Role of ticlopidine for prevention of stroke

Newer agents in antiplatelet therapy: a review

Antiplatelet therapy remains the mainstay in preventing aberrant platelet activation in pathophysiological conditions such as myocardial infarction, ischemia, and stroke. Although there has been significant advancement in antiplatelet therapeutic approaches, aspirin still remains the gold standard treatment in the clinical setting. Limitations in safety, efficacy, and tolerability have precluded many of the antiplatelet inhibitors from use in patients. Unforeseen incidences of increased bleeding risk and recurrent arterial thrombosis observed in patients have hampered the development of superior next generation antiplatelet therapies. The pharmacokinetic and pharmacodynamic profiles have also limited the effectiveness of a number of antiplatelet inhibitors currently in use due to variability in metabolism, time to onset, and reversibility. A focused effort in the development of newer antiplatelet therapies to address some of these shortcomings has resulted in a significant number of potential antiplatelet drugs which target enzymes (phosphodiesterase, cyclooxygenase), receptors (purinergic, prostaglandins, protease-activated receptors, thromboxane), and glycoproteins (αIIbβ3, GPVI, vWF, GPIb) in the platelet. The validation and search for newer antiplatelet therapeutic approaches proven to be superior to aspirin is still ongoing and should yield a better pharmacodynamic profile with fewer untoward side-effects to what is currently in use today.

Antiplatelet Agents: Current Drugs and Future Trends

Antiplatelet drugs in clinical use are discussed in terms of their mechanisms of action and the relevancy of that to the physiology of platelets and the pathophysiology of arterial thrombosis. Current clinical usage is outlined in detail for each drug. Experimental antiplatelet drugs also are discussed.

Orbofiban: an orally active GPIIb/IIIa platelet receptor antagonist

A key role has been established for platelet activation and thrombus formation in the pathogenesis of acute coronary syndromes, and restenosis after percutaneous interventions. Antiplatelet agents that have a wider spectrum of activity than aspirin, and clopidogrel would be expected to provide improved antithrombotic protection. Preclinical studies were used to predict clinical efficacy of orally active GPIIb/IIIa antagonists such as xemilofiban, sibrafiban, lefradafiban, and orbofiban. While clinical trials have shown potent and sustained platelet inhibition, outcomes of trials with these first generation GPIIb/IIIa compounds have been disappointing. The active moiety of orbofiban is a potent and specific inhibitor of fibrinogen binding to GPIIb/IIIa, leading to inhibition of platelet aggregation to a wide variety of agonists. Studies comparing inhibition of aggregation and bleeding suggest that chronic inhibition of platelet aggregation can be achieved without major bleeding side effects. Thrombus formation is prevented in canine models of thrombosis. Orbofiban is approximately 28% bioavailable with a t(1/2) of 18 hr. The high bioavailability, long half-life, and potential safety suggest orbofiban would be suitable for chronic oral administration. Clinical data demonstrate that orally administered orbofiban has the desired pharmacodynamic effect of inhibiting platelet aggregation but does not demonstrate clinical benefit when examined in large-scale trials.

Diagnosis and treatment of ischemic stroke

Most patients who have a stroke are evaluated initially by a primary care physician. For patients to benefit from new stroke therapies that must be initiated within a few hours of stroke onset, primary care physicians must be prepared to diagnose stroke and initiate acute treatment. This article provides information on the rapid and accurate diagnosis and management of patients with acute ischemic stroke. This information is particularly relevant due to the relatively high risk:benefit ratio associated with some acute stroke therapies, such as tissue plasminogen activator. Information is also provided about medical and surgical therapies to prevent subsequent strokes.

Effect of PP1D-1, a synthetic antiplatelet compound, on rabbit platelets

The antiplatelet mechanism of a synthetic compound, 2-chloro-3-methoxycarbonylpropionamido-1,4-naphthoquinone (PP1D-1), was studied by employing washed rabbit platelets in vitro. PP1D-1 concentration-dependently inhibited thrombin (0.1 U/ml)-, platelet-activating factor (2 ng/ml)-, collagen (10 microg/ml)-, arachidonic acid (100 microM)- and U46619 (1 microM)-induced aggregation and ATP release in washed rabbit platelets. The IC50 values of PP1D-1 for aggregation induced by the above inducers are 17.9+/-1.7, 9.8+/-1.1, 3.9+/-0.4, 1.8+/-0.3 and 1.7+/-0.3 microM, respectively. PP1D-1 did not affect platelet thromboxane B2 or prostaglandin D2 formation induced by arachidonic acid, indicating that it did not affect cyclooxygenase and thromboxane synthase activities. PP1D-1 significantly inhibited the formation of inositol 1,4,5-trisphosphate caused by these five platelet stimulators. Moreover, PP1D-1 inhibited the increase in intracellular calcium concentration induced by these agents. On the contrary, PP1D-1 did not inhibit thapsigargin-elevated intracellular calcium concentration in indomethacin-pretreated platelets, indicating it did not influence the effect of thapsigargin. According to these data, PP1D-1 exerts antiplatelet effects mainly by inhibiting phosphoinositide turnover.

