Rapid return of cyclo-oxygenase active platelets in dogs after a single oral dose of aspirin

A single dose of oral aspirin in human subjects inhibits the aggregation response of platelets to arachidonate and other agents for approximately one week after ingestion. In the present study we have evaluated the rate at which cyclo-oxygenase active platelets return to the circulation in humans and dogs and compared the response curves obtained to improvements in cyclo-oxygenase activity produced by the aspirin platelets. After a single dose of aspirin, dog platelet function was compromised for several days. Normal responses to arachidonate and other aggregating agents were restored six days after aspirin, and the pattern of recovery was the same for dogs and human subjects. However, cyclo-oxygenase active platelets returned to the circulation in dogs more rapidly than in humans and chemical competence was restored in both species well before correction of the defective response to aggregating agents. The delay of 1-3 days before return of significant numbers of cyclo-oxygenase active platelets most likely reflects acetylation of bone marrow megakaryocytes by the drug. More rapid return of chemically competent cells in dogs than humans probably relates to the more rapid turnover and shorter life span of canine platelets. The basis for the discrepancy in return of chemical integrity compared to functional activity after aspirin in vivo compared to simultaneous correction of chemistry and function when 10% normal platelets are added to aspirin platelets in vitro remains unresolved.     

Comparison in anesthetized dogs of the anti-aggregatory and hemodynamic effects of prostacyclin and a chemically stable prostacyclin analog, 6a-carba-PGI2 (carbacyclin)

The intravascular anti-aggregatory and systemic and hemodynamic effects of prostacyclin and carbacyclin were compared by intravenous infusion in pentabarbital anesthetized dogs. Ten times as much carbacyclin was needed to produce comparable inhibition of platelet aggregation in the lumen of partially obstructed circumflex coronary arteries. These doses of carbacyclin caused similar decreases in total peripheral resistance as equi-effective anti-aggregatory doses of prostacyclin. There was a trend for the decrease in blood pressure with carbacyclin to be less than that produced by equi-effective anti-aggregatory doses of prostacyclin because carbacyclin caused somewhat greater increases in cardiac output. Changes in heart rate were similar with both substances. During carbacyclin and prostacyclin infusion resistance in normal (unobstructed) coronary arteries decreased. Both substances had comparable effects on pulmonary vascular resistance, right atrial pressure and left ventricular dp/dt at equivalent anti-aggregatory doses both before and after atropine (1 mg/kg) and hexamethonium (5 mg/kg). During 5 to 6 hour infusions of carbacyclin there was no evidence of desensitization of dog platelets to the anti-aggregatory activity. These results show that carbacyclin has a similar spectrum of activity as prostacyclin and is about one-tenth as potent.     

Indomethacin prevents the long-lasting inhibitory effect of aspirin on human platelet cyclo-oxygenase activity

Aspirin and indomethacin do interact with the same site on cyclo-oxygenase. This suggestion is based on in vitro studies on ram seminal vesicles and in vivo drug interaction studies on rat platelets. The purpose of the present study was to ascertain whether the same interaction also occurred after administration of both drugs to human volunteers. Platelet aggregation induced by sodium arachidonate or by collagen, and formation of platelet MDA and TxB2 were measured before, two and 48 hours after ingestion of either indomethacin (50 mg) or aspirin (500 mg) or of both drugs (30 minutes apart). While the inhibitory effect of indomethacin on these parameters was short lasting, that of aspirin persisted for at least 48 hours. However, when both drugs were given concurrently, the long-lasting effect of aspirin was no longer detectable. Since competition at levels other than platelets was unlikely, this study indicates that indomethacin and aspirin inhibit human platelet cyclo-oxygenase by the same basic mechanism. Acetylation of the enzyme appears to be a secondary mechanism which makes the inhibitory effect of aspirin persistent.     

Low dose aspirin,platelet function and prostaglandin synthesis: Influence of epinephrine and alpha adrenergic blockade

The present investigation has evaluated the effects of low doses of oral aspirin on platelet prostaglandin synthesis and function. Whole (80 mg) or half (40 mg) tablets of baby aspirin given to adults had no effect on the response of their platelets to thrombin, ADP and epinephrine, but selectively inhibited aggregation induced by threshold concentrations of arachidonate 16–20 hours after ingestion. Larger amounts of arachidonate overcame the inhibition imposed by low dose aspirin, but not by adult aspirin tablets (600 mg). Epinephrine, in concentrations too low to cause aggregation, restored the sensitivity of aspirin-treated platelets to arachidonate. Studies with a-adrenergic agonists, antagonists and calcium channel blockers demonstrated that the corrective effect of epinephrine was mediated by an a-adrenergic receptor influence on calcium modulation of the platelet membrane.     

