Platelet Prostaglandin Research Articles

Short-term in vivo modifications of platelet NADPH oxidase 2 (NOX2) and prostaglandin F2α in HIV-1 patients on abacavir-based therapies.

The aim of the study was to investigate the in vivo effect of abacavir (ABC) on platelet oxidative stress. We performed a randomized pilot study including 39 HIV-1-infected patients, 17 on zidovudine/lamivudine (ZDV/3TC) and 22 on tenofovir/emtricitabine (TDF/FTC). Ten patients on ZDV/3TC and eight patients on TDF/FTC were randomly allocated to switching the nucleoside backbone to ABC/3TC. At baseline and after 6 months, platelet oxidative stress was assessed by platelet NADPH oxidase 2 (NOX2)-derived peptide (sNOX2-dp), a marker of NOX2 activation, and platelet prostaglandin F2α (8-iso-PGF2α ). Platelet activation was measured by soluble CD40L (sCD40L). At baseline, no differences between ZDV/3TC or TDF/FTC recipients were found. After 6 months, patients switching from ZDV/3TC showed a decrease of sNOX2-dp (from 20.9±5.7 to 12.5±3.8 pg/ml, p=0.002) and 8-iso-PGF2α (from 154.3±41.9 to 122.9±28.0 pmol/l, p=0.025). No effects on platelet oxidative stress biomarkers were observed in subjects from TDF/FTC, who showed a significant increase in blood glucose (p=0.043) and total cholesterol (p=0.027). ABC showed no effect on sCD40L levels in both groups. ABC reduced platelet sNOX2-dp and 8-iso-PGF2α in HIV-1 subjects switching from ZDV/3TC but not in those from TDF/FTC after 6 months. No changes in platelet activation were found in both groups.

HIV-1 induces in vivo platelet activation by enhancing platelet NOX2 activity

HIV-1 patients show increased platelet activation, but the mechanisms involved are not completely clarified. We speculated that HIV-1 might induce in vivo platelet activation by enhancing platelet NOX2-related oxidative stress. We measured soluble CD40 Ligand (sCD40L), a systemic marker of platelet activation, in 36 HIV-1 patients under effective combined antiretroviral therapy (cART) and in 10 naïve HIV-1 subjects. As control, 20 healthy subjects (HS) were included. Platelet oxidative stress was measured by platelet NOX2-derived peptide (sNOX2-dp), p47(phox) translocation to platelet membrane and platelet prostaglandin F2α (8-iso-PGF2α). sCD40L was increased both in HIV-1 naïve and cART patients compared to HS (p < 0.001). Platelet sNOX2-dp and 8-iso-PGF2α were significantly higher in HIV-1 naïve subjects compared to those on cART and to HS, and both were mutually correlated (R = 0.734, p < 0.001). A stepwise multivariable linear regression analysis showed that platelet sNOX2-dp (β: 0.803, p < 0.001), HIV-1 infection (β: 0.146, p = 0.014) and age (β: 0.166, p = 0.001) were independently associated to sCD40L levels. HIV-1 infection is associated with increased platelet oxidative stress, which was related to the activation of NOX2. The independent association between platelet NOX2 activation and plasma levels of sCD40L suggest that in vivo platelet activation may be dependent upon platelet oxidative stress.

B-1. プロスタグランディン前駆体としての多価不飽和脂肪酸分析

B-1. プロスタグランディン前駆体としての多価不飽和脂肪酸分析

Pharmacodynamics of Platelet Inhibition with Aspirin: Feasibility for Detection of Suboptimal Platelet Inhibition

