Magnitude and time course of platelet inhibition with Aggrenox® and Aspirin in patients after ischemic stroke: the AGgrenox versus Aspirin Therapy Evaluation (AGATE) trial

In vitro evaluation of current and novel antivirals in combination against human cytomegalovirus

Human cytomegalovirus (HCMV) can cause severe disease in patients with compromised or immature immune systems. Currently approved pharmacotherapies for the treatment of systemic HCMV infections [ganciclovir (GCV), cidofovir (CDV), foscarnet] are limited by a high incidence of adverse effects and/or the development of drug resistance. Given that many of these drugs have the same viral target (HCMV-encoded DNA polymerase), cross-resistance is relatively common. The primary means to combat drug resistance is combination pharmacotherapy using therapeutics with different molecular mechanisms of action with the expectation that those combinations result in an additive or synergistic enhancement of effect; combinations that result in antagonism can, in many cases, be detrimental to the outcome of the patient. We therefore tested select combinations of approved (GCV, CDV, letermovir (LMV)) and experimental (brincidofovir (BCV), cyclopropavir (CPV), maribavir (MBV), BDCRB) drugs with the hypothesis that combinations of drugs with different and distinct molecular mechanisms of action will produce an additive and/or synergistic enhancement of antiviral effect against HCMV in vitro. Using MacSynergy II (a statistical package that measures enhancement or lessening of effect relative to zero/additive), select drug combination studies demonstrated combination indices ranging from 160 to 372 with 95% confidence intervals greater than zero indicating that these combinations elicit a synergistic enhancement of effect against HCMV in vitro. These data suggest that administration of a viral DNA polymerase inhibitor, MBV, and/or a viral terminase inhibitor in combination has the potential to address the resistance/cross-resistance problems associated with currently available therapeutics.

Human platelet protease-activated receptor-1 responsiveness to thrombin related to P2Y12 inhibition

Dual antiplatelet therapy with aspirin and adenosine diphosphate (ADP) receptor inhibitors significantly improves the outcome of patients with stable coronary heart disease. However, abundant thrombin generation, which is not influenced by this dual antiplatelet therapy, is a major reason for recurrent thromboembolic disease in these patients. We, therefore, assessed in a hypothesis generating study in patients with stable coronary artery disease specifically the relation of responsiveness of the platelet thrombin receptor protease-activated receptor (PAR)-1 to the magnitude of the inhibition of the ADP receptor. PAR-1 regulation was studied prospectively in 86 consecutive patients with stable coronary artery disease treated with aspirin and clopidogrel (67 patients) or prasugrel (19 patients) and correlated the data to ADP inducible platelet reactivity by impedance aggregometry. PAR-1 expression did not differ between patients on aspirin and clopidogrel vs patients on aspirin and prasugrel (P > 0.5). PAR-1 levels were correlated to P-selectin expression (P < 0.0001). The higher the PAR-1 expression the more profound was the in vitro thrombin-inducible platelet activation. However, neither ex vivo PAR-1 expression nor in vitro thrombin-inducible PAR-1 were correlated to ADP-inducible platelet aggregation (P > 0.5). Thus, like in a real life scenario, patients with stable ischemic heart disease on dual antiplatelet therapy may express high levels of PAR-1, which are associated with profound thrombin-inducible platelet activation. This responsiveness cannot be predicted by the magnitude of ADP responsiveness.

Clinical outcomes using a platelet function-guided approach for secondary prevention in patients with ischemic stroke or transient ischemic attack

BACKGROUND AND PURPOSE

Antiplatelet therapy nonresponse is associated with worse clinical outcomes. We studied the clinical outcomes associated with platelet function-guided modifications in antiplatelet therapy in patients with ischemic stroke or transient ischemic attack.

