Nitric oxide reverses aspirin antagonism of t-PA thrombolysis in a rabbit model of thromboembolic stroke

A novel anti-ischemic nitric oxide donor (LA419) reduces thrombogenesis in healthy human subjects

BACKGROUND

Platelet and endothelial production of nitric oxide (NO) is known to be impaired in coronary artery disease patients. Compounds that release NO (e.g. nitrates) have antiplatelet effects, but at supratherapeutic doses with hypotensive side effects.

OBJECTIVES

To investigate the antithrombotic effect on human blood of a novel NO donor (LA419) with known anti-ischemic properties but without hypotensive side effects and to compare with abciximab.

PATIENTS/METHODS

Healthy subjects (n = 8; 32 +/- 3 years) received daily aspirin starting three days prior to the study day. Treatments (LA419 10 and 20 microm, and abciximab 4 microm) were added ex vivo to non-anticoagulated blood, and the antithrombotic properties were assessed by measuring changes in thrombus size from pretreatment baseline in the Badimon perfusion chamber at low and high shear rates. Platelet surface adhesion using a Cone and Platelet Analyzer (CPA) and platelet fibrinogen-receptor activation with flow cytometry were also evaluated.

RESULTS

At low shear rates, LA419 displayed a reduction in thrombus area of 43% +/- 8% (10 microm) and 56% +/- 6% (20 microm), whereas at high shear rates the reductions were 44% +/- 3% (10 microm) and 62% +/- 6% (20 microm). Platelet surface adhesion with the CPA was also reduced. Abciximab exhibited a strong inhibitory effect on thrombus formation, platelet surface adhesion and fibrinogen receptor activation.

CONCLUSIONS

The novel NO donor, LA419, shows a strong antithrombotic effect in human blood, which is comparable to abciximab, especially under high shear rate conditions. Our observations suggest that the availability of an NO donor could prove beneficial in the prevention of thrombotic complications of cardiovascular disease. Further clinical studies are warranted.

A systematic review of nitric oxide donors and L-arginine in experimental stroke; effects on infarct size and cerebral blood flow

BACKGROUND

Nitric oxide (NO) is a candidate treatment for acute ischaemic stroke, however published studies in experimental stroke have given conflicting results.

METHODS

We performed a systematic review of published controlled studies of L-arginine (the precursor for NO) and NO donors in experimental stroke. Data were analysed using the Cochrane Collaboration Review Manager software. Standardised mean difference (SMD) and 95% confidence intervals (95% CI) were calculated.

RESULTS

Altogether, 25 studies(s) were identified. L-Arginine and NO donors reduced total cerebral infarct volume in permanent (SMD -1.21, 95% CI -1.69 to -0.73, p < 0.01, s = 10) and transient models of ischaemia (SMD -0.78, 95% CI -1.21 to -0.35, p < 0.01, s = 7). Drug administration increased cortical CBF in permanent (SMD +0.86, 95% CI 0.52-1.21, p < 0.01, s = 8) but not transient models (SMD +0.34, 95% CI -0.02 to 0.70, p = 0.07, s = 4).

CONCLUSIONS

Administration of NO in experimental stroke reduces stroke lesion volume in permanent and transient models. This may be mediated, in part, by increased cerebral perfusion in permanent models. These data support clinical trials in stroke patients, although the presence of a narrow therapeutic time window may be a limiting factor.

Effects of a novel platelet nitric oxide donor (LA816), aspirin, clopidogrel, and combined therapy in inhibiting flow- and lesion-dependent thrombosis in the porcine ex vivo model

BACKGROUND

Acetylsalicylic acid (ASA), or aspirin, plus clopidogrel is becoming the standard antithrombotic treatment in people with coronary disease. Novel approaches such as the use of platelet-selective nitric oxide (NO) donors may provide additional protection against thrombosis. We evaluated the antithrombotic properties of a novel platelet-selective NO donor (LA816) administered alone and in combination with ASA, clopidogrel, or ASA+clopidogrel.

METHODS AND RESULTS

Thrombogenicity was measured in the porcine experimental model and assessed as platelet-thrombus formation in the ex vivo Badimon perfusion chamber. Pigs were randomly divided into 4 groups: (1) placebo control, (2) clopidogrel, (3) ASA, and (4) ASA+clopidogrel (ASA and clopidogrel were given orally, 10 mg x kg(-1) x d(-1) for 3 d). The animals were anesthetized, heparinized, and catheterized, and the Badimon perfusion chamber was placed in an extracorporeal shunt. After baseline perfusions, all animal groups received the intravenous infusion of LA816 for 2 hours. Platelet aggregation, blood pressure, and heart rate also were evaluated during the experiments. LA816, clopidogrel, and ASA+clopidogrel produced a reduction of approximately 45% on thrombus mass versus placebo control perfusions (P<0.05). Combined treatment of oral ASA+clopidogrel and intravenous LA816 produced a significant further reduction of 25% in platelet deposition (70% from placebo controls; P<0.0001). LA816 intravenous treatment did not modify blood pressure or heart rate.

