Inhibitiory effects of procainamide on rabbit platelet aggregation and thromboxane B_2 production in vitro

目的：研究普鲁卡因胺（ＰＡ）对凝血酶诱导血小板聚集和血栓素Ｂ２（ＴＸＢ２）产生的影响．方法：用比浊法和放射免疫分析法．结果：普鲁卡因胺８５，３４，１３６和５４４μｍｏｌ·Ｌ－１有明显抑制作用．抑制率分别为４５％±３７％，４８％±３２％，８８％±２３％，９２％±１５％和５３％±２４％，６５％±２６％，９０％±６％，９５％±６％．ＰＡ的浓度与血小板聚集和ＴＸＢ２产生的抑制效力之间，以及血小板聚集抑制率和ＴＸＢ２产生的抑制率之间均存在着正相关关系，三者的线性方程和主要参数为：＾Ｙ＝０２０７５Ｘ－４９１５７，ｒ＝０９９８５；＾Ｙ＝０９５４６Ｘ－３４６７２４，ｒ＝０９９２１；＾Ｙ＝０８２０２Ｘ＋１９７０６２，ｒ＝０９９２１．结论：普鲁卡因胺抑制凝血酶诱导血小板聚集和ＴＸＢ２产生．     

当归注射液对冠心病患者血浆前列环素、血栓素 A_2 及血小板聚集的影响

２４例冠心病患者静脉滴注当归注射液治疗后，血浆６－酮－前列腺素Ｆ１α（６－Ｋ）、６－Ｋ／血栓素Ｂ２（ＴＸＢ２）和ＴＸＢ２、血小板最大聚集率均分别显著高于和低于治疗前水平。结果表明，当归注射液有调节前列环素－血栓素Ａ２平衡和抑制血小板聚集的作用。     

Rapid recovery of in vivo prostacyclin formation after inhibition by aspirin

Summary The effect of aspirin on the in vivo formation of prostacyclin and thromboxane A₂ in normal healthy individuals was studied by measuring the urinary excretion of 2,3-dinor-6-keto-PGF₁ and 2,3-dinor-TxB₂ by gas chromatography-mass spectrometry. Administration of 500 mg aspirin twice daily caused a sustained reduction in the excretion of 2,3-dinor-TxB₂ to 10–15% of the predose value, while the excretion of 2,3-dinor-6-keto-PGF₁ was reduced for only about 3 hours after the aspirin dose. The data demonstrate a considerable difference in the inhibitory effect of aspirin on the in vivo synthesis of thromboxane A₂ and prostacyclin.     

The effect of oral administration of dipyrone on the capacity of blood platelets to synthesize thromboxane A2 in man

Summary Platelet aggregation and thromboxane A₂ (TXA₂) production induced by arachidonic acid and collagen were studied in 10 healthy volunteers prior to and at various times after the oral administration of a single dose of 1 g dipyrone. The plasma concentrations of four dipyrone metabolites were also determined. Dipyrone inhibited platelet aggregation and markedly decreased TXA₂ synthesis induced by threshold concentrations of both agonists. Maximal inhibition was noted 1 hour after drug administration and in some subjects it lasted as long as 72 h. At all times the effect of the drug could be abolished by increasing the concentration of the agonist. This is consistant with a competitive inhibitory effect of dipyrone on prostaglandin synthetase activity. The mean plasma concentration of the main dipyrone metabolite methylaminoantipyrine at 1 h was 11 µg/ml. There was no correlation between individual plasma levels and the parameters of platelet function. At 24 h the mean concentration of each of the metabolites studied was up to 1 µg/ml, and these levels, too, did not correlate with the biological effect of the drug.     

Single dose pharmacokinetics and effects on platelet function of the thromboxane receptor blocker BM 13.177

Summary The pharmacokinetics and pharmacodynamic effect on platelet activation of a single 800 mg oral dose of BM 13.177 have been investigated in 8 male volunteers. BM 13.177 disappeared from plasma with a terminal elimination half-life of 0.85 h. 52% of the dose was excreted unchanged in urine. Assuming complete absorption, total clearance was calculated to be 741.3 ml/min and renal celearance to range from 310.4 to 396.9 ml/min. The pharmacodynamic studies were performed ex vivo/in vitro in platelets stimulated either with methyl mercury chloride or with U 46619. Methyl mercury chloride is a platelet activator that requires TXA₂ formation from endogenous arachidonic acid, whereas U 46619 is a stable PGH₂ analogue and thromboxane mimetic at the platelet TXA₂/PGH₂ receptor. A close correlation between the plasma concentration-time profile of BM 13.177 and inhibition of platelet shape change or aggregation was demonstrated.     

