The inhibitory effect of aspirin on platelet and vascular prostaglandins in rats cannot be completely dissociated

Effects of desmopressin on thrombogenesis in aspirin-induced platelet dysfunction

Aspirin causes a coagulation disorder. Desmopressin has haemostatic effects by increasing the plasma levels of coagulation factor VIII and von Willebrand factor. The precise effects of desmopressin on thrombogenesis are not known. In an in vivo model, we investigated the effect of the drug on thrombus formation and platelet function after aspirin use. Male Lewis rats weighing 250-300 g were used. Four groups with 10 animals each were formed: control, aspirin, desmopressin and aspirin + desmopressin. In each animal, the femoral artery was dissected. A thrombogenic vessel injury was created by inverting a full thickness portion of the proximal edge of the incised artery into the lumen. The following parameters were measured: maximum thrombus size, time period until maximum thrombus size was reached and overall platelet function. In addition, the thrombi generated were investigated histologically. Thrombus formation time was significantly shorter with desmopressin compared with the animals treated with aspirin (P < 0.0001) and controls (P = 0.008). Maximum thrombus size was larger in the desmopressin and desmopressin + aspirin groups when compared with the group treated with aspirin only. Overall platelet function was significantly enhanced with desmopressin compared with controls (P = 0.025) and with aspirin (P < 0.0001). The differences were confirmed histologically. In conclusion, desmopressin significantly accelerates thrombus formation in aspirin-treated animals. It can also re-establish thrombus size after the use of aspirin. Overall platelet function is significantly increased by desmopressin.

Selective cyclooxygenase-2 (COX-2) inhibitors reduce anti-Mycobacterium antibodies in adjuvant arthritic rats

Adjuvant arthritis, induced by Mycobacterium butyricum, is an experimental immunopathy that shares many features of human rheumatoid arthritis and, as such, is one of the most widely used models for studying the anti-inflammatory activity of compounds. In rats with adjuvant induced arthritis, IgG antibodies to M. butyricum have been detected and autoantigens that cross react with mycobacteria may be involved in the pathogenesis of adjuvant arthritis. In this study, the anti-inflammatory and immunosuppressive activities of two cyclooxygenase-2 selective inhibitors, flosulide and L-745,337, at doses of 0.1, 1 and 5 mg/kg/day, were examined in adjuvant arthritic rats. After 14 days of treatment, a clear dose-dependent inhibition of plantar edema was seen for both flosulide (ID50 lower than 0.1 mg/kg) and L-745,337 (ID50 = 0.4 mg/kg). Plasma levels of IgG anti-M. butyricum antibodies were also decreased by both drugs. In each case the maximal immunosuppressive effect was observed at doses lower than 5 mg/kg. The non-selective COX-2 inhibitor, indomethacin (1 mg/kg) decreased paw edema by 65% and the levels of IgG anti-M. butyricum by 45%. Neither cyclooxygenase selective inhibitors nor indomethacin decreased the delayed hypersensitivity reaction induced by M. butyricum. Thus, in vivo inhibition of COX-2 inhibited articular swelling and also the humoral immune response to Mycobacterium.

The differential effects of aspirin on platelets, leucocytes and vascular endothelium in an in vivo model of thrombus formation

Unanswered questions remain with regard to the therapeutic use of aspirin and the selective inhibition of thromboxane A2 and prostacyclin in platelets and endothelial cells. In the present study, the effects of aspirin on platelets and endothelial cells in vivo were examined using a helium-neon (He-Ne) laser-induced thrombosis model. Single intravenous injections of aspirin at concentrations of more than 0.5 mg/kg body weight mediated a dose dependent inhibition of thrombus formation in arterioles but not in venules. This antithrombotic effect was optimum after 15 min and declined after 90 min. Potent antithrombotic activity in arterioles was manifest at doses of 2.5 mg/kg to 50 mg/kg, and initial inhibition of thrombogenesis in vivo was most pronounced at high doses. Oral aspirin also inhibited thrombus formation in arterioles but not in venules, although the antithrombotic effects were delayed and prolonged. Maximum inhibition of ex vivo, collagen induced platelet aggregation by aspirin was observed approximately 180 min after intravenous injection. The results demonstrated that, although aspirin might have differential effects on platelets and endothelial cells, potent antithrombotic activity was manifest in arterioles at all concentrations. The findings suggest that the concept of the aspirin dilemma might be ignored for therapeutic purposes in many clinical circumstances. The antithrombotic effects of aspirin were unchanged in granulocyte-depleted animals, indicating that leucocyte-related mechanisms including neutrophil superoxide anion production did not modulate the potency of aspirin in this model.

