The thromboxane A2/prostaglandin endoperoxide receptor antagonist activity of CV-4151, a thromboxane A2 synthetase inhibitor

The thromboxane A2/prostaglandin endoperoxide (TXA2/PGH2) receptor antagonist activity of CV-4151, a potent TXA2 synthetase inhibitor, was examined. CV-4151 inhibited guinea pig and human platelet aggregation induced by U-44069 with IC50 values of 1.2 +/- 0.3 X 10(-5) and 1.9 +/- 0.4 X 10(-5) M, respectively, and inhibited the specific binding of [3H]U-46619 to washed guinea pig and human platelets with IC50 values of 1.2 +/- 0.3 X 10(-6) and 5.1 +/- 1.0 X 10(-6) M, respectively. CV-4151 competitively inhibited the contraction of rabbit aortic strips induced by U-44069 with a pA2 value of 5.90. In experiments in mice in vivo, CV-4151 (1 and 10 mg/kg i.v.) significantly inhibited the thrombocytopenia induced by U-44069 in a dose-dependent manner. These results show that CV-4151 has a distinct TXA2/PGH2 receptor antagonist effect, and that this effect together with its inhibition of TXA2 synthetase could be important for the pharmacological action of this compound.     

Antagonistic action of AA-2414 on thromboxane A2/prostaglandin endoperoxide receptor in platelets and blood vessels

AA-2414, (+/-)-7-(3,5,6-trimethyl-1,4-benzoquinon-2-yl)-7-phenylheptanoi c acid, inhibited the aggregation of guinea pig platelets induced by a prostaglandin endoperoxide (PGH2) analogue, U-44069 and the specific binding of another analogue, [3H]U-46619 to washed guinea pig platelets with IC50 values of 3.1 x 10(-7) and 8.2 x 10(-9) M, respectively. AA-2414 competitively inhibited the contraction of rabbit aorta and pig coronary arteries induced by U-44069 with pA2 values of 8.3 and 9.0, respectively. AA-2414 also inhibited the contraction of rabbit aorta induced by PGF2 alpha (pA2: 7.8) and the contraction of pig coronary arteries induced by PGF2 alpha, PGD2 and 9 alpha,11 beta-PGF2 with pA2 values of 7.8, 8.6 and 7.8, respectively. But, AA-2414 had no effect on the antiaggregatory effect of PGD2 on the aggregation of guinea pig platelets. In experiments with guinea pigs ex vivo, AA-2414 (0.1-1 mg/kg, p.o.) dose-dependently inhibited the platelet aggregation induced by U-44069; the inhibition at a dose of 1 mg/kg was 100% at 1 hr and was 89% even at 24 hr after the administration. The thromboxane (TX) A2/PGH2 receptor antagonistic action of AA-2414 was stereospecific. These results show that AA-2414 is a potent, orally active and long acting TXA2/PGH2 receptor antagonist. In addition, AA-2414 has PGF2 alpha, PGD2 and 9 alpha,11 beta-PGF2 antagonistic effects.     

Pharmacological actions of ICI 180080, a novel thromboxane receptor antagonist

ICI 180080 (5(Z)-7-[2,2-dimethyl-4-(2-hydroxyphenyl)-1,3-dioxan-cis-5-yl] heptenoic acid) potently inhibited contractions of rat and rabbit aortae and guinea-pig trachea elicited by 11,9-epoxymethano PGH2 (U-46619). This antagonism was selective because contractions of aortae to noradrenaline and 5-hydroxytryptamine and trachea to histamine were not antagonized by ICI 180080. Schild analysis of data obtained from experiments on rabbit aortae indicated that this thromboxane receptor antagonism was competitive (pA2 = 7.50, slope = 1.07). Addition of ICI 180080 to human platelet-rich plasma caused dose-related inhibition of U-46619-induced platelet aggregation. This modification of platelet aggregation was not associated with inhibition of thromboxane synthetase, cyclo-oxygenase or lipoxygenase. ICI 180080 did not modify the primary phase of ADP-induced aggregation of human platelets neither did it affect the platelet inhibitory activity of prostacyclin. When dosed orally to anaesthetized guinea-pigs, ICI 180080 (5-50 mg kg-1) caused dose-related inhibition of U-46619-evoked bronchoconstriction. We conclude that ICI 180080 is a potent, selective, competitive, orally active thromboxane antagonist.     

