Mechanism of the prostanoid TP receptor agonist U46619 for inducing emesis in the ferret

U46619 is a potent thromboxane A₂ mimetic with emesis-inducing actions that are mediated via prostanoid TP receptors. We investigated its emetic mechanism of action in more detail using the ferret as model animal. The emesis induced by U46619 (30 μg/kg, intraperitoneal) was antagonized significantly by (+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine hydrochloride (CP-99,994; 1 and 10 mg/kg; P < 0.05) and metoclopramide (0.3 and 3 mg/kg), but not by domperidone (3 mg/kg), sulpiride (0.1 mg/kg), ondansetron (0.1 and 1 mg/kg) alone or combined with droperidol (3 mg/kg), GR125487 (1 mg/kg), promethazine (3 mg/kg), or scopolamine (3 mg/kg); GR 125487 (1 mg/kg) prevented the anti-emetic action of metoclopramide (3 mg/kg). U46619 0.3 μg administered into the fourth ventricle rapidly induced emesis. However, bilateral abdominal vagotomy was ineffective in reducing the emetic response (P > 0.05). Our data suggests that U46619 induces emesis via an extra-abdominal mechanism, probably within the brain. Metoclopramide probably has a mechanism of action to prevent U46619-induced emesis via 5-HT₄ receptor activation and NK₁ tachykinin receptor antagonists could be useful to prevent emesis induced by TP receptor activation in man.     

Actions of prostanoids to induce emesis and defecation in the ferret

Several prostanoids were investigated for their ability to induce emesis and/or defecation and tenesmus in the ferret. The rank order of emetic potency (dose producing four episodes, D4) was: sulprostone (5 microg/kg)>11alpha,9alpha-epoxymethano-15S-hydroxyprosta-5Z,13E-dienoic acid (U46619; 8 microg/kg)>misoprostol (27 microg/kg)>17-phenyl-omega-trinor prostaglandin E2 (53 microg/kg)>prostaglandin E2 (94 microg/kg)>5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropyl) hydantoin (BW245C; 148 microg/kg)>prostaglandin F(2alpha) (13,500 microg/kg). Emesis was also induced by iloprost (D4 not determined) and prostaglandin E2 methyl ester (D4=350 microg/kg). Cicaprost and fluprostenol were virtually inactive; they also failed to modify copper sulphate (100 mg/kg, intragastric)-induced emesis (P>0.05), although cicaprost potentiated apomorphine (0.25 mg/kg, s.c.)-induced emesis (P<0.05). U46619-induced emesis was antagonised by vapiprost (P<0.05). The rank order of potency to produce defecation and tenesmus (dose producing three episodes) was: sulprostone (12 microg/kg)>misoprostol (15 microg/kg)>17-phenyl-omega-trinor prostaglandin E2 (94 microg/kg)>prostaglandin E2 (113 microg/kg)>fluprostenol (158 microg/kg)z.Gt;prostaglandin F(2alpha) (1759 microg/kg); prostaglandin E2 methyl ester also induced defecation (196 microg/kg). Data are discussed in relation to mechanisms involved in emesis and defecation.     

Emetic action of the prostanoid TP receptor agonist, U46619, in Suncus murinus (house musk shrew)