AbstractAbstract 4361 Introduction:Patients with diabetes mellitus and or metabolic syndrome may or may not be maximally protected from future vascular events with aspirin. To be maximally protected at least 90% of platelets should be inhibited by aspirin (aspirinated). Since platelet acetylation by aspirin is limited to the first 30 minutes after ingestion, in conditions associated with increased platelet production the percentage of aspirinated platelets could be diluted to less than 90% with the usual daily dosing. We studied the pharmacodynamics of aspirin inhibition of platelets to evaluate the feasibility of developing a metric that would detect suboptimal aspirin inhibition of platelets. Methods:The slope of the platelet prostaglandin agonist (PPA) stimulated light aggregometry curve was used to gage the degree of platelet inhibition with aspirin.(Schwartz J. Lab Clin Med 2002; 139:227) Seven normal subjects were studied. After observed ingestion of 325mg of aspirin, the degree of platelet inhibition was studied at the following post aspirin time points; 0, 8hrs, 24hrs and 48hrs. Ex-vivo dilution curves were constructed using time 0 as 100% non-aspirinated and 1hr post aspirin as 100% aspirinated under the assumption that over 24hrs 10% of platelets are newly formed and non-aspirinated. Statistical evaluations were performed for each subject studied using both within subject comparisons and considering the group as a whole, between-subject comparisons.Results: Within group comparisons:A linear regression analysis comparing the in-vivo PPA aggregometry slopes with the PPA slopes obtained via ex-vivo dilutions demonstrated that the ex-vivo dilution curves for a particular subject reliably predicted the degree of aspirin inhibition observed in-vivo for that subject at the following time points with their corresponding percentages of aspirinated platelets; 8hrs (97% aspirinated) 24hrs (90% aspirinated) and 48hrs (80% aspirinated) (r2=0.6, p< 0.001). In addition, a direct in group comparison showed no differences between the in-vivo PPA slopes and those obtained with ex-vivo dilutions with 8 hr. and 48 hr. time points (p=0.16 and p=0.31, respectively), but a difference was observed at the 24 hr. time point (p=0.02). Entire group comparisons: Platelet inhibition with aspirin was highly variable among subjects. When analyzed as a group, the ex-vivo dilution curves were not predictive of the slopes obtained in-vivo. The PPA slopes observed in-vivo were significantly different from those obtained from the ex-vivo dilution curves at all of the time points (p<0.05). Conclusions:1. In normal subjects the ex-vivo dilution curve for a particular subject reliably predicts the degree of platelet inhibition observed in-vivo for that subject. 2. The variation in aspirin inhibition among subjects is large and suggests that the development of a metric to detect suboptimal platelet inhibition with aspirin is only possible if the individual subjects serve as their own control. Disclosures:No relevant conflicts of interest to declare.

Clinical Use of Aspirin in Ischemic Heart Disease: Past, Present and Future

Aspirin is an antiplatelet drug, inhibiting the cyclooxygenase activity of platelet prostaglandin H synthase-1 and almost complete suppressing platelet capacity to generate the prothrombotic and proatherogenic thromboxane A2. Antiplatelet therapy with aspirin reduces the risk of serious vascular events by about a quarter in patients who are at high risk because they already have occlusive vascular disease. However, the inhibition of thromboxane-dependent platelet function by aspirin is effective for the prevention of thrombosis, but is also associated with excess bleeding, although the absolute increase in major gastrointestinal or other major extracranial bleeds is an order of magnitude smaller. For secondary prevention of vascular events, the benefits of aspirin therapy substantially exceed the risks. Therefore, aspirin is a cornerstone of antithrombotic therapy in acute coronary syndromes, in chronic ischemic heart disease and in percutaneous coronary intervention. On the other hand, the role of aspirin in primary prevention remains uncertain and it is still debated, because the absolute risk of vascular complications is the major determinant of the absolute benefit of antiplatelet prophylaxis and the reduction in vascular events needs to be weighed against any increase in major bleeds. Future data from ongoing studies will help us to identify people at high vascular risk who take advantage from aspirin therapy for primary prevention or will indicate if specific category of high risk patients, like patients with diabetes, could be better protected from an increase in the frequency of aspirin administration.