Effects of ONO-3708, an antagonist of the thromboxane A2/prostaglandin endoperoxide receptor, on blood vessels

The pharmacological properties of a novel thromboxane A2/prostaglandin endoperoxide receptor antagonist, ONO-3708, on blood vessels were examined in vitro and in vivo. ONO-3708, 10 microM, inhibited the rabbit aorta contractions induced by thromboxane A2, prostaglandin H2, 11,9-epoxymethano-prostaglandin H2 (U-46619) or prostaglandin F2 alpha without affecting the contractions induced by angiotensin II, serotonin or norepinephrine. ONO-3708, at a concentration of 1 to 100 nM, appeared to be a competitive inhibitor of the contractile responses of the canine basilar artery to 9,11-epithio-11,12-methano-thromboxane A2 (STA2), U-46619 and PGF2 alpha, and a non-competitive inhibitor of the contractile responses to 15-hydroperoxy-eicosatetraenoic acid (15-HPETE). In in vivo studies, ONO-3708 (10 and 100 micrograms/kg per min i.v.) ameliorated the decrease in diameter of the basilar artery induced by the i.v. infusion of STA2 (0.1 microgram/kg per min) in cats. Furthermore, infusion of ONO-3708 (10 and 30 micrograms/kg per min i.v.) prevented the cerebral vasospasm in an experimental subarachnoid hemorrhage model in dogs. These results indicate that ONO-3708 is a potent antagonist of the thromboxane A2/prostaglandin endoperoxide receptor in vitro and in vivo and may be of therapeutic use in preventing cerebral vasospasm.     

Stereo-dependent inhibition of human platelet function by the optical isomers of trimetoquinol

The stereoisomers of trimetoquinol [1-(3',4',5'-trimethoxybenzyl)-6–7-dihydroxy-1,2,3,4-tetrahydroisoquinoline; TMQ] were shown to have potent and selective inhibitory effects on human platelet function in vitro. the R(+)-isomer was 12.1-, 12.3-, 39.2-, 82.9- and 36.0-fold more effective than the S(?)-isomer as an inhibitor of aggregation induced by arachidonic acid (AA), collagen, the epoxymethano-PGH2 analogs U44069 and U46619, and thromboxane A; (TxA₂) respectively. the concentrations of the R(+)-isomer that produced 50 percent inhibition (IC₅₀) of platelet aggregation were 4.2, 4.3, 1.4, 0.14 and 0.64 μM using AA, collagen, U44069, U46619, and TxA₂₀ as respective inducers. the graphical approximation of an inhibitory Constant (K_i = 0.13 μM) for the effect of TMQ on U46619-induced aggregation suggested that a competitive-like inhibition was operative. In other experiments, platelet aggregation and serotonin release induced by U46619 were inhibited differentially by the TMQ stereoisomers with nearly identical concentration-response curves. In addition, racemicTMQ blocked the secondary phase of platelet aggregation and serotonin release induced by ADP. These data, together with the ability of the TMQ stereoisomers to selectively inhibit TxA₂-induced aggregation, suggest that TMQ is an inhibitor of endoperoxide or TxA₂ action, e.g. a thromboxane A₂ receptor antagonist.     

