced platelet aggregation and thromboxane generation. Heterogeneity in the way patients respond to aspirin may in part reflect variation in COX-1 genotype.     

Novel assay demonstrates that coronary artery disease patients have heightened procoagulant platelet response

Essentials

• Procoagulant platelets can be detected using GSAO in human whole blood.
• Stable coronary artery disease is associated with a heightened procoagulant platelet response.
• Agonist‐induced procoagulant platelet response is not inhibited by aspirin alone.
• Collagen plus thrombin induced procoagulant platelet response is partially resistant to clopidogrel.

Summary

Background

Procoagulant platelets are a subset of highly activated platelets with a critical role in thrombin generation. Evaluation of their clinical utility in thrombotic disorders, such as coronary artery disease (CAD), has been thwarted by the lack of a sensitive and specific whole blood assay.

Objectives

We developed a novel assay, utilizing the cell death marker, GSAO [(4‐(N‐(S‐glutathionylacetyl)amino)phenylarsonous acid], and the platelet activation marker, P‐selectin, to identify procoagulant platelets in whole blood by flow cytometry.

Patients/Methods

Using this assay, we characterized the procoagulant platelet population in healthy controls and a cohort of patients undergoing elective coronary angiography.

Results

In patients with CAD, compared with patients without CAD, there was a heightened procoagulant platelet response to thrombin (25.2% vs. 12.2%), adenosine diphosphate (ADP) (7.8% vs. 2.7%) and thrombin plus collagen (27.2% vs. 18.3%). The heightened procoagulant platelet potential in CAD patients was not associated with other markers of platelet function, including aggregation, dense granule release and activation of α_2bβ₃ integrin. Although dual antiplatelet therapy (DAPT) was associated with partial suppression of procoagulant platelets, this inhibitory effect on a patient level could not be predicted by aggregation response to ADP and was not fully suppressed by clopidogrel.

Conclusions

We report for the first time that procoagulant platelets can be efficiently detected in a few microliters of whole blood using the cell death marker, GSAO, and the platelet activation marker, P‐selectin. A heightened procoagulant platelet response may provide insight into the thrombotic risk of CAD and help identify a novel target for antiplatelet therapies in CAD.     

Evidence that production of platelet fibrinogen is synchronous with platelet production in the turpentine-induced acute phase response

Fibrinogen is synthesized by both hepatocytes and megakaryocytes, and available evidence indicates that cellular production of this protein is controlled by a single gene. We evaluated platelet and plasma fibrinogen production in rats injected with turpentine to induce the acute phase response. After turpentine injection, plasma fibrinogen levels, as expected, rose to more than double the baseline values within 48 hours and then declined to the upper limit of the normal range in 6 days. When levels were expressed as a percentage of total plasma protein, a rise form 4.2% to 10.0% occurred. Thrombocytosis was not observed until day 4; platelet counts returned to the normal range by day 8. The mean fibrinogen content per platelet did not change significantly (P greater than 0.05) during this period of observation. Thus, in contrast to the abrupt rise in the level of plasma fibrinogen, changes in the blood level of platelet fibrinogen paralleled changes in the platelet count. We interpret these findings to indicate that platelet fibrinogen production is synchronized with platelet production by megakaryocytes and thus suggest that fibrinogen production is regulated differently in these cells than it is in hepatocytes.     

Distribution and dynamic changes of sphingolipids in blood in response to platelet activation

BACKGROUND: Sphingolipids are signaling molecules in a range of biological processes. While sphingosine-1-phosphate (S1P) is thought to be abundantly stored in platelets and released upon stimulation, knowledge about the distribution and function of other sphingolipids in blood is lacking. OBJECTIVES: To analyze the sphingolipid content of blood components with special emphasis on dynamic changes in platelets. METHODS: Blood components from mice and humans were prepared by gradient centrifugation and analyzed by liquid chromatography-mass spectrometry. Additionally, murine platelets were activated in vitro and in vivo. RESULTS: Isolated non-activated platelets of mice were devoid of S1P, but instead contained dihydrosphingosine-1-phosphate (dhS1P), along with a high concentration of ceramide. Activation of platelets in vitro led to a loss of dhS1P and an increase in sphingosine, accompanied by a reduction of ceramide content. Platelet activation in vivo led to an immediate and continuous rise of dhS1P in plasma, while S1P remained stable. The sphingolipid distribution of human blood was markedly different from mice. Human platelets contained dhS1P in addition to S1P. CONCLUSIONS: Mouse platelets contain dhS1P instead of S1P. Platelet activation causes loss of dhS1P and breakdown of ceramide, implying ceramidase activation. Release of dhS1P from activated platelets might be a novel signaling pathway. Finally, the sphingolipid composition of mouse and human blood shows large differences, which must be considered when studying sphingolipid biology.     

