Effect of alternate-day regular and enteric-coated aspirin on platelet aggregation, bleeding time, and thromboxane A2 levels in bleeding-time blood

Sulindac plus a phospholipid is effective for polyp reduction and safer than sulindac alone in a mouse model of colorectal cancer development

Background

Non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin and sulindac are effective for colorectal cancer prevention in humans and some animal models, but concerns over gastro-intestinal (GI) ulceration and bleeding limit their potential for chemopreventive use in broader populations. Recently, the combination of aspirin with a phospholipid, packaged as PL-ASA, was shown to reduce GI toxicity in a small clinical trial. However, these studies were done for relatively short periods of time. Since prolonged, regular use is needed for chemopreventive benefit, it is important to know whether GI safety is maintained over longer use periods and whether cancer prevention efficacy is preserved when an NSAID is combined with a phospholipid.

Methods

As a first step to answering these questions, we treated seven to eight-week-old, male and female C57B/6 Apc^min/+ mice with the NSAID sulindac, with and without phosphatidylcholine (PC) for 3-weeks. At the end of the treatment period, we evaluated polyp burden, gastric toxicity, urinary prostaglandins (as a marker of sulindac target engagement), and blood chemistries.

Results

Both sulindac and sulindac-PC treatments resulted in significantly reduced polyp burden, and decreased urinary prostaglandins, but sulindac-PC treatment also resulted in the reduction of gastric lesions compared to sulindac alone.

Conclusions

Together these data provide pre-clinical support for combining NSAIDs with a phospholipid, such as phosphatidylcholine to reduce GI toxicity while maintaining chemopreventive efficacy.

Acetylsalicylic Acid Daily vs Acetylsalicylic Acid Every 3 Days in Healthy Volunteers: Effect on Platelet Aggregation, Gastric Mucosa, and Prostaglandin E2 Synthesis

Substantial platelet inhibition was observed 3 days after a single administration of acetylsalicylic acid 81 mg to healthy volunteers. Here we investigate prostaglandin E2 (PGE2 ) antrum concentrations and gastrointestinal symptoms in two treatment groups: one receiving losartan and acetylsalicylic acid every day and the other receiving losartan every day and acetylsalicylic acid every 3 days. Twenty-eight healthy volunteers from both sexes received either 50 mg losartan and acetylsalicylic acid 81 mg daily or 50 mg losartan and acetylsalicylic acid 81 every 3 days with placebo on the other days. Therapy was delivered for 30 days for both groups. Gastric endoscopy was performed before and after treatment period. Biopsies were collected for PGE2 quantification. Platelet function tests were carried out before and during treatment and TXB2 release on platelet rich plasma was measured. The every 3 day low-dose acetylsalicylic acid regimen produced complete inhibition of platelet aggregation compared to the daily treatment. Thromboxane B2 release was substantially abolished for both groups during treatment. There was no significant difference on the endoscopic score of both treatment groups after the 30-day treatment (P = .215). There was over 50% suppression of antrum PGE2 content on volunteers receiving acetylsalicylic acid daily (P = .0016), while for the every 3 day dose regimen there was no significant difference between pre and post-treatment antrum PGE2 dosages (P = .4193). Since PGE2 is involved in gastric healing, we understand that this new approach could be safer and as efficient as the standard daily therapy on a long-term basis.