Pharmacological profile of 48740 R.P., a PAF-acether antagonist

The pyrrolo-thiazole derivative 48740 R.P. inhibited the platelet-activating factor (PAF-acether)-induced aggregation of human and rabbit platelets and was poorly effective against ADP- and arachidonic acid-induced platelet aggregation. 48740 R.P. prevented the activation of guinea-pig alveolar macrophages by PAF-acether, and the PAF-acether-induced thromboxane B₂ production from guinea-pig lungs. 48740 R.P. (3 mg/kg i.v.) antagonized selectively in anaesthetized guinea-pigs the bronchoconstriction due to PAF-acether without affecting that due to acetylcholine, histamine, serotonin, thromboxane A₂ analogue U-46,619 and arachidonic acid. A higher dose of 48740 R.P. (10 mg/kg i.v.) was required to block the thrombocytopenia and the leucopenia induced by PAF-acether in the propranolol-treated guinea-pigs. 48740 R.P. (30 mg/kg i.v.) antagonized the PAF-acether effects when bronchoconstriction was induced by aerosolized PAF-acether. 48740 R.P. is a selective antagonist of PAF-acether under in vitro and in vivo conditions.     

Actions of the novel thromboxane A2 receptor antagonist sodium (E)-11-[2-(5,6-dimethyl-1-benzimidazolyl)-ethylidene]-6,11- dihydrodibenz[b,e]oxepine-1-carboxylate monohydrate on smooth muscle preparations.

The effects of KW-3635 (sodium (E)-11-[2-(5,6-dimethyl-1-benzimidazolyl)- ethylidene]-6,11-dihydrodibenz[b,e]oxepine-2-carboxylate monohydrate, CAS 127166-41-0) on smooth muscle preparations were examined. In isolated guinea-pig aorta, KW-3635 competitively inhibited the U-46619 (a thromboxane mimetic) induced contractions (pA2 = 7.74), the effect being more potent than those of sulotroban and daltroban. In canine saphenous vein, KW-3635 also antagonized the U-46619-induced contraction (pA2 = 8.11). In this preparation, solutroban and daltroban, but not KW-3635, exhibited intrinsic agonistic action. KW-3635, even at a high concentration of 10(-5) mol/l did not affect the norepinephrine- or KCl-induced contractions of guinea-pig or rat aorta, prostaglandin (PG)E2- or PGF2 alpha-induced contractions of guinea-pig ileum nor the PGE2-induced contraction of rat fundus. KW-3635 at concentrations higher than its thromboxane A2- (TxA2-)antagonistic one, non-competitively inhibited the PGF2 alpha-induced contractions of guinea-pig aorta (pD2' = 6.23), as was the case with daltroban. The inhibitory effect of KW-3635 (3 x 10(-6) mol/l) on U-46619-induced contractions of guinea-pig aorta persisted for longer than 2 h following washout of the tissue, whereas that of daltroban (10(-5) mol/l completely disappeared at 1 h after the washout. In anesthetized guinea-pigs, KW-3635 at doses of 10 to 1000 micrograms/kg (i.v.) inhibited U-46619 (1 microgram/kg i.v.)-induced pressor responses in a dose-dependent manner. The effect of KW-3635 (0.1 to 1 mg/kg i.v.) persisted for longer than 3 h. These results demonstrate that KW-3635 is a potent and specific TxA2 antagonist without agonistic action in vascular smooth muscles. KW-3635 is considered to be a promising candidate for the treatment of patients with disorders mediated via TxA2.     

Effects of RP and SP diastereoisomers of adenosine 5'-O-(1-thiodiphosphate) on human platelets.