The emetic action of the prostanoid TP receptor agonist, 11alpha,9alpha-epoxymethano-15S-hydroxyprosta-5Z,13E-dienoic acid (U46619; 300 microg/kg, i.p.), was investigated in Suncus murinus. The emetic response was reduced by 76% following bilateral abdominal vagotomy (P<0.001) and by reserpine (5 mg/kg, i.p., 24 h pretreatment; P<0.05) but U46619 administered i.c.v. (30-300 ng) was not emetic, suggesting a peripheral mechanism involving monoamines. However, fenfluramine (5 mg/kg, repeated treatment) and para-chlorophenylalanine (100-400 mg/kg) and ondansetron (0.3-3 mg/kg) were inactive (P>0.05) to reduce U46619-induced emesis precluding a role of 5-HT and 5-HT(3) receptors in the mechanism. Similarly, phentolamine (0.3-3 mg/kg), propranolol (3 mg/kg), and their combination, and metoclopramide (0.3-3 mg/kg), domperidone (0.3-3 mg/kg), droperidol (0.3-3 mg/kg), scopolamine (0.3-3 mg/kg) and promethazine (0.3-3 mg/kg) were inactive (P>0.05) to reduce the retching and vomiting response. However, the tachykinin NK(1) receptor antagonist, (+)-2S,3S(-3-(2-methoxy-5-trifluoromethoxybenzyl)amino-2-phenylpiperidine) (CP-122,721; 1-10 mg/kg) antagonized emesis (P<0.01). In conclusion, U46619-induced emesis appears to be mediated via a predominant peripheral mechanism sensitive to reserpine and is not likely to involve adrenoceptors, dopamine, 5-HT(3), muscarinic or histamine (H(1)) receptors. The action of CP-122,721 to reduce U46619-induced emesis extends the spectrum of anti-emetic action tachykinin NK(1) receptor antagonists to mechanisms involving TP receptors.     

Evidence for partial agonist properties of daltroban (BM 13,505) at TP receptors in the anæsthetized open-chest rat

We sought to determine whether the intrinsic pulmonary hypertensive activity of the purported thromboxane A₂/prostanoid (TP) receptor antagonist, daltroban, was mediated by TP receptors, using the high efficacy TP receptor agonist, U-46619, and the silent TP receptor antagonist, SQ 29,548. In pentobarbitone-anæsthetized, open-chest rats (n = 4–10 per group), non-cumulative injections of U-46619, dose-dependently increased mean pulmonary arterial pressure (MPAP) with an ED₅₀ (geometric mean with 95% confidence limits in parentheses) of 1.4 (1.1– 2.3) μg/kg i.v.. Daltroban increased MPAP in a bell-shaped manner, with an apparent ED₅₀ [29 (21–35)μg/kg i.v.] being 21 fold less potent than that of U-46619. The maximal pulmonary hypertensive responses evoked by daltroban represented about half those induced by U-46619 (25.4 ± 1.0 vs. 12.7 ± 2 mmHg; P < 0.05 between groups). The TP receptor antagonist SQ 29,548 fully antagonized increases in MPAP evoked by equihypertensive doses of U-46619 (1.25 μg/kg) or daltroban (80 μg/kg). Further experiments were carried out to determine whether daltroban antagonized the pulmonary hypertensive responses evoked by the high efficacy agonist, U-46619, or by itself as receptor theory would predict for a partial agonist. Daltroban (10–2500 μg/kg) antagonized, although not fully, U-46619 (20 μg/kg)-evoked pulmonary hypertensive responses, since prominent intrinsic pulmonary hypertensive effects of daltroban were observed in the same range of doses. Furthermore, in contrast to U-46619 (1.25 μg/kg), daltroban (80 μg/kg) failed to evoke a second pulmonary hypertensive response following a previous injection, as would be expected for a partial agonist. Collectively, the results strongly suggest that daltroban behaves as a partial agonist at TP receptors in the pulmonary vascular bed of the rat in vivo.     

Guamanian Suncus murinus responsiveness to emetic stimuli and the antiemetic effects of 8-OH-DPAT

The Japanese Suncus murinus, the house musk shrew, is a small insectivore commonly used in emetic research. The Guamanian S. murinus has not had extensive testing as an emetic model, but it is readily available for use in emetic experiments in the United States, unlike the Japanese Suncus. This study determined that Guamanian S. murinus is an acceptable model for emesis research and its differences from the Japanese strain were examined. Motion and nicotine were used as emetic stimuli and comparable doses of 8-OH-DPAT were used to compare emetic susceptibility to the Japanese strain. The Guamanian strain had decreased susceptibility to motion and increased susceptibility to nicotine as compared to the Japanese, as well as increased sensitivity to 8-OH-DPAT, with lower doses of the recovery drug eliminating retching episodes. The study also determined that Guamanian S. murinus are smaller and more aggressive than the Japanese strain, but just as effective as a model for emetic research.     

