Platelet-active drugs: the relationships among dose, effectiveness, and side effects

Update on the interaction between aspirin and angiotensin-converting enzyme inhibitors

We summarized recent published literature regarding the significance of an interaction between aspirin and angiotensin-converting enzyme (ACE) inhibitors in patients with various cardiovascular diseases. A MEDLINE search (January 1998-July 1999) was performed and abstracts from the 1999 American College of Cardiology and 1998 American Heart Association annual scientific sessions were reviewed to identify pertinent studies. Material for discussion was identified through a MEDLINE search from January 1996-July 1999 and through cited references. The results of several studies added to our understanding of the clinical ramifications of an aspirin-ACE inhibitor interaction, but also introduced questions. These studies are largely contradictory, but do reiterate the possibility of an interaction, if only in certain subsets of patients. Low dosages (< or = 100 mg/day) of aspirin appear to be safer in this regard than higher dosages. The frequency and severity of the interaction and possible predisposing factors await future research.

Prevention of pulmonary thromboembolism by NCX 4016, a nitric oxide-releasing aspirin

We studied the antithrombotic activity of 2-acetoxybenzoate 2-[1-nitroxy-methyl]-phenyl ester (NCX 4016), a novel nitric oxide (NO)-releasing aspirin derivative, in vivo in different animal models of platelet-dependent and independent pulmonary thromboembolism and compared it with that of aspirin. NCX 4016 protected mice from death induced by the intravenous (i.v.) injection of collagen plus epinephrine, of 9,11-dideoxy-11alpha, 9alpha-epoxymethano-prostaglandin F(2alpha) (U46619) and of thrombin while aspirin was only active against collagen plus epinephrine. The drop in platelet count and number of lung emboli were reduced by NCX 4016 more effectively than aspirin. NCX 4016 protected mice also from mechanical pulmonary embolism (i.v. injection of hardened rat red blood cells) while aspirin was ineffective. In rabbits, NCX 4016 significantly reduced the accumulation of [111In]oxine-labeled platelets in the pulmonary vasculature induced by collagen and by thrombin while aspirin produced reductions which were significant only versus collagen. In conclusion, NCX 4016 exerts a more pronounced antithrombotic activity than aspirin in vivo in two different animal species, largely due to a deeper inhibitory effect on platelets. NCX 4016 may represent a better antithrombotic agent than aspirin.

Stroke prevention: anti-platelet and anti-thrombolytic therapy

In patients with TIA or ischemic stroke of noncardiac origin antiplatelet drugs are able to decrease the risk of stroke by 11-15%, and the risk of stroke, MI, and vascular death by 15-22%, but not mortality. Low doses of aspirin (50-325 mg) are as effective as high doses and cause less gastrointestinal side effects. Severe bleeding complications are not dose-dependent. The combination of aspirin with slow release dipyridamole is superior to aspirin alone for stroke prevention. Ticlopidine is superior to aspirin but has slightly more serious adverse effects (neutropenia). It will be replaced by clopidgrel which has a better safety profile. Anticoagulation with an INR between 3.0 and 4.5 is too dangerous. Whether anticoagulation with lower INR is safe and effective is not yet known.

[Aspirin and hemostasis]

Aspirin is one hundred years old, though its use has clearly evolved during the last 25 years. Identifying its action mechanism has allowed us to better understand the antithrombotic impact. Prostaglandin H synthetase (PGHS) is a bifunctional enzyme with cyclooxygenase and peroxydase activities. There are two isoforms: constitutive PGHS-1 and inducible PGHS-2. Aspirin irreversive acetylates the platelet cyclooxygenase involved in the formation of thromboxane A2, a powerful proaggregating agent and vasoconstrictor. More than 95% of inhibition of this synthesis takes place in two to three days using very weak doses of aspirin, on the order of 30 to 50 mg per day. Under some circumstances, this inhibition requires higher dosages. Certain clinical and biological circumstances could lead to a resistance to aspirin, making a readjustment of doses and sometimes complementary explorations necessary. The ISIS 2 study showed in an apparently irrefutable way the entry of aspirin into the antithrombotics arsenal, with a significant risk reduction of vascular death and recurrence of infarctus. Numerous studies have confirmed this efficacy. Consensus studies are based on information showing total coherence between the dose necessary to acetylate the enzyme to inhibit thromboxane A2 platelet production and the clinical antithrombotic effect. Aspirin seems to have a secure place, and it begins the third millennium in relative peace with new extra-platelet potentialities outside the framework of hemostasis and thrombosis.