1 RP and SP diastereoisomers of adenosine 5'-O-(1-thiodiphosphate) ((R)-ADP-alpha-S and (S)-ADP-alpha-S), an adenosine 5'-diphosphate (ADP) analogue, were tested on intact human platelets. 2 Each diastereoisomer induced aggregation, (S)-ADP-alpha-S being 5 times more potent than (R)-ADP-alpha-S but they achieved only 75% of the maximal effect of ADP. 3 Aggregation induced by each diastereoisomer was competitively inhibited by ATP (50 microM). 4 Simultaneous addition of each diastereoisomer inhibited aggregation induced by ADP but not by 11 alpha, 9 alpha-epoxymethano prostaglandin H2, a stable endoperoxide analogue. Both diastereoisomers are therefore partial agonists at the ADP receptor mediating aggregation. 5 Unlike ADP, neither diastereoisomer inhibited prostaglandin E1 (PGE1)-stimulated adenylate cyclase, but each competitively inhibited the effect of aDP, with (S)-ADP-alpha-S again being 5 times more potent than (R)-ADP-alpha-S. 6 These are the first reported examples of ADP analogues to induce platelet aggregation without inhibiting PGE1-stimulated adenylate cyclase.     

Pharmacology: Pharmacological Antagonism of Anchietia salutaris Extracts on the Contraction Induced by Prostaglandin D2 and U46619 in Guinea-pig Lung Parenchymal Strips

Anchietia salutaris tea is traditionally used in Brazil to treat allergies, suggesting it contains compounds with antagonistic activity on the allergic mediators. We have evaluated extracts and semi-purified fractions of Anchietia salutaris as a source of compounds having this type of antagonism on the contraction induced in guinea-pig lung parenchymal strips and on platelet aggregation and shape change. After 10 min pre-incubation dichloromethane extracts containing 30 or 100 μg mL^?1 inhibited the contraction induced by prostaglandin D₂ (PGD₂) in guinea-pig lung parenchymal strips with dose ratios (DR) of 0.76±0.14 and 0.93±0.19, respectively; the amount of inhibition depended both on the concentration and on the time of preincubation (DR after 30 min pre-incubation was 1.21±0.51). The dichloromethane extract and its semi-purified fractions also inhibited the contractions induced by U46619, a more potent, stable, synthetic agonist of thromboxane A₂ (TxA₂) prostanoid (TP) receptors, the receptors acted upon by PGD₂ to produce lung contractions. The dichloromethane extract did not inhibit the lung parenchymal contractions induced by histamine, leukotriene D₄ (LTD₄) or platelet-activating factor (PAF). Platelet aggregation induced by U46619, adenosine 5′-diphosphate (ADP) or PAF was not inhibited by the dichloromethane extract. Indeed, the extract potentiated platelet aggregation induced by low concentrations of these agonists and also potentiated the shape change induced by U46619. These results imply that the dichloromethane extract of Anchietia salutaris and its semi-purified fractions contain an active principle that competitively inhibits TxA₂ TP receptors, the stimulation of which causes lung parenchymal contraction. The inhibition seems to be selective for this receptor subtype, because the extract fails to inhibit platelet aggregation or shape change. This provides additional support of earlier reports suggesting the occurrence of TP receptor subtypes.     

Partial agonist behaviour of adenosine 5'-O-(2-thiodiphosphate) on human platelets.

1 The effects of an adenosine diphosphate (ADP) analogue, adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S), in which a terminal phosphate oxygen has been replaced by sulphur, were studied on human platelets. 2 ADP-beta-S induced platelet aggregation and inhibited prostaglandin E1 (PGE1)-stimulated adenylate cyclase but in both cases was less potent than ADP and did not achieve the same maximal effects. 3 Both actions of ADP could be inhibited by the simultaneous addition of ADP-beta-S (50 microM). 4 Aggregation induced by 11 alpha, 9 alpha-epoxymethano prostaglandin H2 (a stable endoperoxide analogue) was not inhibited by simultaneous addition of ADP-beta-S (50 microM). 5 The behaviour of ADP-beta-S towards human platelets was consistent with it being a partial agonist.     

CGS 12970: a novel, long acting thromboxane synthetase inhibitor.

