Characterization of imidazo[1,5-a]pyridine-5-hexanoic acid (CGS 13080) as a selective thromboxane synthetase inhibitor using in vitro and in vivo biochemical models

Classification models and SAR analysis on thromboxane A2 synthase inhibitors by machine learning methods

Thromboxane A2 synthase (TXS) is a promising drug target for cardiovascular diseases and cancer. In this work, we conducted a structure-activity relationship (SAR) study on 526 TXS inhibitors for bioactivity prediction. Three types of descriptors (MACCS fingerprints, ECFP4 fingerprints, and MOE descriptors) were utilized to characterize inhibitors, 24 classification models were developed by support vector machine (SVM), random forest (RF), extreme gradient boosting (XGBoost), and deep neural networks (DNN). Then we reduced the number of fingerprints according to the contribution of descriptors to the models, and constructed 16 extra models on simplified fingerprints. In general, Model_4D built by DNN algorithm and 67 bits MACCS fingerprints performs best. The prediction accuracy of the model on the test set is 0.969, and Matthews correlation coefficient (MCC) is 0.936. The distance between compound and model (dSTD-PRO) was used to characterize the application domain of the model. In the test set of Model_4D, dSTD-PRO of 91.5% compounds is lower than the corresponding training set threshold (threshold0.90 = 0.1055), and the accuracy of these compounds is 0.983. In addition, the important descriptors were summarized and further analyzed. It showed that aromatic nitrogenous heterocyclic groups were beneficial to improve the bioactivity of TXS inhibitors.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

Eicosanoid production by a differentiated canine colonic epithelial cell line, VNCC

BACKGROUND/AIMS

The lack of pure, proliferative, but not transformed intestinal epithelial cells has impeded progress in understanding their role in chronic intestinal inflammation. To clarify that role, the present study characterized the epithelial cell line VNCC, derived from normal adult dog distal colon.

METHODS

Cells were cultured on plastic and permeable supports for analysis of eicosanoid production (by radioimmunoassay and high-performance liquid chromatography) and transport characteristics (by Ussing chamber short-circuit determinations).

RESULTS

In culture, VNCC formed confluent monolayers and domes, suggesting formation of tight junctions and active solute absorption. When cultured on permeable supports, VNCC developed modest, but variable, transepithelial resistances (563 +/- 94 omega/cm2) with a spontaneous short-circuit current of 5.0 +/- 0.4 microA/cm2. Forskolin caused a prolonged increase in the short-circuit current, inhibited by amiloride but not bumetanide, suggesting that VNCC display 5'-cyclic adenosine monophosphate-stimulated sodium absorption. VNCC incubated with arachidonic acid released a variety of eicosanoids including 6-keto-prostaglandin (PG)F1 alpha, PGE2, thromboxane B2, and PGF2 alpha, but no hydroxyarachidonate metabolites. Bradykinin stimulated VNCC eicosanoid release.

CONCLUSIONS

The ability of VNCC to divide and differentiate in culture, to form polarized monolayers capable of active sodium absorption, and to respond to inflammatory mediators with eicosanoid release makes them a unique tool for the study of the interactions of inflammation on colonocyte function.

Endothelial dysfunction augments myogenic arteriolar constriction in hypertension

To elucidate the underlying reason or reasons for the increased peripheral resistance in hypertension, we investigated the pressure-diameter relation--the myogenic response--of isolated, cannulated arterioles (approximately 50 microns) of cremaster muscle of 12-week-old Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHR), and normal Wistar (NW) rats. All arterioles constricted in response to step increases in perfusion pressure from 20 to 160 mm Hg. This constriction was, however, significantly enhanced from 60 to 160 mm Hg in arterioles of SHR compared with NW or WKY rats. For example, at 80 and 140 mm Hg, respectively, the normalized diameter (expressed as a percentage of the corresponding passive diameter of arterioles of SHR) was 11.8% and 27.6% (P < .05) less compared with those of WKY rats. Endothelium removal eliminated the enhanced pressure-induced tone in SHR. Similarly, indomethacin (10(-5) mol/L, sufficient to block prostaglandin synthesis) or SQ 29,548 (10(-6) mol/L), a thromboxane A2-prostaglandin H2 receptor blocker that inhibited vasoconstriction to the thromboxane agonist U46619, attenuated the enhanced pressure-diameter curve and reversed the blunted dilation to arachidonic acid in SHR. In contrast, the thromboxane A2 synthesis inhibitor CGS 13,080 (5 x 10(-6) mol/L) did not affect the increased pressure-induced tone or the reduced dilation to arachidonic acid in SHR. Thus, the present findings suggest that in early hypertension pressure-induced arteriolar constriction is increased. This seems to be due to an enhanced production of endothelium-derived constrictor factors, primarily prostaglandin H2.

