Decreased Platelet Inhibition Research Articles

Use of Thromboelastography with Platelet Mapping to Identify Prothrombotic Coagulation Profiles in Patients with History of Cardiac Intervention Undergoing Lower Extremity Revascularization.

Patients with concomitant coronary and peripheral artery disease (CAD and PAD) are at significant risk for major adverse limb events (MALEs). Prevention of thrombosis in this population is of paramount importance. Identifying prothrombotic coagulation profiles in this cohort may facilitate targeted thromboprophylaxis. We compared coagulation profiles of those with CAD and PAD to those with PAD alone during the perioperative period of lower extremity revascularization. Patients undergoing lower extremity revascularization underwent thromboelastography-platelet mapping (TEG-PM) analysis preoperatively and at serial intervals for up to 6 months. Coagulation profiles of patients with significant CAD (defined as history of coronary artery bypass graft or percutaneous coronary intervention) and PAD were compared with those with PAD alone. MALE in the postoperative period was recorded. Four hundred seventy-seven TEG-PM samples from 114 patients were analyzed; 28.1% had a history of significant CAD. The incidence of atrial fibrillation was higher in this group. The significant CAD group had lower ADP-platelet inhibition, higher ADP-platelet aggregation, and greater maximum clot strength compared with patients with PAD alone. Patients with significant CAD were more frequently on full-dose anticoagulation, but less frequently on dual antiplatelet therapy; 28.1% of patients with significant CAD developed postoperative MALE compared with 22.9% of patients with PAD alone (p = 0.40). For both groups, patients who developed postoperative MALE demonstrated greater ADP-platelet aggregation and lower ADP-platelet inhibition. Patients with a history of significant CAD undergoing lower extremity revascularization demonstrated prothrombotic TEG-PM profiles, less frequent use of dual antiplatelet therapy, and greater rates of full-dose anticoagulation. Decreased platelet inhibition was also associated with postoperative MALE. This study underscores the potential utility of viscoelastic assays for coagulation profiling in complex cardiovascular patients.

Abstract 14286: Comparison of Enteric Coated Aspirin With Plain Aspirin on Effectiveness and Safety in the ADAPTABLE Trial

Introduction: Many clinicians recommend enteric coated aspirin (EC-ASA) to decrease gastrointestinal bleeding (GIB) in secondary cardiovascular (CV) protection even though studies suggest decreased platelet inhibition with EC-ASA compared to uncoated “plain” formulations (P-ASA). ADAPTABLE was a large, pragmatic study, showing no difference in ischemic events or bleeding between 81-mg and 325-mg ASA and provides a large population to assess effectiveness and safety of EC-ASA versus P-ASA in CV patients. Methods: Participants (pts) from ADAPTABLE were re-grouped according to type and dose of reported ASA use at baseline. The primary effectiveness endpoint was the composite of myocardial infarction (MI), stroke, or death from any cause and the primary safety endpoint was major bleeding events (hospitalization for a bleeding event with use of a blood product or intracranial hemorrhage) analyzed between groups by ASA type and within groups by dose using multivariable Cox proportional hazards. Results: 15,076 pts were randomized to 81-mg or 325 -mg ASA were followed for a median of 26.2 months of whom 10,678 reported ASA type at baseline. Of pts reporting ASA type used, 69% used EC-ASA and were slightly older, more likely to be in the 81-mg cohort, be nonsmokers, have slightly more coronary artery disease, and have a slightly lower prior MI rate (Table). No other significant differences between the 2 cohorts were noted. There was no significant difference in the effectiveness or safety outcomes between the EC-ASA and P-ASA cohorts (Table). Within EC-ASA and P-ASA, ASA dose had no effect on effectiveness (interaction p = 0.41) or safety (interaction p = 0.07). Conclusion: Enteric coating did not significantly affect the primary effectiveness or safety endpoints in ADAPTABLE, regardless of ASA dose. More research is needed to confirm whether enteric coated aspirin formulations or newer formulations will improve ischemic and bleeding outcomes in patients with CV disease.