CONCLUSIONS

Acute NO donation with LA816, without modifying hemodynamic parameters, provides the same inhibitory effect as that obtained with chronic treatment with clopidogrel+ASA. Moreover, LA816 provides platelet inhibitory effects in addition to those of the combined blockade of cyclooxygenase and P2y(12) receptor.

Aspirin and the fibrinolytic response

Apart from the anti-inflammatory action, aspirin (ASA) by inhibiting thromboxane A2 synthesis, decreases platelets activity and possesses the antithrombotic action. However, an ASA effect on fibrinolysis has not been yet finally established. Menon [Lancet 1 (1970) 364] reported increased fibrinolytic response in patients treated with high doses of ASA and this observation started a series of studies to find the relation between aspirin and fibrinolysis. This review comprises the results of those studies, divided into in vitro and in vivo, animal and human experiments. The results of our animal studies are also included. Data survey shows that the ASA effect on fibrinolysis depends on experimental conditions, the dose and the time of drug administration. The results of our study indicate the essential role of plasma components in the fibrinolysis regulation by ASA.

The potential of nitric oxide therapeutics in stroke

The therapeutic modulation of the nitric oxide (NO) system has generated considerable interest as a new way for managing many disease processes. In stroke, a useful strategy is to increase NO availability and thereby exploit its beneficial antiplatelet, antiatherosclerotic, haemodynamic and neuroprotective properties. Pharmacologically, this can be achieved by providing NO substrate, using NO donors or by upregulating nitric oxide synthase. Alternatively, one can reduce NO availability by inhibiting NO synthase and thereby limiting its pro-inflammatory and neurotoxic properties. This article reviews developments in NO-related therapeutics for treatment of stroke, with a particular emphasis on compounds that are in the clinical research and development pipeline. Although the routine use of NO therapeutics for the prevention or treatment of stroke cannot currently be recommended, we are evidently at an exciting stage in their pharmacological development. Definitive randomised controlled trials in stroke patients are required as a matter of urgency.

Combining antiplatelet and thrombolytic therapies for stroke

Pharmacological therapy for acute nonhaemorrhagic stroke has become a reality over the last 5 years. Mechanistically, both thrombolytic (tissue plasminogen activator and urokinase) and antiplatelet (aspirin) monotherapy have demonstrated efficacy. However, unintended actions limit the extent of clinical improvement in each circumstance. For example, in addition to excess bleeding, tissue plasminogen activator therapy has been associated with complement activation, neuronal toxicity and laminin degradation, while aspirin may reduce nitric oxide synthase activity and cerebral blood flow. Attention is now directed toward improving the therapeutic index for each class of agents. Generally, while thrombolytic therapy is focused on developing agents with greater fibrin specificity and safety (that is, a reduction in intracranial haemorrhage rate), the development of antiplatelet agents is primarily focused on achieving greater potency. The latter is being investigated by combining agents with different mechanisms (aspirin and dipyridamole, aspirin and clopidogrel) as well as agents designed to block the glycoprotein IIb/IIIa receptor, the final common pathway for platelet aggregation. Thus, combination therapy using both thrombolytic and antiplatelet agents will further attempt to improve the therapeutic index by increasing potency and improving the safety profile. Anecdotal case studies support the merits of this approach and are consistent with the data reported for myocardial ischaemia and interventional strategies. It is anticipated that drug therapy directed at both thrombolytic and antiplatelet targets will ultimately result in a widened therapeutic window that will allow acute stroke therapy to be administrated to a much greater number of patients than is currently possible.

Influence of nitric oxide on the in vitro antiaggregant effect of ticlopidine

The aim of this study was to evaluate the influence of leukocyte nitric oxide (NO) production on the antiplatelet aggregant effect of aspirin and ticlopidine. This in vitro study was done with platelets (platelet-rich plasma, PRP) and polymorphonuclear leukocytes (PMNLs) separated from samples of human blood. Collagen-induced platelet aggregation and calcium-dependent NO production by PMNL were quantified. The inhibition of NO production in PRP significantly reduced the antiaggregant affect of aspirin (IC50 2.64-fold greater), whereas it had no significant effect on the effect of ticlopidine (IC50 1.03-fold greater). Incubating PMNL in PRP increased the antiaggregant effect of both aspirin (IC50 5.09-fold lower) and ticlopidine (IC50 10.16-fold lower). The inhibition of NO production in PMNL significantly reduced the antiaggregant effect of both aspirin (IC50 2.21-fold greater) and ticlopidine (IC50 3.26-fold greater). Both drugs increased leukocyte NO production. The concentration of aspirin that raised NO production by 50% was greater than 1000 microM, whereas the concentration of ticlopidine that led to this effect was 9.14 +/- 0.87 microM. We conclude that the effect of ticlopidine on leukocyte NO production may constitute an addition mechanism to the IIb/IIIa glycoprotein complex inactivation in the inhibition of platelet activation.