Time-dependent inhibition of platelet cyclo-oxygenase by indomethacin is slowly reversible

Indomethacin has been charterized as a time-dependent, irreversible inhibitor of cyclo-oxygenase, yet its effects on human platelets have been found to be reversible . To understand this apparent contradiction, we have investigated the kinetics of recovery of platelet thromboxane production after a single dose of indomethacin. The inhibition of platelet thromboxane production was greater than would be expected from the levels of indomethacin found in the plasma suggesting that the time-dependent inhibition occurs . Yet recovery of platelet thromboxane production was faster than expected for the irreversible inhibitor, with 50% of control values being regained within 24 hours after ingestion of the drug. When platelets were isolated and resuspended in homologous drug-free plasma, slow recovery of thromboxane production was seen to occur with 50% of control activity regained in 100 minutes. This recovery was much slower than that seen from a competitive inhibitor of cyclo-oxygenase, ibuprofen. Ibuprofen-treated platelets recovered nearly completely immediately on being resuspended in drug-free plasma. When microsomes were isolated from platelets, then treated with indomethacin, no time-dependent recovery of activity was seen. The recovery of cyclo-oxygenase after indomethacin inhibition appears to be limited to the unperturbed enzyme in this natural milieu.     

Time-dependent inhibition of platelet cyclooxygenase by indomethacin is slowly reversible

Indomethacin has been characterized in vitro as a time-dependent, irreversible inhibitor of cyclo-oxygenase, yet its effects on human platelets have been found to be reversible in vivo. To understand this apparent contradiction, we have investigated the kinetics of recovery of platelet thromboxane production after a single dose of indomethacin. The inhibition of platelet thromboxane production was greater than would be expected from the levels of indomethacin found in the plasma suggesting that the time-dependent inhibition occurs in vivo. Yet recovery of platelet thromboxane production was faster than expected for an irreversible inhibitor, with 50% of control values being regained within 24 hours after ingestion of the drug. When platelets were isolated and resuspended in homologous drug-free plasma, slow recovery of thromboxane production was seen to occur with 50% of control activity regained in 100 minutes. This recovery was much slower than that seen from a competitive inhibitor of cyclo-oxygenase, ibuprofen. Ibuprofen-treated platelets recovered nearly completely immediately on being resuspended in drug-free plasma. When microsomes were isolated from platelets, then treated with indomethacin, no time-dependent recovery of activity was seen. The recovery of cyclo-oxygenase after indomethacin inhibition appears to be limited to the unperturbed enzyme in its natural milieu.     

Indomethacin prevents the long-lasting inhibitory effect of aspirin on human platelet cyclo-oxygenase activity

Aspirin and indomethacin do interact with the same site on cyclo-oxygenase. This suggestion is based on studies on ram seminal vesicles and drug interaction studies on rat platelets. The purpose of the present study was to ascertain whether the same interaction also occurred after administration of both drugs to human volunteers.Platelet aggregation induced by sodium arachidonate or by collagen, and formation of platelet MDA and TxB₂ were measured before, two and 48 hours after ingestion of either indomethacin (50 mg) or aspirin (500 mg) or of both drugs (30 minutes apart).While the inhibitory effect of indomethacin on these parameters was short lasting, that of aspirin persisted for at least 48 hours. However, when both drugs were given concurrently, the long-lasting effect of aspirin was no longer detectable. Since competition at levels other than platelets was unlikely, this study indicates that indomethacin and aspirin inhibit human platelet cyclo-oxygenase by the same basic mechanism. Acetylation of the enzyme appears to be a secondary mechanism which makes the inhibitory effect of aspirin persistent.     

Prevention of blockage of partially obstructed coronary arteries with prostacyclin correlates with inhibition of platelet aggregation.