Decreased Platelet Inhibition by Thienopyridines in Hyperuricemia

PurposeHyperuricemia carries an increased risk of atherothrombotic events in acute coronary syndrome (ACS) patients undergoing percutaneous coronary intervention (PCI). This may at least in part be due to inadequate P2Y12 inhibition. The aim of this study was to prospectively investigate the potential association between hyperuricemia and decreased platelet inhibition by P2Y12 antagonists.MethodsLevels of uric acid as well as on-treatment residual platelet reactivity in response to adenosine diphosphate (ADP) were assessed in 301 clopidogrel-treated patients undergoing elective angioplasty and stenting, and in 206 prasugrel- (n = 118) or ticagrelor-treated (n = 88) ACS patients following acute PCI. Cut-off values for high on-treatment residual ADP-inducible platelet reactivity (HRPR) were based on previous studies showing an association of test results with clinical outcomes.ResultsHyperuricemia was significantly associated with increased on-treatment residual ADP-inducible platelet reactivity in clopidogrel- and prasugrel-treated patients in univariate analyses and after adjustment for differences in patient characteristics by multivariate regression analyses. In contrast, ticagrelor-treated patients without and with hyperuricemia showed similar levels of on-treatment residual platelet reactivity to ADP. HRPR occurred more frequently in clopidogrel- and prasugrel-treated patients with hyperuricemia than in those with normal uric acid levels. In contrast, hyperuricemic patients receiving ticagrelor did not have a higher risk of HRPR compared with those with normal uric acid levels.ConclusionHyperuricemia is associated with decreased platelet inhibition by thienopyridines but a normal response to ticagrelor. It remains to be established if lowering uric acid increases the antiplatelet effects of clopidogrel and prasugrel in hyperuricemic patients with HRPR.

Clopidogrel Resistance in Lower Extremity Arterial Endovascular Interventions.

Antiplatelet pharmacotherapy for endovascular interventions has been widely adopted, with clopidogrel being one of the most common agents prescribed. A fraction of patients is resistant to clopidogrel resulting in decreased platelet inhibition despite adequate use. This finding is often termed high on-treatment platelet reactivity (HPR) and may lead to decreased patency in lower extremity arterial endovascular interventions. Current literature on HPR with lower extremity arterial endovascular interventions is limited to only a few studies. Resistance to clopidogrel is largely a result of CYP2C19 enzyme loss of function alleles. Several tests are available to measure clopidogrel resistance but light transmittance aggregometry remains the gold standard, yet direct genetic testing may be more reliable. One-year patency rates following lower extremity arterial endovascular interventions in patients with clopidogrel resistance (HPR) range between 35%-83% whereas those with the proper response to clopidogrel range between 73%-100%. Patients with decreased CYP2C19 activity show a significant decrease in one-year patency of endovascular femoropopliteal interventions (35% vs. 73%; p=0.006). Among patients tested for platelet function after in-stent thrombosis, up to 53% are resistant to clopidogrel. Lack of robust data limits our ability to predict patency in lower extremity arterial interventions for patients with HPR, but there is little doubt that longer patency seems to favor non-HPR patients. Large population, prospective trials are needed to guide our practice.