The recovery of platelet cyclooxygenase activity explains interindividual variability in responsiveness to low‐dose aspirin in patients with and without diabetes

See also Lordkipanidze M, Harrison P. Aspirin twice a day keeps new COX‐1 at bay. This issue, pp 1217–9. Summary Background. Interindividual variability in response to aspirin has been popularized as ‘resistance’. We hypothesized that faster recovery of platelet cyclooxygenase‐1 activity may explain incomplete thromboxane (TX) inhibition during the 24‐h dosing interval. Objective. To characterize the kinetics and determinants of platelet cyclooxygenase‐1 recovery in aspirin‐treated diabetic and non‐diabetic patients. Patients/Methods. One hundred type 2 diabetic and 73 non‐diabetic patients on chronic aspirin 100 mg daily were studied. Serum TXB₂ was measured every 3 h, between 12 and 24 h after a witnessed aspirin intake, to characterize the kinetics of platelet cyclooxygenase‐1 recovery. Patients with the fastest TXB₂ recovery were randomized to aspirin 100 mg once daily, 200 mg once daily or 100 mg twice daily, for 28 days and TXB₂ recovery was reassessed. Results and Conclusions. Platelet TXB₂ production was profoundly suppressed at 12 h in both groups. Serum TXB₂ recovered linearly, with a large interindividual variability in slope. Diabetic patients in the third tertile of recovery slopes (≥ 0.10 ng mL^?1 h^?1) showed significantly higher mean platelet volume and body mass index, and younger age. Higher body weight was the only independent predictor of a faster recovery in non‐diabetics. Aspirin 100 mg twice daily completely reversed the abnormal TXB₂ recovery in both groups. Interindividual variability in the recovery of platelet cyclooxygenase activity during the dosing interval may limit the duration of the antiplatelet effect of low‐dose aspirin in patients with and without diabetes. Inadequate thromboxane inhibition can be easily measured and corrected by a twice daily regimen.     

Aspirin and the in vitro linear relationship between thromboxane A2‐mediated platelet aggregation and platelet production of thromboxane A2

Summary. Background: Currently, ‘aspirin resistance’, the anti‐platelet effects of non‐steroid anti‐inflammatory drugs (NSAIDs) and NSAID‐aspirin interactions are hot topics of debate. It is often held in this debate that the relationship between platelet activation and thromboxane (TX) A₂ formation is non‐linear and TXA₂ generation must be inhibited by at least 95% to inhibit TXA₂‐dependent aggregation. This relationship, however, has never been rigorously tested. Objectives: To characterize, in vitro and ex vivo, the concentration‐dependent relationships between TXA₂ generation and platelet activity. Method: Platelet aggregation, thrombi adhesion and TXA₂ production in response to arachidonic acid (0.03–1 mmol L^?1), collagen (0.1–30 μg mL^?1), epinephrine (0.001–100 μmol L^?1), ADP, TRAP‐6 amide and U46619 (all 0.1‐30 μmol L^?1), in the presence of aspirin or vehicle, were determined in 96‐well plates using blood taken from naïve individuals or those that had taken aspirin (75 mg, o.d.) for 7 days. Results: Platelet aggregation, adhesion and TXA₂ production induced by either arachidonic acid or collagen were inhibited in concentration‐dependent manners by aspirin, with logIC₅₀ values that did not differ. A linear relationship existed between aggregation and TXA₂ production for all combinations of arachidonic acid or collagen and aspirin (P < 0.01; R² 0.92; n = 224). The same relationships were seen in combinations of aspirin‐treated and naïve platelets, and in blood from individuals taking an anti‐thrombotic dose of aspirin. Conculsions: These studies demonstrate a linear relationship between inhibition of platelet TXA₂ generation and TXA₂‐mediated aggregation. This finding is important for our understanding of the anti‐platelet effects of aspirin and NSAIDs, NSAID–aspirin interactions and ‘aspirin resistance’.     