Coronary arteries (circumflex or left anterior descending) of anesthetized dogs were partially obstructed to approximately 5% of the normal lumen size by fitting a plastic cylinder around the vessel. Under these conditions, blood flow in the artery was not maintained but, instead, gradually declined over a few minutes until the vessel was completely blocked. Shaking the plastic obstructor restored blood flow temporarily, however, flow gradually declined again to zero. Sometimes flow was spontaneously restored by immediate increases that occurred at irregular intervals while, on other occasions, blood flow had to be restored by shaking the obstructor every time the rate declined to near zero. Intravenous infusion of prostacyclin (PGI2) at 15 to 150 ng/kg/min reversed and prevented the blockage of the coronary arteries. The efficacy of PGI2 in preventing blockage correlated with inhibition of ADP-induced platelet aggregation in platelet rich plasma prepared from blood samples withdrawn from the dogs during PGI2 infusion. Other coronary vasodilators, nitroglycerin and PGE2, that have no antiaggregatory effects, failed to prevent blockage whereas PGE1 and indomethacin, which do block aggregation, also prevented blockage of the vessels. PGI2 or its precursor, PGH2, dripped topically on the obstructed site prevented the blockage of the artery. This local effect of IGI2 could be obtained with amounts too small to cause systemic inhibition of platelet aggregation. The results show that PGI2 prevents blockage of partially obstructed coronary arteries and this effect correlates with inhibition of platelet aggregation. Furthermore, the data suggest that locally produced PGI2 may have a local antiaggregatory effect without inhibiting platelet aggregation in the general circulation.     

High concentrations of exogenous arachidonate inhibit calcium mobilization in platelets by stimulation of adenylate cyclase.

1. Exposure of platelets to exogenous arachidonic acid results in aggregation and secretion, which are inhibited at high arachidonate concentrations. The mechanisms for this have not been elucidated fully. In our studies in platelet suspensions, peak aggregation and secretion occurred at 2-5 microM-sodium arachidonate, with complete inhibition around 25 microM. 2. In platelets loaded with quin2 or fura-2, the cytoplasmic Ca2+ concentration, [Ca2+]i, rose in the presence of 1 mM-CaCl2 from 60-80 nM to 300-500 nM at 2-5 microM-arachidonate, followed by inhibition to basal values at 25-50 microM. Thromboxane production was not inhibited at 25 microM-arachidonate. Cyclic AMP increased in the presence of theophylline, from 3.5 pmol/10(8) platelets in unexposed platelets to 8 pmol/10(8) platelets at 50 microM-arachidonate; all platelet responses were inhibited with doubling of cyclic AMP contents. 3. The adenylate cyclase inhibitor 2',5'-dideoxyadenosine attenuated the inhibitory effect of arachidonate, suggesting that it is mediated by increased platelet cyclic AMP and that it is unlikely to be due to irreversible damage to platelets. 4. Aspirin or the combined lipoxygenase/cyclo-oxygenase inhibitor BW 755C did not prevent the inhibition by arachidonate of either [Ca2+]i signals or aggregation induced by U46619. 5. Thus high arachidonate concentrations inhibit Ca2+ mobilization in platelets, and this is mediated by stimulation of adenylate cyclase. High arachidonate concentrations influence platelet responses by modulating intracellular concentrations of two key messenger molecules, cyclic AMP and Ca2+.     

Prostacyclin-like activity in rat vascular tissues. fast, long-lasting inhibition by treatment with lysine acetylsalicylate

Both arterial and venous tissues obtained from normal rats released prostacyclin (PGI₂)-like activity, as marked by its potent inhibitory effect on platelet aggregation. Intraperitoneal or intravenous administration of a single dose of a soluble lysine salt of acetylsalicylic acid (L-ASA, 1–400 mg/kg) resulted in abolition or substantial reduction of prostacyclin-like activity released from rat vasculature. The inhibitory effect of L-ASA was evident one minute after its i.v. administration to the animals, persisted for at least 24 hours and was still detectable (in venous tissues only) 168 hours later. Venous tissues were inhibited by doses of L-ASA as low as 1 mg/kg, whereas arterial tissues were not inhibited by doses of L-ASA lower than 10 mg/kg. This difference may possibly be related to the lower prostacyclin-like activity shown by rat venous tissues compared to arterial ones.It is suggested that L-ASA or part of its molecule may bind to and inhibit cyclo-oxygenase in the blood vessel wall in a manner similar to the acetylation of platelet cyclo-oxygenase.     

