Effects Of Antiplatelet Agents Research Articles

The effects of antiplatelet agents on platelet–leukocyte aggregations in patients with acute cerebral infarction

Studies have shown that platelet-leukocyte aggregates (PLA) are sensitive to platelet activation which might exist before the onset of cerebral infarction. In this study, we investigated the formation of PLA in patients with cerebral infarction and the effects of antiplatelet agents on PLA. The level of soluble P-selectin, C-reaction protein, platelet aggregation rate and leukocyte-platelet aggregations were measured in 40 patients with acute cerebral infarction and 20 normal controls. The 40 patients were randomly assigned to two treatment groups: aspirin group (n = 20) and clopidogrel group (n = 20). Both groups were monitored for Scandinavian stroke scale (SNSS), soluble P-selectin, serum C-reaction protein, platelet aggregation rate and PLA before and after the treatment. Flow cytometry was used to detect the levels of PLA in the blood. The percentage of platelet-monocyte aggregates (PMA) in patients with cerebral infarction was significantly increased compared with the controls (P < 0.001), which was positively correlated to soluble P-selectin, C-reaction protein and platelet aggregation rate (P < 0.05). After the treatment, the levels of PMA and platelet aggregation rate were decreased in both groups (P < 0.05). The level of PMA and platelet aggregation rate in the clopidogrel group was significantly lower than that in the aspirin group (P < 0.05). PMA are a sensitive biomarker to platelet activation in patients with cerebral infarction. In addition, although both aspirin and clopidogrel lowered the level of PMA, clopidogrel is a more effective treatment than aspirin in inhibiting platelet activation.

Characterization of Canine Platelet Activation Induced by Platelet Activating Factor (PAF)

Characteristics of the signal transduction in Platelet activating factor (PAF)-activated platelets and effects of anti-platelet agents on this response were investigated in vitro for potential therapeutic applications in canine endotoxemia. Blockade of the PAF receptor by a specific blocker has the strongest inhibitive effect on the PAF-induced platelet reactions. The response was also inhibited by either Ca(2+) channel blockers or prostaglandin E(1).

Carotid endarterectomy for stroke prevention revisited

Since its original description, more than 50 years ago, carotid endarterectomy (CEA) has been challenged in its success in achieving adequate stroke prevention among both symptomatic and asymptomatic patients with cervical carotid stenosis. CEA remains the most common vascular surgical operation performed today, however, its future has been called into question with the introduction of percutaneous carotid angioplasty and stenting, more effective antiplatelet agents (i.e., clopidogrel), cholesterol-lowering agents (i.e., statins) and angiotensin-converting enzyme inhibitors. The focus of this article is to review the notable trials substantiating the efficacy of CEA, indications for surgery and technical components that have refined expected favorable outcomes.

Cost–effectiveness of antiplatelet therapy for secondary stroke prevention

Antiplatelet therapy is recommended over anticoagulants for the secondary prevention of vascular death in patients with noncardioembolic ischemic stroke or transient ischemic attack based upon the 2006 American Heart Association/American Stroke Association guidelines for the prevention of stroke and the National Stroke Association guidelines for the management of transient ischemic attack. Aspirin is commonly used as a cornerstone antiplatelet agent considering its mild but definite prevention benefit and low costs. Other antiplatelet strategies that are currently recommended include extended-release dipyridamole plus low-dose aspirin (Aggrenox®, Asasantin®) and clopidogrel. In this brief review, we evaluate the cost–effectiveness of antiplatelet agents for secondary stroke prevention to better understand the socioeconomical value of the recommended agents.

Influence of Preparative Procedures on Assay of Platelet Function and Apparent Effects of Antiplatelet Agents