Excitatory action of prostanoids on the ferret isolated vagus nerve preparation

We have investigated the actions of various prostanoid receptor agonists on an isolated preparation of the ferret cervical vagus using a grease-gap extracellular recording technique. The potency ranking for depolarization was BW245C (5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropyl) hydantoin; DP-selective, EC50=0.14 microM)>prostaglandin E2 (nonselective EP agonist)>U-46619 (11alpha, 9alpha-epoxymethano-15S-hydroxyprosta-5Z,13E-dienoic acid; TP agonist)>prostaglandin F2alpha (FP receptor agonist). Sulprostone (EP1/EP3-selective), fluprostenol (FP-selective) and cicaprost and iloprost (both IP-selective) had minimal effects. It is likely that DP, EP2/EP4 and TP receptors are present on the vagal fibres of the ferret.     

5-Hydroxytryptamine is emetogenic in the house musk shrew,Suncus murinus

Summary The emetic effects of 5-hydroxytryptamine (5-HT) and 5-HT₃ receptor agonists were investigated in the house musk shrew, Suncus murinus. 5-Hydroxytryptamine (5-HT; i.p., i.v., s.c.) and 2-methyl-5-HT (2-Me-5HT; i.p.) but not 5-hydroxyindoleacetic acid (i.p.) or 5-ethoxytryptamine (i.p.) induced emesis with very short latency. Tropisetron (ICS 205-930, a 5-HT₃ receptor antagonist, s.c.) blocked the emesis induced by 5-HT (10 mg/kg, i.p.) and 2-Me-5-HT (5 mg/kg, i.p.) with respective ID₅₀ values of 7.8 and 70.9 g/kg. Pindolol (5-HT₁ receptor antagonist) and ketanserin (5-HT₂ receptor antagonist) were about 100 times less potent than tropisetron. The emesis induced by 5-HT was prevented by surgical vagotomy but not by pretreatment with a combination of atropine (0.1 mg/kg, s.c.) and hexamethonium (10 mg/kg, s.c.). These results clearly indicate that 5-HT is emetogenic probably through a stimulation of peripheral 5-HT₃ receptors. Send offprint requests to N. Matsuki at the above address     

Involvement of Hypothalamic Glutamate in Cisplatin-Induced Emesis in Suncus murinus (House Musk Shrew)

We investigated the effect of cisplatin on glutamate release in the hypothalamus of Suncus murinus measured by brain microdialysis. Dialysis samples were collected every 20 min for 1 h before and 3 h after the cisplatin (30 mg/kg, i.p.) administration with the animals also being observed for the development of emesis. Cisplatin increased glutamate levels within 1 h and this was closely associated with the occurrence of emesis. Pretreatment with the 5-HT₃– receptor antagonist ondansetron (2 mg/kg, i.p.) inhibited both the emesis and the increased glutamate levels. These results suggest that hypothalamic glutamate is involved in cisplatin-induced emesis in Suncus murinus.     

Regional differences in the responses to prostanoids of circular muscle from guinea-pig isolated intestine

The effects of prostaglandins (PGs) D₂, E₂, F_2α, an epoxymethano analogue of PGH₂ (U-46619), prostacyclin (PGI₂), 6-keto-PGF_lα and thromboxane (Tx) B₂ were tested on spirally-cut strips of guinea-pig isolated ileum or colon. In the ileum no prostanoid exerted a marked effect on the resting tissue, but PGD₂, PGE₂ or PGI₂ 1 μg ml^?1 inhibited submaximal contraction to KC1. U-46619 1 μg ml^?1 either inhibited or increased contractions to KC1, but PGF_2α, 6-keto-PGF_1α or TxB₂ 1 μg ml^?1 had no significant effect. PGE₂ relaxed colonic strips whereas the other prostanoids caused contraction, except for TxB₂ which had no effect. The PG antagonist SC-19220 blocked colonic contractions to the prostanoids, and a residual inhibitory effect of PGD₂, U-46619 or PGI₂ was demonstrated by the reduction of submaximal contractions to acetylcholine. Our results suggest that prostanoid receptors mediating inhibitory responses of circular muscle predominate in the ileum, whereas in the colon both excitatory and inhibitory prostanoid receptors occur.     