Decreased platelet inhibition by P2Y12 receptor blockers in anaemia

Anaemic patients undergoing angioplasty and stenting are at an increased risk of ischaemic events, which may be caused by an inadequate response to antiplatelet therapy with adenosine diphosphate (ADP) P2Y12 inhibitors. In the current study, we investigated the associations between anaemia and on-treatment platelet reactivity in clopidogrel-treated (group 1, n=306) and prasugrel-/ticagrelor-treated (group 2, n=109) patients undergoing elective and acute angioplasty with stent implantation, respectively. Monocyte-platelet aggregate (MPA) formation was determined by flow cytometry in both groups. On-treatment residual platelet reactivity in response to ADP was assessed by light transmission aggregometry (LTA) in both groups, and by the VerifyNow P2Y12 assay and the Impact-R in group 1. P-selectin expression was measured by flow cytometry in group 2. In both groups, anaemia was associated with significantly higher MPA formation in response to ADP (both P≤.02). Moreover, by LTA maximal aggregation in response to ADP was significantly higher in patients with anaemia in both groups (both P<.05), and anaemic patients in group 1 had a significantly higher on-treatment platelet reactivity by the VerifyNow P2Y12 assay and the Impact-R than those without anaemia (both P<.001). In group 2, significantly higher platelet surface expression of P-selectin was seen in anaemia after stimulation with ADP (P=.02). Anaemia is associated with decreased platelet inhibition by ADP P2Y12 receptor antagonists after elective and acute percutaneous interventions with stent implantation. However, due to inconsistencies between different platelet function tests additional data are needed to clarify the role of anaemia for platelet inhibition.

Genetic polymorphisms of CYP2C19*2 and ABCB1 C3435T affect the pharmacokinetic and pharmacodynamic responses to clopidogrel in 401 patients with acute coronary syndrome

Backgrounds and objectivesClopidogrel, an inhibitor of platelet ADP P2Y12 receptors, plays an important role in the prevention of stent thrombosis. However, some patients do not attain adequate antiplatelet effects. Studies have shown that the genetic variation in CYP2C19*2 is associated with an impaired response to clopidogrel. This study was designed to investigate the genetic variants of 21 genes involving in the absorption, metabolism, and pharmacodynamics of clopidogrel. The effects of these genes on the plasma level of clopidogrel and its metabolites (active clopi-H4 and inactive CLPM) and platelet reactivity were also studied. Methods and results401 acute coronary syndrome (ACS) patients received either a 300mg loading dose following 75mg maintenance dose daily or a 75mg maintenance dose daily of clopidogrel. The inhibition of platelets was assessed using light transmittance aggregometry. Plasma concentrations of clopidogrel as well as its active (clopi-H4) and inactive (CLPM) metabolites were measured using HPLC-MS-MS method. Among 21 genes, the carriers of CYP2C19*2 were associated with lower exposure to its active (clopi-H4) and inactive (CLPM) metabolites (both P<0.05 vs. non-carriers) and thus decreased platelet inhibition (P<0.05 vs. non-carriers). Notably, the carriers of ABCB1 C3435T were associated with lower levels of plasma clopidogrel and its active (clopi-H4) and inactive (CLPM) metabolites (all P<0.05 vs. non-carriers) which also correlated with subsequently decreased platelet inhibition (P<0.05 vs. non-carriers). There were no obvious effects of other studied genes on clopidogrel. ConclusionsCYP2C19*2 is a determinant for the formation of the active metabolite of clopidogrel and its antiplatelet effects. Meanwhile, ABCB1 C3435T plays an important role in intestinal absorption of clopidogrel which further affects the exposure to the active metabolite of clopidogrel and platelet aggregation.

CYP2C19 Genetic Testing Should Not Be Done in All Patients Treated With Clopidogrel Who Are Undergoing Percutaneous Coronary Intervention