The effect of vasodilators on aspirin-induced antagonism of t-PA thrombolysis

Although i.v. t-PA has proven successful in reducing neurologic deficits in acute ischemic stroke, the disadvantages of a narrow therapeutic time window and the failure of thrombolysis in more than 50% of patients treated have necessitated an examination of adjuvant therapies to improve the rate of thrombolysis. Experimentally, the combination of aspirin therapy with t-PA has resulted in a paradoxical antagonism of thrombolysis. Reversal of this antagonism with nitric oxide (NO) donors suggested that aspirin may inhibit/ antagonize NO-related mechanisms. Using this rabbit model of thromboembolic stroke, this hypothesis is now expanded to compare two clinically relevant anti-hypertensive agents, atenolol (NO-dependent) and hydralazine (NO-independent), for their ability to improve t-PA-mediated clot lysis following aspirin pre-treatment. Thirty rabbits (10 per group) were pre-treated with aspirin (20mg kg(-1), i.v.) and then randomized to receive either vehicle, atenolol (20 microg kg(-1) h(-1), i.v.) or hydralazine (10 microg kg(-1) min(-1), i.v.) beginning 30 min following autologous clot embolization. All rabbits then received t-PA (6.3 mg kg(-1), i.v.) beginning 1 h after embolization, with completion of the protocol 4 h after embolization. Aspirin therapy reduced regional cerebral blood flow (rCBF) from 82.8m +/- 4.7 to 62.5 +/- 6.6 (n = 30; p = 0.0005). In the aspirin control group only 30% (3 of 10) rabbits demonstrated complete clot lysis, whereas the combined atenolol (60%) and hydralazine (70%) groups experienced a clot lysis rate of 65% (13 of 20 rabbits), similar to clot lysis rates previously observed with t-PA alone. In a separate series of experiments, all agents able to reverse aspirin antagonism of thrombolysis demonstrated an improvement in rCBF, suggesting a common mechanism for this diverse group of agents in reversing aspirin's antagonism of thrombolysis.

Evaluating the efficacy of citicoline in embolic ischemic stroke in rats: neuroprotective effects when used alone or in combination with urokinase

The combination of thrombolysis with neuroprotection, because of different mechanisms, would be expected to show better results when used after onset of focal ischemia. In this study we report our experience with the neuronal protective effects of citicoline alone and in combination with urokinase in a model of focal ischemia. Both medications were injected 2 h after onset of a focal occlusion of the middle cerebral artery (MCA) in rats. Focal ischemia was produced with embolization of a clot into the origin of the MCA. This produces a large infarction involving the cortex and the basal ganglia. Animals were observed for neuronal deficts at 2 and 24 h after surgery and were sacrificed 72 h after onset of ischemia. Saline-treated animals showed a large infarction involving the cerebral cortex and basal ganglion in most animals (volume 33.1 +/- 9.7%). Animals treated with citicoline alone were divided in two groups. The first group of animals were treated with a single injection (300 mg/kg, ip) of the medication 2 h after the arterial occlusion. The second group was treated with the active medication intermittently (3 x 300 mg/kg, ip) over a 72-h period. There was a significant decrease in the neuronal damage in the cortex in the animals treated with citicoline (single dose, 20.9 +/- 9.7%, P = 0.01; intermittent injection, 18.9 +/- 11.4%, P < 0.008). The last experiment evaluated the usefulness of the combination of citicoline with intraarterial urokinase. The combination showed significantly more protection than with urokinase or citicoline alone (volume 13.6 +/- 9.1%, P < 0.001). We conclude from our experiments that citicoline may offer significant neuronal protection that may be further enhanced with the addition of a thrombolytic agent.