Ligand binding specificities of the eight types and subtypes of the mouse prostanoid receptors expressed in Chinese hamster ovary cells

1. Eight types and subtypes of the mouse prostanoid receptor, the prostaglandin D (DP) receptor, the prostaglandin F (FP) receptor, the prostaglandin I (IP) receptor, the thromboxane A (TP) receptor and the EP₁, EP₂, EP₃ and EP₄ subtypes of the prostaglandin E receptor, were stably expressed in Chinese hamster ovary cells. Their ligand binding characteristics were examined with thirty two prostanoids and their analogues by determining the K_i values from the displacement curves of radioligand binding to the respective receptors.
2. The DP, IP and TP receptors showed high ligand binding specificity and only bound their own putative ligands with high affinity such as PGD₂, BW245C and BW868C for DP, cicaprost, iloprost and isocabacyclin for IP, and S-145, I-BOP and GR 32191 for TP.
3. The FP receptor bound PGF_2α and fluprostenol with K_i values of 3–4 nM. In addition, PGD₂, 17-phenyl-PGE₂, STA₂, I-BOP, PGE₂ and M&$\grave{B}$-28767 bound to this receptor with K_i values less than 100 nM.
4. The EP₁ receptor bound 17-phenyl-PGE₂, sulprostone and iloprost in addition to PGE₂ and PGE₁, with K_i values of 14–36 nM. 16,16-dimethyl-PGE₂ and two putative EP₁ antagonists, AH6809 and SC-19220, did not show any significant binding to this receptor. M&B-28767, a putative EP₃ agonist, and misoprostol, a putative EP₂/EP₃ agonist, also bound to this receptor with K_i values of 120 nM.
5. The EP₂ and EP₄ receptors showed similar binding profiles. They bound 16,16-dimethyl PGE₂ and 11-deoxy-PGE₁ in addition to PGE₂ and PGE₁. The two receptors were discriminated by butaprost, AH-13205 and AH-6809 that bound to the EP₂ receptor but not to the EP₄ receptor, and by 1-OH-PGE₁ that bound to the EP₄ but not to the EP₂ receptor.
6. The EP₃ receptor showed the broadest binding profile, and bound sulprostone, M&B-28767, GR63799X, 11-deoxy-PGE₁, 16,16-dimethyl-PGE₂ and 17-phenyl-PGE₂, in addition to PGE₂ and PGE₁, with K_i values of 0.6–3.7 nM. In addition, three IP ligands, iloprost, carbacyclin and isocarbacyclin, and one TP ligand, STA₂, bound to this receptor with K_i values comparable to the K_i values of these compounds for the IP and TP receptors, respectively.
7. 8-Epi-PGF_2α showed only weak binding to the IP, TP, FP, EP₂ and EP₃ receptor at 10 μM concentration.

     

Inhibitory effect of vapiprost on contractile responses of isolated dog renal arteries to thromboxane A2 analogue,U46619

1. Cumulative administrations of U46619, a thromboxane A₂ analogue, and prostaglandin (PG) F_2α produced concentration-dependent contractions of isolated dog renal arterial preparations, which were significantly and concentration-dependently inhibited by vapiprost.2. A bolus administration of U46619 or PGF_2α produced sustained contracture of these preparations, which was concentration-dependently relaxed by cumulative vapiprost.3. Results indicate that vapiprost inhibits U46619- and PGF_2α-induced dog renal arterial contractions through antagonism for so-called TP receptors.     

Latest discoveries in prostaglandin receptor modulators

Prostanoids (prostaglandins and the thromboxanes) are cyclooxygenase products derived from C-20 unsaturated fatty acids. Since arachidonic acid is the most abundant among these precursor fatty acids in mammals, including humans, the series 2 prostanoids are predominantly formed in the body. Thus, cyclooxygenases metabolise arachidonate to five primary prostanoids: PGE₂, PGF_{2 α}, PGI₂, TXA₂ and PGD₂. These lipid mediators interact with specific members of a family of distinct G-protein-coupled prostanoid receptors, designated EP, FP, IP, TP and DP, respectively. As a review focused on latest discoveries and developments of novel TXA₂ modulators patented from January 1997 to December 2000 has been recently published, this paper specifically focuses on newly developed IP, DP, FP and EP modulators patented for three years, from January 1998 to December 2001. Indeed, because prostaglandins are involved in a large series of pathophysiological processes, a classification of these modulators has been proposed, based on the type of receptors activated or inhibited. Their pharmacological profile is also described.     