CGS 12970 is a potent selective inhibitor of human platelet thromboxane synthetase in vitro (IC50 = 12 nM). It is four orders of magnitude less potent as an inhibitor of sheep seminal vesicle cyclooxygenase, bovine aorta prostacyclin synthetase and human leucocyte 15-lipoxygenase. The compound inhibited collagen-induced thromboxane B2 production by human platelets in vitro without an effect on the accompanying platelet aggregation induced by collagen, ADP, platelet activating factor, thrombin, arachidonic acid or the prostaglandin mimetic, U 46619. Administration of CGS 12970 to rats inhibited collagen-induced thromboxane B2 production but had no effect on platelet aggregation ex vivo. It also had no effect on platelet aggregation induced by thrombin or on plasma clotting times. A single oral dose of 1 or 3 mg kg-1 to rabbits inhibited thromboxane B2 production in clotting blood ex vivo for 12 or 24 h respectively. CGS 12970 inhibited thromboxane B2 production in vivo induced by intravenous administration of collagen to rats or calcium ionophore to guinea-pigs. In both cases there was a concomitant elevation of immunoreactive 6-keto-prostaglandin F1 alpha but no effect on the induced thrombocytopenia. As with other thromboxane synthetase inhibitors, CGS 12970 prolonged tail bleeding time in the rat. However, CGS 12970 was not as potent as other thromboxane synthetase inhibitors in this test. In addition to these usual effects of thromboxane synthetase inhibitors, CGS 12970 inhibited the thrombocytopenia induced by the Forssman reaction or ADP administration. In these tests the effect of the compound was of short duration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ONO-3708, an antagonist of the thromboxane A2/prostaglandin endoperoxide receptor, on platelet aggregation and thrombosis

The beneficial effects of an antagonist of the thromboxane A2/prostaglandin endoperoxide receptor, 7-[2 alpha,4 alpha-(dimethylmethano)-6 beta-(2-cyclopentyl-2 beta- hydroxyacetamido)-1 alpha-cyclohexyl]-5(Z)-heptenoic acid (ONO-3708) on thrombosis were examined. ONO-3708 at 0.1-3 microM inhibited the human platelet aggregation induced by thromboxane A2, prostaglandin H2, collagen, ADP (secondary phase) and epinephrine (secondary phase) without affecting prostanoid synthesis and the content of cyclic AMP in platelets. The in vivo effects, on coronary thrombosis in this case, were examined in two canine models. ONO-3708, 3 to 300 micrograms/kg i.v., prevented dose dependently the coronary thrombosis induced by partial obstruction of the coronary artery. ONO-3708, 3 micrograms/kg per min i.v., significantly prevented electrically stimulated coronary thrombosis without affecting systemic blood pressure and heart rate. These results indicate that the thromboxane A2/prostaglandin endoperoxide receptor could play an important role in the pathogenesis of thrombosis and that ONO-3708 may have therapeutic advantages in preventing thrombosis.     

Pharmacological characterization of cinnamophilin, a novel dual inhibitor of thromboxane synthase and thromboxane A2 receptor.

1. The pharmacological effects of cinnamophilin, a new lignan, isolated from Cinnamomum philippinense, was determined in vitro in human platelet, rat isolated aorta and guinea-pig isolated trachea and in vivo in mice and guinea-pigs. 2. Cinnamophilin inhibited dose-dependently human platelet-rich plasma (PRP) aggregation induced by arachidonic acid (AA), collagen and U-46619 with IC50 of 5.0 +/- 0.4, 5.6 +/- 0.6 and 3.0 +/- 0.4 microM, respectively. The second wave of ADP- or adrenaline-induced platelet aggregation was inhibited by cinnamophilin, while the first wave was only slightly inhibited by cinnamophilin above 30 microM. 3. Cinnamophilin was found to be a thromboxane A2 (TXA2) receptor blocking agent in human platelet, rat aorta and guinea-pig trachea as revealed by its competitive antagonism of U-46619-induced aggregation of human-PRP, contraction of rat aortic rings and guinea-pig tracheal rings with pA2 values of 7.3 +/- 0.2, 6.3 +/- 0.1 and 5.2 +/- 0.2, respectively. 4. [3H]-inositol monophosphate formation and the rise of intracellular Ca2+ caused by U-46619 in human platelet was suppressed by cinnamophilin (10 microM). 5. Cinnamophilin induced a dose-dependent inhibition of thromboxane B2 (TXB2) formation, while the prostaglandin E2 (PGE2) formation was increased. Cinnamophilin did not affect unstimulated platelet adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. When the platelets were challenged with AA, a dose-dependent rise in cyclic AMP was observed. Dazoxiben (a pure TX synthase inhibitor) and SQ 29548 (a pure TXA2 receptor antagonist) did not affect cyclic AMP levels in AA-treated platelets.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of bronchoconstriction and of thrombocytopenia induced by collagen in the guinea pig