Recent developments of thromboxane modulators

Thromboxane A₂ (TXA₂) is a labile product formed from arachidonic acid by the way of cyclooxygenase and thromboxane synthase. An overproduction of TXA₂ has been detected in a series of diseases where this eicosanoid is assumed to contribute to the underlying pathomechanisms by its potent stimulation of platelet aggregation and smooth muscle contraction. Indeed, literature supports participation of TXA₂ in several physiopathological mechanisms such as tumour growth and metastasis, pre-eclampsia, asthma, pulmonary hypertension and in the progression of ischaemic injury after coronary artery occlusion. This evidence provides a strong rationale for pursuing TXA₂ blocking strategies in drug development. Therefore, TXA₂ receptor antagonists, thromboxane synthase inhibitors and drugs which combine both activities have been developed. Of the 28 thromboxane modulator patents reviewed from January 1997 to December 2000, 16 relate to thromboxane receptor antagonists (TXRAs), seven to thromboxane synthase inhibitors and five to agents combining both activities. Of these patents, 19 claim new chemical structures, five claim a new use, one claims combination therapy and three claim processes. This article focuses on latest discoveries and developments of novel TXA₂ modulators described in these patents.     

Effect of treatment at night with S-1452, a thromboxane A2 receptor antagonist,on the morning rise in platelet aggregation

Summary It is well known that platelet aggregation shows a morning rise, which may contribute to the increase in the onset of ischaemic heart diseases during the morning period. The present study was undertaken to determine whether nocturnal dosage with S-1452, a thromboxane AZ receptor antagonist, would blunt the morning rise in platelet aggregability.S-1452 50 mg or placebo were given orally to 8 healthy subjects at 10.00 h (day trial) or 22.00 h (night trial) according to a cross-over design. Plasma concentrations of S-1452 and its metabolites, bisnor-( + )-S-145 and tetranor-(+ )-S-145, and platelet aggregation were determined during the 12-hour period following the dose. Mean plasma concentrations of S-1452, bisnor-( + )-S-145 and tetranor-(+ )-S-145 during the absorption phase were lower after the nocturnal dose than after the morning dose. The maximum plasma concentration and area under the plasma concentration-time curve of the compounds were also lower and the time to the maximum concentration were delayed after the treatment at night. A morning rise in platelet aggregation was observed following placebo treatment. The inhibitory effect of S-1452 on platelet aggregation was observed at 3 hours and persisted for up to 9 h in both trials.The results suggest that S-1452 is absorbed more slowly after the nocturnal dose than after the morning dose. However nocturnal treatment with 50 mg S-1452 may blunt the morning rise in platelet aggregability.     

噻氯匹定对血小板血栓烷B_2生成的影响

口服噻氯匹定(ticlopidine)能抑制胶原诱导的血小板血栓烷B_2(TXB_2)生成。大剂量(500mg/d)可使TXB_2生成很快降低,停药1wk后恢复正常。给小剂量(250mg/d)时TXB_2的减少出现较晚,恢复亦快。噻氯匹定对ADP诱导的血小板TXB_2生成也有显著抑制作用。噻氯匹定的抗血小板作用至少有部分与抑制花生四烯酸的代谢有关。     

Relaxation of carbocyclic thromboxane A2-induced contractions of isolated coronary arteries by nifedipine

Summary ± Carbocyclic thromboxame A₂ (2.9×10^–7 mol/l) contracted isolated rings of cat coronary artery. These contractions could be partly inhibited by low concentrations of nifedipine (50% inhibitory concentration —IC₅₀=6.8×10^–8 mol/l) and completely relaxed by papaverine. Relaxation of thromboxane-induced coronary spasm may contribute to the effectiveness of calcium antagonists in variant angina, which may be caused or exacerbated by thromboxane A₂ release from platelets.     