[Bone marrow aplasia related to ticlopidine]

The authors report two cases of bone marrow aplasia observed 2 months after initiation of a treatment with ticlopidine. The outcome was favorable after discontinuation of therapy. The frequency of this severe drug-induced complication seems to have been underestimated. The absolute necessity of a careful haematological survey during the first 3 months of therapy is pointed out.

The effect of oral antiplatelet agents on tissue plasminogen activator-mediated thrombolysis in a rabbit model of thromboembolic stroke

OBJECTIVE

The success of thrombolytic therapy in acute stroke relies on timely reperfusion. The current study examines the efficacy of antiplatelet agents as adjuvants for thrombolytic therapy.

METHODS

Using an established rabbit model of clot embolization and a randomized blinded design, rabbits (n = 8 in each group) were orally pretreated daily for 5 days with adjuvant aspirin (1 mg/kg of body weight or 20 mg/kg), ticlopidine (100 mg/kg), or vehicle (sodium carbonate). On the 6th day, tissue plasminogen activator (6.3 mg/kg administered intravenously over 2 h), was initiated 1 hour after embolization.

RESULTS

In all groups, cerebral blood flow (CBF) was reduced to < 10 ml/100 g/min immediately after clot embolization. After the initiation of tissue plasminogen activator (t-PA), there was significant restoration of CBF in the control (t-PA only) and ticlopidine groups (P < 0.05) only. Restoration of CBF generally correlated with brain infarct size (percent hemisphere, mean +/- standard error of the mean), which was 18.0 +/- 7.0 in the t-PA only group versus 11.0 +/- 3.3, 26.5 +/- 5.8, and 21.5 +/- 3.4 in the ticlopidine, low-dose aspirin, and high-dose aspirin groups, respectively (ticlopidine versus aspirin, P < 0.05). Clot lysis was identical in the control and ticlopidine groups, with 6 of 8 animals demonstrating complete clot lysis. Aspirin antagonized clot lysis in a dose-related manner, with low-and high-dose aspirin groups noting clot lysis in four of eight and two of eight animals, respectively.

CONCLUSIONS

Pretreatment with ticlopidine significantly reduced brain infarct size when compared with aspirin treatment (P < 0.05). Moreover, whereas ticlopidine treatment did not affect clot lysis or CBF relative to t-PA alone, aspirin therapy resulted in antagonism of clot lysis and was associated with a more modest restoration of blood flow. This study provides a background for a more comprehensive understanding of the balance of thrombogenicity and thrombolysis and may assist in the development of novel therapies to expedite cerebrovascular patency and reduce ischemic and reperfusion-mediated neuronal injury.

Anti-platelet effects of clopidogrel in rat middle cerebral artery thrombosis model

We have developed a model whereby the middle cerebral artery (MCA) in an experimental animal can be occluded by photochemical reaction between rose bengal and green light which causes endothelial injury followed by platelet adhesion, aggregation and formation of a platelet and fibrin-rich thrombus at the site of photochemical reaction. Using this model, we investigated the effect of clopidogrel, an analogue of ticlopidine which is a potent inhibitor of adenosine 5'-diphosphate (ADP)-induced platelet aggregation. Oral clopidogrel (3-10mg/kg) inhibited ex-vivo platelet aggregation induced by ADP, thrombin or the thromboxane A2 mimetic, and U46619, when platelets had been primed with low concentration of phorbol myristate acetate. At these doses, clopidogrel significantly (P < 0.001) prolonged the time to produce thrombotic occlusion of the MCA and induced a significant (P < 0.001) reduction in the size of ischaemic cerebral damage examined 24 hours after photochemical reaction. The results suggest that ADP has a key role in the thrombotic occlusion of the MCA in this model. Clopidogrel may be beneficial in the prevention of arterial thrombosis.