A contemporary viewpoint on ‘aspirin resistance’

Aspirin is an irreversible inhibitor of platelet prostaglandin synthase activity, and is the most widely prescribed drug for the secondary prevention of cardiovascular disease. In recent years, clinical and laboratory evidence has shown significant individual variability in the response to aspirin and its link to clinical outcome. The term ‘aspirin resistance’ has been introduced to describe situations when clinical or ex-vivo effects of aspirin are less than expected. The accumulating evidence of increased risk of major adverse clinical events (MACE) associated with ‘aspirin resistance’ in the settings of acute coronary syndrome (ACS), stroke, and peripheral arterial disease has stimulated the search for ways of overcoming aspirin resistance. Existence of the link between high on-treatment platelet reactivity and atherothrombotic events suggests the common mechanisms for atherosclerosis progression and thrombotic complications with the platelets, being a key cellular interface between coagulation and inflammation.This review article provides a contemporary view on ‘aspirin resistance’ and discusses its definition, clinical importance, and possible mechanisms in light of recent data on the role of platelets in atherothrombosis.

Amount of Platelet Inhibition Produced When Initiating Aspirin Therapy: Comparison of Two Doses 81 Vs. 325

Abstract 1250 Introduction:Patients with occlusive arterial disease are commonly treated with different daily doses of aspirin usually either 81 or 325mg. This treatment practice continues despite studies showing equal protection from future vascular events with daily aspirin doses that exceed 81mg. We evaluated the hypothesis that the amount of platelet inhibition observed 96 hours after initiating aspirin therapy with 3 daily doses of 81mg would be equivalent to that observed using a dose of 325mg. Methods:The amount of platelet inhibition produced when initiating daily aspirin therapy was evaluated using the slope of the platelet prostaglandin agonist (PPA) stimulated light aggregometry curve at 4 time points: time 0 before the first dose, 2 hours after the first dose, 24 hours after the first dose (just before the second dose) and 96 hours after the first dose (24 hours after the 3rd dose). After a 7 day wash out period, during which subjects were instructed not to take aspirin or other NSAIDs, the subjects repeated the study using a 325mg aspirin dose. Because of the large variation in reactivity of platelets among people, each normal subject served as their own control and the results were evaluated using a paired “t” test. We plan to have studied a minimum of 12 normal subjects by the time of the ASH meeting. Results:To date 6 normal subjects were studied. 96 hours after initiating aspirin therapy, 24 hours after the 3rd aspirin dose, there was no difference between the PPA slopes observed for the 81mg=36 and the 325mg=28 (p=.28). Immediate platelet inhibition observed in the 24 hours after initiating aspirin therapy was far greater with the 325mg than the 81mg dose. Two hours after the 81mg dose the PPA slope was 77 compared to the 325mg slope of 12 (p=.000). Similarly, the slopes 24 hours after initiating aspirin were 68 for the 81mg dose and 35 for the 325mg dose (p=.006). The time 0 slopes were also statistically different 81mg=91 and 325=78 (p=.024). Conclusions:1. Initiation of aspirin therapy with a dose of 325mg dose produces a more immediate inhibition of platelet function with maximal platelet inhibition observed 2 hours after the first dose. 2. Platelet inhibition with an aspirin dose of 81mg occurs more gradually with maximal platelet inhibition observed 24 hours after the 3rd dose at 96 hours. 3. Whether daily aspirin therapy is initiated with a daily dose of 81mg or 325mg, the amount of platelet inhibition observed 96 hours after starting aspirin therapy, 24 hours after the 3rd dose, is equivalent. Comment:This data supports the clinical observations that protection from future vascular events is equivalent with either 81 or 325mg daily dosing and suggests that the current practice of increasing the dose of aspirin in patients who experience a subsequent event may not be beneficial. Disclosures:No relevant conflicts of interest to declare.