Clopidogrel when added to aspirin reduces major vascular events in patients undergoing percutaneous coronary intervention (PCI).1 Recent reports have suggested that common genetic variants involving hepatic cytochrome P450 system enzymes that convert clopidogrel to its active metabolite are associated with an increased risk of cardiovascular events. Specifically, patients who are carriers of 1 or more loss-of-function CYP2C19 alleles (including the *2 and *3 alleles) have reduced conversion of clopidogrel to its active metabolite, decreased platelet inhibition, and an increased risk of myocardial infarction, death, and stent thrombosis compared with noncarriers. Based on these findings and on related pharmacokinetic and pharmacodynamic data (NCT01123824), the United States Food and Drug Administration (FDA) has issued a “black box” warning of reduced effectiveness of clopidogrel in patients who are carriers of 2 loss-of-function alleles (so-called poor metabolizers) and has suggested that affected individuals receive a higher dose of clopidogrel or an alternative antiplatelet agent. This warning has led some investigators to conclude that all patients undergoing PCI with planned clopidogrel therapy should undergo CYP2C19 genetic testing. Response by Sibbing, Bernlochner, and Kastrati on p 521 In this report, we critically review the evidence for routine CYP2C19 testing in patients undergoing PCI according to established criteria for the implementation of a screening test in clinical practice. ### Summary of the Evidence Linking CYP2C129 Loss-of-Function Alleles to Clopidogrel Response and Cardiovascular Risk Clopidogrel is a prodrug that must undergo 2-step hepatic metabolism by enzymes of the CYP system to form the active moiety that inhibits the platelet P2Y12 receptor.2–4 Common loss-of-function variants involving the CYP2C19 gene have been conclusively demonstrated to influence clopidogrel active metabolite levels and levels of platelet inhibition5–7 (Figure 1), and an increasing number of clinical reports link loss-of-function alleles with adverse clinical outcomes.7–12 Figure 1. Hypothesized causal chain leading from loss-of-function (LOF) allele carriage to increased ischemic events …

Sulfonylureas and on-clopidogrel platelet reactivity in type 2 diabetes mellitus patients

Clopidogrel is a prodrug that needs to be converted in vivo by several cytochrome (CYP) P450 iso-enzymes to become active. Both clopidogrel and the oral hypoglycemic drug class sulfonylureas are metabolized by the iso-enzyme CYP2C9. The objective of the study was to evaluate the relationship of sulfonylureas and on-clopidogrel platelet reactivity in type 2 diabetes mellitus patients undergoing elective coronary stent implantation. In this prospective, observational study, on-clopidogrel platelet reactivity was quantified using adenosine diphosphate (ADP)-induced light transmittance aggregometry in 139 type 2 diabetes mellitus patients undergoing elective coronary stent implantation treated with clopidogrel and aspirin. High on-clopidogrel platelet reactivity was defined as >70.7% platelet reactivity to 20 µmol/L ADP. A total of 53 patients (38.1%) were on concomitant treatment with sulfonylureas. The remaining 86 patients were on other hypoglycemic drugs. On-clopidogrel platelet reactivity was significantly higher in patients with concomitant sulfonylurea treatment as compared to patients without concomitant sulfonylurea treatment (for 5 µmol/L ADP: 46.0% ± 11.8 vs. 40.6% ± 16.0; p = 0.035, adjusted p = 0.032 and for 20 µmol/L ADP: 64.6% ± 10.8 vs. 58.7% ± 15.5; p = 0.019, adjusted p = 0.017). The concomitant use of sulfonylureas was associated with a 2.2-fold increased risk of high on-clopidogrel platelet reactivity (OR 2.2, 95% CI 1.1–4.7, p = 0.039 and after adjustment for confounders: ORadj 2.0, 95% CI 1.0–5.7, p = 0.048). Concomitant treatment with sulfonylureas might be associated with decreased platelet inhibition by clopidogrel in type 2 diabetes mellitus patients on dual antiplatelet therapy undergoing elective coronary stent implantation.

Platelet Activity Associated with Concomitant Use of Clopidogrel and Proton Pump Inhibitors in Children with Cardiovascular Disease