Action of (R)-sila-venlafaxine and reboxetine to antagonize cisplatin-induced acute and delayed emesis in the ferret

The chemotherapeutic drug cisplatin is associated with severe gastrointestinal toxicity that can last for several days. A recent strategy to treat the nausea and emesis includes the combination of a 5-HT₃ receptor antagonist, a glucocorticoid, and an NK₁ receptor antagonist. The present studies explore the use of the selective noradrenaline reuptake inhibitors, (R)-sila-venlafaxine, (R,R)-reboxetine and (S,S)-reboxetine to prevent cisplatin (5 mg/kg, i.p.)-induced acute (0-24 h) and delayed (24-72 h) emesis in ferrets. The positive control regimen of ondansetron and dexamethasone, both at 1 mg/kg/8 h, reduced acute and delayed emesis by 100 (P < 0.001) and 61% (P < 0.05). (R)-sila-venlafaxine at 5 and 15 mg/kg/4 h reduced acute emesis by 86 (P < 0.01) and 66% (P < 0.05), respectively and both enantiomers of reboxetine at 1 mg/kg/12 h also reduced the response by ∼ 70-90% (P < 0.05). Out of the reuptake inhibitors, only (R)-sila-venlafaxine at 15 mg/kg/4 h was active to reduce delayed emesis (a 57% reduction was observed (P < 0.05)); its terminal plasma levels were positively correlated with an inhibition of emesis during the delayed phase (P < 0.05). (R)-sila-venlafaxine was also examined against a higher dose of cisplatin 10 mg/kg, i.p. (3 h test) and it dose-dependently antagonized the response (maximum reduction was 94% at 10 mg/kg, p.o.; P < 0.01) but it was ineffective against apomorphine (0.125 mg/kg, s.c.) and ipecacuanha (2 mg/kg, p.o.)-induced emesis (P > 0.05). In conclusion, the studies provide the first evidence for an anti-emetic potential of noradrenaline reuptake inhibitors to reduce chemotherapy-induced acute and delayed emesis.     

Anti-emetic Effect of Mosapride Citrate Hydrate,a 5-HT4 Receptor Agonist,on Selective Serotonin Reuptake Inhibitors (SSRIs)-Induced Emesis in Experimental Animals

Although selective serotonin reuptake inhibitors (SSRIs) are widely used to treat depression, they frequently cause gastrointestinal adverse effects, such as nausea and emesis. In the present study, we investigated the anti-emetic effect of mosapride, a 5-HT₄ receptor agonist, on SSRIs-induced emesis in Suncus murinus and dogs. We also examined the effect of mosapride on SSRIs-induced delay in gastric emptying and increase in gastric vagal afferent activity in rats. Oral administration of paroxetine, but not its subcutaneous administration, dose-dependently caused emesis in both animals. Mosapride inhibited paroxetine-induced emesis in Suncus murinus and dogs with ID₅₀ values of 7.9 and 1.1 mg/kg, respectively. The anti-emetic effect of mosapride was partially inhibited by SB207266, a selective 5-HT₄ antagonist. Intragastric administration of paroxetine increased gastric vagal afferent discharge in anesthetized rats. Mosapride failed to suppress this increase. On the other hands, mosapride improved the delay in gastric emptying caused by paroxetine in rats. We have shown in this study that oral administration of SSRIs causes emesis and activates gastric vagal afferent activity in experimental animals and that mosapride inhibits SSRIs-induced emesis, probably via improvement of SSRIs-induced delay in gastric emptying. These findings highlight the promising potential of mosapride as an anti-emetic agent.     