Mechanism of action of p-chlorobiphenyl on the inhibition of platelet aggregation.

p-Chlorobiphenyl (1-50 microM) concentration-dependently inhibited the aggregation and release reaction of rabbit washed platelets induced by arachidonic acid and collagen, but not those induced by platelet-activating factor (PAF), U46619 and thrombin. The IC50 values of p-chlorobiphenyl on the arachidonic acid and collagen-induced platelet aggregation were 2.9 +/- 0.5 and 12.8 +/- 2.3 microM, respectively. The formation of both platelet thromboxane B2 and prostaglandin D2 caused by arachidonic acid was inhibited by p-chlorobiphenyl concentration-dependently. In myo-[3H]inositol-labeled and fura-2-loaded platelets. [3H]inositol monophosphate generation and the rise in intracellular Ca2- stimulated by arachidonic acid were inhibited by p-chlorobiphenyl. In human platelet-rich plasma, p-chlorobiphenyl and indomethacin prevented the secondary aggregation and blocked ATP release from platelets induced by adenosine 5'-diphosphate and adrenaline without affecting the primary aggregation. It is concluded that p-chlorobiphenyl may be a cyclo-oxygenase inhibitor and its antiplatelet action is mainly due to the inhibition of thromboxane formation.

Long-lasting inhibition of platelet prostaglandin but normal vascular prostacyclin generation following sulphinpyrazone administration to rats.

Long-lasting inhibition of platelet prostaglandin but normal vascular prostacyclin generation following sulphinpyrazone administration to rats.

Diabetic Platelets Are Not Resistant to Aspirin Inhibition

AbstractAbstract 3324A daily aspirin in patients with myocardial infarctions (MI) effectively reduces the risk of occlusive vascular disease by 25%. This protective effect is reduced or absent in diabetics possibly because aspirin induced inhibition of platelet function is diminished. We tested the hypothesis that the amount of aspirin induced inhibition of platelets in diabetics is decreased. Methods:Each subject was evaluated at 2 time points: off-aspirin and 2-hours after observed ingestion of a 325mg aspirin tablet. Platelets were stimulated in a light aggregometer with platelet prostaglandin agonist (PPA). The slope of the PPA aggregation curve decreases with aspirin inhibition and is a sensitive measure of the amount of aspirin induced platelet inhibition (Schwartz J. Lab Clin Med 2002;139:227). Light aggregometry with arachidonic acid was used to verify that the patients had or had not taken aspirin. Maximal aspirin induced decrease in platelet function was defined as the difference between the PPA aggregation curve slopes at the 2 time points, off-aspirin and 2-hours post aspirin ingestion. 187 adult subjects (62% male 38% female) divided into four groups were studied, MI patients without diabetes n=98, diabetics with an MI n=41, diabetics without an MI n=9, and normal subjects n=39. Results:Comparisons of the PPA slopes of the 4 groups showed that diabetics had higher slopes than non-diabetics. Platelets from diabetics with and without an MI had significantly (p<0.05) increased off aspirin platelet reactivity (PPA slope) compared to normals. At the 2 hr time point patients with diabetes and an MI had a significantly higher PPA slope than the MI without diabetes group. Maximum aspirin induced decrease in platelet function was negatively correlated with age (r=-0.32, p=0.002). When controlled for age, there was no difference in the maximum decrease in platelet function among the 4 groups. Conclusions:1. Platelets from diabetic patients with and without MI have increased platelet reactivity to PPA. 2. The maximum decrease in platelet function observed in diabetic patients with and without an MI was similar to that observed in MI patients without diabetes and in normals. This suggests that the decreased protection from future vascular events observed with aspirin in diabetic patients is not because of an intrinsic resistance to aspirin inhibition of platelets. Disclosures:No relevant conflicts of interest to declare.

Selective activation of the prostaglandin E2 receptor subtype EP2 or EP4 leads to inhibition of platelet aggregation