Inhibition of prostaglandin D2 clearance in rat hepatocytes by the thromboxane receptor antagonists daltroban and ifetroban and the thromboxane synthase inhibitor furegrelate

Prostanoids, i.e. prostaglandins and thromboxane, regulate liver-specific functions both in homeostasis and during defense reactions. For example, prostanoids are released from Kupffer cells, the resident liver macrophages, in response to the inflammatory mediator anaphylatoxin C5a, and mediate an enhanced glucose output from hepatocytes as energy supply. In perfused rat livers, the thromboxane receptor antagonist daltroban enhanced C5a-induced prostanoid overflow and reduced glucose output. It was the aim of this study to elucidate whether daltroban interfered with prostanoid release from Kupffer cells or prostanoid clearance by hepatocytes, and/or whether it directly influenced prostanoid-dependent glucose metabolism in these cells. In perfused rat livers, daltroban enhanced prostaglandin (PG)D(2) overflow not only after infusion of C5a (15-fold), but also after PGD(2) (10-fold). Neither daltroban nor another receptor antagonist, ifetroban, or the thromboxane synthase inhibitor furegrelate enhanced prostanoid release from Kupffer cells. In contrast, all inhibitors reduced clearance, i.e. uptake and degradation, of PGD(2) by hepatocytes: within 5 min uptake of 1 nmol/L PGD(2) was reduced from 43+/-5 fmol (controls) to 22+/-6 fmol (daltroban), 24+/-6 fmol (ifetroban) and 21+/-6 fmol (furegrelate). PGD(2) in the medium was reduced to 39+/-7% in the controls, but remained at 93+/-9%, 93+/-11% and 60+/-3% in the presence of the inhibitors. PGD(2)-dependent glucose output in the perfused liver or activation of glycogen phosphorylase in isolated hepatocytes remained unaffected by daltroban. These data clearly demonstrate that the thromboxane-inhibitors reduced PGD(2) clearance by hepatocytes, presumably by inhibition of prostanoid transport into the cells. In contrast, they did not interfere with PGD(2)-dependent glucose metabolism, suggesting an independent mechanism for the inhibition of glucose output from the liver.     

糖尿病大鼠阴茎海绵体内PGI2，TXA2含量变化的研究

目的：探讨前列环素（PGI2）与血栓塞A2（TXA12）在糖尿病性阳萎发病过程中的作用,方法：以四氧嘧啶诱导的四月龄雄性Wistar大鼠为糖尿病模型,模型成功后3周,以放射免疫分析方法检测糖尿病组及对照组大鼠阴茎海绵体内PGI2和TXA2含量。结果：糖尿病大鼠阴茎海绵体内PCI2含量较对照组明显下降（P＜0．01）,TXA2含量增高（P＜0．05）,结论：PGI2与TXA2比较失衡是糖尿病性阳萎的病因之一。     

Effects of naproxen on the in vivo synthesis of thromboxane and prostacyclin in man

Summary The effect of a single oral dose of 500 mg naproxen on the synthesis in vivo of thromboxane A₂ and prostacyclin was studied in healthy volunteers. The synthesis of the prostanoids was assessed by measuring the urinary excretion of the metabolites 2,3-dinor-TxB₂ and 2,3-dinor-6-keto-PGF₁, respectively, using stable isotope dilution assays based on gas chromatography — mass spectrometry.Naproxen caused significant inhibition of the excretion of both metabolites for about two days. The reduction of the thromboxane metabolite was more pronounced (75% inhibition) than that of the prostacyclin metabolite (about 50% inhibition).The data support the idea that naproxen causes reversible inhibition of cyclooxygenase.     

Thromboxane synthetase inhibitors differentially antagonize thromboxane receptors in vascular smooth muscle

Summary A number of thromboxane (Tx) synthetase inhibitors have been found to prevent thromboxane release in acute cardiopulmonary disorders. However, little is known about Tx receptor antagonism by these substances. Imidazole, UK-37,248 and pinane thromboxane A₂ (PTA₂) were tested in isolated perfused cat coronary arteries, spirally cut rabbit pulmonary artery strips, and in rabbit and cat platelets for their ability to antagonize vasoconstrictor and aggregatory effects of a stable Tx agonist, carbocyclic thromboxane A₂ (CTA₂). Imidazole, at concentrations that completely inhibit thromboxane synthesis, failed to antagonize the vasoconstrictor effects of CTA₂ in both systems. UK-37,248, at 10 to 1000 g/ml, failed to inhibit CTA₂-induced coronary constriction but at 1 g/ml reduced rabbit pulmonary artery constriction by 80±8% (P<0.0005). In comparison, 1 M PTA₂ completely prevented PTA₂-induced constriction in both coronary and pulmonary arteries. PTA₂ did not antagonize KCl and angiotensin II responses. In platelets, PTA₂ but not UK-37,248 prevented arachidonate induced aggregation in rabbit and cat platelets. Some of the beneficial effects of thromboxane synthetase inhibitors may be associated with thromboxane receptor antagonism. Inhibitors structurally similar to TxA₂ appear to have greater thromboxane receptor antagonistic activity. This additional activity may be of importance in therapeutics of coronary and pulmonary disorders.Supported in part by a grant from the W. W. Smith Charitable Trust     