Effect of low dose aspirin on thrombus formation at arterial and venous microanastomoses and on the tissue microcirculation

In free flap/replantation surgery, failure is usually associated with thrombotic occlusion of a microvascular anastomosis (risk zone I) or, on occasion, flow impairment in the microcirculation of the transferred or replanted tissue (risk zone II). The objective of this study is to describe the effect of low dose aspirin on blood flow at both risk zones in microvascular surgery. Risk zone I: In rat femoral arteries and veins, thrombus formation was measured at the anastomoses using transillumination and videomicroscopy. Forty male Wistar rats were assigned in equal numbers to four groups: either arterial or venous injury with either aspirin (5 mg/kg systemically) or saline treatment. We found that aspirin significantly reduces thrombus formation at the venous anastomosis (p = 0.001). Risk zone II: In the isolated rat cremaster muscle downstream from an arterial anastomosis, we measured capillary perfusion, arteriolar diameters, and the appearance of platelet emboli for 6 hours in the muscle microcirculation. Sixteen male Wistar rats in two equal groups received either aspirin (5 mg/kg systemically) or saline. We found that in aspirin-treated animals, capillary perfusion is significantly (p = 0.002) improved, whereas arteriolar diameters and emboli only slightly increased. In conclusion, low dose aspirin inhibits anastomotic venous thrombosis and improves microcirculatory perfusion in our rat model. These studies provide quantitative data confirming and clarifying the beneficial effects of low dose aspirin in microvascular surgery.

The mechanism of action of KBT-3022, a new antiplatelet agent

1. The mechanism of action of a new antiplatelet agent, KBT-3022 (ethyl 2-[4,5-bis(4-methoxyphenyl)thiazol-2-yl]pyrrol-1-ylacetate) and its active main metabolite, desethyl KBT-3022, was investigated. 2. KBT-3022 and desethyl KBT-3022 inhibited cyclooxygenase from ovine seminal gland with IC50 values of 0.69 and 0.43 microM, respectively. 3. At concentrations higher than those required for cyclooxygenase inhibition, desethyl KBT-3022 inhibited cAMP-phosphodiesterase, specific binding of U46619, and release of phosphatidic acid from thrombin-stimulated platelets. 4. Oral administration of KBT-3022 inhibited the production of thromboxane B2 during blood coagulation more potently than the production of 6-keto-prostaglandin F1 alpha from aortic strips in guinea pigs. 5. These findings suggest that KBT-3022 may inhibit platelet activation principally via the inhibition of cyclooxygenase by desethyl KBT-3022.

The role of antiplatelet therapy in acute coronary syndromes and for secondary prevention following a myocardial infarction

Acute Coronary Syndromes: Therapy with aspirin is recommended for all patients with acute myocardial ischemic syndromes unless contraindications for its use is present. None of the studies thus far have conclusively established evidence for a selective dose of aspirin. Until conclusive evidence exists, aspirin in doses of 81 mg (children's tablet) to 325 mg (adult tablet) are recommended. Ticlopidine may prove to be an attractive alternate choice in those who cannot take aspirin. According to the ACC/AHA task force recommendations, patients with acute myocardial infarction receiving thrombolytic therapy should receive both heparin and aspirin. Aspirin is to be administered in a dose of 160 mg daily. Heparin can be discontinued after 2 days if the patient's clinical course remains uncomplicated. Aspirin should be continued indefinitely. An alternative strategy in those who cannot take aspirin is to switch to warfarin before hospital discharge with a view toward long-term anticoagulant therapy. Secondary Prevention: Aspirin in a dose of 325 mg daily is recommended for all survivors of an acute myocardial infarction. No benefit derives from the addition of dipyridamole. The role of sulfinpyrazone remains undefined. Warfarin is an effective antithrombotic alternative to aspirin for secondary prevention after a myocardial infarction. However, aspirin is cheaper to administer and follow up when compared with warfarin. From available information, aspirin appears to be an adequate antithrombotic agent in patients who have near-normal left-ventricular function, the elderly, patients with coexisting cerebrovascular or peripheral vascular disease, and those with contraindications for anticoagulants. Warfarin should be preferred in high-risk patients with anterior or apical myocardial infarction, left-ventricular dysfunction with or without a mural thrombus, and those with associated atrial fibrillation. A randomized study assessing aspirin versus warfarin for secondary prevention after myocardial infarction is being initiated to determine the relative efficacy and safety of these drugs in secondary prevention after myocardial infarction.

Single-dose effect of enteric-coated aspirin on platelet function and thromboxane generation in middle-aged men

OBJECTIVE

To evaluate the effect of a single dose of enteric-coated aspirin (ECA) in three different dosages on platelet function and thromboxane generation in middle-aged men.

DESIGN AND METHODS

In a nonblind, nonplacebo-controlled, crossover study, a single dose of ECA (50, 250, or 1000 mg) was given in a tablet form to a group of healthy, middle-aged men. Ten subjects, aged 50-67 years, volunteered to participate in this study. Platelet functions including bleeding time, platelet aggregation, adenine nucleotides, beta-thromboglobulin, platelet factor 4, thromboxane generation, and aspirin measurement were determined.

RESULTS

Before ECA ingestion, the intracellular adenine nucleotides (adenosine triphosphate, adenosine diphosphate) were decreased, and both beta-thromboglobulin and platelet factor 4 were increased. These observations suggested that platelets were activated in vivo in middle-aged men. These findings returned to normal within 8 hours after the ingestion of ECA, and maintained normal for at least two days. Bleeding time was significantly prolonged at 8 and 24 hours compared with that before ingestion of ECA 1000 mg (p < 0.05). The generation of platelet thromboxane was maximally inhibited by approximately 40 percent in the samples 8 hours after ECA ingestion. Abnormal values of adenine nucleotides, beta-thromboglobulin, and platelet factor 4 returned to normal within 8 hours. Arachidonic acid-induced platelet aggregation was inhibited compared with that before treatment (p < 0.01) and the inhibitory effect was maintained for at least three days. Adenosine diphosphate- and epinephrine-induced aggregations were less inhibited than those induced by arachidonic acid. Inhibitory effects of ECA on platelet aggregation were dose dependent.

