Role of thromboxane A2 synthetase inhibitors in the treatment of patients with bronchial asthma

Druggable Prostanoid Pathway

Prostanoids (prostaglandins, prostacyclin and thromboxane) belong to the oxylipin family of biologically active lipids generated from arachidonic acid (AA). Protanoids control numerous physiological and pathological processes. Cyclooxygenase (COX) is a rate-limiting enzyme involved in the conversion of AA into prostanoids. There are two COX isozymes: the constitutive COX-1 and the inducible COX-2. COX-1 and COX-2 have similar structures, catalytic activities, and subcellular localizations but differ in patterns of expression and biological functions. Non-selective COX-1/2 or traditional, non-steroidal anti-inflammatory drugs (tNSAIDs) target both COX isoforms and are widely used to relieve pain, fever and inflammation. However, the use of NSAIDs is associated with various side effects, particularly in the gastrointestinal tract. NSAIDs selective for COX-2 inhibition (coxibs) were purposefully designed to spare gastrointestinal toxicity, but predisposed patients to increased cardiovascular risks. These health complications from NSAIDs prompted interest in the downstream effectors of the COX enzymes as novel drug targets. This chapter describes various safety issues with tNSAIDs and coxibs, and discusses the current development of novel classes of drugs targeting the prostanoid pathway, including nitrogen oxide- and hydrogen sulfide-releasing NSAIDs, inhibitors of prostanoid synthases, dual inhibitors, and prostanoid receptor agonists and antagonists.

Leukotriene Receptor Antagonists and Antiallergy Drugs

As one of the candidates of the therapeutic strategy for asthma in addition to inhaled corticosteroids (ICS), leukotriene receptor antagonists (LTRAs) are known to be useful for long-term management of asthma patients complicated by allergic rhinitis (AR) or exercise-induced asthma (EIA). Currently available LTRAs are pranlukast hydrate, zafirlukast, and montelukast. These LTRAs have a bronchodilator action and inhibit airway inflammation, resulting in a significant improvement of asthma symptoms, respiratory function, inhalation frequency of as-needed inhaled β2-agonist, airway inflammation, airway hyperresponsiveness, dosage of ICSs, asthma exacerbations, and patients' QOL. Although cys-LTs are deeply associated with the pathogenesis of asthma, LTRAs alone are less effective compared with ICS. However, the effects of LTRAs in combination with ICS are the same as those of LABAs in combination with ICS in steroid-naïve asthmatic patients. Concerning antiallergy drugs other than LTRAs, some mediator-release suppressants, H1 histamine receptor antagonists (H1RAs), thromboxane A2 (TXA2) inhibitors/antagonists, and Th2 cytokine inhibitor had been used mainly in Japan until the late 1990s. However, the use of these agents rapidly decreased after ICS/long acting beta agonist (LABA) combination was introduced and recommended for the management of asthma in the early 2000s. The effectiveness of other antiallergic agents on asthma management seems to be quite limited, and the safety of oral antiallergic agents has not been demonstrated in fetuses during pregnancy. Further effectiveness studies are needed to determine the true value of these orally administered agents in combination with ICS as an anti-asthma treatment.

Revisiting the usefulness of thromboxane-A2 modulation in the treatment of bronchoconstriction in asthma

Airway smooth muscle (ASM) is the effector cell in the bronchoconstrictory pathway. It is believed that the bronchoconstriction present in asthma is associated with changes in the airway milieu that affect ASM excitation-contraction coupling and Ca(2+)-handling. Asthmatics also react differently to ventilatory mechanical strain. Deep inspiration (DI), which produces bronchodilation in healthy individuals, is less effective in asthmatics, and even enhances bronchoconstriction in moderate to severely affected patients. Our laboratory has previously studied the mechanotransductory pathway of airway stretch-activated contractions (Rstretch) leading to DI-induced bronchoconstriction. We demonstrated the ability of agonists acting through thromboxane A2 (TxA2) receptors to amplify airway Rstretch responses. Despite the involvement of excitatory prostanoids in bronchoconstriction, clinical trials on treatments targeting TxA2-synthase inhibition and TP-receptor antagonism have produced mixed results. Studies in Western populations produced mostly negative results, whereas studies performed in Asian populations showed mostly positive outcomes. In this review, we discuss the role of TxA2-synthase inhibition and TP-receptor antagonism in the treatment of asthmatics. We present information regarding variations in study designs and the possible role of TP-receptor gene polymorphisms in previous study outcome discrepancies. Perhaps future studies should focus on asthmatic patients with DI-induced bronchoconstriction in particular, planting the seed for the individualized treatments for asthmatics.