Effects of fenflumizole on aggregation ex vivo of human platelets and formation of thromboxane B2 and 6-keto-prostaglandin-F_{1\alpha }

Summary Fenflumizole (2-(2,4-difluorophenyl)-4,5-bis(4-methoxyphenyl)imidazole), a new non-steroidal anti-inflammatory drug, was given to healthy subjects in single oral doses of 0.1, 1 and 2 mg/kg. The effect of the drug was followed for up to 8 h by repeated tests of arachidonic acid-induced platelet aggregation and was related to its concomitant plasma concentration. Fenflumizole reversibly inhibited platelet aggregation and the degree of inhibition was found to be linearly correlated with the log plasma concentration. There was depression of the formation of thromboxane B₂ and 6-keto-prostaglandin F₁ (the stable metabolites of thromboxane A₂ and prostacyclin) in clotted whole blood measured by radioimmunoassay after fenflumizole 1 mg/kg. This effect was directly related to the concentration of the drug in plasma, the maximum effect being reached at fenflumizole concentrations >200 ng/ml. EC₅₀-values for inhibition of the formation of thromboxane B₂ and 6-keto-prostaglandin were approximately 20 and 40 ng/ml, respectively. The results suggest that orally administered fenflumizole is a potent inhibitor of platelet aggregation and prostanoid formation.     

阿司匹林对大鼠局灶性脑缺血-再灌注损伤的保护作用及机制

目的观察阿司匹林对大鼠脑缺血-再灌损伤的保护作用。方法线栓法制作局部脑缺血2 h、再灌24 h模型，观察阿司匹林6和60 mg·kg^-1对脑损伤面积、水肿程度及血前列环素I₂（PGI₂）/血栓素A₂（TXA₂）、一氧化氮（NO）、内皮素（ET）及脑组织丙二醛（MDA）、超氧化物歧化酶（SOD）和ATP含量的影响。结果两个剂量阿司匹林均明显缩小再灌引起的脑损伤范围，减轻水肿，提高血PGI₂/TXA₂。以高剂量效果显著且明显降低脑组织中MDA，提高ATP。低剂量可明显降低血浆NO，增加ET。结论阿司匹林对脑缺血-再灌损伤有明显保护作用，其机制可能与提高PGI₂/TXA₂、减少氧自由基和提高ATP含量有关。     

Imbalanced synthesis of cyclooxygenase-derived thromboxane A2 and prostacyclin compromises vasomotor function of the thoracic aorta in Marfan syndrome

BACKGROUND AND PURPOSE: Thoracic aortic dissection is a life-threatening complication of Marfan syndrome, a connective tissue disorder caused by mutations in the gene encoding fibrillin-1. We have demonstrated that nitric oxide-mediated endothelial-dependent relaxation is impaired in the thoracic aorta in Marfan syndrome. In the present study, we determined whether the cyclooxygenase (COX)-pathway is involved in the compromised aortic vasomotor function. EXPERIMENTAL APPROACH: Thoracic aortae from mice at 3, 6 and 9 months of age, heterozygous for the Fbn1 allele encoding a cysteine substitution (Fbn1 (C1039G/+), 'Marfan', n=35), were compared with those from age-matched controls (n=35). KEY RESULTS: Isometric force measurement revealed that preincubation with indomethacin, a non-specific COX inhibitor, but not valeryl salicylate, a specific COX-1 inhibitor, improved the phenylephrine-induced contractions (at 6 months, EC(50) and E(max) were increased 4.5-fold and by 45%, respectively) in Marfan aortae. Sensitivity to acetylcholine-induced relaxation was improved 10-fold. Blockade of the thromboxane-endoperoxide receptor by SQ-29548 did not affect phenylephrine-mediated contractions in Marfan aortae, although they did respond to the thromboxane analogue, U46619. From 6 months on, phenylephrine-induced secretion of prostacyclin and thromboxane A(2) in Marfan aortae was 200% and 40%, respectively, of those in controls. Reduced COX-1 expression was detected in Marfan aortae at 3 and 9 months, whilst COX-2 expression was increased from 3 months on. CONCLUSIONS AND IMPLICATIONS: The compromised vasomotor function in Marfan thoracic aortae is associated with an imbalanced synthesis of thromboxane A(2) and prostacyclin resulting from the differential protein expression of COX-1 and COX-2.     