METHODS

From January 2005 to August 2007, 324 patients with ischemic stroke underwent platelet function testing using platelet aggregometry. Aspirin nonresponse was defined as a mean platelet aggregation ≥20% with 0.5 mg/mL arachidonic acid and/or ≥70% with 5 μmol/L adenosine diphosphate. Clopidogrel nonresponse was defined as a mean platelet aggregation ≥40% with 5 μmol/L adenosine diphosphate. A modification was any increase in antiplatelet therapy occurring after testing. Clinical outcomes were compared between patients with and without platelet function-guided antiplatelet therapy modifications using univariate and propensity score-adjusted analyses.

RESULTS

In patients with ischemic stroke or transient ischemic attack, 43% (n=128) and 35% (n=54) were nonresponders to aspirin and clopidogrel, respectively. After platelet function testing, antiplatelet therapy was increased in 23% of patients (n=73). After propensity score matching (n=61 in each group), antiplatelet therapy modification was associated with significantly increased rates of death, ischemic events, or bleeding (hazard ratio, 2.24; 95% CI, 1.12-4.47; P=0.02) compared with no modification in antiplatelet therapy and a trend toward increased bleeding (hazard ratio, 3.56; 95% CI, 0.98-12.95; P=0.05). No differences in ischemic events were observed.

CONCLUSIONS

Platelet function-guided modification in antiplatelet therapy after an ischemic stroke or transient ischemic attack was associated with significantly higher rates of adverse clinical outcomes.

Aspirin and clopidogrel hyporesponsiveness and nonresponsiveness in patients with coronary artery stenting

Patients undergoing coronary artery stenting receive an antiplatelet regimen to reduce the risk of antithrombotic complications. Current guidelines recommend the use of acetyl salicylic acid (aspirin) and clopidogrel as evidenced by large clinical trials. There has been a concern about variable responses of patients to aspirin and clopidogrel which may predispose them to subacute stent thrombosis or late stent thrombosis. Up to 25% of patients with acute myocardial infarction (AMI) undergoing percutaneous coronary intervention (PCI) were found to have hyporesponsiveness or resistance to clopidogrel which may predispose them to recurrent events. Dual antiplatelet regimen is a standard therapy in these patients and there is always a concern about variable responses to aspirin and clopidogrel predisposing them to acute coronary syndrome (ACS). Prevalence of this hyporesponsiveness or resistance may be due to noncompliance, genetic mutations, co-morbid situations and concomitant use of other drugs. This issue is of considerable importance in the era of coronary drug eluting stents when a long-term dual antiplatelet regimen is needed. This paper is a review for clinicians taking care of such patients with hyporesponsiveness or nonresponsiveness to dual antiplatelet regimen.

Antiplatelet profiles of the fixed-dose combination of extended-release dipyridamole and low-dose aspirin compared with clopidogrel with or without aspirin in patients with type 2 diabetes and a history of transient ischemic attack: a randomized, single-blind, 30-day trial

BACKGROUND

Clopidogrel, aspirin (ASA), and the fixed-dose combination of extended-release dipyridamole and ASA (ER-DP+ASA) are widely used in post-stroke regimens.

OBJECTIVE

This study compared serial changes in multiple biomarkers of platelet activation with ER-DP+ASA and clopidogrel with or without ASA in patients with type 2 diabetes mellitus and a history of transient ischemic attack (TIA).

METHODS

This was a randomized, single-blind pilot study conducted at an outpatient center in the United States. Eligible patients were aged 40 years and had a diagnosis of type 2 diabetes and a history of TIA. Patients were allocated to receive ER-DP+ASA 200/25 mg BID, clopidogrel 75 mg/d, or clopidogrel 75 mg/d plus ASA 81 mg/d. Multiple platelet bio-markers were assessed at baseline, day 15, and day 30 using aggregometry, cartridge-based platelet function analyzers, and flow cytometry. The primary end point was the change in platelet receptor expression after 30 days of therapy. Compliance and tolerability were monitored by measuring plasma dipyridamole levels and recording all episodes of headache and vomiting.