CONCLUSIONS

Our study indicates that platelets are activated in middle-aged men and that a single dose of ECA 50 mg is safe and can inhibit thromboxane synthesis and platelet aggregation. These results suggest that a daily dose of ECA 50 mg may be useful for blocking platelet activation and preventing thrombosis.

Aspirin and other antiplatelet drugs in the prophylaxis of thrombosis

Aspirin is of proven value as an antithrombotic drug. In unstable angina it reduces the risk of death and myocardial infarction by half. After a myocardial infarction it reduces the risk of death by about 10% and of coronary incidence (coronary death or definite myocardial infarction) by about 25%. These effects appear to be additive with those of beta-blocking drugs. Aspirin also reduces the risk of occlusion of aortocoronary saphenous vein grafts by about half. In transient cerebral ischaemia, aspirin may reduce the risk of stroke and death by 50%. In most clinical trials to date the daily dose of aspirin ranges from 325 mg to 1400 mg. Interest in very low doses of aspirin (less than 60 mg daily) is considerable but has yet to be translated into proven clinical benefit. Dipyridamole has not been shown to be effective as an antithrombotic when used alone. Its antiplatelet action ex vivo may be enhanced by combination with aspirin but clinical trials have shown relatively little advantage of the combination over aspirin alone. Sulphinpyrazone has not become established as a first line antithrombotic drug. Epoprostenol is useful in extracorporeal circulations to prevent platelet consumption and possibly in severe inoperable peripheral vascular disease.

Effect of preoperative antiplatelet drugs on vascular prostacyclin synthesis

Patients undergoing aortocoronary bypass using autogenous saphenous veins were randomly divided into three comparable groups. Group 1 (n = 10) acted as a control, Group 2 (n = 14) received 80 mg of aspirin at midnight before the operation, and Group 3 (n = 12) received 80 mg of aspirin and 75 mg of dipyridamole at midnight and an additional 75-mg dose of dipyridamole at 6 AM. The purpose was to determine which regimen would maximally inhibit platelet function without depressing vascular prostacyclin synthesis. Serum thromboxane A2, saphenous vein wall and aortic wall prostacyclin, platelet aggregation, and bleeding time were measured in all patients. None was restarted on a regimen of aspirin or dipyridamole postoperatively. Aspirin alone and in combination with dipyridamole significantly inhibited thromboxane A2 and platelet aggregation in all treated patients but spared venous prostacyclin synthesis. Aortic prostacyclin synthesis was partially inhibited in both treated groups. Chest tube drainage was comparable in all three groups. These results indicate that the combination of aspirin and dipyridamole offers no measurable advantage over aspirin alone in the perioperative period.

Low dose aspirin does not prevent fibrinolytic response to venous occlusion

Interest in the antithrombotic potential of low-dose aspirin is based on its ability to inhibit thromboxane (Tx)A2-related platelet function with concomitant sparing of vascular prostacyclin (PGI2) production. The aim of this study was to investigate the effect of low-dose aspirin (20 mg daily for 7 days) on the increase in fibrinolytic activity in healthy volunteers after venous occlusion. We also tested the effect of high-dose aspirin (650 mg X 2), of salicylate (569 mg X 2) and of indobufen (200 mg X 2), a new cyclo-oxygenase inhibitor unrelated to salicylates. Low-dose aspirin reduced serum TxB2 generation by about 90% and suppressed arachidonate-induced platelet aggregation. In contrast, fibrinolytic activity, measured by the euglobulin lysis area and the euglobulin lysis time, was not significantly affected. Both high-dose aspirin and indobufen significantly inhibited TxB2 generation and the rise in fibrinolytic activity induced by venous occlusion, without affecting the pre-occlusion values. Salicylate did not significantly affect any parameter studied. Besides offering a favorable solution to the "aspirin dilemma" related to the TxA2/PGI2 balance, low-dose aspirin might leave intact the fibrinolytic capacity of the vessel wall.

Miconazole inhibition of platelet aggregation by inhibiting cyclooxygenase

Platelet dysfunction was found in rabbits to which a dose of miconazole nitrate (1.6 mg/kg body wt) therapeutic for human subjects had been given intravenously. The present experiments were conducted to elucidate the mechanism of inhibitory effects of miconazole on platelet function. After administration of a single dose of miconazole, rabbit platelet aggregation induced by collagen and sodium arachidonate was inhibited significantly for approximately 24 hr. On the other hand, hypertriglycemia, one of the major side effects of this drug, was not seen during 2 days of observations, nor were any other outstanding manifestations observed. In in vitro experiments, miconazole nitrate (10 microM) also significantly inhibited rabbit and human platelet aggregation (P less than 0.01). Biochemical analyses revealed that the stimulant-induced formation of prostaglandin E2 (PGE2) and thromboxane B2 (TXB2), metabolites via cyclooxygenase, was inhibited by miconazole nitrate in both human and rabbit platelets in vitro. PGE2 production was decreased dose-dependently with the increase of miconazole concentration (10 to 100 microM), and the decrease was in parallel with a decrease of TXB2 production. In addition, malondialdehyde (MDA) production of human and rabbit platelets induced by exogenous arachidonate and collagen was also inhibited significantly by miconazole. Chromatographic studies showed that the amount of 12-L-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE), a metabolite via lipoxygenase, was increased markedly in accordance with the miconazole-induced decrease of TXB2 and 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) formation in both human and rabbit platelets. These results indicate that miconazole nitrate inhibits platelet cyclooxygenase, without affecting the stimulant-induced release of arachidonic acid from platelet phospholipids. Use of this drug in the treatment of systemic fungal infection appears to be increasing. Careful attention should be paid to the inhibitory effects of miconazole on platelet function, especially in the case of intravenous treatment.