EFFECTS OF DIETARY FIBRE ON DIMETHYLHYDRAZINE-INDUCED CHANGES IN PROSTANOID CONCENTRATIONS IN RAT COLONIC MUCOSA

1. This study was designed to elucidate the effects of guar gum, a dietary fibre, on changes in prostanoid contents induced by 1,2-dimethylhydrazine, a carcinogenic agent, in rat colonic mucosa. 2. Prostanoid contents were determined using high performance liquid chromatography; five prostanoids, namely 6-keto-prostaglandin F_1α, prostaglandin F_2α, prostaglandin E₂, prostaglandin D₂ and thromboxane B₂, were detected. 3. Four subcutaneous injections of dimethylhydrazine, 60 mg/kg every 6 days, increased the mucosal concentrations of prostaglandin E₂ and thromboxane B₂ by approximately 50%. Other prostanoids did not change significantly throughout the experiments. 4. In rats treated with dimethylhydrazine and a fibre diet a significant increase in thromboxane B₂ content was not observed, although a significant increase in prostaglandin E₂ content was observed. These effects were observed in rats fed with fibre diet over 20 days but not observed in rats fed with fibre diet over 10 days. 5. From these results and the report that aspirin use at low doses is effective in the reduction of the risk of fatal colonic cancer, inhibiting thromboxane B₂ synthesis by fibre diet might he involved in the protective effect against the occurrence of colonic cancer.     

The CCKA receptor antagonist devazepide inhibits the effect of apomorphine on vasopressin release in pigs

• 1.1. A transient increase in plasma vasopressin concentrations represents a physiological correlate of nausea in animals that vomit.
• 2.2. The CCK_A receptor antagonist devazepide has previously been shown to inhibit vasopressin release induced in pigs by intravenous (i.v.) CCK.
• 3.3. This study investigated whether devazepide (70 μg/kg i.v.) would affect vasopressin secretion induced in pigs (n = 6) by the emetic drug apomorphine (25 μg/kg i.v.).
• 4.4. Apomorphine stimulated vasopressin release in the 30 min period following injection; this effect was prevented by prior administration of devazepide.
• 5.5. The results suggest that CCK_A receptor antagonists may have the ability to prevent nausea and/or emesis.

     

Mechanical Effects of Some Prostanoids on Isolated Human Oesophageal Submucosal Veins

Abstract: The effects of prostaglandins E₁, E₂, F_2α, prostacyclin, and the thromboxane A₂-mimic U46619 were investigated on isolated human oesophageal submucosal veins from the oesophageal body and the oesophagogastric junction. U46619 most potently, but also PGF_2α produced venocontraction without differences between preparations from the oesophageal body and the oesophagogastric junction. PGE₁ and prostacyclin caused relaxation of vessels precontracted with U46619 (10^-9 M). PGE₂ induced either contraction or relaxation, but biphasic effects in the same vessel segment were not seen. Indomethacin 10^-6 M inhibited the contractile responses to both noradrenaline and K⁺ (124 mM), suggesting that the agonists induced synthesis or release of vasoconstrictor prostanoids. Prostanoids exert potent mechanical effects in submucosal oesophageal veins and may be of physiological importance.     

PARTICIPATION OF PROSTANOIDS IN CHEMICAL ACTIVATION OF THE PERICARDIAL PRESSOR REFLEX IN DOGS

1. Experiments were performed on anaesthetized, open-chest dogs to determine the reflex effects on systemic blood pressure and heart rate produced by stimulation of the parietal pericardium with bradykinin, prostacyclin, prostaglandin E₂ (PGE₂), prostaglandin D₂ (PGD₂) and arachidonic acid. 2. Pericardial application of bradykinin (1 μg) consistently elicited reflex increases in blood pressure and heart rate, whereas application of prostanoids or arachidonic acid in doses up to 10 μg failed to produce any cardiovascular responses. 3. Indomethacin, applied either directly to the parietal pericardium (1 μg/ml) or given intravenously (5 mg/kg) caused a long lasting reduction of the reflex responses to bradykinin. The reflex effects of bradykinin could be temporarily restored by treatment of the pericardium with either prostacyclin (0.1 μg/min) or PGE₂ (0.1 μg/min). PGD₂ (0.1-1 μg/min) did not influence the bradykinin induced pericardial reflex. 4. Superfusion of arachidonic acid (3 μg/min) over the pericardium amplified the reflex effects of bradykinin when given before, but not when given after indomethacin treatment. 5. The results indicate that locally formed prostanoids, specifically prostacyclin and PGE₂, can facilitate activation of the pericardial pressor reflex by bradykinin. The findings may be relevant to the changes in cardiovascular activity occurring during pericardial inflammation.     