Nonsurgical bleeding diathesis in anemic thrombocytopenic patients: role of temperature, red blood cells, platelets, and plasma-clotting proteins

Research at the Naval Blood Research Laboratory (Boston, MA) for the past four decades has focused on the preservation of red blood cells (RBCs), platelets (PLTs), and plasma-clotting proteins to treat wounded servicemen suffering blood loss. We have studied the survival and function of fresh and preserved RBCs and PLTs and the function of fresh and frozen plasma-clotting proteins. This report summarizes our peer-reviewed publications on the effects of temperature, RBCs, PLTs, and plasma-clotting proteins on the bleeding time (BT) and nonsurgical blood loss. The term nonsurgical blood loss refers to generalized, systemic bleeding that is not corrected by surgical interventions. We observed that the BT correlated with the volume of shed blood collected at the BT site and to the nonsurgical blood loss in anemic thrombocytopenic patients after cardiopulmonary bypass surgery. Many factors influence the BT, including temperature; hematocrit (Hct); PLT count; PLT size; PLT function; and the plasma-clotting proteins factor (F)VIII, von Willebrand factor, and fibrinogen level. Our laboratory has studied temperature, Hct, PLT count, PLT size, and PLT function in studies performed in non-aspirin-treated and aspirin-treated volunteers, in aspirin-treated baboons, and in anemic thrombocytopenic patients. This monograph discusses the role of RBCs and PLTs in the restoration of hemostasis, in the hope that a better understanding of the hemostatic mechanism might improve the treatment of anemic thrombocytopenic patients. Data from our studies have demonstrated that it is important to transfuse anemic thrombocytopenic patients with RBCs that have satisfactory viability and function to achieve a Hct level of 35 vol percent before transfusing viable and functional PLTs. The Biomedical Excellence for Safer Transfusion (BEST) Collaborative recommends that preserved PLTs have an in vivo recovery of 66 percent of that of fresh PLTs and a life span that is at least 50 percent that of fresh PLTs. Their recommendation does not include any indication that preserved PLTs must be able to function to reduce the BT and reduce or prevent nonsurgical blood loss. One of the hemostatic effects of RBC is to scavenge endothelial cell nitric oxide, a vasodilating agent that inhibits PLT function. In addition, endothelin may be released from endothelial cells, a potent vasoconstrictor substance,to reduce blood flow at the BT site. RBCs, like PLTs at the BT site, may provide arachidonic acid and adenosine diphosphate to stimulate the PLTs to make thromboxane, another potent vasoconstrictor substance and a PLT-aggregating substance. At the BT site, the PLTs and RBCs are activated and phosphatidyl serine is exposed on both the PLTs and the RBCs. FVa and FXa, which generate prothrombinase activity to produce thrombin, accumulate on the PLTs and RBCs. A Hct level of 35 vol percent at the BT site minimizes shear stress and reduces nitric oxide produced by endothelial cells. The transfusion trigger for prophylactic PLT transfusion should consider both the Hct and the PLT count. The transfusion of RBCs that are both viable and functional to anemic thrombocytopenic patients may reduce the need for prophylactic leukoreduced PLTs, the alloimmunization of the patients, and the associated adverse events related to transfusion-related acute lung injury. The cost for RBC transfusions will be significantly less than the cost for the prophylactic PLT transfusions.

Aggregometry detects platelet hyperreactivity in healthy individuals

Aggregometry is widely used to assess platelet function, but its use in identifying platelet hyperreactivity is poorly defined. We studied platelet aggregation in 359 healthy individuals using the agonists adenosine diphosphate (ADP), epinephrine, collagen, collagen-related peptide, and ristocetin. We also assessed the reproducibility of these assays in 27 subjects by studying them repeatedly on at least 4 separate occasions. Healthy subjects exhibited considerable interindividual variability in aggregation response to agonists, especially at concentrations lower than those typically used in clinical laboratories. For each agonist tested at these submaximal concentrations, a small proportion of individuals demonstrated an unusually robust aggregation response. Subjects who exhibited such in vitro hyperreactivity to one agonist tended to demonstrate a similar response to others, suggesting that hyperreactivity is a global characteristic of platelets. Epinephrine and collagen-related peptide were especially reliable and efficient in detecting hyperreactivity. For epinephrine, excellent reproducibility persisted for up to 3 years, and hyperreactivity was associated with female sex and higher fibrinogen levels (P < .02). We recommend these assays as appropriate candidates for future studies requiring accurate assessment of increased platelet reactivity. These include clinical studies to improve risk assessment for arterial thrombosis, as well as genetic studies to establish determinants of the hyperreactive platelet phenotype.