Evidence for a bidirectional prostaglandin endoperoxide shunt between platelets and the bovine coronary artery

While platelets have been shown to be capable of supplying prostaglandin (PG) H2 to endothelial cells in culture for PGI2 synthesis, endothelial cells have been shown unable to supply PGH2 to platelets for thromboxane (TX) A2 synthesis. We incubated rings of the bovine coronary artery (BCAR) with human platelets treated with aspirin (to inhibit cyclooxygenase) or CGS 13080 (to inhibit TXA2 synthase) in the presence of 20 microM arachidonic acid. BCAR, with damaged endothelium, produced significantly less PGI2 than that with intact endothelium. However, co-incubation with CGS 13080-treated platelets resulted in an increase in PGI2 independent of endothelium, demonstrating a shunt of PGH2 from platelets to BCAR. Co-incubation of BCAR with aspirin-treated platelets resulted in a net increase in TXA2 demonstrating a shunt of PGH2 from BCAR to platelets. Employing [14C]PGH2 as substrate, BCAR with and without intact endothelium produced similar amounts of 6-keto-[14C]PGF1 alpha. Likewise, homogenates (50 micrograms protein) of intimal and subintimal regions of BCAR and BCAR converted similar amounts of PGH2 to 6-keto-PGF1 alpha. These data suggest that vascular production of PGH2 is more dependent on an intact endothelium than is the conversion of PGH2 to PGI2. These data also suggest a potential for a bidirectional exchange of PGH2 between platelets and vascular wall during platelet-vascular wall interactions.

The role of thromboxane A2 (TxA2) in allergic cutaneous reactions and the effect of (E)-3-[p-(1H-imidazol-1-ylmethyl) phenyl]-2-propenoic acid hydrochloride (OKY-046), a TxA2 synthetase inhibitor

To study the role of thromboxane A2 (TxA2) in cutaneous allergic reactions, the effect of (E)-3-[p-(1H-Imidazol-1-ylmethyl)phenyl]-2-propenoic acid hydrochloride (OKY-046), a selective TxA2 synthetase inhibitor, on cutaneous reactions in rats and mice was studied. Simultaneously, the effect of 9,11-methanoepoxy-prostaglandin H2 (U-46619), a stable analogue of TxA2, on capillary permeability in mouse and rat skin was investigated. Passive cutaneous anaphylaxis (PCA) in mouse ear was clearly inhibited by OKY-046 but not by indomethacin. The inhibitory action of OKY-046 was not influenced by pretreatment with indomethacin. Moreover, prostaglandin I2, which accumulated as a result of the inhibition of TxA2 synthetase, did not affect the PCA. But, the dye leakages caused by histamine, serotonin and leukotriene C4 in mouse ear were clearly inhibited by OKY-046. In addition, OKY-046 inhibited rat reversed cutaneous anaphylaxis, but its inhibitory action was not affected by pretreatment with indomethacin. Contrary to the above results, rat footpad passive Arthus reaction and mouse footpad tuberculin delayed hypersensitivity reaction were not affected by OKY-046. Additionally, U-46619 did not cause an increase of capillary permeability in either mouse and rat skin. These results suggest a slight role of TxA2 in cutaneous allergic reactions in mice and rats and the efficacy of OKY-046 on Type I and II reactions regardless of the inhibition of TxA2 synthetase activity.