Mechanism of action of the platelet function inhibitor from Vipera russelli siamensis snake venom

Human platelet aggregation induced by ADP, adrenaline, collagen or thrombin was inhibited by the venom inhibitor. Heating reduced both its phospholipase A2 enzymatic and anti-aggregatory activities, although not in parallel. The inhibitor caused significant dose-related inhibitory effects on the clot retraction of rabbit platelet-rich plasma caused by thrombin, while platelet malondialdehyde formation stimulated by thrombin was not affected. Furthermore, the venom inhibitor increased basal cyclic AMP levels in platelets, while cyclic GMP content was slightly lowered, but not in a dose-dependent manner. In addition, microscopic study revealed that the cytoskeleton was disordered after treatment of platelets with the venom inhibitor. The platelets lost their discoid form, while the ultrastructural changes of platelet aggregation induced by ADP were blocked. It is concluded that increasing platelet cyclic AMP and the disorder of the cytoskeleton may be the mechanism of action of the venom inhibitor on platelet function.

Antihemostatic and antithrombotic effects of XC386

XC386 prolonged the tail bleeding time in the conscious mice. This effect was dose-dependent and persisted for at least six hours after the oral administration. XC386 was effective in preventing ADP-induced acute pulmonary thromboembolic death in mice at dose of 100 mg/kg. Aspirin and indomethacin had no effect on this model. XC386 also reduced the mortality rate in collagen- induced thromboembolic death at the same dose as aspirin and indomethacin (200 mg/kg). All three drugs caused no significant protection in endotoxin shock. XC386 was found to suppress collagen-induced platelet aggregation, but did not affect blood coagulation. In conclusion, XC386 was proved to be as effective as aspirin and indomethacin in preventing the death of acute pulmonary thromboembolism.

Antihemostatic and antithrombotic effects of capsaicin in comparison with aspirin and indomethacin

Capsaicin prolonged the tail bleeding time in the conscious mice. This effect was dose-dependent and maximal effect was observed at 3 hr after the oral administration. Capsaicin was effective in preventing death caused by ADP-induced acute pulmonary thromboembolism in mice at dose of 25 mg/kg, while aspirin and indomethacin had no effect at 200 mg/kg. Capsaicin also reduced the mortality in collagen- and sodium arachidonate-induced thromboembolic death at dose of 25 and 50 mg/kg, respectively, and aspirin and indomethacin were also effective in these models but only when the dose was higher than 200 mg/kg. Capsaicin, aspirin or indomethacin could not protect mice from endotoxin shock. Capsaicin was found to suppress platelet aggregation markedly, but did not affect blood coagulation. In conclusion, capsaicin was proved to be more effective than aspirin and indomethacin in preventing the death of acute pulmonary thromboembolism, and this effect could be due to its inhibition on platelet aggregation.

Antiplatelet effect of capsaicin

Capsaicin was found to be a potent inhibitor of platelet aggregation and release reaction. It inhibited the aggregation of rat platelet induced by collagen and thrombin, but only slightly reduced those of AA and A23187. The IC50 on collagen-induced platelet aggregation was about 85 micrograms/ml. Less inhibition was observed in the aggregation of platelet-rich plasma. Increase of the calcium concentration could not overcome the inhibitory effect. Washing of the capsaicin-pretreated platelets only partially reversed the inhibition. Capsaicin also inhibited ATP release induced by thrombin and A23187 in the presence of EDTA. MDA and TXB2 formation were markedly inhibited by capsaicin in platelets challenged by collagen, thrombin and A23187. In AA-stimulated platelets, MDA formation was slightly decreased and TXB2 formation was not affected. Capsaicin showed more marked inhibition in the presence of CP/CPK, indomethacin or a combination of both. Capsaicin reduced the hemolysis of RBCs induced by hydrogen peroxide or hypotonicity. It was concluded that capsaicin had some membrane stabilizing property and this might lead to the interference of the activation of phospholipase A2.

XC 386: a new antiplatelet agent

XC386 was a new antiplatelet compound, which inhibited the aggregation and release reaction of rat platelet-rich plasma induced by collagen. This inhibition was dose-dependent and the IC50 was calculated to be 1 mM on collagen-induced aggregation. In washed platelets, the aggregations induced by ADP and collagen were much more markedly inhibited by XC386 than those induced by thrombin, A23187 and arachidonate. High calcium (4 to 8 mM) could not antagonize the inhibition. XC386 did not alter the malondialdehyde (MDA) and thromboxane B2 (TXB2) levels of resting platelets. But MDA formation induced by collagen, thrombin and A23187, and TXB2 formation induced by collagen and thrombin were significantly inhibited, while both formations induced by arachidonate were not changed. Combination of indomethacin or CP/CPK and XC386 enhanced markedly the inhibitory effect of XC386 on collagen- or A23187-induced aggregation. It was concluded that XC386 might inhibit platelet aggregation before the step of arachidonic acid release by phospholipase A2.