Correlation between in vitro aggregation and thromboxane A2 production in fresh, liquid-preserved, and cryopreserved human platelets: effect of agonists, pH, and plasma and saline resuspension

BACKGROUND

Some of the tests used to assess the quality of fresh and preserved platelets (PLTs) include PLT number, PLT morphology, pH of the PLT medium, PLT response to hypotonic stress, and PLT aggregation to agonists. This study was performed to assess the function of fresh and preserved PLTs by their response to aggregation and their production of thromboxane A2 after in vitro stimulation with agonists.

STUDY DESIGN AND METHODS

PLTs isolated by apheresis procedures were stored at 22 degrees C for as long as 5 days and then frozen with 6 percent dimethyl sulfoxide, stored at -80 degrees C, thawed, washed, and resuspended in medium. The effects of agonists and the pH and composition of the medium on PLT aggregation and PLT production of thromboxane A2 after stimulation were measured.

RESULTS

The agonists and the pH and composition of the medium affected both the aggregation response and the production of thromboxane A2 by the fresh and preserved PLTs. PLT aggregation response to arachidonic acid (AA) and adenosine diphosphate (ADP) was significantly lower in the cryopreserved PLTs than in the fresh and preserved PLTs. After stimulation with AA and ADP, the cryopreserved PLTs produced more thromboxane than did the fresh and liquid-preserved PLTs.

CONCLUSIONS

The agonists and the pH and composition of the medium affected the response to aggregate and produce thromboxane in vitro in both the fresh and the liquid-preserved PLTs. PLT thromboxane A2 production may be a better in vitro test than PLT aggregation to assess PLT function in vivo.

Aspirin for the prevention of cardiovascular events in the elderly

Aspirin (acetylsalicylic acid), the most widely used antiplatelet drug, is clinically effective for the prevention of vascular ischaemic events. Very few primary or secondary prevention trials address the benefit-risk ratio of aspirin in the elderly. In secondary prevention, it is generally accepted that the beneficial effect of aspirin in the general patient population, demonstrated by randomised controlled trials, can be extrapolated to the elderly. Elderly patients are at relatively high risk for the development of vascular disease and might also be expected to derive substantial benefit from regular aspirin administration. However, there is no consensus about the definition of elderly and no specific prospective trial conducted in elderly subjects is available. Retrospective studies in the elderly found that the benefit provided by aspirin in older patients was similar or increased compared with younger individuals. In primary prevention, the potential benefit of antiplatelet agents must be balanced against the risk of bleeding, which is higher in older patients. The risk-benefit trade-off from the use of low-dose aspirin in the elderly is not yet established and caution should be exercised when using aspirin in primary prevention. In conclusion, aspirin should only be given for primary and secondary prevention in the elderly after a comprehensive evaluation of an individual patient's thrombotic and haemorrhagic risk has been conducted.

Aspirin in the prophylaxis of coronary artery disease

Aspirin has been used for more than 100 years, but its mechanisms of action have only been understood in the past 20 years. Aspirin interferes with arachidonic acid metabolism in platelets and endothelial cells and thereby reduces thromboxane A2 and prostacyclin. It also has other mechanisms of action, including anti-inflammatory roles, protection from oxidative stress, enhancement of fibrinolysis, and suppression of plasma coagulation and platelet-dependent inhibition of thrombin generation. It has been used for primary and secondary prevention of myocardial ischemia, and for primary and secondary prevention of cerebrovascular ischemia. We review the 5 pivotal studies relating to primary prevention for cardiovascular risk and the many studies relating to secondary prevention of myocardial ischemia. We also review the utility of aspirin in primary prevention of myocardial infarction and stroke. We conclude that aspirin is one of the most potent drugs ever discovered and that its effects extend well beyond those of cycloxoxygenase enzyme inhibition. Aspirin treatment does not preclude control of underlying and comorbid conditions such as diabetes mellitus, hypertension, and dyslipidemia. For most patients, a daily dose of 325 mg is optimal. Patients must understand the potential for gastrointestinal upset and hemorrhagic complications. The utility of aspirin is greater in coronary artery disease prevention than in cerebrovascular prevention.