The effect of selective activation of platelet prostaglandin (PG) E2 receptor subtype EP2 or EP4 on platelet aggregation remains to be determined. In platelets prepared from wild-type mice (WT platelets), high concentrations of PGE2 inhibited platelet aggregation induced by U-46619, a thromboxane receptor agonist. However, there was no significant change in the inhibitory effect of PGE2 on platelets lacking EP2 (EP2-/- platelets) and EP4 (EP4-/- platelets) compared with the inhibitory effect on WT platelets. On the other hand, AE1-259 and AE1-329, agonists for EP2 and EP4, respectively, potently inhibited U-46619 -induced aggregation with respective IC50 values of 590 ± 14 and 100 ± 4.9 nM in WT platelets, while the inhibition was significantly blunted in EP2-/- and EP4-/- platelets. In human platelets, AE1-259 and AE1-329 inhibited U-46619-induced aggregation with respective IC50 values of 640 ± 16 and 2.3 ± 0.3 nM. Notably, the inhibitory potency of AE1-329 in human platelets was much higher than that in murine platelets, while such a difference was not observed in the inhibitory potency of AE1-259. AE1-329 also inhibited adenosine diphosphate-induced platelet aggregation, and the inhibition was almost completely blocked by AE3-208, an EP4 antagonist. In addition, AE1-329 increased intracellular cAMP concentrations in a concentration- and EP4-dependent manner in human platelets. These results indicate that selective activation of EP2 or EP4 can inhibit platelet aggregation and that EP4 agonists are particularly promising as novel anti-platelet agents.

Inhibition of Platelet Function by Aspirin in Insulin Dependent Diabetics without Vascular Disease.

Abstract 4172Diseases like diabetes that are associated with increased platelet production are reported to have decrease aspirin (ASA) induced platelet inhibition. ASA inhibition of platelet function is mediated via acetylation of platelet cylooxygenase, which is completed approximately 30 minutes after ingestion. Thereafter production of new non-acetylated platelets increases the proportion of normal platelets gradually reversing ASA platelet inhibition. We tested the hypothesis that insulin dependent diabetics (type 1) without clinical evidence of occlusive vascular disease have less aspirin platelet inhibition than normal controls. The amount of ASA (325mg) platelet inhibition was measured using the slope (S) of platelet prostaglandin agonist (PPA) stimulated light aggregation curve that decreases after ASA. Aggregation was measured at 3 time points: before ASA ingestion, time 0 (T0); 2 hours post ASA (T2); and 24 hours after ASA (T24). Percent recovery from aspirin inhibition was calculated using the formula (T24–T2)/ (T0-T2) x100. Eight adult insulin dependent diabetic patients without clinical evidence of vascular disease and who were not taking aspirin were compared with 11 nondiabetic controls. 24 hours after aspirin ingestion diabetic patients recovered 10% of their maximal ASA platelet inhibition compared to 13% in controls (p=0.69). We conclude that recovery of platelet function from ASA in insulin dependent diabetics without clinical vascular disease occurs at the same rate as normal controls, suggesting that the previously reported decrease in platelet inhibition in diabetic patients may be related to increased platelet production associated with clinically evident occlusive vascular disease. Disclosures:No relevant conflicts of interest to declare.

Non-compliance is the predominant cause of aspirin resistance in chronic coronary arterial disease patients

BackgroundOur previous publication showed that 9% of patients with a history of myocardial infarction MI. could be labeled as aspirin resistant; all of these patients were aspirin resistant because of non-compliance. This report compares the relative frequency of aspirin resistance between known compliant and non-compliance subjects to demonstrate that non-compliance is the predominant cause of aspirin resistance.MethodsThe difference in the slopes of the platelet prostaglandin agonist (PPA) light aggregation curves off aspirin and 2 hours after observed aspirin ingestion was defined as net aspirin inhibition.ResultsAfter supposedly refraining from aspirin for 7 days, 46 subjects were judged non-compliant with the protocol. Of the remaining 184 compliant subjects 39 were normals and 145 had a past history of MI. In known compliant subjects there was no difference in net aspirin inhibition between normal and MI subjects. Net aspirin inhibition in known compliant patients was statistically normally distributed. Only 3% of compliant subjects (2 normals and 5 MI) had a net aspirin inhibitory response of less than one standard deviation which could qualify as a conservative designation of aspirin resistance. A maximum of 35% of the 191 post MI subjects could be classified as aspirin resistant and/or non-compliant: 9% aspirin resistant because of non-compliance, 23% non-compliant with the protocol and possibly 3% because of a decreased net aspirin inhibitory response in known compliant patients.ConclusionOur data supports the thesis that the predominant cause of aspirin resistance is noncompliance.