Stroke: antithrombin versus antiplatelet therapy

The success of thrombolytic therapy for acute stroke has demonstrated that neurologic outcome can be improved with timely treatment. However, the severely restricted use of thrombolytics has reinforced the need to develop alternative and complementary therapies. Antithrombin and antiplatelet agents represent promising therapeutic approaches for stroke management. Antiplatelet therapy has modestly improved outcome in both acute stroke (aspirin) and in secondary stroke prevention (aspirin with or without dipyridamole; adenosine receptor antagonists), although bleeding and other adverse events associated with antithrombin therapy have largely negated their potential benefit. These findings have prompted innovative solutions to the pharmacokinetic and pharmacodynamic challenges that are crucial to advancing these strategies for acute, primary and secondary stroke therapy. Currently, inhibitors of the platelet surface glycoprotein IIb/IIIa (GP IIb/IIIa, fibrinogen) receptor are being examined in clinical trials while antithrombin therapies focus on thrombin antagonists and inhibitors as well as inhibitors of Factor Xa. Further advances in stroke treatment will include combination therapies. Additionally, the successful design of future drug therapies will result from a more complete understanding of the activity of these agents not only on platelet function and the coagulation cascade, but also for their effects on the endothelium and within the brain parenchyma. The sum of these activities will allow for the maintenance of cerebral blood flow, blood-brain barrier integrity and neuronal function.

Antiplatelet therapy in acute cerebral ischemia

BACKGROUND

Improved recognition of stroke signs and symptoms has paralleled the development of pharmacological strategies that may be examined to reduce stroke mortality and morbidity. Presently, tissue plasminogen activator is the only therapy that significantly improves outcome in acute stroke, with no agent demonstrating a significant reduction in mortality.

SUMMARY OF REVIEW

Antiplatelet agents are a heterogenous class of drugs that have been successfully used for more than 2 decades in secondary stroke prevention. These agents include aspirin, with or without dipyridamole, and more recently, the adenosine antagonists ticlopidine and clopidogrel. However, studies of the use of antiplatelet agents within 48 hours of the ictus have examined only aspirin. Only 1 study, the Multicentre Acute Stroke Trial-Italy (MAST-I), entered patients within 6 hours of the ictus. These data suggest that an improvement in mortality may be related to the speed of administration. No significant adverse events were noted with early antiplatelet monotherapy. However, MAST-I did note a significant increase in early mortality in patients receiving aspirin plus streptokinase, a finding not adequately explained by an increase in the intracranial hemorrhage rate.

CONCLUSIONS

The use of antiplatelet therapy in acute stroke, clinical or experimental, has only recently received attention. It is likely that the use of antiplatelet agents for acute stroke therapy will be less restrictive than that currently seen for thrombolytics. Future studies should include an examination of those agents that have previously demonstrated efficacy in secondary stroke prevention, most notably, aspirin. The recognition that all platelet stimuli share a final common pathway that is dependent on the surface glycoprotein IIb/IIIa (fibrinogen) receptor has resulted in the development of various agents which block this receptor and are currently the focus for clinical trials. The role of nitric oxide in stroke therapy will depend on minimizing the hypotensive side effects of this agent. Stroke models are needed to provide preliminary data on the efficacy of antiplatelet therapy, especially as relates to the interaction of antiplatelet agents with thrombolytics.

Aspirin use and incident stroke in the cardiovascular health study. CHS Collaborative Research Group

BACKGROUND AND PURPOSE

Randomized clinical trials testing aspirin in relatively low-risk, middle-aged people have consistently shown small increases in stroke associated with aspirin use. We analyzed the relationship between the regular use of aspirin and incident ischemic and hemorrhagic stroke among people aged 65 years or older participating in the Cardiovascular Health Study.

METHODS

We conducted a multivariate analysis of incident stroke rates in a prospectively assessed, observational cohort of 5011 elderly people followed for a mean of 4.2 years.

RESULTS

Participants had a mean age of 72 years, and 58% were women. Twenty-three percent used aspirin frequently, and 17% used aspirin infrequently at study entry. Frequent aspirin use was associated with an increased rate of ischemic stroke compared with nonusers (relative risk= 1.6; 95% confidence interval [CI], 1.2 to 2.2; P=0.001). After adjustment for other stroke risk factors, women who used aspirin frequently or infrequently at study entry had a 1.8-fold (95% CI, 1.2 to 2.8) and 1.6-fold (95% CI, 0.9 to 3.0) increased risk of ischemic stroke, respectively (P<0.01, test for trend), compared with nonusers. In men, aspirin use was not statistically significantly associated with stroke risk. Findings were similar when aspirin use in the years before the incident stroke was used in the modeling. Aspirin use at entry was also associated with a 4-fold (95% CI, 1.6 to 10.0) increase in risk of hemorrhagic stroke for both infrequent and frequent users of aspirin (P=0.003).

CONCLUSIONS

Aspirin use was associated with increased risks of ischemic stroke in women and hemorrhagic stroke overall in this elderly cohort, after adjustment for other stroke predictors. The possibility exists of confounding by reasons for aspirin use rather than cause and effect. Whether regular aspirin use increases stroke risk for elderly people without cardiovascular disease can only be determined by randomized clinical trials.