In adults with acute coronary syndrome, decreased platelet inhibition associated with concomitant use of clopidogrel and proton pump inhibitors (PPI) has been reported. To evaluate platelet activity associated with PPI + clopidogrel vs. clopidogrel alone in children enrolled in the Platelet Inhibition in Children On cLOpidogrel (PICOLO) trial of clopidogrel in children with a cardiac condition at risk for arterial thrombosis. Patients 0-24 m randomized to active therapy in the PICOLO trial were included in the present analysis. Platelet aggregation inhibition at baseline and steady state were evaluated in patients taking clopidogrel + PPI vs. clopidogrel only in the overall cohort and sub-group of clopidogrel responders. A total of 49 patients were included (44 clopidogrel only, five clopidogrel + PPI); median age 38 days (interquartile range [IQR] 17-157 days). The majority of patients in each group had undergone systemic-to-pulmonary artery shunt. Compared with the clopidogrel group, patients in the clopidogrel + PPI group had a trend toward lower percent inhibition of maximum extent of platelet aggregation overall (median 6%, IQR 0-44% vs. 49%, IQR 19-63%, P= 0.09), and a significant reduction in the clopidogrel responders sub-group (median 25%, IQR 3-45% vs. 53%, IQR 38-65%, P= 0.04). There was no difference in percent inhibition of rate of platelet aggregation. Concomitant use of PPI + clopidogrel may be associated with decreased platelet inhibition in children with cardiac disease. Further study in a larger population and assessment of associated clinical outcomes is warranted.

Intra-individual variability in clopidogrel responsiveness in coronary artery disease patients under long term therapy

Clopidogrel responsiveness (CR) following a loading dose (LD) predicts thrombotic events after percutaneous coronary interventions (PCI). Some of the mechanisms involved in large inter-individual variability in CR may be varied. We therefore postulated that there may be an intra-individual variability in CR. Two hundred and one patients receiving long-term therapy with aspirin and clopidogrel after drug-eluting stents PCI were prospectively included in this monocentre study along with any patient re-admitted within 12 months post-PCI. Platelet reactivity (PR) inhibition was assessed by the vasodilator phosphoprotein (VASP) index following a 600 mg loading dose of clopidogrel on each admission to determine CR (VASP 1 during the first admission and VASP 2 during re-admission). DeltaVASP = VASP 2 –VASP 1 was used to study intra-individual variability in CR. We observed that the response to a 600 mg LD of clopidogrel was poorly correlated within an individual (kappa = 0.33; p < 0.001 (n = 201)). Although most patients had increased platelet inhibition at the time of readmission, 35.3% of patients exhibited a decreased platelet inhibition despite chronic clopidogrel therapy and a 600 mg reload. Quartiles analysis of DeltaVASP demonstrated that insulin-treated diabetes was associated with decreased CR over time (p = 0.03). In addition to the large inter-individual variability in clopidogrel responsiveness, there is large intra-individual variability. Decreased clopidogrel responsiveness despite long-term clopidogrel therapy could be a trigger for recurrent thrombotic events.

Letter by Lin et al Regarding Article, “Triple Versus Dual Antiplatelet Therapy in Patients With Acute ST-Segment Elevation Myocardial Infarction Undergoing Primary Percutaneous Coronary Intervention”

To the Editor: We read with interest the work by Chen et al,1 which reported that triple antiplatelet therapy seems to be superior to dual antiplatelet therapy in patients with ST-segment elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention with drug-eluting stents. Chen et al1 reported the mortality benefits of triple antiplatelet therapy obtained mainly within 30 days after STEMI, reflecting the importance of the addition of cilostazol to aspirin and clopidogrel before stenting. Meanwhile, …