RESULTS

The study enrolled 60 consecutive patients (20 per treatment arm), all of whom completed the study. There were no significant differences between treatment arms, although the ER-DP+ASA group had a numerically greater mean age, higher proportion of men, and a greater prevalence of vascular disease and smoking compared with the other groups. There were no deaths or serious adverse events during the study, including symptoms attributable to cerebral ischemia, worsening of diabetes, or cerebral or systemic bleeding. Three patients in the ER-DP+ASA group and 1 in the clopidogrel plus ASA group reported headache during the first several days of therapy; 1 patient in the clopidogrel monotherapy group experienced transitory nausea and vomiting. ER-DP+ASA was associated with a significantly delayed (day 30) reduction in expression of glyco-protein (GP) Ilb/IIIa activity (P = 0.02), platelet-endothelial cell adhesion molecule 1 (PECAM-1) (P = 0.03), GP Ib (P = 0.001), vitronectin (P = 0.001), P-selectin (P = 0.001), lysosome-associated membrane protein 1 (P = 0.001), and cluster of differentiation 40 ligand (P = 0.01), as well as significant inhibition of the intact (P = 0.01) and cleaved (P = 0.01) epitopes of protease-activated receptor 1. Clopidogrel monotherapy, on the other hand, was associated with significant inhibition of adenosine diphosphate-induced platelet aggregation (P = 0.001), closure-time prolongation (P = 0.01), and reduction in measurements on the rapid platelet function assay-ASA at day 15 (P = 0.001). Expression of PECAM-1 (P = 0.03) and GP IIb/IIIa activity (P = 0.01) was reduced at day 15 in clopidogrel-treated patients. The addition of ASA to clopidogrel was associated with significant inhibition of collagen-induced platelet aggregation (P = 0.001) and diminished formation of platelet-monocyte microparticles at days 15 (P = 0.02) and 30 (P = 0.03).

CONCLUSIONS

In these patients with type 2 diabetes and a history of TIA, patterns of platelet inhibition differed significantly according to whether treatment was with ER-DP+ASA or clopidogrel with or without ASA. The antiplatelet activity of clopidogrel was more potent and occurred earlier (15 days), whereas ER-DP+ASA was associated with moderate downregulation of multiple activation-dependent platelet receptors that occurred later (30 days).

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((R)), Asasantin((R))) 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.

Combination Antiplatelet Therapy with Aspirin and Clopidogrel: The Role of Antecedent and Concomitant Doses of Aspirin

BACKGROUND

Numerous randomized studies have shown that the combination of clopidogrel with aspirin yields better clinical outcomes than monotherapy in patients with acute vascular events. However, the impact of the aspirin dose on the antiplatelet potency of clopidogrel is unclear. We sought to compare the antiplatelet profile of aspirin 81 mg (n = 252) versus aspirin 325 mg (n = 459) before and during conventional clopidogrel loading (300 mg), and/or clopidogrel maintenance (75 mg/daily) therapy.

METHODS

Secondary post hoc analysis of an existing dataset consisting of 711 patients after coronary stenting (n = 601) and ischemic stroke (n = 110) treated previously with aspirin for at least 1 month, and then with aspirin + clopidogrel for at least 7 days was performed. Platelet assessments include conventional and whole blood aggregometry, rapid cartridge-based analyzers, and expression of platelet/endothelial cell adhesion molecule-1, P-selectin, and GPIIb/IIIa activity by flow cytometry measured before and after addition of clopidogrel.

RESULTS

There was a small but consistent yet non-significant trend towards more potent platelet inhibition with aspirin 325 mg compared to aspirin 81 mg for every platelet activation parameter before addition of clopidogrel. However, after loading and/or 1 week of chronic treatment with clopidogrel + aspirin, measured platelet parameters became very similar between the groups, and identical for collagen-induced aggregation and PFA-100 analyzer readings.

CONCLUSIONS

Before addition of clopidogrel, aspirin 325 mg has a tendency to provide stronger platelet inhibition than aspirin 81 mg. However, when clopidogrel and aspirin are used in combination, the higher aspirin dose does not translate into superior antiplatelet action. Given that the existing body of evidence supports the comparable efficacy and, particularly, superior safety of lower versus higher doses of aspirin, aspirin 81 mg should be the dose used in combination with clopidogrel.