Effects of OKY-046, a selective thromboxane synthetase inhibitor, on blood pressure and thromboxane synthesis in spontaneously hypertensive rats

The effects of OKY-046, a specific thromboxane (TX) synthetase inhibitor, on blood pressure, urinary TX excretion, TX synthesis in blood platelets, kidney slices and aortic strips, were evaluated in adult spontaneously hypertensive rats (SHR). OKY-046 was dissolved in drinking water at concentrations of 1, 10, 100 mg/dl. The average intakes of OKY-046 were 1.4 +/- 0.1, 13.0 +/- 1.1, and 147 +/- 12 mg/kg/day, in rats who took 1, 10, 100 mg/dl of OKY-046 solutions for drinking water, respectively. The systolic blood pressure was significantly decreased by 34 mmHg only with the high dose of OKY-046 (147 mg/kg/day). OKY-046 suppressed the platelet aggregability to ADP and the release of TX B2, a stable metabolite of TX A2, from blood platelets in a dose-dependent fashion. Urinary excretion of TX B2 decreased significantly in both groups treated with moderate (13.0 mg/kg/day) and high doses of OKY-046 (147 mg/kg/day). The release of TX B2 from kidney slices was decreased only by the high dose of OKY-046, while the release of TX B2 from aortic strips was not changed even by the high dose of OKY-046. OKY-046 had no effect on urinary excretion of 6-keto-prostaglandin F1 alpha, a stable metabolite of prostacyclin, or, on its release from the kidney slices and aortic strips. These results suggest that the effect of OKY-046 on TX synthesis has organ specificity and that the antihypertensive effect of this drug in SHR is related to reduced renal TX synthesis.     

Prostacyclin-like activity in rat vascular tissues. Fast, long-lasting inhibition by treatment with lysine acetylsalicylate.

Both arterial and venous tissues obtained from normal rats released prostacyclin (PGI2)-like activity, as marked by its potent inhibitory effect on platelet aggregation. Intraperitoneal or intravenous administration of a single dose of a soluble lysine salt of acetylsalicyclic acid (L-ASA, 1-400 mg/kg) resulted in abolition or substantial reduction of prostacyclin-like activity released from rat vasculature. The inhibitory effect of L-ASA was evident one minute after its i.v. administration to the animals, persisted for at least 24 hours and was still detectable (in venous tissues only) 168 hours later. Venous tissues were inhibited by doses of L-ASA as low as 1 mg/kg, whereas arterial tissues were not inhibited by doses of LA-ASA lower than 10 mg/kg. This difference may possibly be related to the lower prostacyclin-like activity shown by rat venous tissues compared to arterial ones. It is suggested that L-ASA or part of its molecule may bind to and inhibit cyclo-oxygenase in the blood vessel wall in a manner similar to the acetylation of platelt cyclo-oxygenase.     

Ozone inhibits prostacyclin synthesis in pulmonary endothelium

The effects of ozone on lung arachidonate metabolism in-vitro were studied in cultured bovine pulmonary endothelial cells exposed for 2 hours to ozone in concentrations up to 1.0 ppm. A concentration-dependent decrease in prostacyclin synthesis was found (90% decrease at the highest ozone level of 1.0 ppm). The inhibition of prostacyclin synthesis was not due to a decreased release of arachidonic acid from membrane lipids. We also examined the hypoxic pulmonary vasoconstrictive response to 10% oxygen inhalation in anesthetized dogs in-vivo after exposure to 1.0 ppm ozone for 1 hour. Pulmonary vascular resistance was significantly increased after ozone exposure, similar to the findings in dogs given indomethacin (15 mg/kg). The percentage change in the hypoxic pulmonary pressor response was similar between the ozone exposure and indomethacin-treated groups, although due to the variance of the pulmonary vascular resistance values during hypoxia the results did not reach statistical significance. These results suggest that ozone inhalation affects pulmonary endothelial arachidonate metabolism in-vivo as well as in-vitro.     

Differential salicylate-aspirin interaction on vascular prostacyclin and platelet thromboxane synthesis in patients undergoing saphenectomy

We studied the ability of salicylate to counteract the effect of aspirin on platelet thromboxane synthesis and prostacyclin formation in venous tissue in patients undergoing saphenectomy. A single intravenous dose of 40 mg aspirin completely blocked thromboxane formation and reduced prostacyclin to about 43% of the control values. When salicylate (1000 mg po) corresponding in anesthetized subjects to blood levels of 25.9 +/- 5 micrograms/ml was administered before aspirin, vascular prostacyclin was no longer inhibited, whereas platelet thromboxane was still significantly blocked. These results suggest that the combination of salicylate with aspirin at an appropriate dose ratio may result in almost complete dissociation of the drug's effect on platelets and vessels in man.     

The effects of an infusion of prostacyclin on their endotoxin shock in rabbits.