A comparison of every-third-day versus daily low-dose aspirin therapy on serum thromboxane concentrations in healthy men and women

Aspirin's antithrombotic effect is mediated predominately by inhibition of platelet cyclooxygenase-1, leading to a decline in serum thromboxane A2 concentrations. We performed a placebo-controlled, randomized, double-blind trial to determine whether aspirin could be given at 3-day intervals and still achieve potent serum thromboxane inhibition. One hundred nine healthy men and women with no recent exposure to aspirin and no contraindications to its use participated. Subjects received 325 mg, 81 mg, or 40 mg of plain aspirin every third day, with placebo on other days; 81 mg of aspirin every day; or placebo every day. Serum concentrations of thromboxane B2 (the metabolite of thromboxane A2) were measured at 3-day intervals during a 31-day treatment period, as well as 4, 7, and 14 days after treatment ended. Serum thromboxane B2 concentrations were nearly identical during treatment with 325 mg of aspirin every third day or 81 mg of aspirin per day (86% inhibition [84%, 89%] and 85% inhibition [73%, 96%], respectively). An aspirin dose of 81 mg every third day was nearly as potent (74% inhibition [70%, 79%]), whereas 40 mg of aspirin every third day achieved only 50% inhibition (40%, 60%). Every-third-day low-dose aspirin regimens (325 and 81 mg) deserve comparison with daily low-dose aspirin regimens in controlled clinical trials because the former regimens could prove to have equal efficacy with reduced toxicity.

Effects of very low dose daily, long-term aspirin therapy on gastric, duodenal, and rectal prostaglandin levels and on mucosal injury in healthy humans

BACKGROUND & AIMS

The safety of low-dose daily aspirin therapy in the gastrointestinal tract is uncertain. Our objectives were to evaluate the long-term effects of very low daily aspirin doses in the gastrointestinal tract and effects on platelet-derived serum thromboxane levels in volunteers.

METHODS

Subjects were randomized to receive 10 mg (n = 8), 81 mg (n = 11), or 325 mg (n = 10) aspirin daily for 3 months. Before administration of aspirin, all subjects underwent gastroduodenoscopy, and most underwent proctoscopy for assessment of mucosal injury and prostaglandin content. After 1.5 and 3 months, subjects again underwent gastroduodenoscopy and, at 3 months, another proctoscopy.

RESULTS

Each aspirin dose (even 10 mg) significantly reduced gastric mucosal prostaglandin levels, to approximately 40% of the baseline value. All three doses also induced significant gastric injury, and 325 mg caused duodenal injury. Three subjects developed gastric ulcers, 1 while taking 10 mg/day of aspirin. Furthermore, aspirin at 81 mg/day and 325 mg/day (but not 10 mg/day) significantly reduced duodenal mucosal prostaglandin levels to approximately 40% of the baseline value. Only 325 mg of aspirin per day significantly reduced rectal mucosal prostaglandin levels to approximately 60% of the baseline value. Serum thromboxane levels were inhibited 62%, 90%, and 98% with 10, 81, and 325 mg of aspirin.

CONCLUSIONS

The findings explain aspirin's predominant gastric toxicity and question the safety of even 10 mg of aspirin daily.