Calcium channel blockers reduce the antiplatelet effect of clopidogrel

Objectives Because of the known CYP3A4 inhibition by calcium-channel blockers (CCBs), we hypothesized that there might be a drug-drug interaction between clopidogrel and dihydropyridines in patients with coronary artery disease. Background Clopidogrel is activated by CYP3A4, which also metabolizes CCBs of the dihydropyridine class. Methods Responsiveness to clopidogrel was assessed by the vasodilator-stimulated phosphoprotein (VASP) phosphorylation assay and aggregometry in 200 patients with coronary artery disease undergoing percutaneous coronary intervention. Results The platelet reactivity index (PRI) (in the VASP assay, normal range 69% to 100%) was higher in patients receiving both clopidogrel and CCBs (61%) as compared with patients receiving clopidogrel without CCBs (48%). The absolute difference was 13% (95% confidence interval: 6% to 20%; p = 0.001), and the relative difference approached 21%. A decreased platelet inhibition by clopidogrel (PRI >69%) was seen in 40% of patients with concomitant CCB treatment and in 20% of patients without concomitant treatment (chi-square test, p = 0.008). Intake of CCB remained an independent predictor of reduced platelet inhibition by clopidogrel after adjustment for cardiovascular risk factors. Adenosine diphosphate-induced platelet aggregation was 30% higher in patients on concomitant CCB treatment compared with patients without CCBs (p = 0.046). Moreover, intake of CCBs was associated with adverse clinical outcome. In vitro incubation with CCBs (nimodipine, verapamil, amlodipine, and diltiazem) did not alter the PRI or the adenosine diphosphate–induced platelet aggregation of patients taking clopidogrel. This finding indicates that the negative effect occurs in vivo, conceivably at the level of the CYP3A4 cytochrome. Conclusions Coadministration of CCBs is associated with decreased platelet inhibition by clopidogrel.

Calcium-Channel Blockers Reduce the Antiplatelet Effect of Clopidogrel

Because of the known CYP3A4 inhibition by calcium-channel blockers (CCBs), we hypothesized that there might be a drug-drug interaction between clopidogrel and dihydropyridines in patients with coronary artery disease. Clopidogrel is activated by CYP3A4, which also metabolizes CCBs of the dihydropyridine class. Responsiveness to clopidogrel was assessed by the vasodilator-stimulated phosphoprotein (VASP) phosphorylation assay and aggregometry in 200 patients with coronary artery disease undergoing percutaneous coronary intervention. The platelet reactivity index (PRI) (in the VASP assay, normal range 69% to 100%) was higher in patients receiving both clopidogrel and CCBs (61%) as compared with patients receiving clopidogrel without CCBs (48%). The absolute difference was 13% (95% confidence interval: 6% to 20%; p = 0.001), and the relative difference approached 21%. A decreased platelet inhibition by clopidogrel (PRI >69%) was seen in 40% of patients with concomitant CCB treatment and in 20% of patients without concomitant treatment (chi-square test, p = 0.008). Intake of CCB remained an independent predictor of reduced platelet inhibition by clopidogrel after adjustment for cardiovascular risk factors. Adenosine diphosphate-induced platelet aggregation was 30% higher in patients on concomitant CCB treatment compared with patients without CCBs (p = 0.046). Moreover, intake of CCBs was associated with adverse clinical outcome. In vitro incubation with CCBs (nimodipine, verapamil, amlodipine, and diltiazem) did not alter the PRI or the adenosine diphosphate-induced platelet aggregation of patients taking clopidogrel. This finding indicates that the negative effect occurs in vivo, conceivably at the level of the CYP3A4 cytochrome. Coadministration of CCBs is associated with decreased platelet inhibition by clopidogrel.

Abstract 1716: Calcium Channel Blockers Reduce the Antiplatelet Effect of Clopidogrel

Due to the known CYP3A4 inhibition by calcium channel blockers, we hypothesized that there might be a pharmacodynamic interaction between clopidogrel and dihydropyridines in patients with coronary artery disease (CAD). Clopidogrel is activated by CYP3A4 which also metabolizes calcium channel blockers of the dihydropyridine class. Methods: Responsiveness to clopidogrel was assessed by the vasodilator stimulated phosphoprotein (VASP) phosphorylation assay and aggregometry in 200 patients with coronary artery disease (CAD) undergoing percutaneous coronary intervention (PCI). The platelet reactivity index (PRI in the VASP assay, normal range 69–100%) was higher in patients on both clopidogrel and calcium channel blockers (61%) as compared to patients on clopidogrel without calcium channel blockers (48%). The absolute difference was 13% (95%CI: 6 –20%; p = 0.001) and the relative difference approached 21%. A high post-treatment platelet reactivity (PRI &gt; 69%) was seen in 40% of patients with concomitant calcium channel blocker treatment compared to 20% without (X2-test: p = 0.008). Intake of calcium channel blocker remained an independent predictor of high platelet reactivity after adjustment for cardiovascular risk factors. This corresponded to an increased platelet aggregation of similar magnitude (p &lt; 0.05). In vitro incubation with calcium channel blockers (nimodipine, verapamil, amlodipine and diltiazem) did not alter the PRI or the ADP-induced platelet aggregation of patients on clopidogrel, indicating that the interaction occurs in vivo, conceivably at the level of the CYP3A4 cytochrome. Co-administration of calcium channel blockers is associated with a decreased platelet inhibition by clopidogrel.