30 rabbits received an infusion of lipopolysaccharide B (75 micrograms/kg.h) over 4 hours (groups E, EI, EA; n = 10 each). Saline was given to a control group (C; n = 8). In group EI, prostacyclin (PGI2; 500 ng/kg.min) was given simultaneously to endotoxin. Into group EA animals, aspirin (20 mg/kg) was injected before the endotoxin infusion was started. PGI2 and aspirin both improved survival of animals (6/10 each vs. 2/10 in group E). The drop of platelet counts was significantly reduced by PGI2, while leukocyte depletion was similar in all endotoxin groups. PGI2 preserved the functional capacity of platelets as indicated by collagen stimulated aggregation and thromboxane formation. PGI2 but not aspirin significantly reduced renal fibrin deposition.     

Effects of a stable prostacyclin analogue on platelet activity and on host immunocompetence in mice.

The stable prostacyclin (PGI2) analogue, iloprost, is a potent inhibitor of both tumor cell-induced platelet aggregation and of experimental metastasis in mice. To explore possible mechanisms of antimetastatic effect of iloprost, we measured the effect of this drug on both platelet aggregation and immunocompetence in the mouse. Iloprost (4 x 10(-8) M) inhibited platelet aggregation as induced by a mixture of collagen and epinephrine for at least 180 minutes of incubation, and completely reversed platelet aggregation when added during the second wave of aggregation. In addition, aggregation of platelets obtained from iloprost-treated mice (0.2 mg/kg) was completely inhibited for at least 90 minutes of incubation. Moreover, iloprost pretreatment in vivo counteracted tumor cell-induced thrombocytopenia. Thus, mouse platelets were equally sensitive to the inhibitory effect of iloprost on aggregation as platelets of other species including humans. Effects of iloprost on parameters of host immunocompetence that may influence tumor growth and metastasis formation were also evaluated. Iloprost treatment increased significantly macrophage cytostasis to tumor cells, natural killer (NK) lytic activity of spleen cells and T-cell mediated cytotoxicity ex vivo. These results suggested that the antimetastatic effect of iloprost in the mouse may be attributable to multiple mechanisms including inhibition of platelet aggregation and stimulation of certain host immune functions.     

Slow intravenous administration of low dose aspirin inhibits both vascular and platelet cyclooxygenase activity: an experimental study in the rat

Aspirin irreversibly inhibits cyclooxygenase, thus preventing thromboxane (Tx)A2 production in platelets and prostacyclin in vascular cells. While it is generally accepted that the inhibitory effect of low dose aspirin is cumulative on platelet cyclooxygenase, it is still a matter of debate whether a similar phenomenon also occurs on vascular cyclooxygenase. We have measured in anesthetized rats the inhibitory effect of two doses of aspirin (2.5 and 5.0 mg/kg), given intravenously either as a bolus or as a continuous infusion (for 30 min), on platelet TxB2 and 6-ketoprostaglandin F1 alpha generation by different vascular segments. Aspirin significantly inhibited both platelet and vascular cyclooxygenase independently of the rate of drug administration. The aspirin peak plasma levels at the end of bolus injection was about 170 times higher than the average level measured during the slow infusion (1.21 +/- 0.15 micrograms/ml). At this concentration aspirin did not affect in vitro either platelet or vascular cyclooxygenase activity. Thus the inhibitory effect of aspirin on both platelet and vascular cyclooxygenase seems to be related to total exposure of the enzyme to the drug rather than to the maximal drug concentration attainable in the systemic circulation. These findings may be relevant to the current debate on optimal conditions for the biochemical selectivity of aspirin as an antithrombotic drug.     

The effects of continuous infusions of prostacyclin-Na (epoprostenol-sodium) on platelet counts, ADP-induced aggregation, and cyclic AMP levels in normal volunteers