Identifying determinants of variability to tailor aspirin therapy

Once-daily, low-dose aspirin is a cornerstone in the prophylaxis and treatment of cardiovascular diseases. Aspirin ‘resistance’ still lacks definition, a standardized reference assay, underlying mechanisms, clinical impact or efficacy of alternative antiplatelet drugs. Aspirin response in several studies has been measured by different platelet function tests, not always reflecting aspirin pharmacodynamics, thus generating significantly heterogeneous results. The EMA indicated serum thromboxane B₂ as the only valid surrogate assay to study different aspirin formulations. Rather than resistance, recent studies focused on sources of intra- and inter-individual variability in response to aspirin, based on pharmacokinetic and/or pharmacodynamic mechanisms. Drug interactions, diabetes, conditions of increased platelet output, obesity and aging can potentially increase the variability of aspirin response. Preliminary studies testing different aspirin regimens showed that twice-daily low doses were more effective than once-daily higher aspirin doses on surrogate end points of platelet inhibition. Large studies are needed to test new disease-tailored, low-dose aspirin regimens.     

The chaperone protein HSP47: a platelet collagen binding protein that contributes to thrombosis and hemostasis

Essentials

• Heat shock protein 47 (HSP47), a collagen specific chaperone is present on the platelet surface.
• Collagen mediated platelet function was reduced following blockade or deletion of HSP47.
• GPVI receptor regulated signalling was reduced in HSP47 deficient platelets.
• Platelet HSP47 tethers to exposed collagen thus modulating thrombosis and hemostasis.

Summary

Objective

Heat shock protein 47 (HSP47) is an intracellular chaperone protein that is vital for collagen biosynthesis in collagen secreting cells. This protein has also been shown to be present on the surface of platelets. Given the importance of collagen and its interactions with platelets in triggering hemostasis and thrombosis, in this study we sought to characterize the role of HSP47 in these cells.

Methods and Results

The deletion of HSP47 in mouse platelets or its inhibition in human platelets reduced their function in response to collagen and the GPVI agonist (CRP‐XL), but responses to thrombin were unaltered. In the absence of functional HSP47, the interaction of collagen with platelets was reduced, and this was associated with reduced GPVI‐collagen binding, signalling and platelet activation. Thrombus formation on collagen, under arterial flow conditions, was also decreased following the inhibition or deletion of HSP47, in the presence or absence of eptifibatide, consistent with a role for HSP47 in enhancing platelet adhesion to collagen. Platelet adhesion under flow to von Willebrand factor was unaltered following HSP47 inhibition. Laser‐induced thrombosis in cremaster muscle arterioles was reduced and bleeding time was prolonged in HSP47‐deficient mice or following inhibition of HSP47.

Conclusions

Our study demonstrates the presence of HSP47 on the platelet surface, where it interacts with collagen, stabilizes platelet adhesion and increases collagen‐mediated signalling and therefore thrombus formation and hemostasis.     

Influence of high‐density lipoprotein and paraoxonase‐1 on platelet reactivity in patients with acute coronary syndromes receiving clopidogrel therapy

Summary. Background: The paraoxonase activity of the enzyme paraoxonase‐1 (PON‐1) associated with high‐density lipoprotein (HDL) may significantly influence clopidogrel’s antiplatelet and clinical efficacy as a result of its involvement in the clopidogrel biotransformation to the pharmacologically active thiol metabolite. We evaluated the possible relationships of HDL levels as well as PON‐1 activities and the Q192R genotype with clopidogrel’s antiplatelet efficacy in acute coronary syndrome (ACS) patients. Methods and results: The platelet aggregation, P‐selectin expression and platelet/leukocyte conjugates as well as the clopidogrel response variability (evaluated by the VASP phosphorylation test and expressed as platelet reactivity index, PRI) were assessed in 74 ACS patients undergoing percutaneous coronary intervention (PCI) in relation to the PON‐1 Q192R genotype and to serum HDL‐cholesterol levels, and PON‐1 (paraoxonase and arylesterase) activities. Patients were loaded with 600 mg of clopidogrel followed by 75 mg per day. HDL‐cholesterol levels and PON‐1 activities at baseline (before clopidogrel loading) were not altered at 5‐ and 30‐day post‐clopidogrel loading, whereas baseline platelet activation parameters were significantly attenuated. At 5 days, 17 patients were clopidogrel non‐responders (PRI: 64.2 ± 11.1%). HDL‐cholesterol was inversely associated with platelet activation parameters independently on platelet response variability to clopidogrel whereas a negative association between platelet activation parameters and paraoxonase activity was observed in patients adequately responding to clopidogrel but not in clopidogrel non‐responders. Similarly, the platelet activation markers were significantly higher in PON‐1 Q192Q genotype carriers compared with those having one or two R alleles only in patients adequately responding to clopidogrel. Conclusions: PON‐1 is an important determinant of clopidogrel antiplatelet efficacy only in patients adequately responding to clopidogrel. These findings may be clinically important in ACS patients receiving clopidogrel therapy, especially the first days after the episode.