Effects of nonsteroidal antiinflammatory drugs on platelet function and systemic hemostasis

Aspirin and nonaspirin nonsteroidal antiinflammatory drugs (NSAIDs) inhibit platelet cyclooxygenase, thereby blocking the formation of thromboxane A2. These drugs produce a systemic bleeding tendency by impairing thromboxane-dependent platelet aggregation and consequently prolonging the bleeding time. Aspirin exerts these effects by irreversibly blocking cyclooxygenase and, therefore, its actions persist for the circulating lifetime of the platelet. Nonaspirin NSAIDs inhibit cyclooxygenase reversibly and, therefore, the duration of their action depends on specific drug dose, serum level, and half-life. The clinical risks of bleeding with aspirin or nonaspirin NSAIDs are enhanced by the concomitant use of alcohol or anticoagulants and by associated conditions, including advanced age, liver disease, and other coexisting coagulopathies.

The effect of regular and enteric-coated aspirin on bleeding time, thromboxane, and prostacyclin

We compared the effect of different aspirin schedules, dosages, and formulations on various bleeding time parameters including bleeding time, plasma and total blood volume, and levels of the stable metabolites of thromboxane A2 (TXA2) and prostacyclin (PGI2) (respectively, TXB2 and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha)) to determine the optimal dosage and formulation of aspirin to inhibit TXA2 production while sparing PGI2. In a randomized, parallel study, 52 healthy male volunteers (62 independent observations) with no history of bleeding disorders were given 80 mg or 325 mg of regular aspirin, or 325 mg of enteric-coated aspirin to ingest daily (14 pills) or every other day (7 pills) for a continuous 14 day period. Bleeding times were performed on day 1 before aspirin, 6 h after aspirin on day 1, and before aspirin on day 14. Bleeding times, plasma volume, and total volume increased significantly from before aspirin to after 6 h and 14 days (p < 0.0001 for all parameters) for all aspirin formulations. For day 1 before aspirin ingestion to 6 h later, both TX and PGI2 (p < 0.008) decreased significantly. 6 h after ingestion of aspirin on day 1 to day 14, both TX and PGI2 levels also significantly decreased (p < 0.0001). There was a highly significant decrease in PGI2 production on every other day aspirin schedules (p = 0.0001) particularly with 80 mg of aspirin, while the decrease in PGI2 production on daily aspirin was not significant (p = 0.10). The most favourable ratio of 6-keto-PGF1 alpha to TXB2 occurred with 80 mg daily.(ABSTRACT TRUNCATED AT 250 WORDS)

Objective evidence of aspirin use in both ulcer and nonulcer upper and lower gastrointestinal bleeding

To obtain the best evidence for nonsteroidal anti-inflammatory drug (NSAID) use in gastrointestinal (GI) bleeding, a detailed patient history was supplemented with objective tests of aspirin use, i.e., high-performance liquid chromatography of plasma and platelet cyclo-oxygenase inhibition, which detect aspirin intake within 24 and 120 hours, respectively. Seventy-one patients consecutively admitted for upper or lower GI bleeding and 138 age- and sex-matched controls were studied. Five bleeders were excluded for confounding factors, e.g., warfarin. Of the other 66 bleeders, 45 had upper GI bleeding (28 from peptic ulcer, 14 from duodenal ulcer, and 14 from gastric ulcer) and 21 lower GI bleeding. Evidence of current NSAID use (of which 89% was aspirin) was found in 80% of bleeders vs. 24.3% of controls (P less than 0.0001), for an odds ratio of 13.7 (95% confidence interval, 6.39-27.27). The cyclo-oxygenase test uncovered 21.5% more aspirin users than history alone. Severity of bleeding was not different in acetylsalicylic acid users. The surprisingly high association of current intake of NSAIDs, especially aspirin, with nonulcer GI bleeding including colonic bleeding, changes the conventional view of the following hierarchy of the risk: NSAID----peptic ulcer----bleeding to: NSAIDs----GI bleeding. This view has important implications for current ulcer cotherapy prophylactic strategies, which could fail to prevent greater than 50% of GI bleeding episodes.