Collagen injected to guinea pigs i.v. increased the pulmonary resistance to inflation (bronchoconstriction) and induced thrombocytopenia. Immune platelet depletion protected against the effects of collagen, and has been shown not to prevent bronchoconstriction induced by the prostaglandin/thromboxane A2 precursor arachidonic acid. Use of inhibitors demonstrated that histamine, serotonin, acetylcholine and bradykinin were not involved with the effects of collagen in the guinea pig. Aspirin and indomethacin inhibited collagen-induced bronchoconstriction completely and thrombocytopenia partly, supporting the hypothesis that the former is prostaglandin cyclo-oxygenase dependent, whereas the latter has a thromboxane A2-independent mechanism as well. Carrageenan, heparin and reserpine inhibited the in vivo effects of collagen to various extents, but their precise mechanism of action could not be discovered. Collagen-induced bronchoconstriction is strictly platelet and thromboxane A2-dependent.     

Specific but noncompetitive inhibition by 2-alkylthio analogues of adenosine 5'-monophosphate and adenosine 5'-triphosphate of human platelet aggregation induced by adenosine 5'-diphosphate.

Some 2-alkylthio derivatives of adenosine 5'-monophosphate (AMP), adenosine 5'-monophosphorothioate (AMPS) and adenosine 5'-triphosphate (ATP) were examined as inhibitors of human platelet aggregation. 2-Methylthio-AMP, 2-ethylthio-AMP, 2-(pentan-l-yl)thio-AMP, 2-ethylthio-AMPS, 2-methylthio-ATP and 2-ethylthio-ATP (100 microM) each inhibited aggregation induced by adenosine 5'-diphosphate (ADP) but not by 11 alpha, 9 alpha-epoxymethano prostaglandin H2, a stable endoperoxide analogue. Log dose-response curves to ADP in the absence and presence of each inhibitor were not parallel and the inhibition could not be overcome by high concentrations of ADP. The ATP analogues achieved greater inhibition of aggregation induced by ADP (5 microM) than did the AMP analogues. The order of potency of the AMP analogues was 2-ethylthio-AMPS greater than 2-ethylthio-AMP greater than 2-(pentan-l-yl)thio-AMP greater than 2-methylthio-AMP, and 2-methylthio-ATP was more potent than 2-ethylthio-ATP. These 2-alkylthio substituted analogues of AMP and ATP are specific but noncompetitive inhibitors of ADP-induced human platelet aggregation.     

Differential regulation of the cyclooxygenase pathway in starch elicited rat peritoneal macrophages by prostaglandin E2, U-44069, a stable endoperoxide analogue and dibutyryl-cyclic AMP

The basal and carrageenin-stimulated release of thromboxane (TX) B2, the stable product of TXA2, 6-ketoPGF1 alpha, the stable breakdown product of prostacyclin (PGI2) and prostaglandin (PG) E2 from 24 h starch elicited rat peritoneal macrophages was inhibited by dibutyryl-cyclic AMP (db-cAMP). PGE2 also inhibited the release of TXA2 and 6-keto-PGF1 alpha whereas the stable endoperoxide analogue, U-44069, stimulated PGE2 and 6-keto PGF1 alpha release but inhibited TXB2 release. The effects of all three mediators tested were related to an increase of M? intracellular cAMP content.     