Previous studies have shown that anticoagulants alter platelet reactivity and the apparent effects of antiplatelet agents. This study was conducted to identify the impact of methods of preparation of blood samples on an assay of platelet function and the effects of antiplatelet agents. The activation of platelets was identified with the use of flow cytometry in response to thrombin (1 and 10 nmol/L), adenosine diphosphate (0.2 and 1 micromol/L), platelet activating factor (1 nmol/L), and convulxin (1 and 10 ng/ml). Antiplatelet effects were assessed after the addition in vitro of tirofiban (50 ng/ml) and cangrelor (10 nmol/L). Results were compared in whole blood and platelet-rich plasma (PRP) anticoagulated with corn trypsin inhibitor (32 microg/ml, a specific inhibitor of factor XIIa). The fraction of young platelets was quantified with thiazole orange, which identifies ribonucleic acid. The activation of platelets was consistently less in PRP compared with whole blood. Activation in PRP was 23 +/- 15% that in whole blood for thrombin, 65 +/- 26% for adenosine diphosphate, 40 +/- 20% for platelet activating factor, and 49 +/- 25% for convulxin (p <0.01 for each comparison). The fraction of young platelets in PRP was 39 +/- 11% that in whole blood (p <0.001). The effects of antiplatelet agents varied with agonist and antiplatelet agent but were generally greater in PRP compared with whole blood (p <0.05). In conclusion, platelet reactivity is lower and the effects of antiplatelet agents are greater and potentially misleading in PRP compared with whole blood. The accuracy of platelet function testing may be improved by performance in whole blood.

Enzyme Replacement in Fabry Disease: The Essence Is in the Kidney

In this issue, Banikazemi and colleagues' landmark trial on Fabry disease provides a crucial path toward lowering the morbidity of this disease. Among other findings, they determined that hemizygou...

Increased basal platelet activity, plasma adiponectin levels, and diabetes mellitus are associated with poor platelet responsiveness to in vitro effect of aspirin

Introduction Aspirin is one of the most effective antiplatelet agents and is now commonly used to prevent vascular events. In some patients, however, recurrent vascular events have been demonstrated despite aspirin therapy. Our objective was to characterize individuals showing poor response to in vitro effect of aspirin, using PFA-100. Methods One hundred sixty-eight healthy male subjects were analyzed. We assessed platelet function tests, including PFA-100, whole blood aggregation, and optical platelet aggregation. Also measured were hemostatic and other parameters including von Willebrand factor (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), soluble vascular adhesion molecule-1 (sVCAM-1), high sensitive C-reactive protein (hs-CRP), and adiponectin. Poor responders were defined as having a collagen/epinephrine-induced closure time (CEPI-CT) under 250 s with PFA-100 when incubated with 10 μM aspirin, whereas good responders were defined as having a CEPI-CT of more than 250 s. Results and conclusions PFA-100 tests revealed that 40 subjects (24%) were poor responders (PR) and 128 (76%) were good responders (GR). Poor responsiveness was significantly associated with (1) higher basal platelet activities in PFA-100, as well as in whole blood aggregation and aggregometer;(2) increased level of adiponectin (8.8 ± 4.1 μg/mL [PR] vs 7.3 ± 2.9 μg/mL [GR], p = 0.010);and (3) the presence of diabetes mellitus (17.5% [PR] vs 4.7% [GR], p = 0.009). Importantly, whereas 24% of the subjects showed insufficient inhibition in PFA-100 when incubated with 10 μM aspirin, almost all subjects showed maximum inhibition with 30 μM aspirin. These observations suggest that higher doses of aspirin might overcome aspirin resistance.

Clopidogrel in the treatment of ischaemic heart disease

Clopidogrel is an effective antiplatelet agent that has undergone rigorous assessment in the setting of ischaemic heart disease over the last decade. There is extensive evidence for the use of this drug in patients undergoing percutaneous coronary intervention, in those with stable ischaemic heart disease and also in those with acute coronary syndromes. This article examines the use of clopidogrel in patients with ischaemic heart disease.

Effects of Antiplatelet Agents on Outcomes for Elderly Patients With Traumatic Intracranial Hemorrhage

Effects of Antiplatelet Agents on Outcomes for Elderly Patients With Traumatic Intracranial Hemorrhage

The Contributions of the P4-P2 Positions in PAR1 and 4 for Thrombin Recognition and Cleavage.