Increased platelet sensitivity among individuals with aspirin resistance – Platelet aggregation to submaximal concentration of arachidonic acid predicts response to antiplatelet therapy

Aspirin 'resistance' (AR) is a phenomenon of uncertain etiology describing decreased platelet inhibition by aspirin. We studied whether (i) platelets in AR demonstrate increased basal sensitivity to a lower degree of stimulation and (ii) platelet aggregation with submaximal stimulation could predict responses to aspirin. Serum thromboxane B(2) (TxB(2)) levels and platelet aggregation with light transmission aggregometry (LTA) were measured at baseline and 24 hours after 325 mg aspirin administration in 58 healthy subjects. AR was defined as the upper sixth of LTA (> or = 12%) to 1.5 mM AA. Baseline platelet aggregation with submaximal concentrations of agonists [ADP 2 microM, arachidonic acid (AA) 0.75 mM, collagen 0.375 and 0.5 microg/ml] was greater in AR subjects compared with non-AR subjects, but not with higher concentrations (ADP 5 microM and 20 microM, AA 1.5 mM and collagen 1 microg/ml). Post-aspirin platelet aggregation was elevated in AR subjects with both submaximal and maximal stimulation. Baseline and post-aspirin serum TxB(2) were higher in AR subjects and decreased further with ex-vivo COX-1 inhibition, suggesting incompletely suppressed COX-1 activity. Pre-aspirin platelet aggregation to 0.75 AA demonstrated a dichotomous response with 29/58 subjects having aggregation < or = 15% and 29/58 subjects having aggregation > or = 75%. In the high aggregation group 28% had AR compared to 6% in the non-AR group (p = 0.04). In conclusion, platelets in AR subjects demonstrate increased basal sensitivity to submaximal stimulation, which could predict responses to antiplatelet therapy.

Decreased platelet inhibition by nitric oxide in two brothers with a history of arterial thrombosis.

Highly reactive oxygen species rapidly inactivate nitric oxide (NO), and endothelial product which inhibits platelet activation. We studied platelet inhibition by NO in two brothers with a cerebral thrombotic disorder. Both children had hyperreactive platelets, as determined by whole blood platelet aggregometry and flow cytometric analysis of the platelet surface expression of P-selectin. Mixing experiments showed that the patients'platelets behaved normally in control plasma; however, control platelets suspended in patient plasma were not inhibited by NO. As determined by flow cytometry, in the presence of plasma from either patient there was normal inhibition of the thrombin-induced expression of platelet surface P-selectin by prostacyclin, but not NO. Using a scopoletin assay, we measured a 2.7-fold increase in plasma H2O2 generation in one patient and a 3.4-fold increase in the second patient, both compared woth control plasma. Glutathione peroxidase (GSH-Px) activity was decreased in the patients' plasmas compared with control plasma. The addition of exogenous GSH-Px led to restoration of platelet inhibition by NO. These data show that, in these patients' plasmas, impaired metabolism of reactive oxygen species reduces the bioavailability of NO and impairs normal platelet inhibitory mechanisms. These findings suggest that attenuated NO-mediated platelet inhibition produced by increased reactive oxygen species or impaired antioxidant defense may cause a thrombotic disorder in humans.