Effects of aspirin dosage and time of administration on arterial prostacyclin production and platelet aggregation in rats

Previously we reported that electrically-induced carotid artery thrombosis in anesthetized rats was prevented by 3.3 or 10 mg/kg aspirin (ASA) given i.v. 10 min before injury but not by 1.7, 20 or 100 mg/kg and protection was lost by delaying injury to 20 min (Haemostasis 13:42, 1983). Here, collagen-induced platelet aggregation and arterial prostacyclin-generating activity, measured by RIA for 6 keto-PGF1 alpha and by human platelet aggregation bioassay, were studied ex vivo after i.v. ASA to anesthetized rats. In all cases where platelet aggregation was inhibited less than 50%, no protection had been observed (1.7 mg/kg at 10 min, 3.3 at 20 min, 20 at 10 min). In the two cases where protection had been observed, platelet aggregation was inhibited by about 75% or more and in one, prostacyclin activity was about 50% of normal (3.3 mg/kg at 10 min). Thus in five of six dose-time combinations tested, antithrombotic protection could be explained by a requirement for about 50% of normal prostacyclin activity and about 75% of inhibition of collagen aggregation. Aberrant findings are discussed in the light of knowledge of salicylate/aspirin competition for cyclooxygenase.

Inhibition of human platelet cyclo-oxygenase activity by sulfinpyrazone and three of its metabolites

Sulfinpyrazone and three of its major metabolites were compared in vitro for their inhibitory effect on human platelet cyclo-oxygenase activity. Sulfinpyrazone appeared to be about 15-20 times less potent than its sulfide metabolite (G25671) and 6-7 times less potent than the other two compounds, the sulfone metabolite (G31442) and p-hydroxysulfide (G33378). All four compounds were apparently competitive inhibitors of platelet cyclo-oxygenase activity. Comparison of the potency of sulfinpyrazone and its metabolites, as determined in the present study and the plasma levels previously measured in man, indicates that sulfinpyrazone and G33378 were not potent enough to be effective in man. G31442 showed inhibitory potency slightly lower than its corresponding plasma levels, whereas G25671 was effective at concentrations well below those found in human plasma. This study supports the hypothesis that sulfinpyrazone metabolites (in particular the sulfide) rather than the drug itself affect platelet function when administered therapeutically.

Bleeding on patients with autoimmune thrombocytopenic purpura and normal platelet count

80% of patients with chronic autoimmune thrombocytopenic purpura (ATP) may reach normalization of platelet count after steroids or splenectomy. In some of these cases a bleeding tendency may still persist and this has been attributed to abnormalities of platelet function resulting from the effect of platelet-bound antibodies. We have studied 49 ATP patients in order to assess the frequency and the pattern of this immune thrombopathia, and the correlation with the levels of platelet associated IgG (PAIgG). Only 3 patients (6%) had prolonged bleeding time and persistence of mucocutaneous haemorrhages, whereas 36% presented an altered platelet aggregation pattern. Platelet serotonin content, platelet and plasma beta-thromboglobulin concentration, and malondialdehyde generation after thrombin stimulus were found also altered, but the findings were not clearly correlated with the concentration of PAIgG.

The metabolism of arachidonic acid by platelets in nephrotic syndrome

The production of malondialdehyde (MDA) and thromboxane B2 (TxB2) by platelets following an arachidonic acid (AA) challenge was greater in nephrotic platelet rich plasma (PRP) than in normal PRP. The uptake of 14C-AA, and its subsequent conversion to 14C-TxB2 following a thrombin stimulus, was also greater in nephrotic than normal PRP. Normal plasma diminished the MDA production by nephrotic platelets. The addition of albumin to nephrotic PRP, or, the intravenous infusion of albumin in quantities sufficient to correct hypoalbuminemia also diminished the excessive production of prostaglandin metabolites by nephrotic platelets. The platelet aggregate ratio (PAR), which measures circulating platelet aggregates, was abnormal during the acute phase of nephrotic syndrome but reverted to normal following remission. These data indicate that hypoalbuminemia is associated with increased AA metabolism by platelets and suggest that platelet "hyperactivity" may contribute to the proclivity toward thrombosis observed in nephrotic syndrome.

Lotifazole (F 1686), a non-steroidal anti-inflammatory agent with an unusual pharmacological spectrum

Lotifazole (F 1686) - 4-phenyl-2-(2',2',2-trichloroethoxycarboxamido) thiazole - has a range of anti-inflammatory activities in animals that differs from the activities of classic non-steroidal drugs. It reduces carrageenin-induced oedema in rats, UV-induced erythema in guinea pigs, and Arthus pleurisies in rats only at high doses. It does not affect Freund's-adjuvant polyarthritis, and it only slightly affects passive skin anaphylaxis in rats and anaphylactic shock in guinea pigs. Lotifazole does not greatly inhibit prostaglandin synthesis. However, at low doses and after various conditions of treatment, F 1686 reduces PPD- and Bordetella- pertussis-induced delayed-hypersensitivity pleurisy in guinea pigs and rats, respectively, and contact hypersensitivity reactions to picryl chloride and oxazolone in mice. Its action on the two models of delayed-hypersensitivity pleurisy is reflected in a decrease of the pleural exudate and of the number of mononuclear cells in the focus of inflammation. At active doses, Lotifazole does not cause changes in the differential leukocyte count in normal animals. It appears, furthermore, to be a T-lymphocyte stimulant.