Two year Cottbus reinfarction study with 30 mg aspirin per day

All 701 heart infarction patients admitted to 15 hospitals in the district of Cottbus between 1981 and 1983 were randomly administered 30, 60 or 1000 mg aspirin daily according to the territorial affiliation of their local hospitals. The physical and drug therapy during the 2 years follow-up was highly standardized; deviations--as far as they occurred--were documented. Lower all-cause mortality was statistically demonstrated in patients over 60 and a lower fatal reinfarction rate in patients over 50 as well as in men. Deaths and fatal reinfarctions were significantly lower among patients with a history of angina pectoris, marked ST-depression, with an infarction location except for the posterior wall and among hypercholesterolemic patients. The preventive effect of 60 mg aspirin daily was less than that of 30 mg in comparison to the 1000 mg group. Side effects were seen in 4 and 8% (first and second year), respectively, of the patients administered 30 mg aspirin as opposed to 22 and 17% in patients allocated 1000 mg. We conclude that the optimum dose of aspirin for preventing reinfarctions could be as low as 30 mg daily.

The effect of enteric-coated aspirin on the morning increase in platelet activity

In vitro platelet aggregability to adenosine diphosphate (ADP) and epinephrine increases in the morning, as does the frequency of myocardial infarction. A single-blind, randomized, cross-over study of 15 healthy males was conducted to determine: (1) if other measures of platelet activity show a morning increases and (2) if aspirin eliminates any increases in platelet activity detected. Subjects received 325 mg of enteric-coated aspirin (ECA) or placebo. During placebo therapy,platelet thromboxane A2 production (following collagen stimulation) increased significantly after the subjects got up, as did platelet aggregability to ADP, epinephrine, and collagen. ECA markedly reduced baseline platelet thromboxane A2 production and eliminated the increase after the subjects got up. It also abolished biphasic aggregation in response to epinephrine and ADP (thereby eliminating the morning increase in aggregability to these agents), lengthened collagen lag time, reduced synergistic aggregation to combined agonists, was effective on day 2, and did not alter increases of tissue plasminogen activator that occurred following the subjects' arising. If aspirin prevents myocardial infarction by its antiplatelet action, as seems likely, the preferential reduction of morning infarction observed in the Physicians' Health Study, and the demonstration that aspirin eliminates the morning increase in platelet activity, suggest that the morning increase in myocardial infarction is due in part to a concurrent relatively modest increase in platelet activity.

Circadian variation of acute myocardial infarction and the effect of low-dose aspirin in a randomized trial of physicians

Increased platelet aggregation in the morning and upon assuming an upright posture may account at least in part for the observed circadian variation in onset of acute myocardial infarction. The Physicians' Health Study, a randomized, double-blind, placebo-controlled trial of alternate-day aspirin intake (325 mg) among 22,071 US male physicians, afforded the opportunity to assess this circadian pattern and examine whether it is altered by aspirin therapy. During a 5-year period of follow-up, 342 cases of nonfatal myocardial infarction were confirmed, of which the time of onset was available in 211 (62%). The placebo group showed a bimodal circadian variation in onset of myocardial infarction with a primary peak between 4:00 AM and 10:00 AM (p less than 0.001). In the aspirin group, however, this circadian variation was minimal (p = 0.16), due primarily to a marked reduction in the morning peak of infarction. Specifically, aspirin was associated with a 59.3% reduction in the incidence of infarction during the morning waking hours, compared with a 34.1% reduction for the remaining hours of the day. The greater reduction was observed during the 3-hour interval immediately after awakening, a period with a risk of infarction twice that of any other comparable time interval (p less than 0.001). Aspirin intake was associated with a mean reduction in the incidence of infarction of 44.8% over the entire 24-hour cycle. These data support the hypothesis that increased platelet aggregability in the morning and upon arising contributes to the occurrence of myocardial infarction and that aspirin reduces the risk of infarction by inhibiting platelet aggregation during these critical periods.