Fenamates may antagonize the actions of prostaglandin endoperoxides in human myometrium.

1 The prostaglandin endoperoxide analogue U-46619 potently contracted human isolated myometrium, suggesting that prostaglandin H2 (PGH2) may be a major stimulant of myometrial contractions. 2 Sodium mefenamate, flufenamate or meclofenamate 2 microgram/ml greatly reduced contractions of the myometrium induced by the PGH2 analogue. 3 Flufenamate, but not the other two drugs, also significantly inhibited contractions to acetylcholine. 4 Sodium meclofenamate 2 microgram/ml did not consistently antagonize contractions to PGF2alpha. 5 The relief of dysmenorrhoea by fenamates may be explained both by inhibition of PG synthesis, and by antagonism of contractions to PGH2 produced by incompletely blocked PG synthesis.     

The 5-Lipoxygenase Inhibitory Activity of Zileuton in In Vitro and In Vivo Models of Antigen-Induced Airway Anaphylaxis

Summary: Leukotrienes are biologically active lipid mediators capable of producing airway inflammation, hyperresponsiveness and bronchoconstriction. The first enzyme in the metabolic pathway of arachidonic acid leading to the leukotrienes is 5-lipoxygenase (5-LO). A selective and potent 5-LO inhibitor, zileuton (N-1(1-benzo[b]thien-2-ylethyl)-N-hydroxyurea, A-64077) was evaluated in models of airway anaphylaxis, where leukotrienes are a major component. In vitro, zileuton inhibited antigen-induced contractions of guinea-pig tracheal strips (GPTS) from actively sensitized animals with an IC₅₀ of 6 μM. Similar results were obtained in human bronchial strips passively sensitized to IgE. Zileuton had little or no effect on contractions elicited by acetylcholine, prostaglandin D₂ (PGD₂), or the thromboxane agonist, U-44069. In anesthetized sensitized guinea-pigs pretreated with meclofenamic acid and mepyramine, a single aerosol exposure of antigen produced a substantial decrease in dynamic lung compliance (C_dyn). These profound changes in lung function were dose-dependently inhibited by orally administered zileuton (ED₅₀=12 mg/kg). These results demonstrate that zileuton is a potent, selective inhibitor of in vitro contraction of GPTS and antigen-induced bronchoconstriction in vivo. These data also confirm the participation of 5-LO products in these models of airway anaphylaxis and suggest the usefulness of the guinea-pig for identifying and characterizing the pulmonary effects of 5-LO inhibitors.     

GR32191, a highly potent and specific thromboxane A2 receptor blocking drug on platelets and vascular and airways smooth muscle in vitro.