Thrombohaemorrhagic complications in 101 cases of myeloproliferative disorders: relationship to platelet number and function

A series of 101 consecutive patients with chronic myeloproliferative disorders including polycythaemia vera, chronic myelogenous leukaemia, idiopathic myelofibrosis and essential thrombocythaemia have been studied. The aim was to establish the incidence of thrombotic and haemorrhagic complications and the possible role played by platelet number and function. The total incidence of haemostatic complications was 21% and the platelet functional tests investigated (platelet aggregation, generation of malondialdehyde, endogenous serotonin, beta-thromboglobulin and platelet coagulant activity) were of little help for predicting these clinical complications.

Sensitivity of rabbit platelet and aortic cyclooxygenase to inhibition by aspirin

It has been suggested that aspirin (ASA) would be more effective as an antithrombotic agent if employed in a dose which inhibits platelet thromboxane synthesis selectively, sparing vascular synthesis of prostaglandin I2 (PGI2). We have studied the effect of ASA concentration on rabbit platelet and aortic cyclooxygenase activity in vitro and the effect of administration of varying doses of ASA to rabbits on the cyclooxygenase activity of these tissues ex vivo. We also measured plasma levels of thromboxane B2 (TXB2) and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) after infusion of arachidonic acid into rabbits pretreated with varying doses of ASA. Concentrations or doses of ASA required for 50% inhibition were about 10 times greater for the arterial enzyme than for the platelet enzyme in the in vitro and ex vivo studies. However, the dose required for complete inhibition of the platelet enzyme was 1 mg/kg and this dose inhibited the vascular enzyme by 62%. We conclude that meaningful selective inhibition of cyclooxygenase activity of the two tissues is difficult to achieve in the rabbit. Since doses of ASA which are antithrombotic appear to be high enough to almost totally inhibit vascular PGI2 synthesis, PGI2 synthesis may be a relatively unimportant mechanism for prevention of thrombosis.

Effects of dilazep on human platelets and rat vascular tissue: in vitro studies on platelet aggregation, and arachidonic acid oxidation

The "in vitro" effects of Dilazep on platelet aggregation and arachidonic acid oxidation in platelets and vessel wall were studied. Dilazep exerted a clearcut inhibitory effect on platelet aggregation induced by several stimuli. In addition, the compound completely suppressed malondialdehyde production by platelets elicited by collagen and thrombin, in a dose-dependent fashion. The evaluation of Dilazep effects on the in vitro release of prostacyclin-like material by rat aortic tissue did not provide any indication about an interference of the drug on the generation of endogenous antiaggregatory material.

Low-dose aspirin (ASA) renders human platelets more vulnerable to inhibition of aggregation by prostacyclin (PGI2)

Pre-treatment of human, platelet-rich plasma with concentrations of aspirin that produced 50% or less inhibition of aggregation induced by collagen, arachidonic acid or adenosine diphosphate, significantly increased the % inhibition of platelet aggregation by a low concentration of authentic prostacyclin or by prostacyclin-like activity generated by incubation of rat aorta rings in human platelet-poor plasma. Similarly a single aspirin tablet (325 mg) taken orally by human volunteers significantly increased the sensitivity of their platelets to inhibition of aggregation by authentic prostacyclin (8.1 X 10(-10) M) for 2-48 h after ingestion. Statistical significance was lost at 72 h but the trend was still evident. These results support the contention that low doses of aspirin may be efficacious in the therapy of arterial thromboembolism since this could preserve some arterial prostacyclin-generating activity which might be sufficient to inhibit adhesion and aggregation of the aspirin-treated platelets.

Triphenyltin fluoride in vitro inhibition of rabbit platelet collagen-induced aggregation and ATP secretion and blockade of arachidonic acid mobilization from membrane phospholipids

Recent studies have demonstrated that triphenyltin fluoride (TPTF) inhibits collagen-induced aggregation and ATP secretion of rabbit platelets in vivo [S. Manabe and O. Wada, J. Toxic. Sci. 6, 236 (1981)]. The aim of the present investigation was to test the effects in vitro of TPTF on platelet aggregation and to elucidate the mechanism of the inhibitory action by studying the release and metabolism of arachidonic acid and the cyclic AMP contents of rabbit platelets treated in vitro with TPTF. Although no inhibitory effect of TPTF was found on sodium arachidonate-induced platelet aggregation and ATP secretion, TPTF inhibited both reactions induced by collagen. Triphenylarsine and triphenylantimony did not inhibit, even at a concentration of 10(-3) M. The anti-aggregating concentration (IC50) of TPTF was 6.0 x 10(-6) M against collagen. TPTF had no inhibitory effect on the conversion of exogenous arachidonic acid to malondialdehyde (MDA) by platelets, while the collagen-induced production of arachidonate metabolites [MDA, 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and thromboxane B2] was remarkably inhibited by TPTF. Furthermore, TPTF apparently inhibited the collagen-induced release of arachidonic acid from platelets, although the formation of phosphatidic acid was not inhibited. Total cyclic AMP content after TPTF exposure was not changed significantly. These results indicate that TPTF inhibited the collagen-induced arachidonic acid release from platelet phospholipids, presumably by acting on phospholipase A2. Furthermore, it seems unlikely that the inhibition of arachidonic acid release by TPTF can be explained by the level of cyclic AMP in platelets.