Involvement of endothelial thromboxane A2 in the vasoconstrictor response induced by 15-E2t-isoprostane in isolated human umbilical vein

The present study was undertaken to evaluate the contractile response of several E- and F-ring isoprostanes (IsoP) in human umbilical vein (HUV) and to investigate the role of the endothelium on the effect of 15-E_2t-IsoP, the most potent vasoconstrictor isoprostane, in human vessels. HUV rings with or without endothelium were suspended in an organ bath for recording the isometric tension in response to different agonists. The inhibitors to be evaluated were applied 30 min before the addition of the agonist. All of the compounds tested produced concentration-dependent contractions when tested on HUV rings with endothelium. Although these compounds were equieffective, significant differences were observed in their potency, with U46619 being the most potent followed by 15-E_2t-IsoP > 15-E_1t-IsoP = 15-F_2t-IsoP > 15-F_1t-IsoP = 9-epi-15-F_2t-IsoP in descending rank order of potency. 15-E_2t-IsoP was the most potent of the isoprostanes evaluated and, therefore, the one employed in the present study. When intact endothelium HUV rings were used, 15-E_2t-IsoP-induced contraction was unaffected by the endothelin-converting enzyme inhibitor, phosphoramidon (10 μM), suggesting that short-term endothelin-1 release is not involved in this response. However, the non-selective cyclooxygenase (COX) inhibitor, indomethacin (10 and 30 μM), and the COX-2 selective inhibitor, NS-398 (3, 10 and 30 μM) produced inhibitory effects on 15-E_2t-IsoP-induced contraction of HUV rings with endothelium. These results indicate that COX-derived contractile prostanoids are involved in this effect. Furthermore, the apparent pK _b values estimated for indomethacin (5.5) and NS-398 (5.4) suggest that the prostanoids involved are derived from the COX-2 isoenzyme pathway. On HUV rings with endothelium, the phospholipase A₂ inhibitor, oleyloxyethyl phosphorylcholine (30 and 100 μM), induced an inhibitory effect on 15-E_2t-IsoP-induced contraction, suggesting that the phospholipase A₂ pathway is also involved in this effect. In addition, the thromboxane A₂ synthase inhibitor furegrelate (10 and 30 μM) also inhibited 15-E_2t-IsoP-induced contraction of HUV rings with endothelium, indicating that thromboxane A₂ is one of the contractile prostanoids involved in this response. Endothelium denudation clearly diminished the vasoconstrictor potency of 15-E_2t-IsoP, demonstrating that the endothelium releases a vasoconstrictor factor in response to 15-E_2t-IsoP. The absence of an inhibitory effect at the highest concentration of furegrelate (30 μM) on 15-E_2t-IsoP-induced contraction of HUV rings without endothelium suggested that endothelium is the source of thromboxane A₂. We conclude that prostanoids derived from the COX-2 isoenzyme pathway participate in 15-E_2t-IsoP-induced vasoconstriction of isolated HUV rings. Our results also indicate that endothelial thromboxane A₂ is one of the prostanoids involved in this effect.     

Stretch-induced contraction of rabbit isolated pulmonary artery and the involvement of endothelium-derived thromboxane A2

1. The mechanism of stretch-induced contraction of the intrapulmonary artery of rabbit was studied with special regard to the endothelium-dependence and production of prostanoids.
2. Isolated intrapulmonary artery of rabbits in ring form produced contraction when stretched slowly up to 180% of its initial muscle length (=100%) at a rate of 0.44 mm s⁻¹, with a stimulus period of 5 min.
3. The stretch-induced contraction was attenuated by the mechanical removal of the endothelium, inhibitors of cyclo-oxygenase such as aspirin and indomethacin, [1S-[1α,2α(Z),3α,4α]]-7-[3-[[2-[(phenylamino) carbonyl] hydrazino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid (SQ 29,548), which is a thromboxane A₂/prostaglandin H₂ receptor antagonist, or by ozagrel, an inhibitor of thromboxane A₂ synthase.
4. Biochemical assay indicated that the production of thromboxane B₂, a stable metabolite of thromboxane A₂, was increased 17 times in response to stretch only when the endothelium was intact. The production of thromboxane B₂ was also inhibited by aspirin or ozagrel.
5. The production of 6-keto prostaglandin F_1α, a stable metabolite of prostacyclin, was also increased in response to stretch in the preparation with intact endothelium. However, ozagrel showed no apparent effect on the production of 6-keto prostaglandin F_1α.
6. These results suggest that a mechanical stimulus like stretch can act on endothelial cells of rabbit pulmonary artery to cause contraction by activation of arachidonic acid metabolism via the cyclo-oxygenase pathway and subsequent release of thromboxane A₂ and/or an increase in the ratio of thromboxane A₂/prostacyclin.