Current concepts for a drug-induced inhibition of formation and action of thromboxane A2

Urinary and plasma metabolites of thromboxane A2 (TxA2) indicate an increased TxA2 synthesis in a number of diseases, whereby TxA2 is assumed to contribute to the underlying pathomechanisms by its profound effects on platelet aggregation and smooth muscle contraction. In some clinical situations the increment in TxA2 biosynthesis is accompanied by an increased formation of prostacyclin (PGI2) which is one of the most potent inhibitors of platelet activation and smooth muscle contraction. Therefore, drugs are being developed which suppress the formation or action of TxA2 without interfering with its functional antagonist PGI2. Low doses of acetylsalicyclic acid (ASA) preferentially inhibit cyclooxygenase activity in platelets and the synthesis of TxA2 in vivo. However, neither low doses (approximately 300 mg/day) nor very low doses spare the formation of PGI2 completely. Despite its limited selectivity, very low dose ASA (approximately 40 mg/day) provides an attractive perspective in TxA2 pharmacology. Although thromboxane synthase inhibitors selectively suppress TxA2 biosynthesis PGH2 can accumulate instead of TxA2 and substitute for TxA2 at their common TxA2/PGH2 receptors. Thromboxane synthase inhibitors can only exert platelet-inhibiting and vasodilating effects if PGH2 rapidly isomerizes to functional antagonists like PGI2 that can be formed from platelet-derived PGH2 by the vessel wall. TxA2/PGH2 receptor antagonists provide a specific and effective approach for inhibition of TxA2. These inhibitors do not interfere with the synthesis of PGI2 and other prostanoids but prevent TxA2 and PGH2 from activating platelets and inducing smooth muscle contractions. Most of the available TxA2/PGH2 receptor antagonists produce a competitive antagonism that can be overcome by high agonist concentrations. Since in certain disease states very high local TxA2 concentrations are to be antagonized, non-competitive receptor antagonists may be of particular interest. Some recent TxA2/PGH2 receptor antagonists produce such a non-competitive type of inhibition due to their low dissociation rate constant. As a consequence, agonists like TxA2 or PGH2 only reach a hemiequilibrium state at their receptors, previously occupied by those antagonists. A combination of a thromboxane synthase inhibitor with a TxA2/PGH2 receptor antagonist presents a very high inhibitory potential that utilizes the dual activities of the synthase inhibitor to increase PGI2 formation and of the receptor antagonist to antagonize PGH2 and TxA2. Such combinations or dual inhibitors, combining both moieties in one compound, prolong the skin bleeding time to a greater extent than thromboxane synthase inhibitors and even more than low dose ASA or TxA2/PGH2 receptor antagonists.

Aspirin in chronic cardiovascular disease and acute myocardial infarction

The ability of low-dose aspirin to irreversibly inhibit platelet-dependent cyclooxygenase provides a biologic mechanism to explain why this drug may decrease the risk of thrombotic cardiovascular events. Observational epidemiologic studies, both case-control and cohort, have suggested that aspirin might reduce the risk of cardiovascular disease by approximately 20 to 30%. An overview of 25 randomized trials of aspirin among individuals with a history of prior cardiovascular disease demonstrated that those receiving aspirin experienced a significant 25% reduction in the occurrence of "important vascular events," an endpoint that combines nonfatal myocardial infarction (MI), nonfatal stroke, and cardiovascular death. There were also significant 32% reductions in subsequent nonfatal MI, 27% reductions in nonfatal stroke, and 15% reductions in vascular mortality. Thus, individuals with a history of MI, stroke, transient ischemic attack, or unstable angina clearly benefit from aspirin. The Second International Study of Infarct Survival (ISIS-2) sought to determine if benefits would accrue if aspirin was given within the first 24 hours of suspected evolving MI. The aspirin group experienced a significant 23% reduction in 5-week vascular mortality compared with those receiving placebo. Aspirin was also associated with significantly fewer reinfarctions and strokes. Thus, among those with suspected evolving MI, aspirin significantly reduces the risk of reinfarction, stroke, and vascular mortality. These analyses indicate that aspirin is of proven value in the therapy of most patients who have survived MI, stroke, or unstable angina, as well as those evolving a suspected MI.(ABSTRACT TRUNCATED AT 250 WORDS)