The angiotensin converting enzyme breakdown product of bradykinin, bradykinin 1–5 (RPPGF), inhibits thrombin induced human or mouse platelet aggregation by preventing proteolysis of PAR1 and PAR4 by binding adjacent to the thrombin cleavage site and thus preventing activation of these receptors (Am. J. Physiol. 285:H183–H193, 2003; J. Pharmacol. Exp. Ther. 311:492–501, 2004; FEBS Lett. 579:25–29, 2005). Alanine scanning mutagenesis determined that RPPGF binds to Pro46 at the P2 position of PAR4 to block thrombin cleavage. New studies determined the amino acids required for thrombin to bind and cleave PAR4. Wild type PAR4 exodomain is cleaved by thrombin with a Km of 17 μM, kcat of 3.5 s−1 and kcat/Km of 2x105 M−1 s−1. In contrast, PAR4 exodomain in which the P2 (PAR4-P46A) is changed to alanine, is not efficiently cleaved with 10 nM alpha thrombin. Alteration of PAR4's P4 position (e.g., PAR4-P44A) does not influence the PAR4's rate of cleavage. The ability of the PAR4 exodomain to inhibit thrombin proteolysis of H-D-Phe-Pip-pNA was also determined as an independent measure of the thrombin/PAR4 interaction. Wild type PAR4 exodomain and PAR4-P44A are competitive inhibitors of thrombin hydrolysis of the chromogenic substrate with a Ki of 23 ± 6 micromolar and 19.6 ± 4 μM respectively. In contrast, PAR4-P46A and PAR4-P44A/P46A have a Ki > 300 μM nd the nature of the inhibition changes from competitive to noncompetitive. Taken together, these data demonstrate that Pro46 of PAR4 is important for alpha thrombin to bind and orient PAR4 in its active site for efficient cleavage. Further studies examined the role of the combined amino acids in the P4 to P3 positions of PAR4 and PAR1 to contribute to the rate of thrombin cleavage. Chimeric molecules were prepared in which the P4 and P3 positions of PAR4 (ProAla) are replaced with those from PAR1 (LeuAsp) to generate PAR4-LD. The reciprocal chimera was also made (PAR1-PA). PAR4-LD is proteolyzed by 10 nM alpha thrombin more efficiently than PAR4-wt. Alternatively, the rate of PAR1-PA proteolysis by alpha thrombin is less efficient than PAR1-wt but better than PAR4-wt due to the presence of the exosite I binding region in the PAR1 exodomain. However, if the exosite I binding domain is removed from PAR1-PA the rate of cleavage is like that seen with PAR4-wt exodomain. These data indicate PAR1 is cleaved by alpha thrombin at a faster rate than PAR4 due to the amino acids in the P4 and P3 positions as well as the exosite I binding region. In sum, these data demonstrate that the P2 position of PAR4 is the most important amino acid in thrombin binding to the region adjacent to the thrombin cleavage site. However the combined P4 and P3 positions are also important determinants of the rate of receptor cleavage. These data indicate that the amino acids around the thrombin cleavage site of both PAR4 and 1 influence its rate of cleavage. Designing thrombin receptor activation antagonists directed to the thrombin cleavage site on PAR1 and 4 should be effective anti-platelet agents.

Monitoring of platelet activation and function after ischemic stroke Thrombozytenaktivität und -funktion nach ischämischem zerebralem Insult

Abstract Assessment of platelet activation after ischemic stroke could be clinically valuable if platelet markers existed that predict the risk of recurrent events and reflect the effect of antiplatelet therapy. Recently developed techniques such as the detection of activation-dependent neoantigens on the platelet surface by flow cytometry, the platelet function analyzer (PFA), or whole blood aggregometry represent methods that in the future could become helpful tools in stroke care. Studies showed an increased expression of activation-dependent markers such as CD62p (p-selectin) or CD63 after ischemic stroke, however, for these and other parameters it remains unclear whether an increased expression at any time point after stroke heralds an increased risk of future events. Sufficiently powered studies on the predictive value of platelet activation parameters and on effects of antiplatelet agents on these parameters after stroke are required in order to validate the potential clinical usefulness of these recently developed techniques.

Antiplatelet therapy in ischemic stroke

Stroke is the third leading cause of death and the leading cause of disability in the developed world. Atherothrombosis is the underlying condition that results in events leading to ischemic stroke and vascular death. Antiplatelet therapy is commonly used for both acute stroke and in secondary prevention. Numerous trials and meta-analyses have left little doubt that antiplatelet therapy effectively reduces stroke risk in patients with prior stroke or transient ischemic attack. Current antiplatelet agents include acetylsalicylic acid, clopidogrel, ticlopidine and extended release dipyridamole with low doses of acetylsalicyclic acid (aspirin). The optimum doses of antiplatelet drugs depend upon several variables, such as genetic and environmental factors, so that clinical and laboratory response for dosage varies for each patient. Recently, the correlation between the laboratory-measurable effect of antiplatelet agents and the clinical effectiveness on the mortality of ischemic stroke and cardiovascular patients has been documented. Due to the side effect of bleeding with different antithrombotic drugs, their future employment will be determined in combination with low dosages of each component. Laboratory-controlled, tailored drug therapy will be needed for long-lasting secondary prevention of ischemic stroke.