Six male volunteers received either 0 (buffer), 2.5 or 5.0 ng/kg/min PGI2 X Na for 72 hrs. Various platelet parameters were monitored for an additional 72 hrs. Each morning, for seven consecutive days, and +1 and +6 hrs after the termination of the infusion, blood was drawn and platelet rich plasma (PRP) was prepared. The PRP was immediately exposed to 100 ng/ml PGI2 X Na, and the subsequent increase in platelet cyclic AMP was measured by radioimmunoassay. Aggregation in response to 2 or 4 microM ADP was measured simultaneously. Three volunteers returned for a second 72 hr infusion of 5.0 ng/kg/min PGI2 X Na. After 72 hrs, the infusion rate was gradually "tapered off" over a 12 hr period at which time the infusion was terminated. The sensitivity of the PRP to ADP-induced aggregation was recorded before, during, and after the "tapering off" regimen. Platelet counts were not altered by any of the infusions. The responsiveness of the platelet adenylate cyclase to exogenous PGI2 X Na was inversely related to the concentration of PGI2 X Na infused. Desensitization occurred and was more severe after 72 hrs of infusion than after either 24 or 48 hrs. For example, after 72 hrs at 5.0 ng/kg, platelets lost approximately 50% of their responsiveness to PGI2. ADP-induced aggregation was not significantly inhibited ex vivo by the infusion of 2.5 ng/kg/min PGI2. During the infusion of 5.0 ng/kg/min PGI2, ADP-induced aggregation was inhibited at 24 and 48 hrs, but by 72 hrs, the platelets began to respond to ADP more like control cells even though the PGI2 X Na infusion was continuing. When the infusion was abruptly terminated a hyperaggregable response (rebound) to exogenous ADP was observed. In subjects where the 5.0 ng/kg/min infusion was gradually "tapered off" over a 12 hr period, there was no evidence of platelet hyperaggregability at the time points studied.     

Prevention of blockage of partially obstructed coronary arteries with prostacyclin correlates with inhibition of platelet aggregation

Coronary arteries (circumflex or left anterior descending) of anesthetized dogs were partially obstructed to approximately 5% of the normal lumen size by fitting a plastic cylinder around the vessel. Under these conditions, blood flow in the artery was not maintained but, instead, gradually declined over a few minutes until the vessel was completely blocked. Shaking the plastic obstructor restored blood flow temporarily, however, flow gradually declined again to zero. Sometimes flow was spontaneously restored by immediate increases that occurred at irregular intervals while, on other occasions, blood flow had to be restored by shaking the obstructor every time the rate declined to near zero. Intravenous infusion of prostacyclin (PGI₂) at 15 to 150 ng/kg/min reversed and prevented the blockage of the coronary arteries. The efficacy of PGI₂ in preventing blockage correlated with inhibition of ADP-induced platelet aggregation in platelet rich plasma prepared from blood samples withdrawn from the dogs during PGI₂ infusion. Other coronary vasodilators, nitroglycerin and PGE₂, that have no antiaggregatory effects, failed to prevent blockage whereas PGE₁ and indomethacin, which do block aggregation, also prevented blockage of the vessels. PGI₂ or its precursor, PGH₂, dripped topically on the obstructed site prevented the blockage of the artery. This local effect of PGI₂ could be obtained with amounts too small to cause systemic inhibition of platelet aggregation. The results show that PGI₂ prevents blockage of partially obstructed coronary arteries and this effect correlates with inhibition of platelet aggregation. Furthermore, the data suggest that locally produced PGI₂ may have a local antiaggregatory effect without inhibiting platelet aggregation in the general circulation.     

Inhibition of platelet thromboxane A2 synthase activity by sodium 5-(3'-pyridinylmethyl)benzofuran-2-carboxylate

This report outlines the activity of a new thromboxane synthase inhibitor sodium, 5-(3-pyridinylmethyl)-2-benzofurancarboxylate, (U-63557A). U-63557A is a potent inhibitor of the thromboxane synthase in human platelets in vitro, as well as in rhesus monkey platelets ex vivo. A single oral dose of 3.0 mg/kg U-63557A inhibits the platelet thromboxane synthase in rhesus monkeys approximately 80% for at least 12 hrs. U-63557A has been administered to monkeys twice a day, (10 mg/kg) for 14 days, without evidence of drug tachyphylaxis or rebound. U-63557A does not inhibit thrombin-stimulated PGI2 biosynthesis in human endothelial cells, the 5-lipoxygenase in human neutrophils, or the cyclo-oxygenase in a variety of test systems. In anesthetized dogs, U-63557A injected i.v. at 0.1 to 5 mg/kg prevented the blockage of stenosed coronary arteries caused platelet aggregation. Similar effects were obtained by oral administration of 1-5 mg/kg. The thromboxane synthase inhibitor was more efficacious than cyclooxygenase inhibitors and equal to PGI2 in efficacy. Under appropriate conditions the protective effects of U-63557A could be reversed by i.v. cyclooxygenase inhibitors suggesting that its efficacy depended in part on endogenous PGI2 formation. Due to its specificity, oral activity, and extended duration of action, U-63557A is a promising compound for the evaluation of the role of thromboxane synthase in a variety of pathophysiological states.