Some biochemical platelet patterns in stable coronary patients

Patients affected by stable angina pectoris whose platelets were characterized by ultrastructural alterations, were investigated in some biochemical changes of their platelets. A decrease of P.A.R. (platelet aggregate ratio), an increase of MDA production in particular conditions, nevertheless a stability of cAMP content in platelets, even after thrombin stimulation, were found.

Platelet thromboxane synthetase inhibitors with low doses of aspirin: possible resolution of the "aspirin dilemma"

Selective pharmacological inhibition of thromboxane A2 synthesis did not prevent arachidonate-induced aggregation of human platelets in vitro. Prevention was instead achieved by a combination of thromboxane A2 inhibitors with low concentrations of aspirin. The latter partially reduced the proaggregatory cyclooxygenase products that accumulated when thromboxane A2 synthesis was blocked. The aspirin concentrations did not affect per se either platelet aggregation or prostacyclin synthesis in cultured human endothelial cells. The combination of thromboxane synthetase inhibitors with low doses of aspirin may offer greater antithrombotic potential than either drug alone.

Arachidonate metabolism in cultured human vascular endothelial cells. Evidence for two prostaglandin synthetic pathways sensitive to acetylsalicylic acid

The effect of acetylsalicylic acid on endothelial prostaglandin synthesis was measured in the presence of exogenous and endogenous substrates. In both types of measurement, a rate of inhibition was found similar to that observed for acetylsalicylic acid inhibition of cyclooxygenase activity in platelets. After withdrawal of acetylsalicylic acid, a rapid restoration of cyclooxygenase activity was observed when exogenous [1-14C]arachidonate was used as a substrate (50% of initial activity at 5 h). However, when endogenous substrate, released after phospholipase activation induced by thrombin treatment of the cells, was used to test cyclooxygenase activity, only partial restoration of enzymic activity was observed (30% after 48 h). Phospholipase activity, measured by the release of free fatty acids, was not inhibited by acetylsalicylic acid. Measurement of turnover times by incubating the cells with cycloheximide revealed a short turnover time for the enzymic activity tested with exogenous [1-14C]arachidonate (2.3 h) and a relatively long turnover time for the cyclooxygenase activity tested with endogenous substrate released after thrombin treatment of the cells (54 h). These results suggest that at least two pools of cycloxygenase are involved in endothelial prostaglandin synthesis.

Failure of thromboxane A2 blockade to prevent attacks of vasospastic angina

Thromboxane A2 (TxA2), released by aggregating platelets, has been proposed as a potential mediator of coronary vasospasm. We studied six patients with variant angina, a clinical syndrome due to coronary vasospasm, and one patient with frequent recurrent episodes of transient ST-segment depression at rest in whom the spasm was demonstrated angiographically. All patients underwent continuous ECG monitoring for 2 days before and 2 days after a single, low, i.v. dose of aspirin (2 mg/kg), which reduced TxB2 (the stable metabolite of TxA2) to less than 3% of the control values. There were 129 transient ischemic episodes during control and 146 after aspirin, when platelet TxB2 was reduced to negligible levels. The duration, severity and incidence of symptomatic episodes were not significantly affected by TxA2 blockade. We conclude that platelet TxA2 is probably not responsible for the initiation of coronary vasospasm.

Effect of acetylsalicylic acid on platelet aggregation and thromboxane B2 production in flowing aortic blood in the rat studied with a filter loop technique

A method of measuring in vivo platelet function using a filter loop technique has been used to study platelet aggregation in response to Adenosine Diphosphate (ADP) and Sodium Arachidonate infusion in flowing aortic blood in the rat. ADP infusion produced reversible platelet aggregation in vivo with no change in thromboxane B2 (TXB2) levels whereas sodium arachidonate infusion resulted in virtually irreversible aggregation with a rise in TXB2 levels. Oral Acetylsalicylic Acid (ASA) in doses of 1-100 mg/kg had no effect on ADP induced aggregation but prevented platelet aggregation in vivo induced by sodium arachidonate and the concomitant rise in TXB2 levels.

Dosage frequency for suppression of platelet function by low dose aspirin therapy

A study of platelet aggregation and MDA production after an oral dose of 300 mg aspirin indicated that partial recovery of platelet function occurred when approximately one third of the circulating platelets had been replaced by new (uninhibited) platelets. In vitro studies on mixtures of normal and aspirin inhibited platelets indicated partial restoration of platelet aggregation and thromboxane B2 production with as little as 10% of normal platelets in some subjects. Restoration of full function required a higher proportion of normal platelets. There was considerable variation between subjects. These data suggest that complete suppression of platelet functions in all normal subjects requires daily administration of the drug.