Methodologic considerations in the design and conduct of randomized trials: the U.S. Physicians' Health Study

For treatments whose hypothesized risk reductions are small to moderate in size, on the order of 10%-30%, observational epidemiologic studies may not be reliable, since the amount of uncontrolled confounding could easily be as great as the likely reduction in risk. In such cases, randomized trials may be the only epidemiologic design strategy able to provide clear answers to research questions. However, the choice of design features and proper conduct of a randomized trial are crucial to the ability of the study to provide reliable results. In planning a study, five principal issues should be considered: design efficiency, completeness of follow-up, level of compliance, size of the probable risk reduction, and accumulation of sufficient numbers of outcome measures. Careful attention to these design considerations will enhance a trial's ability to provide definitive research results. The importance of these issues can be illustrated by examining the design and conduct of the ongoing Physicians' Health Study, a randomized, double-blind, placebo-controlled trial of aspirin in the reduction of cardiovascular disease and beta-carotene in the prevention of cancer.

Final report on the aspirin component of the ongoing Physicians' Health Study

The Physicians' Health Study is a randomized, double-blind, placebo-controlled trial designed to determine whether low-dose aspirin (325 mg every other day) decreases cardiovascular mortality and whether beta carotene reduces the incidence of cancer. The aspirin component was terminated earlier than scheduled, and the preliminary findings were published. We now present detailed analyses of the cardiovascular component for 22,071 participants, at an average follow-up time of 60.2 months. There was a 44 percent reduction in the risk of myocardial infarction (relative risk, 0.56; 95 percent confidence interval, 0.45 to 0.70; P less than 0.00001) in the aspirin group (254.8 per 100,000 per year as compared with 439.7 in the placebo group). A slightly increased risk of stroke among those taking aspirin was not statistically significant; this trend was observed primarily in the subgroup with hemorrhagic stroke (relative risk, 2.14; 95 percent confidence interval, 0.96 to 4.77; P = 0.06). No reduction in mortality from all cardiovascular causes was associated with aspirin (relative risk, 0.96; 95 percent confidence interval, 0.60 to 1.54). Further analyses showed that the reduction in the risk of myocardial infarction was apparent only among those who were 50 years of age and older. The benefit was present at all levels of cholesterol, but appeared greatest at low levels. The relative risk of ulcer in the aspirin group was 1.22 (169 in the aspirin group as compared with 138 in the placebo group; 95 percent confidence interval, 0.98 to 1.53; P = 0.08), and the relative risk of requiring a blood transfusion was 1.71. This trial of aspirin for the primary prevention of cardiovascular disease demonstrates a conclusive reduction in the risk of myocardial infarction, but the evidence concerning stroke and total cardiovascular deaths remains inconclusive because of the inadequate numbers of physicians with these end points.

A study of bleeding time in 120 long-term aspirin trial patients

The bleeding time was measured in 120 patients participating in a longterm randomised double-blind trial of aspirin in thromboembolic prophylaxis (UK-TIA aspirin Study). In 70 patients taking aspirin 300 mg or 1,200 mg daily for a mean duration of 35 months the bleeding time averaged 228 seconds. In comparison with 30 patients randomised to placebo and not taking aspirin whose bleeding time averaged 217 seconds, there was no significant difference. Stratification of bleeding time estimation by duration of treatment suggested no significant trend in either placebo or aspirin groups over several years. These results suggest that the longterm trials of aspirin should be looked at again from the point of view of efficacy of treatment by time from randomisation.