Gastroduodenal tolerability of medium dose enteric-coated aspirin: a placebo controlled endoscopic study of a new enteric-coated formulation versus regular formulation in healthy volunteers

We compared, in a cross-over study, the toxicity of 300 mg enteric-coated aspirin with regular aspirin used for the prevention of cardiovascular events. In terms of endoscopic haemorrhagic lesions, enteric-coated aspirin is less gastrotoxic than regular aspirin.

[Pharmacology of aspirin]

Aspirin (acetylsalicylic acid) is the best-known salicylate and belongs to the non steroid anti-inflammatory drug class. Despite wide use being made since more than 100 years, knowledge about mechanism of action and therapeutic issues continually evolves. The main mechanism of action is prostaglandin synthesis inhibition. This is achieved through inhibition of prostaglandin endoperoxide synthase (PGHS) or cyclooxygenase (COX) synthesis. Most of the therapeutic uses of aspirin are explained by this mechanism. Nevertheless aspirin uses change as time goes by: if the main one during the first fifty years was an analgesic, anti-pyretic and anti-inflammatory one, the last fifty years saw aspirin being used mainly as an anti-thrombotic agent, in primary and secondary thrombo-embolic prevention. Better knowledge of mechanism of action points today at, on one hand, more selective and therefore better tolerated molecules, and, on the other hand, at new therapeutic applications, such as anti-cancer and neurodegenerative diseases prevention.

Reversible ageusia as an effect of clopidogrel treatment

In two patients we observed a loss of taste during clopidogrel treatment which was reversible.

Newer antithrombotic strategies in the initial management of non-ST-segment elevation acute coronary syndromes

OBJECTIVE

To review the place in therapy of currently available antithrombotic agents in the non-ST-segment elevation acute coronary syndromes, that is, unstable angina and non-Q-wave myocardial infarction (MI). Recommendations are made based on currently available data.

DATA SOURCE

English-language clinical studies, position statements, and review articles pertaining to the management of unstable angina and non-Q-wave MI with currently available products.

STUDY SELECTION

Selection of prospective clinical studies was limited to those focusing on the management of the non-ST-segment elevation acute coronary syndromes, unstable angina, and non-Q-wave MI.

DATA SYNTHESIS

It has yet to be determined which combination of agents (dalteparin, enoxaparin, lepirudin, clopidogrel, ticlopidine, abciximab, eptifibatide, tirofiban) and procedural strategies most significantly reduces mortality and serious events in these patients. The relevant pathophysiology, diagnostic criteria, and risk-stratifying procedures are reviewed in context with information from clinical studies regarding currently available agents for the management of non-ST-segment elevation acute coronary syndromes.

CONCLUSIONS

A large number of new therapeutic classes and agents are available for the treatment of unstable angina and non-Q-wave MI. Although the diagnoses of unstable angina or non-Q-wave MI identify risk, treatment decisions are often based on the presence or absence of ST-segment elevations. Limited prospective evidence delineates the proper utilization of resources to best manage these patients. Efforts should be aimed at identifying particular patients who will best benefit from recently available therapies.

Antiplatelet and anticoagulant therapy in the secondary prevention of ischemic heart disease

Antiplatelet and anticoagulant medications play a major role in the secondary prevention of ischemic heart disease. Numerous trials have demonstrated their clinical benefits. Newer agents, such as clopidogrel, have challenged aspirin's role as the premier medication for secondary prevention. Much remains to be learned, however, about the merits of these different drug classes, relative to one another and in combination.