     

NP-313, 2-acetylamino-3-chloro-1,4-naphthoquinone, a novel antithrombotic agent with dual inhibition of thromboxane A(2) synthesis and calcium entry

BACKGROUND AND PURPOSE

1,4-Naphthoquinones exhibit antiplatelet activity both in vivo and in vitro. In the present study, we investigated the antiplatelet effect of a novel naphthoquinone derivative NP-313, 2-acetylamino-3-chloro-1,4-naphthoquinone and its mechanism of action.

EXPERIMENTAL APPROACH

We measured platelet aggregation, Ca²⁺ mobilization, thromboxane B2 formation and P-selectin expression and examined several enzymatic activities. Furthermore, we used the irradiated mesenteric venules in fluorescein sodium–treated mice to monitor the antithrombotic effect of NP-313 in vivo.

KEY RESULTS

NP-313 concentration-dependently inhibited human platelet aggregation induced by collagen, arachidonic acid, thapsigargin, thrombin and A23187. NP-313 also inhibited P-selectin expression, thromboxane B₂ formation and [Ca²⁺]_i elevation in platelets stimulated by thrombin and collagen. NP-313 at 10 µM inhibited cyclooxygenase, thromboxane A₂ synthase, and protein kinase Cα, whereas it did not affect phospholipase A₂ or phospholipase C activity. In the presence of indomethacin and an adenosine 5-diphosphate scavenger, NP-313 concentration-dependently inhibited thrombin- and A23187-induced [Ca²⁺]_i increase through its inhibitory effects on Ca²⁺ influx, rather than blocking Ca²⁺ release from intracellular stores. NP-313 also inhibited thapsigargin-mediated Ca²⁺ influx through store-operated calcium channel but had no effect on Ca²⁺ influx through store-independent calcium channel evoked by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol. Nevertheless, it had little effect on cyclic AMP and cyclic GMP levels. Also, intravenously administered NP-313 dose-dependently inhibited the thrombus occlusion of the irradiated mesenteric vessels of fluorescein-pretreated mice.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results indicate that NP-313 exerts its antithrombotic activity through dual inhibition of thromboxane A₂ synthesis and Ca²⁺ influx through SOCC.     

Pharmacokinetics and pharmacodynamics of terbogrel, a combined thromboxane A2 receptor and synthase inhibitor, in healthy subjects

AIMS: To characterize the pharmacokinetics of terbogrel, a new combined thromboxane A2 (TxA2) receptor and synthase inhibitor, in healthy human subjects after single or multiple oral administration. METHODS: Forty-eight healthy male subjects received a single oral dose (10, 25, 50, 100, 150 or 200 mg) of terbogrel or placebo and 32 different subjects received one of the following treatments twice daily for 7 days: 50, 100 or 150 mg terbogrel, placebo, or once-a-day 330 mg acetylsalicylic acid. RESULTS: Terbogrel was well tolerated without obvious adverse effects following either a single oral dose or administration over 7 days. Plasma drug concentrations were dose-linear and there was no accumulation over 7 days. There was a dose-dependent blockade of TxA2 receptors and of inhibition of thromboxane synthase activity with values for IC50 of 12 ng ml(-1) and 6.7 ng ml(-1), respectively. At the highest dose tested (150 mg) there was almost complete inhibition of thomboxane synthase and thromboxane receptor occupancy. Even at trough concentrations, receptor occupancy remained above 80% and thromboxane synthase was still completely inhibited. These two activities were associated with a dose-dependent inhibition of platelet aggregation (>80% at the 150 mg dose of terbogrel) and enhanced prostacyclin production. CONCLUSIONS: Terbogrel is a potent agent having two distinct, complimentary pharmacodynamic actions, namely inhibition of thromboxane synthase and antagonism of the TxA2 receptor. The antithrombotic effect of terbogrel was dose-dependent and was associated with enhanced prostacyclin production. Terbogrel is an attractive candidate for long-term antithrombotic therapy.