Adding aspirin to clopidogrel after TIA and ischemic stroke

Antiplatelet therapy is effective for reducing the risk of recurrent stroke and other serious vascular events in patients with recent TIA and ischemic stroke. Effective antiplatelet agents include aspirin, ticlopidine, clopidogrel, dipyridamole, and the combination of aspirin and dipyridamole. The combination of aspirin and clopidogrel is more effective than aspirin in patients with acute coronary syndrome but is more hazardous than clopidogrel alone in patients with recent TIA and ischemic stroke. Further trials are needed to determine whether the combination of aspirin and clopidogrel may have a role immediately after TIA and ischemic stroke in patients with symptomatic large artery atherothromboembolism and continued for approximately 3 months before switching to less hazardous antiplatelet regimens.

Pharmacology of CS-747 (prasugrel, LY640315), a Novel, Potent Antiplatelet Agent with in Vivo P2Y12Receptor Antagonist Activity

CS-747 (prasugrel, LY640315) is a member of the thienopyridine class of oral platelet aggregation inhibitors that includes ticlopidine and clopidogrel. A single oral administration of CS-747 produced a dose-related inhibition of platelet aggregation in rats that was approximately 10- and 100-fold more potent than that of clopidogrel and ticlopidine, respectively. The antiaggregatory effect of CS-747 was evident at 30 minutes and lasted until 72 hours after dosing, indicating fast onset and long duration of action. CS-747 showed more potent antithrombotic activity compared with clopidogrel and ticlopidine with the same rank order as the antiaggregatory potencies. Combined administration of CS-747 with aspirin to rats produced substantially greater inhibition of both platelet aggregation and thrombus formation compared with each agent alone. The antiplatelet action of CS-747 is due to irreversible and selective blockade of platelet P2Y (12) adenosine diphosphate (ADP) receptors by its active metabolite R-138727. In phase I studies, a single oral dose of CS-747 (30 and 75 mg) produced > 50% inhibition of ADP-induced platelet aggregation, with rapid onset (1 hour) and long duration (> 48 hours) of action. In healthy volunteers, once-daily administration of 10 mg CS-747 for 10 days showed significant cumulative inhibition of platelet aggregation from 2 days after the first dose until at least 2 days after the final dose. Studies conducted to date indicate that CS-747 is a highly effective antiplatelet and antithrombotic agent and is anticipated to be effective in the treatment of atherothrombotic and other ischemic vascular diseases.

Effects of Antiplatelet Agents on Outcomes for Elderly Patients With Traumatic Intracranial Hemorrhage

Recent literature on elderly patients with traumatic intracranial hemorrhage receiving preinjury antiplatelet agents shows a mortality rate of 47%. In a retrospective analysis, patients older than 50 years presenting to the hospital over the past 4 years with traumatic intracranial hemorrhage and the use of aspirin, clopidogrel, or a combination were compared with a control group that had hemorrhage but no antiplatelet medications. Patient demographics, mechanism of injury, and injury scores were recorded. No significant differences were found between the 90 study patients and the 89 control subjects in terms of demographics, mechanism of injury, Injury Severity Score, Glasgow Coma Score, or hospital length of stay. Patients receiving antiplatelet therapy had significantly more comorbid conditions (71% vs. 35%; p < 0.001). In this series, 21 study patients and 8 control patients died (23% vs. 8.9%; p = 0.016). Age older than 76 years and a Glasgow Coma Score lower than 12 were correlated significantly with increased mortality. The use of antiplatelet agents with elderly trauma patients significantly increases the risk of mortality when head injury involves intracranial hemorrhage.

Intracranial Bleeding Associated with Clopidogrel

Clopidogrel is an effective antiplatelet agent and is useful in the prevention of stroke, myocardial infarction, and related vascular death in patients with vascular disease. Intracranial hemorrhage related with using antithrombotic agents, including clopidogrel can occur. We report a case of a 68-year-old-man who developed intracranial bleeding following clopidogrel usage that, until now such a complication has not been yet reported.