Antithrombotic efficacy of the vitamin K antagonist fluindione in a human Ex vivo model of arterial thrombosis : effect of anticoagulation level and combination therapy with aspirin

Thrombin is a main mediator of arterial thrombus formation, and its inhibition is an important antithrombotic strategy. However, the place of vitamin K antagonists among the different therapeutic strategies for preventing arterial thrombus formation is still debated. We studied the antithrombotic efficacy of the vitamin K antagonist fluindione in a human ex vivo model of arterial thrombosis and determined whether aspirin enhances fluindione efficacy. Ten healthy male volunteers were randomly assigned to receive fluindione, alone or in combination with aspirin (325 mg/d). Fluindione was given at increasing doses to give a stable international normalized ratio (INR) between 1.5 and 2.0 and between 2.1 and 3.0. We induced arterial thrombus formation ex vivo by exposing collagen- or tissue factor (TF)-coated coverslips in a parallel-plate perfusion chamber to native blood for 3 minutes at an arterial wall shear rate of 2600 s(-1). Platelet and fibrin deposition were measured by immunoenzymatic methods. Fluindione inhibited thrombus formation on TF-coated coverslips in a dose-dependent manner by 50% and 80% at INR 1.5 to 2.0 and INR 2.1 to 3.0, respectively (P<0.05). Fluindione in combination with aspirin inhibited TF-induced thrombus formation in a comparable manner. Collagen-induced thrombus formation was not reduced in subjects treated by fluindione. It was reduced by 50% to 60% in those treated with fluindione plus aspirin, regardless of the level of anticoagulation (P<0.05). Thus, the effectiveness of fluindione for preventing arterial thrombosis is dependent on the nature of the thrombogenic trigger. Fluindione is very effective in preventing TF- but not collagen-triggered thrombus formation. Aspirin enhances the antithrombotic effectiveness of fluindione, because combined treatment interrupts both TF- and collagen-induced thrombus formation.

Haemostatic function, arterial disease and the prevention of arterial thrombosis

Recent years have seen the expansion of information linking raised plasma levels of individual clotting factors and evidence of disturbances of fibrinolytic activity with the risk of thrombotic manifestations of arterial disease, both in community-based, apparently healthy populations and in patients with known atherosclerosis. Some of these prothrombotic changes in the haemostatic system may result partly from underlying chronic inflammation or acute infection and may, in turn, contribute substantially to the thrombotic risk which accompanies these underlying processes. The importance of the coagulation system in the pathogenesis of arterial thrombosis is further illustrated by the benefit in the Thrombosis Prevention Trial of low-intensity, dose-adjusted warfarin in the primary prevention of ischaemic heart disease. Clinical trials of bezafibrate, which is being used for its fibrinogen-lowering as well as lipid-modifying properties, are in progress.

Therapeutic benefit. Aspirin revisited in light of the introduction of clopidogrel

BACKGROUND

Antiplatelet agents are widely recognized for their efficacy in reducing the occurrence of vascular events in patients with atherothrombotic disease. Aspirin is currently considered to be the "reference standard" antiplatelet agent and is recommended by the American Heart Association for use in patients with a wide range of manifestations of cardiovascular disease on the basis of its high benefit-to-risk and benefit-to-cost ratios. Recently, clopidogrel (Plavix, Bristol-Myers Squibb Co), another antiplatelet agent, was approved by the Food and Drug Administration for many of the same indications as aspirin.

SUMMARY OF REVIEW

Because physicians will be faced with deciding whether to switch from the well-established practice of recommending aspirin for use in patients with atherothrombotic disease, both aspirin and clopidogrel are compared with respect to the primary factors that influence such decisions (ie, their relative efficacy, safety, cost, and convenience of use).

CONCLUSIONS

Based on the available evidence, aspirin is preferred for the majority of stroke or myocardial infarction patients at risk of recurrent atherothrombotic events. Clopidogrel may, however, provide valuable therapeutic benefit over aspirin in patients with peripheral arterial disease and in stroke or myocardial infarction patients for whom aspirin treatment is contraindicated or for whom aspirin fails to achieve the desired therapeutic effect.