Aspirin resistance: truth or dare

Acetylsalicylic acid, or aspirin (ASA), is widely used in patients with cardiovascular disease to prevent acute ischemic events. However, platelet response to ASA is not equal in all individuals, and a high variability in the prevalence of ASA resistance is reported in the literature (0.4-83%). Actually, ASA resistance is poorly understood; this stems from the fact that its definition is unclear, its presence can be evaluated by a number of assays that are not equivalent, and its prevalence may vary widely based on the population studied. This article (1) exposes the difficulties in defining ASA resistance; (2) discusses the mechanisms by which ASA resistance may occur; (3) presents the characteristics that may put patients at greater risk of exhibiting ASA resistance; and (4) discusses the clinical impact of ASA resistance in patients requiring chronic therapy.

The action of dipyridamole to prevent thrombosis: practical implications for the treatment and prevention of stroke

Aggrenox (Boehringer Ingelheim, Ingelheim, Germany), a novel combination of low-dose aspirin with dipyridamole, represents a safe and promising combination alternative for mild but sustained platelet inhibition, and reduction of both arterial and venous thrombi occurrences. In a large, well-controlled randomized trial (ESPS-2 ) evaluating antiplatelet agents for stroke prevention, Aggrenox was twice as effective as monotherapy with either aspirin or dipyridamole. There is an increasing body of evidence that a delicate strategy with Aggrenox provides modest inhibition of platelet activity, especially in a chronic, long-term setting. Mild platelet inhibition beyond conventional aggregation may represent a substantial advantage over aggressive antiplatelet regimens for the treatment, and especially for secondary prevention, of cerebrovascular ischemic events. Although there is no doubt that the concept of inhibiting platelets is vital for the treatment of vascular ischemic disease in general and ischemic stroke and transient ischemic attack (TIA) in particular, the optimal degree of such inhibition still remains an unsolved mystery. It seems that the concepts of "the more, the better" and "one size fits all" may no longer be valid for ideal antiplatelet protection in such high-risk populations. Without routine individual laboratory assessment of platelet function, mild regimens have the advantage of being more suitable for the majority of patients and will contribute substantially to the success of dipyridamole. Conversely, if we can determine baseline platelet status and intelligently apply therapy based on platelet activity in each particular patient, clinical outcomes may be better. Avoiding excessive bleeding risks after aggressive strategies in patients with normal or already decreased platelet function, but targeting those who exhibit activated platelets, may improve risk stratification and save lives. Therefore, Aggrenox should be considered a drug of choice to prevent the second stroke. Eliminating, or at least minimizing, the most frequent side effect, namely transitory headaches at the beginning of therapy with Aggrenox, will benefit patients and increase the use of this agent. Should the PRoFESS (Prevention Regimen For Effectively avoiding Second Strokes) trial show an advantage in event reduction with Aggrenox over clopidogrel, the increase will be especially dramatic. In short, based on current evidence most guidelines include Aggrenox as a first-line option for secondary prevention of ischemic stroke or TIA, and some recent versions suggest it may be preferable in other clinical scenarios.

Dipyridamole Decreases Protease-Activated Receptor and Annexin-V Binding on Platelets of Poststroke Patients with Aspirin Nonresponsiveness

BACKGROUND

Although controversial, the phenomenon of aspirin resistance (AR) has been correlated in some small studies with poor clinical outcomes in patients with coronary artery disease. Even less is known regarding the role of AR in the post stroke population. The reason for and the underlying mechanism of AR is unknown. We hypothesized that excessive formation of thrombin on the platelet surface may contribute to this phenomenon and assessed how dipyridamole affects multiple platelet and thrombin generation biomarkers in AR patients after ischemic stroke.

METHODS

Whole blood samples from 20 post stroke AR patients were pretreated with dipyridamole, simulating the therapeutic range, and then incubated for 45 min at 37 degrees C. Platelet characteristics were assessed by aggregometry, cartridge-based analyzer, and receptor expression by flow cytometry. Markers of thrombin generation were measured in the autologous plasma by ELISA.