Non-Compliance Is the Predominant Cause of Aspirin Resistance.

Classifying patients as resistant to aspirin is an arbitrary designation with up to 60% of patients reported to be aspirin resistant. Our previous publication showed that 16 or 9% of 192 patients with a history of M.I. could be labeled as aspirin resistant and that all of these patients were aspirin resistant because of non-compliance. This report compares the relative frequency of aspirin resistance between known compliant and non-compliance subjects to demonstrate that non-compliance is the predominant cause of aspirin resistance. Platelet prostaglandin agonist (PPA) stimulated light aggregometry was used to measure amount of aspirin induced platelet inhibition and arachidonic acid (AA) stimulated platelet aggregation to classify platelets as either aspirin inhibited or not. After not taking aspirin for 7 days blood for aggregation studies was drawn, off aspirin time point, which was followed by observed ingestion of 365mg of aspirin. Two hours after observed aspirin ingestion aggregation studies were repeated, on aspirin time point. The difference in the slopes of the PPA light aggregation curves on and off aspirin was defined as the net aspirin response. After supposedly refraining from aspirin for 7 days, 45 subjects were judged non-compliant with the protocol and removed from the study. Of the remaining 185 subjects 39 were normals and 146 had a past history of myocardial infarction (MI). There was no difference in net aspirin response between normal and MI subjects. The net aspirin response in known compliant patients was statistically normally distributed. Only 4% of complaint subjects (2 normals and 5 MI) had a net aspirin response of less than one standard deviation which would qualify as a conservative designation of aspirin resistance. Of the 230 enrolled subjects 16 were noncompliant with their daily prescribed aspirin, 45 were non-compliant with the protocol not to take aspirin for 7 days and one patient had NANSAID interference with aspirin's effect. In total a maximum of 30% of the 230 patients could be classified as aspirin resistant and or non-compliant with 9% aspirin resistant because of non-compliance 18% non-compliant with the protocol and only 3% because of a decreased net aspirin response in known compliant patients. We conclude that:1.the predominant cause of aspirin resistance is noncompliance;2.net aspirin response in compliant patients is normally distributed;3.in compliant patients no difference in net aspirin response is observed between normals and MI patients; and4.PPA stimulated light aggregometry measured off aspirin and 2 hours after observed aspirin ingestion is a useful technique for measuring net aspirin response.

The thrombolytic effect of aspirin in animal model

The aspirin induced platelet aggregation has been reported to be mediated through the inhibition of platelet prostaglandin synthesis. This compound has also been recently reported to stimulate nitric oxide synthesis in platelets. Since nitric oxide has been reported to produce fibrinogen/fibrinolytic effect, investigation was carried out to determine fibrinolytic effect of in vivo exposure of platelets to aspirin in normal volunteers on the fibrinolysis of the clotted platelet-rich plasma in vitro. The thrombolytic effect of aspirin in situ was also carried out by injecting aspirin solution in the mice with ADP induced formed thrombi in the coronary artery. It was found that the clotted platelet-rich plasma prepared from the volunteers (n = 10, F = 5, M = 5) who ingested 150 mg aspirin, began to undergo spontaneous and progressive fibrinolysis for 200 min at 37 degrees C with the generation of fibrin degradation products in the lysate. No such fibrinolysis could be seen in control experiments. When platelet thrombi were produced in the coronary artery of mice by injecting ADP, and these animals subsequently received intravenous injection of aspirin (4 muM final), they not only survived (P < 0.0001, n = 10) the thrombogenic assault but the lysis of the platelet thrombi was also noted in the post mortem examination. The thrombolytic effect of aspirin was found to be comparable to that of streptokinase in these animals. Aspirin, through the stimulation of NO synthesis, may produce thrombolysis in vivo.