1. The thromboxane A2 (TP)-receptor blocking activity and specificity of action of GR32191 ([1R-[1 alpha(Z),2 beta,3 beta,5 alpha]]-(+)-7-[5-([1,1'-biphenyl] -4-ylmethoxy)-3-hydroxy-2-(1-piperidinyl)cyclopentyl]-4-heptoni c acid has been evaluated in human platelets and various smooth muscle preparations, both vascular and non-vascular, from a range of species including man. 2. Utilising a platelet counting method to assess aggregation the drug was found to antagonise, in a surmountable manner, human platelet aggregation produced by the TP-receptor agonists, U-46619, EP171 and SQ26655, in whole blood and physiological buffer, with pA2 values of approximately 8.3 and 8.7 in the two media respectively. In the presence of GR32191 the rate of aggregation induced by U-46619 was slowed. 3. The effect of GR32191 upon U-46619-induced platelet shape change and aggregation in platelet-rich plasma was evaluated utilising a turbidometric technique. Both shape change and aggregation were antagonised by GR32191. At relatively high concentrations of the drug a slowing of aggregation and shape change to U-44619 was seen and an unsurmountable antagonism became apparent. 4. The action of GR32191 upon human platelets was specific with platelet aggregation induced by adenosine 5'-diphosphate, platelet activating factor, vasopressin and adrenaline and the inhibitory effects of prostacylin (PGI2), prostaglandin D2 (PGD2) and N-ethylcarboxamide-adenosine (NECA) being unaffected by concentrations of the drug as high as 10 microM. Furthermore, at concentrations of up to 100 microM, the drug itself produced no shape change or aggregation, of human platelets. 5. GR32191 also specifically and potently antagonised in a competitive, surmountable manner the contractile actions of U-46619 upon human vascular smooth muscle and antagonised U-46619-induced contractions of vascular and airways smooth muscle preparations from rat, dog, guinea-pig and rabbit with varying potency. This is discussed in terms of possible heterogeneity of TP-receptors. 6. GR32191 therefore represents a highly potent and specific TP-receptor blocking drug. This profile of action, coupled to its long duration of effect in man described elsewhere, make it an ideal drug tool for elucidating the physiological and pathophysiological role of thromboxane A2.     

Antiplatelet effects of the novel thromboxane A2 receptor antagonist sodium (E)-11-[2-(5,6-dimethyl-1-benzimidazolyl)-ethylidene]-6,11- dihydrodibenz[b,e] oxepine-2-carboxylate monohydrate.

The effects of KW-3635 (sodium (E)-11-[2-(5,6-dimethyl-1-benzimidazolyl)- ethylidene]-6,11-dihydrodibenz[b,e] oxepine-2-carboxylate monohydrate, CAS 127166-41-0) on platelet aggregation were examined. In human washed platelets, KW-3635 shifted the concentration-aggregation curves for U-46619, a thromboxane A2 (TxA2) mimetic, to the right. The pA2 value for KW-3635 was 8.8 +/- 0.10, while those for sulotroban and daltroban were 6.31 +/- 0.18 and 7.75 +/- 0.07, respectively. In human platelet rich plasma (PRP), KW-3635 at 10(-8) mol/l to 10(-6) mol/l inhibited the aggregations induced by U-46619 (1 mumol/l) or collagen (1.5 micrograms/ml). However, KW-3635 at up to 10(-5) mol/l did not affect the primary phase of platelet aggregation induced by adenosine diphosphate or epinephrine. KW-3635 at 10(-5) mol/l did not affect the antiaggregatory effects of the prostaglandins PGI2, PGE1 and PGD2. These results indicate that KW-3635 is a potent and selective TxA2 receptor antagonist. The TxA2 antagonistic effects of KW-3635 were compared with that of daltroban in PRP from various animals species. The effects of KW-3635 on platelet aggregation were species-dependent and KW-3635 exhibited the most prominent activity in human platelets. The activities of KW-3635 in mouse and rabbit PRP were much less potent. In PRP from guinea-pigs, dogs, cats and rats, KW-3635 exhibited moderate anti-aggregatory effects. In the guinea-pig PRP, KW-3635 at 10(-7) mol/l to 3 x 10(-6) mol/l inhibited both the platelet aggregation and the concomitant adenosine triphosphate secretion in a concentration-dependent manner, the effect being more potent than those of sulotroban and daltroban. In the experiments on the platelet aggregation ex vivo in guinea-pigs, KW-3635 at oral doses of 3 and 10 mg/kg inhibited the aggregations induced by U-46619 (1, 3 mumol/l), collagen (3, 6, 9 micrograms/ml) and arachidonate (50, 100 mumol/l). The effects lasted for longer than 7 h following oral administration. These results indicate that KW-3635 is a specific and orally active TxA2 receptor antagonist. KW-3635 is expected to be a drug useful for the treatment of patients with thrombotic disorders.