Inhibitory effect of clopidogrel, vapiprost and argatroban on the middle cerebral artery thrombosis in the rat

This study investigated the roles of thromboxane A2 (TXA2), ADP and thrombin in middle cerebral artery (MCA) thrombosis in the rat. The rat MCA was occluded by a thrombus induced by the photochemical reaction of rose bengal by green light that causes endothelial damage followed by platelet adhesion, aggregation and formation of a platelet and fibrin-rich thrombus at the site of the photochemical reaction. Vapiprost, a specific TXA2-receptor antagonist; clopidogrel, which has the thienopyridine structure of ticlopidine and is a more potent inhibitor of ADP-induced platelet aggregation than ticlopidine; argatroban, a specific thrombin inhibitor; or heparin was administered intravenously before rose bengal injection. The MCA local blood flow was monitored by a laser Doppler flowmeter. The MCA was occluded by thrombus about 5 min after the initiation of the photochemical reaction. Vapiprost, clopidogrel and argatroban all significantly prolonged the time taken for the thrombotic occlusion of the MCA, but in this respect, heparin was ineffective. Our observations suggest that vapiprost and clopidogrel are useful antithrombotic agents against platelet and fibrin-rich thrombi. The effect of argatroban is attributable to inhibition of thrombin-induced platelet activation and fibrin generation. The thrombosis model described in this study is useful for understanding the mechanism(s) of thrombogenesis in the rat MCA and may be applied to other mammalian species.

Antithrombotic agents in cerebral ischemia

The choice of antithrombotic agent in cerebral ischemia depends on the pathogenesis: thrombosis, embolism, or hemorrhage. Antiplatelet agents are considered most beneficial in thrombotic stroke, anticoagulants are most effective in cardioembolic stroke; antithrombotic agents are generally contraindicated in hemorrhagic stroke. A meta-analysis of 18 trials documented a 23% reduction in stroke risk with antiplatelet agents; aspirin is typically the antiplatelet agent of choice for stroke prevention. There are no definitive data regarding the optimal aspirin dose for stroke prevention and this issue remains controversial. Ticlopidine is the most effective antiplatelet agent, but its adverse effect profile restricts its use. Anticoagulants are highly effective for preventing cardioembolic stroke, but their effectiveness in non-cardioembolic stroke is uncertain because of lack of trial data. Results of the ongoing Warfarin/Aspirin Recurrent Stroke Study (warfarin [INR 1.8-2.8] vs aspirin [325 mg/day]) may clarify this issue. There is renewed interest in thrombolytics because recent data indicate that reperfusion within a few hours of stroke onset appears to be effective in preventing neuronal damage. In addition, when given within 6 hours of stroke onset, thrombolytics appear to be relatively safe. Several direct thrombin inhibitors are being evaluated. Experimentally, hirudin, hirulog, D-Phe-L-Pro-L-Arg-CH2Cl (PPACK), and argatroban are clearly more effective than heparin in inhibiting platelet deposition and thrombus formation, and also show promise in preventing reocclusion after thrombolysis for both experimental thrombotic and embolic stroke. However, the risk of hemorrhage in patients with cerebrovascular disease is unknown for these agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Ticlopidine and fatal aplastic anemia in an elderly woman

OBJECTIVE

To report a case of aplastic anemia that developed during ticlopidine treatment.

CASE SUMMARY

An 84-year-old woman was started on ticlopidine for secondary stroke prevention. Within six weeks of initiating ticlopidine therapy she developed aplastic anemia. She was hospitalized and received empiric antibiotics, antifungal agents, blood transfusions, platelets, and granulocyte colony-stimulating factor. The patient died on day 76 after beginning ticlopidine.

DISCUSSION

Hematologic effects such as neutropenia, thrombocytopenia, agranulocytosis, thrombotic thrombocytopenic purpura, and pancytopenia have been described with the use of ticlopidine. Previous case reports have associated ticlopidine with the development of aplastic anemia.

CONCLUSIONS

Ticlopidine can produce fatal hematologic adverse effects, and its use should be reserved as second-line therapy.

Antiplatelet therapy following cardiac valve replacement-a comparative study of aspirin and ticlopidine

The antiplatelet agents, aspirin and ticlopidine, are widely used to prevent thromboembolism following cardiac valve replacement. To compare the clinical effects of each platelet inhibitor, a daily dose of ticlopidine 300 mg was given to 50 patients who underwent aortic valve or mitral valve replacement with an average age of 56.9 years over a mean 52.6 months after surgery. 50 more patients with an average age of 50.2 years were given a daily dose of aspirin 81 mg over a mean 51.3 months after surgery. Warfarin was given to maintain thrombotest values at 10 to 25% (PT-INR at 1.6-3.0). The incidence of thromboembolism was low in both groups; 1.0/100 patient years in the ticlopidine group and 1.9 in the aspirin group. Hemorrhagic complications, hematuria and ecchymosis, showed an incidence of 2.9 in the ticlopidine group and 2.3 in the aspirin group. Slight increases in GOT and GTP were observed in 4 and 18% of cases and elevated total cholesterol and neutral fat in 2 and 18% of cases. No adverse reactions were reported. With the exception of a significant decrease in ADP-induced platelet aggregation in patients who took ticlopidine, there were no significant differences observed between the two groups.