Changes in platelet function, hemostasis, and prostaglandin expression after treatment with nonsteroidal anti-inflammatory drugs with various cyclooxygenase selectivities in dogs

Changes in platelet function, hemostasis, and prostaglandin expression after treatment with nonsteroidal anti-inflammatory drugs with various cyclooxygenase selectivities in dogs

Minimal Frequency of Aspirin Resistance after Observed Aspirin Ingestion.

Poor compliance is an important cause of aspirin resistance. Our goal was to investigate amount of aspirin induced inhibition of platelets independent of compliance. We investigated the net amount of aspirin induced inhibition of platelets by comparing Platelet Prostaglandin Agonist (PPA) stimulated platelet aggregations before, off aspirin, with the platelet inhibition that occurred 2 hours after observed aspirin ingestion. 217 post-myocardial infarction patients who had taken aspirin for at least 30 days were evaluated at two time points. The first time point was 7 days after they had signed informed consent to stop taking aspirin as well as nonsteroidal anti-inflammatory drugs (NSAIDs). The second time point was 2 hrs after they were observed to ingest 325mg of aspirin. Platelets were evaluated using 2 different agonists in a light aggregometer. Arachidonic acid (AA) was used as a qualitative check to see if the patients at the first time point had really refrained from taking aspirin and NSAIDs for 7 days and PPA which mainly activates platelets via the prostaglandin pathway and can detect degrees of aspirin induced platelet inhibition. The slope of the PPA stimulated aggregation curve was used to measure aspirin induced platelet inhibition. The net aspirin response was calculated by determining the paired difference between the PPA slopes at the two time points. Compliance with the directive of not taking aspirin or NSAIDs before the first time point was checked with AA stimulated aggregometry and 45 patients were judged to be noncompliant. We defined aspirin resistance as a minimal change in PPA slope of 15 or less. Of the patients who had complied with the directive not to take aspirin or NSAIDs for 7 days as determined by AA aggregometry only 7 (3.2%) were aspirin resistant. The net aspirin response was directly related to the PPA slope off aspirin (p<0.001). When the 45 patients who were judged by AA aggregometry to be noncompliant with the directive to refrain from taking aspirin or NSAIDs were removed from the analysis, the direct relationship between the slope of the off aspirin PPA aggregation and the net aspirin response remained significant (p<0.001). The amount of platelet inhibition in the compliant group 42±16 was significantly larger than the platelet inhibition in the noncompliant group 36±16 (p=0.02). We conclude that: 1. a small percent of patients, 3.2%, whose net amount of aspirin induced platelet inhibition was less than 15 may be resistant to aspirin because of mechanisms unrelated to compliance and 2. The net amount of aspirin induced inhibition of platelets is directly related to the slope of the “off” aspirin PPA stimulated aggregation curve.

Relationships between Risk Factors for Coronary Artery Disease and Aspirin Resistance.

Aspirin prevents or delays atherogenesis and subsequent arterial occlusions. Patients who are relatively resistant to aspirin have a higher rate of vascular occlusive events. We evaluated the hypothesis that in patients with a previous myocardial infarction (MI) relative resistance to aspirin was related to risk factors for coronary heart disease (CHD). We studied 192 patients with a history of at least 1 MI including 136 males and 64 females. Mean age was 62. Compliance was assured by studying patients 2 hours after they were observed to ingest 325mg aspirin and confirmed with a qualitative assay (arachidonic acid 1.0mM stimulated light aggregometry) that detected aspirin inhibition of platelet aggregation in all of the study patients. To assess amount of aspirin inhibition of platelet aggregation (aspirin resistance) platelet prostaglandin agonist (PPA) 30mM stimulated light aggregation was performed (Schwartz J. Lab Clin Med 2002; 139:227). A CHD risk assessment score that includes diabetes, HDL, total cholesterol, high blood pressure (HBP) smoking and age was used as a measure of 10 year risk for CHD. Statistical comparisons were evaluated using a univariate general linear model. A significant relationship was observed between platelet resistance to aspirin inhibition and increased CHD risk (p=0.03). Evaluation of individual CHD risk factors showed that diabetic patients were more resistant to aspirin as demonstrated by an increased slope of aggregation (p=0.02) a decreased time to 50% aggregation (p=0.04) and a linear relationship between an increase in fasting blood sugar and an increased aggregation slope (p=0.05). A higher (BMI) (p=0.05) or a lower HDL (p=0.02) were also related to aspirin resistance. Although decreased aspirin inhibition of platelets was observed in patients who smoked or had HBP, these differences were not significant. We conclude that in patients with CHD a relative resistance to aspirin inhibition of platelet aggregation correlates with a commonly used risk assessment equation and with the presence of diabetes, increasing BMI or decreasing HDL. This suggests the hypothesis that CHD patients who have a higher risk for CHD, or those who have diabetes or obesity or a decreased HDL, may benefit from additional platelet inhibiting medications.