Response:

Response:

Aspirin for Stroke Prevention Taken in the Evening?

To the Editor: We read the article by Yip et al1 with great interest, in which they demonstrated that platelet activation significantly increases in acute ischemic stroke and subtantially decreases thereafter. The lesser long-term pharmacodynamic potency of aspirin relative to clopidogrel raises the prospect of the need for more effective antiplatelet agents or a synergistic combination therapy for stroke prevention in the future.1 Their results are very impressive and raise some ideas, particularly associated with the prevention’s procedures. According to World Health Organization data, in 1996 4.6 million people in the world died because of cerebrovascular manifestation of atherothrombosis.2 In Hungary, nearly 18 000 people suffer stroke every year, half of whom die within a year. In cases of both cardiovascular and cerebrovascular diseases the significant decrease of morbidity/mortality can only be achieved by increasing the effect of prevention. A major form of secondary prevention is the administration of drugs inhibiting the aggregation of platelets. Aspirin is one of the most common, useful, and inexpensive tools for prevention. The effect of aspirin on platelets is irreversible lasting for the duration of the platelet’s life span (≈10 days). Aspirin-mediated inhibition of platelet function occurs within 60 minutes of ingestion.3 The incidence of stroke assessed by onset of clinical symptoms exhibits a marked circadian variation with a peak period during the morning. Stroke usually occurs unexpectedly and more frequently in early morning …

Serial changes in platelet activation in patients after ischemic stroke: role of pharmacodynamic modulation.

Enhanced platelet activity has previously been reported in the acute phase after ischemic stroke. We tested the hypothesis that activated platelets (expressed by CD62p) are substantially increased in the acute stage after a stroke and decrease thereafter, and that antiplatelet therapies can suppress CD62p expression. We serially examined platelet CD62p expression using flow cytometry after acute ischemic stroke in 87 consecutive patients. The CD62p expression was also evaluated in 20 healthy volunteers and 33 at-risk control subjects. CD62p expression was significantly higher in the acute phase after ischemic stroke than in normal and at-risk control subjects (both P<0.0001). CD62p expression decreased to a significantly lower level on day 21, and to a substantially lower level on day 90. CD62p expression was not significantly suppressed by warfarin. However, CD62p expression was significantly suppressed by aspirin treatment (P=0.024) and more substantially suppressed by clopidogrel (P<0.0001) on day 90. Furthermore, only clopidogrel treatment (P=0.0016) was significantly independently associated with decreased CD62p expression on day 90. Platelet activation was significantly increased in acute ischemic stroke and substantially decreased thereafter. The lesser long-term pharmacodynamic potency of aspirin relative to clopidogrel raises the prospect of the need for more effective antiplatelet agents or a synergistic combination therapy for stroke prevention in the future.

Application of ex vivo flow chamber system for assessment of stent thrombosis.

Factors influencing platelet accumulation around stents were to be investigated by an ex vivo flow chamber system. Platelet accumulations on collagen surfaces under flow conditions were augmented in the presence of stents, especially at sites downstream from coil stents. Densitometric analysis revealed that 4.9+/-0.8 times more platelets accumulated downstream from coil stents than were formed downstream from tube stents (P<0.01), suggesting that stent morphology is an important determinant factor of its thrombogenicity. Platelet accumulations around stents were significantly inhibited by a combination of ticlopidine and aspirin, whereas aspirin alone produced only modest inhibition. Anti-glycoprotein IIb/IIIa (abciximab) inhibited platelet accumulation around stents in a dose-dependent manner, whereas the antibody blocking von Willebrand factor binding to glycoprotein Ib(alpha), which had been shown to inhibit platelet thrombus formation under high shear rates, did not inhibit the accumulation downstream from the coil stents. Our results suggest that the important characteristics of in vivo stent thrombosis, ie, augmented platelet accumulation with coil stents and the strong antithrombotic effect of the combination antiplatelet agents and an anti-glycoprotein IIb/IIIa, can be reproduced in ex vivo perfusion model. We conclude that an ex vivo perfusion system is useful in the assessment of the thrombogenicity of various stents and in the screening of effective antiplatelet agents.