ZD9583, an Orally Effective Thromboxane A2 Synthase Inhibitor and Receptor Antagonist with a Sustained Duration of Action in Rat and Dog

The thromboxane A₂ (TXA₂) synthase inhibitory activity and the TXA₂ receptor (TP-receptor) blocking action of ZD9583 ((4Z)-6-[(2S,4S,5R)-2-(1-[2-cyano-4-methylphenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hex-4-enoic acid) has been evaluated in-vitro by use of whole blood and platelets from man, and ex-vivo by use of platelets and whole blood from rats and dogs. ZD9583 caused concentration–dependent inhibition of human platelet microsomal TXA₂ production with an IC50 of 0.017 ± 0.003 μm; this inhibition was associated with an increase in prostaglandin E₂ (PGE₂) and prostaglandin F_2α (PGF_2α) formation. ZD9583 also inhibited collagen-stimulated TXA₂ synthesis in whole blood from man, rat and dog giving IC50 values of 0.027 ± 0.005, 0002 ± 0.006 and 0.013 ± 0.01 μm, respectively. The drug did not modify platelet cyclooxygenase activity as inhibition of thromboxane B₂ (TXB₂) formation was associated with a concomitant increased synthesis of prostaglandin D₂ (PGD₂), PGE₂ and PGF_2α. ZD9583 had little effect on cultured human umbilical vein endothelial cell prostacyclin synthase giving an IC50 of 24.2 ± 4.9 μm. In-vitro ZD9583 caused concentration-dependent inhibition of U46619-induced aggregation responses of platelets from man, rat and dog, yielding apparent log A₂ values of 8.7 ± 0.12, 8.8 ± 0.2 and 9.3 ± 0.2, respectively. The drug was selective; at concentrations up to 100 μm it did not affect 5-hydroxytryptamine or the primary phases of adenosine diphosphate and adrenaline-induced aggregation. ZD9583 (100 μm) did not, furthermore, modify the platelet inhibitory effects of PGD₂, prostaglandin E₁ (PGE₁) and prostacyclin. Oral administration of ZD9583 (3–10 mg kg^?1) to both rats and dogs caused dose-dependant inhibition of collagen-stimulated TXA₂ production ex-vivo which persisted for up to 12 h. The drug also caused profound TXA₂ receptor blockade in both species for in excess of 12-h after an oral dose of 3 mg kg^?1. ZD9583 (3 mg kg^?1, p.o.), when administered to dogs over a five-day period at 12 h intervals, did not cause either tachyphylaxis or an accumulation of effect. We conclude that ZD9583 is a potent, selective, orally active thromboxane synthase inhibitor and TXA₂ receptor antagonist.     

The action of prostanoid receptor agonists and antagonists on smooth muscle and platelets.

1. Prostanoid receptors have been characterized in a range of guinea-pig and rat smooth muscle and platelets, according to the scheme of Coleman et al., (1985a), using agonists (prostaglandin D2 (PGD2), PGE1, PGE2, 16,16 dimethyl PGE2, PGF2 alpha, PGI2 and U46619) and putative selective antagonists (AH 6809 and SQ 29,548). 2. The ileum possesses EP1- and IP-receptors; the trachea, EP1-EP2- and TP-receptors; the oesophageal muscularis mucosa, EP1- and TP-receptors; the aorta and the portal vein TP-receptors. The rat colon possesses IP-, FP- and TP-receptors. 3. Guinea-pig platelets possess both IP and DP receptors mediating an inhibition of aggregation and TP receptors mediating proaggregation responses. No evidence was found for the presence of EP1-, EP2- or FP-receptors. 4. Misoprostol and fenprostalene were characterized using the above preparations. Misoprostol acts as a selective EP1-agonist, and has little or no DP, FP, IP and TP activity. In the trachea precontracted with carbachol no evidence of EP2-receptor stimulation was observed. Fenprostalene possesses FP and TP activity but no EP1, EP2, DP or IP activity.