Inhibition of human blood platelet aggregation and the stimulation of nitric oxide synthesis by aspirin

Incubation of platelet-rich plasma with 80 μM aspirin that resulted in the inhibition of both the secondary phase of ADP induced platelet aggregation and prostaglandin synthesis simultaneously stimulated the production of NO in platelets. Furthermore it was also found that the treatment of platelet-rich plasma either with 80 μM ibuprofen or salicylic acid, like aspirin, which inhibited the secondary phase of platelet aggregation by ADP and prostaglandin synthesis, also stimulated the production of NO in the absence of added ADP. However the inhibition of prostaglandin synthesis by ibuprofen or salicylic acid, unlike aspirin, was transient in nature. Incubation of washed platelets with any of these three compounds also stimulated NO synthesis indicating that the effect of these compounds was not mediated through plasma proteins. The in vitro effect of aspirin on the increase of NO in platelets could also be demonstrated by in vivo exposure of platelets to the compound. It was concluded that either a temporary or a lasting inhibition of prostaglandin synthesis by these inhibitors resulted in the synthesis of NO in resting platelets. Since NO is a potent inhibitor of platelet aggregation the inhibition of platelet aggregation, by these compounds may not be the consequence of the inhibition of prostaglandin synthesis alone, but could also be related, at least partly, to the stimulated synthesis of NO by these inhibitors.

Why Are Some Individuals Resistant to the Cardioprotective Effects of Aspirin?

It has been slightly more than 100 years since the synthesis, development, and commercialization of acetylsalicylic acid, the most widely consumed drug in the world. It is estimated that 35 000 kg are consumed daily in the United States and 6000 kg in the UK.1 Who would have ever thought that this widely consumed drug would achieve significant uses other than as an analgesic, antipyretic, and anti-inflammatory. The first report of a possible antithrombotic effect of aspirin appeared in 1953 in the Mississippi Valley Medical Journal .2 Dr Craven noticed that the patients who took Aspergum had a tendency to bleed more easily. He concluded that aspirin must be thinning the blood and because thrombosis of the coronary arteries led to myocardial infarction, if his patients took aspirin, they might be less prone to experiencing a myocardial infarction. See p 1650 These prescient observations were succeeded by the discovery by numerous investigators that aspirin could significantly decrease platelet function. In 1971, the mechanism by which aspirin inhibited platelet function began to unfold when Smith and Willis3 demonstrated that aspirin inhibited platelet prostaglandin synthesis. It was subsequently shown that aspirin irreversibly acetylated serine-529 close to the active site of the fatty acid cyclooxygenase (COX).4 In the case of the anucleate platelet, the enzyme is rendered inactive for its lifetime. Samuelsson and co-workers5 demonstrated that thromboxane A2 along with prostaglandin H2 were the arachidonic acid metabolites responsible for activation of the platelet. Thromboxane A2 is also a very effective vasoconstrictor and mitogenic substance. After the discovery of thromboxane A2, numerous studies followed that investigated its potential role in thrombotic cardiovascular diseases. Platelet thromboxane A2 synthesis is increased in acute myocardial infarction, unstable angina, thrombolysis therapy, percutaneous transluminal coronary artery angioplasty, and …