Effect of thromboxane synthase inhibition on the thrombolytic action of tissue-type plasminogen activator in a rabbit model of peripheral arterial thrombosis

The thrombolytic efficacy of recombinant tissue-type plasminogen activator (tPA) in the presence and absence of a thromboxane synthase inhibitor was studied in a model of femoral artery thrombosis in the anesthetized rabbit. The thrombus was formed by injection of thrombin and whole blood into an isolated segment of the femoral artery. After 30 min of stable thrombotic occlusion of the femoral artery, sodium heparin (300 U/kg, i.v.) was administered and tPA was infused locally to the site of the thrombus for 30 min at 0.01, 0.10 or 1.0 microgram/kg/min. In other experiments, CGS 13080, a selective thromboxane synthase inhibitor, was administered at a dose of 2 mg/kg i.v., 5 min before tPA was infused and at the end of the 30 min tPA infusion. Pretreatment with CGS 13080 resulted in a shorter time to tPA-induced reperfusion, greater incidence of reperfusion and increased the magnitude of femoral artery blood flow achieved after effective thrombolysis. Furthermore, pretreatment with CGS 13080 resulted in a greater than 10-fold enhancement in the effective dose of tPA. These data indicate that thromboxane synthase inhibition may be beneficial as an adjunct to thrombolytic therapy with tPA.

Output and effects of thromboxane produced by the liver perfused with phorbol myristate acetate

The capacity of the perfused rat liver to produce thromboxane after stimulation by phorbol myristate acetate was examined. A total of 109 +/- 20 and 155 +/- 28 pmol/g liver were found in the perfusate and in the bile, respectively, after 40 min. The amount of thromboxane recovered in the perfusate and in the bile accounted for 12.6% of the production calculated from the same number of Kupffer cells in primary cultures, indicating that a major part of thromboxane was taken up and inactivated by hepatocytes. The effect of endogenously synthesized thromboxane on the liver was assessed by using CGS 13080, a thromboxane synthase inhibitor, or BM 13.177, a thromboxane receptor antagonist. 20 nM CGS 13080 in the perfusate inhibited the synthesis of thromboxane and at the same time the elevation of portal pressure and glycogenolysis following administration of phorbol 12-myristate 13-acetate (PMA). The thromboxane receptor antagonist BM 13.177 did not inhibit the synthesis of thromboxane, but reduced the PMA-related elevation of portal pressure and glycogenolysis to the same extent (greater than 60%) as CGS 13080. Sodium nitroprusside, a vasodilator, inhibited the rise in portal pressure caused by PMA to the same extent as CGS 13080 or BM 13.177 but reduced the increase in glycogenolysis only by 25%. These results indicate that thromboxane released by stimulated Kupffer cells of the liver elevates portal pressure and glycogenolysis in the perfused rat liver, although by different mechanisms.

Characterization of CGS 8515 as a selective 5-lipoxygenase inhibitor using in vitro and in vivo models

CGS 8515 inhibited 5-hydroxyeicosatetraenoic acid (5-HETE) and leukotriene B4 synthesis in guinea pig leukocytes (IC50 = 0.1 microM). The compound did not appreciably affect cyclooxygenase (sheep seminal vesicles), 12-lipoxygenase (human platelets), 15-lipoxygenase (human leukocytes) and thromboxane synthetase (human platelets) at concentrations up to 100 microM. CGS 8515 inhibited A23187-induced formation of leukotriene products in whole blood (IC50 values of 0.8 and 4 microM, respectively, for human and rat) and in isolated rat lung (IC50 less than 1 microM) in vitro. The selectivity of the compound as a 5-lipoxygenase inhibitor was confirmed in rat whole blood by the 20-70-fold separation of inhibitory effects on the formation of leukotriene from prostaglandin products. Ex vivo and in vivo studies with rats showed that CGS 8515, at an oral dose of 2-50 mg/kg, significantly inhibited A23187-induced production of leukotrienes in whole blood and in the lung. The effect persisted for at least 6 h in the ex vivo whole blood model. CGS 8515, at oral doses as low as 5 mg/kg, significantly suppressed exudate volume and leukocyte migration in the carrageenan-induced pleurisy and sponge models in the rat. Inhibitory effects of the compound on inflammatory responses and leukotriene production in leukocytes and target organs are important parameters suggestive of its therapeutic potential in asthma, psoriasis and inflammatory conditions.