RESULTS

Pretreatment of blood with dipyridamole resulted in 22-26% diminished expression of intact PAR-1 receptor (p=0.021 and p=0.024) and 28-31% decrease of annexin V binding (p=0.031 and p=0.02) after incubation with 2 microg/ml and 4 microg/ml of dipyridamole, respectively. Platelet aggregation and thrombin generation markers were not affected in vitro by dipyridamole.

CONCLUSIONS

Dipyridamole may be capable of overcoming increased prothrombinase complex formation and be in part able to compensate for AR in patients with moderate carotid stenosis. This phenomenon may explain the clinical advantages of Aggrenox, known to reduce ischemic events in post stroke patients as proven in clinical trials, though an additional antithrombotic benefit beyond the platelet inhibition by aspirin alone.

The Effect of Dipyridamole on Vascular Cell-Derived Reactive Oxygen Species

Platelet and vascular stimulation leads to release of reactive oxygen species (ROS) that are known to influence vascular reactivity and thrombosis. Dipyridamole is a vasodilator and platelet inhibitor that has previously been shown to have direct antioxidant properties. The antioxidant effects of dipyridamole on vascular cell-derived ROS are not known; therefore, dipyridamole was incubated with endothelial cells and platelets and cellular redox status and release of endogenous ROS were assessed. Dipyridamole decreased intracellular basal ROS generation from endothelial cells as measured by DCFDA (2',7'-dichlorodihydrofluorescein diacetate) oxidation. Incubation of endothelial cells with dipyridamole also attenuated t-butylhydroperoxide-induced oxidative stress. Using a redox-sensitive fluorescent dye, dipyridamole improved cellular activity after treatment with t-butylhydroperoxide. Incubation with dipyridamole did not alter platelet release of nitric oxide or hydrogen peroxide but significantly attenuated superoxide release. Using flow cytometry and confocal microscopy, dipyridamole decreased platelet ROS generation. Dipyridamole also suppressed platelet-soluble CD40 ligand release. In summary, at therapeutically relevant concentrations, dipyridamole suppresses the formation of ROS in platelets and endothelial cells and improves cellular redox status. These data suggest that the redox-dependent properties of dipyridamole have a direct effect on vascular cells.

Adverse Effects and Drug Interactions of Antithrombotic Agents Used in Prevention of Ischaemic Stroke

Stroke is the third most common cause of death in the US. Primary prevention of stroke can be achieved by control of risk factors including hypertension, diabetes mellitus, elevated cholesterol levels and smoking. Approximately one-third of all ischaemic strokes occur in patients with a history of stroke or transient ischaemic attack (TIA). The mainstay of secondary prevention of ischaemic stroke is the addition of medical therapy with antithrombotic agents to control the risk factors for stroke. Antithrombotic therapy is associated with significant medical complications, particularly bleeding.Low-dose aspirin (acetylsalicylic acid) has been shown to be as effective as high-dose aspirin in the prevention of stroke, with fewer adverse bleeding events. Aspirin has been shown to be as effective as warfarin in the prevention of noncardioembolic ischaemic stroke, with significantly fewer bleeding complications. Ticlopidine may be more effective in preventing stroke than aspirin, but is associated with unacceptable haematological complications. Clopidogrel may have some benefit over aspirin in preventing myocardial infarction, but has not been shown to be superior to aspirin in the prevention of stroke. The combination of clopidogrel and aspirin may be more effective than aspirin alone in acute coronary syndromes, but the incidence of adverse bleeding is significantly higher. Furthermore, the combination of aspirin with clopidogrel has not been shown to be more effective for prevention of recurrent stroke than clopidogrel alone, while the rate of bleeding complications was significantly higher with combination therapy. The combination of aspirin and extended-release dipyridamole has been demonstrated to be more effective than aspirin alone, with the same rate of adverse bleeding complications as low-dose aspirin. When selecting the appropriate antithrombotic agent for secondary prevention of stroke, the adverse event profile of the drug must be taken into account when assessing the overall efficacy of the treatment plan.