Platelet function in rheumatoid arthritis

Platelet function tests were evaluated in 20 patients with rheumatoid arthritis (RA), 9 of whom displayed thrombocytosis (greater than 400,000 platelets/microliter). Most patients had an enhanced sensitivity to collagen and epinephrine induced aggregation, as compared with a reference group. In contrast, malondialdehyde (MDA) production, an index of platelet arachidonic acid metabolism, was in the normal range for all the patients except 3, who were LE phenomenon positive. The hypersensitivity of platelets to aggregating stimuli was particularly marked in thrombocytotic patients, who also showed the most striking biochemical and clinical abnormality. These findings indicate the crucial role of platelets in the development and self-sustaining of RA.

Salicylate-aspirin interaction in the rat. Evidence that salicylate accumulating during aspirin administration may protect vascular prostacyclin from aspirin-induced inhibition

Aspirin inhibits cyclooxygenase, thus preventing thromboxane A2 production in blood platelets and prostacyclin in vascular cells. Aspirin is rapidly hydrolyzed to salicylate in the circulation. The objectives of this study were (a) to evaluate whether administration of salicylate, though ineffective by itself, prevents the inhibitory effect of aspirin on platelet and/or vascular cyclooxygenase activity; (b) to verify whether salicylate accumulating in blood after aspirin administration interferes with the pharmacological activity of further doses of aspirin. Pretreatment of rats with sodium salicylate (25-100 mg/kg i.p.) resulted in dose-related prevention of the effect of a subsequent dose of aspirin (2.5-10 mg/kg i.v.) on both platelet and vascular cells. Sodium salicylate appeared to amplify the greater response of platelets to aspirin compared with vessel wall. Pretreatment of rats with repeated high doses of aspirin (200 mg/kg) resulted after 24 h in blood salicylate levels (150-200 microgram/ml) that significantly prevented the inhibitory effect of a subsequent dose of aspirin on newly synthesized vascular prostacyclin. Blood salicylate levels obtained after 36 or 48 h (less than 50 microgram/ml) were too low to blunt aspirin's effect. The interference with aspirin of its major endogenous metabolite should be borne in mind when interpreting results obtained with high dose aspirin or during repeated administration of this drug.

Differential effects of itanoxone--a new hypolipidemic and hypouricemic drug--on platelet and vascular prostaglandin generation in rats

Itanoxone ((chloro-2'-diphenyl)-4-oxo-4 methylene 2-butyric acid), a newly developed, hypolipidemic and hypouricemic compound with moderate anti-inflammatory activity, showed a short-lived, dose-dependent (20--200 mg/kg, orally), apparently competitive inhibition of platelet malondialdehyde (MDA), stimulated by either thrombin or arachidonic acid. Repeated doses did not result in any cumulative effect. At doses which completely blocked MDA production, itanoxone also inhibited thrombin-stimulated thromboxane B2 production in platelets but had no measurable effect on vascular prostacyclin generation. Pretreatment with itanoxone partially prevented the inhibitory effect of aspirin on both platelet and vascular prostaglandin synthesis. This suggests that itanoxone--like aspirin--acts at the level of cyclo-oxygenase but with greater selectivity on the platelet enzyme. This pharmacological activity is of great theoretical interest for the potential use of this compound as an antithrombic drug.

Differential inhibition by low-dose aspirin of human venous prostacyclin synthesis and platelet thromboxane synthesis

The capacity of venous tissue for prostacyclin synthesis was determined in 68 patients undergoing surgery for removal of varicose veins. A single dose of aspirin (81 mg or 300 mg) taken 14 h preoperatively strongly inhibited its synthesis, and the effect of 300 mg was still evident 48 h after ingestion. A single dose of 40 mg aspirin taken 14 h preoperatively had no effect on prostacyclin synthesis. The capacity of blood platelets to synthesise thromboxane (measured as malondialdehyde) was determined in volunteers before and at various times after ingestion of 300 mg or 40 mg aspirin. Both doses had an inhibitory effect that lasted for at least 96 h. The length of time for which the amount of thromboxane synthesised was insufficient to support platelet aggregation and the platelet release reaction depended on both the donor and the dose of aspirin. If prostacyclin and thromboxane are important in the pathogenesis of thrombosis, then doses of aspirin much lower than those used previously should be tested. The long-lasting effect of 300 mg aspirin on both venous tissue and platelets indicates that this dose is unlikely to produce a favourable prostacyclin/thromboxane balance.

In vivo platelet hyperactivity and factor VIII related antigen increase long after myocardial infarction. A controlled effect of sulfinpyrazone

In the Anturane Reinfarction Italian Study the trend of some specific and quantitative haemostatic parameters is being investigated in different series of patients balanced for sulfinpyrazone and placebo. In a series of young male patients who had had myocardial infarction 6 months previously, it has been shown that the placebo treatment subsample presented shortened platelet production time (PPT), increased levels of plasma beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4), and increased factor VIII related antigen (VIII:RAg) compared with a matched control group. A close correlation between plasma concentration of VIII:RAg and PPT was evidenced in the same treatment subsample. The sulfinpyrazone treatment subsample presented normalized PPT accompanied by reduction of VIII:RAg but not of beta-TG or PF4 levels.