An alternate mechanism for regulation of leukotriene production in leukocytes: studies with an anti-inflammatory drug, sodium diclofenac

Sodium diclofenac, a potent cyclooxygenase inhibitor, was recently shown to inhibit arachidonic acid conversion to leukotriene products in human leukocytes. This activity was confirmed by radioimmunoassay in calcium ionophore A 23187-stimulated leukocytes isolated from the rat peritoneal cavity and human peripheral blood. Studies with rat peritoneal leukocytes revealed that this effect was not mediated by inhibition of 5-lipoxygenase or phospholipase A2, but rather through modulation of arachidonic acid uptake and release. The potency of this effect was dependent upon cell type; macrophages being more sensitive to the drug than neutrophils. In leukocytes treated with sodium diclofenac, arachidonic acid released from phospholipids in response to A 23187 challenge was reincorporated into triacylglycerols. The drug enhanced the spontaneous uptake of arachidonic acid into the cellular triacylglycerol pool and, in this manner, decreased the availability of intracellular arachidonic acid. Therefore, sodium diclofenac, in addition to inhibition of cyclooxygenase, regulates leukotriene production of inflammatory cells by a mechanism mediated in part through the redistribution of arachidonic acid in lipid pools.

Stimulation of prostacyclin synthesis in rats treated with phenobarbital

The effect of phenobarbital treatment on the synthesis of prostacyclin in coronary vascular microsomes was studied in Sprague Dawley rats. It was found that the treatment increased the synthesis of prostacyclin by nearly 100%. The treatment also resulted in an increase in HDL and a decrease in LDL in the serum. In vitro effects of HDL and LDL on the microsomal synthesis of prostacyclin showed that the synthesis was stimulated by HDL and inhibited by LDL. Hence it appears that the increase in prostacyclin synthesis resulting from phenobarbital treatment was at least partly due to increased level of HDL and decreased level of LDL in the serum.

Thromboxane synthetase inhibition with CGS 13080 improves coronary blood flow after streptokinase-induced thrombolysis

The focus of this investigation was to examine the potential beneficial effects of the selective thromboxane synthetase inhibitor CGS 13080 (imidazo [1,5-a] pyridine-5-hexanoic acid) on coronary blood flow after streptokinase-induced thrombolysis. Thrombotic occlusion of the circumflex coronary artery was produced by electrolytic (100 microA anodal current) injury to the intimal surface of the circumflex coronary artery at the site of a noncircumferential stenosis in dogs anesthetized with pentobarbital to have open-chest surgery. Intracoronary streptokinase, 6000 IU/kg in 3 ml saline solution, was infused at 0.05 ml/min for 1 hour, beginning 30 minutes after the formation of an occlusive thrombus. The animals were assigned randomly to two groups. In group I (n = 10) the intravenous infusion of vehicle was begun simultaneously with the intracoronary administration of streptokinase and continued for 2 hours after thrombolysis had been achieved. The animals in group II (n = 10) received intravenous CGS 13080 (1 mg/kg/hr) along with intracoronary streptokinase. Infarct size was assessed by a dual perfusion technique with Evans blue and triphenyltetrazolium stains to demarcate the normally perfused myocardium from the area at risk and the infarct zone within the risk region. The two groups did not differ with respect to baseline coronary blood flow, time to the development of coronary artery thrombotic occlusion, or time to achieve thrombolysis. Oscillations in coronary blood flow were more frequent in group I than in group II (group I, 9 +/- 2.2; group II, 4.4 +/- 0.8 oscillation/min X 100, p less than 0.05 [mean +/- SEM)].(ABSTRACT TRUNCATED AT 250 WORDS)

Macrophage eicosanoid formation is stimulated by platelet arachidonic acid and prostaglandin endoperoxide transfer

The present study was designed to determine whether platelets transfer arachidonic acid or prostaglandin endoperoxide intermediates to macrophages which may be further metabolized into cyclooxygenase products. Adherent peritoneal macrophages were prepared from rats fed either a control diet or an essential fatty acid-deficient diet, and incubated with a suspension of washed rat platelets. Macrophage cyclooxygenase metabolism was inhibited by aspirin. In the presence of a thromboxane synthetase inhibitor, 7-(1-imidazolyl)heptanoic acid, immunoreactive 6-ketoprostaglandin F1 alpha formation was significantly increased 3-fold. Since this increase was greater (P less than 0.01) than that seen with either 7-(1-imidazolyl)heptanoic acid-treated platelets or aspirin-treated macrophages alone, these results indicate that shunting of endoperoxide from platelets to macrophages may have occurred. In further experiments, macrophages from essential fatty acid-deficient rats were substituted for normal macrophages. Essential fatty acid-deficient macrophages, depleted of arachidonic acid, produced only 2% of the amount of eicosanoids compared to macrophages from control rats. When platelets were exposed to aspirin, stimulated with thrombin, and added to essential fatty acid-deficient macrophages, significantly more immunoreactive 6-ketoprostaglandin F1 alpha was formed than in the absence of platelets. This increased macrophage immunoreactive 6-ketoprostaglandin F1 alpha synthesis, therefore, must have occurred from platelet-derived arachidonic acid. These data indicate that in vitro, in the presence of an inhibition of thromboxane synthetase, prostaglandin endoperoxides, as well as arachidonic acid, may be transferred between these two cell types.

CGS 15435A, a thromboxane synthetase inhibitor with an extended duration of action: a comparison with dazoxiben

CGS 15435A, a novel thromboxane (Tx) synthetase inhibitor (5-chloro-1-methyl-2-(3-pyridyl)-3-indolhexanoic acid HCl), had a selectivity for Tx synthetase 100,000-fold greater than that for cyclooxygenase, PGI2 synthetase and lipoxygenase enzymes. In conscious beagles, 1 h following a single 3 mg/kg p.o. dose, serum TxB2 was inhibited 95% by CGS 15435 and 82% by dazoxiben (DAZ). Unlike the short acting Tx synthetase inhibitor DAZ, CGS 15435A significantly inhibited TxB2 formation 4, 6, 12 and 24 h after dosing. Serum levels of 6-keto PGF1 alpha and PGE2 were significantly increased following the administration of either drug. CGS 15435A and DAZ were further examined in a model with known Tx involvement. Thrombotic sudden death, produced in anesthetized rabbits by injection of 0.75 mg/kg arachidonic acid (AA) i.v. resulted in a 45% fall in the platelet count and 0% survival. Pretreatment with DAZ (8.6 mumol/kg i.v.) at 0.25 or 2 h pre-AA resulted in 3 and 42% thrombocytopenia and 100 and 0% survival respectively. CGS 15435A (8.6 mumol/kg i.v.) prevented the increases in plasma TxB2 levels, thrombocytopenia and sudden death with pretreatment at 0.25 h (0% thrombocytopenia and 100% survival) or 24 h (11% thrombocytopenia and 83% survival) before AA. These data indicate that CGS 15435A is a potent and selective Tx synthetase inhibitor with a long duration of action, and suggest that the compound could be useful in chronic, non-symptomatic indications of Tx involvement.

Pharmacological studies on the TXA2 synthetase inhibitor (E)-3-[p-(1H-imidazol-1-ylmethyl)phenyl]-2-propenoic acid (OKY-046)

The effects of (E)-3-[p-(1H-imidazol-1-ylmethyl)phenyl]-2-propenoic acid (OKY-046) on thromboxane A2 (TXA2) synthetase in vitro and on experimental animal models of sudden death and cerebral infarction were studied. IC50 values of OKY-046 for the TXA2 synthetase of human, rabbit, dog and guinea pig washed platelets were 0.004, 0.004, 0.26 and 2.4 microM, respectively. OKY-046 at concentrations up to 1 mM, however, did not inhibit prostacyclin (PGI2) synthetase from bovine aorta microsomes or cyclooxygenase and PGE2 isomerase from sheep seminal vesicle microsomes. Similarly, platelet 12-lipoxygenase was not affected by OKY-046. Evidence for a re-direction of arachidonate metabolism from thromboxane synthesis toward PGI2 synthesis was obtained using rat peritoneal cells. Namely, OKY-046 increased PGI2 production accompanied by an inhibition of TXA2 production at a concentration of more than 1 microM. OKY-046 at a dose of 0.1 mg/kg (i.v.) in dogs inhibited the aortic and mesenteric arterial contraction of rabbit induced by the addition of arachidonate to extracorporated blood of the dogs. OKY-046 at a dose of 0.3 mg/kg (i.v.) prevented the arachidonate-induced sudden death and also decreased the incidence of cerebral infarction induced by injection of arachidonate into the internal carotid artery in rabbits. Aspirin also decreased the incidence of cerebral infarction at a dose of 30 mg/kg (i.v.). These results suggest that OKY-046 may be valuable for the treatment of cerebrovascular and cardiovascular diseases associated with vasoconstriction and thrombosis due to TXA2.

Spectral studies on structure-activity relationships of thromboxane synthase inhibitors

Thromboxane A2 synthase is a cytochrome P450-type enzyme and its interaction with imidazole or pyridine-based inhibitors could be studied by absolute and difference spectroscopy with the solubilized as well as the purified enzyme. Nitrogenous bases shift the 418-nm Soret absorption by 4-6 nm to the red and among them the best inhibitors of enzyme activity showed a stoichiometric binding to the enzyme. The structural and energetic prerequisites for such high binding affinities were primarily the liganding of the basic nitrogen to the hemin but also the attachment of a hydrophobic carboxylic side chain to the active site at an about 1 nm distance from the nitrogen. In addition, the side chain seemed to be oriented almost parallel to the plane of the heme. If this geometry was changed, a decrease in affinity was observed and if the ligand binding was sterically hindered, a spectral shift to a five-coordinated complex absorbing at 390 nm occurred. This is best explained by the displacement of an endogenous oxygen ligand, presumably water, from the sixth coordination position of the heme. From these results it can be concluded that the inhibitors mimic the binding of prostaglandin H2 (PGH2) with its carboxylic group at the carboxyl side chain and the endoperoxide oxygen atom at C9 as previously reported. The methyl side chain of PGH2 does not seem to play a role in the formation of the enzyme-substrate complex.

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. 8 CGS 12970 had no effect on the thrombocytopenia associated with the Arthus reaction which distinguishes it from cyclo-oxygenase inhibitors. It also had no effect on thrombus formation on a cotton thread in an arteriovenous shunt in the rat.

Ionophore-stimulated lipoxygenase activity and histamine release in a cloned murine mast cell, MC9

A cloned murine mast cell line designated MC9 expresses a 5-lipoxygenase activity when stimulated with the ionophore A23187. Upon addition of 0.5 microM ionophore, MC9 cells produce 270 +/- 43 pmoles 5-HETE, 74 +/- 40 pmoles 5,12 diHETEs and 65 +/- 31 pmoles LTC4/10(6) cells from 37 microM exogenously added [1-14C]arachidonic acid in two minutes. 5-HETE and 5,12-diHETES, including LTB4 were identified by GC/MS whereas LTC4 was confirmed by HPLC mobility, bio-assay, RIA and enzymatic transformation. The principal cyclooxygenase products were PGD2 and TxB2 (8.5 +/- 2.4 and 5.4 +/- 1.2 pmoles/10(6) cells respectively). Prostanoids were identified by comigration with authentic standards on two-dimensional thin layer chromatograms. Production of arachidonic acid lipoxygenase metabolites stimulated with ionophore proved relatively insensitive to removal of extracellular Ca+2 and chelation by EGTA. In addition, MC9 5-lipoxygenase required only low micromolar amounts of exogenous arachidonic acid for maximal activity. Whereas production of arachidonic acid metabolites lasted only two to five minutes, histamine release stimulated with ionophore was not initiated until 5 minutes (12 +/- 3% cellular histamine) and continued for 30 minutes (37 +/- 7% cellular histamine). Although these cells metabolize arachidonic acid differently from the classic peritoneal-derived mast cell, they resemble subpopulations found in certain tissues (such as mucosa) and should be useful in understanding the biochemistry of mast cell mediator release.

The effects of a new thromboxane synthetase inhibitor, CGS-13080, in man

CGS-13080 is a new potent selective inhibitor of thromboxane synthetase. This study reports the results of a safety and efficacy study of single oral doses in normal healthy volunteers. The compound was well tolerated by all subjects without evidence of any adverse reactions. Serum thromboxane B2 levels (the stable metabolic product of thromboxane A2) were significantly reduced after administration of the compound, with the maximal effect of a 99 per cent reduction occurring at 0.5 and 1 hour after administration. There was a concomitant increase in PGE2 and 6-keto-PGF1 alpha (the stable metabolic product of PGI2), suggesting a shunting of cyclic endoperoxide metabolism. The apparent half-life of the compound is about 1 hour, but return to 50 per cent of the original thromboxane B2 levels was achieved between 4 and 6 hours. Platelet aggregation to collagen and bleeding times failed to demonstrate significant changes after drug administration. Bleeding times showed a slight increase 2 hours after administration of the compound.