Lipid mediator metabolic profiling demonstrates differences in eicosanoid patterns in two phenotypically distinct mast cell populations

Mast cells are inflammatory cells that play key roles in health and disease. They are distributed in all tissues and appear in two main phenotypes, connective tissue and mucosal mast cells, with differing capacities to release inflammatory mediators. A metabolic profiling approach was used to obtain a more comprehensive understanding of the ability of mast cell phenotypes to produce eicosanoids and other lipid mediators. A total of 90 lipid mediators (oxylipins) were characterized using liquid chromatography-tandem mass spectrometry (LC-MS/MS), representing the cyclooxygenase (COX), lipoxygenase (LO), and cytochrome P450 (CYP) metabolic pathways. In vitro-derived murine mucosal-like mast cells (MLMC) and connective tissue-like mast cells (CTLMC) exhibited distinct mRNA expression patterns of enzymes involved in oxylipin biosynthesis. Oxylipins produced by 5-LO and COX pathways were the predominant species in both phenotypes, with 5-LO products constituting 90 ± 2% of the CTLMCs compared with 58 ± 8% in the MLMCs. Multivariate analyses demonstrated that CTLMCs and MLMCs secrete differing oxylipin profiles at baseline and following calcium ionophore stimulation, evidencing specificity in both a time- and biosynthetic pathway-dependent manner. In addition to the COX-regulated prostaglandin PGD(2) and 5-LO-regulated cysteinyl-leukotrienes (e.g., LTC(4)), several other mediators evidenced phenotype-specificity, which may have biological implications in mast cell-mediated regulation of inflammatory responses.

Prostanoids as pharmacological targets in COPD and asthma

COPD (Chronic Obstructive Pulmonary Disease) and bronchial asthma are two severe lung diseases which represent a major problem of world public health. Leukotrienes and prostanoids play an important role in the pathogenesis of pulmonary diseases. Prostanoids: prostaglandins (PGs) and thromboxane A2 (TXA2), the cyclooxygenase metabolites of arachidonic acid are implicated in the inflammatory cascade that occurs in asthmatic airways. Recently, the roles played by isoprostanes or prostaglandin-like compounds nonenzymatically generated via peroxidation of membrane phospholipids by reactive oxygen species, in particular F2-isoprostanes, in pulmonary pathophysiology have been highlighted. This article aims to provide an overview of the role of prostanoids and isoprostanes in the pathogenesis of COPD and asthma and to discuss the pharmacological strategies developed in prevention and/or treatment of these pathologies.

Bupivacaine Inhibits Thromboxane A2-Induced Vasoconstriction in Rat Thoracic Aorta

Plasma levels of thromboxane A2 (TXA2), an inflammatory mediator inducing platelet aggregation, bronchoconstriction, and vasoconstriction, are increased in the perioperative period. A major role in the pathogenesis of perioperative thromboembolic and ischemic syndromes is attributed to this prostanoid. Local anesthetics (LA) inhibit signaling of TXA2 receptors expressed in cell models. Therefore, we hypothesized that LA may inhibit vasoconstriction induced by the TXA2 analog U46619 in rat thoracic aorta. Rings (3-mm length) of the rat thoracic aorta were mounted in organ baths and isometric contractile force was measured. Rings, with or without endothelium, were incubated for 60 min in bupivacaine (10(-6) or 10(-5) M) or Krebs-Henseleit solution (control group) and subsequently exposed to cumulative concentrations of U46619 (10(-10) to 10(-6) M). The reversibility of the TXA2-induced vasoconstriction by bupivacaine was also studied. Pretreatment of rings with bupivacaine concentration-dependently diminished TXA2-induced contraction in rat aortic rings. We found no significant differences in relaxing effect of bupivacaine between rings with and without endothelium. Contraction in rings established with U46619 could not be reversed by cumulative concentrations of bupivacaine. Bupivacaine inhibited carbachol-induced vascular relaxation. This study provides experimental evidence that bupivacaine is an endothelium-independent inhibitor of TXA2-induced vasoconstriction of rat thoracic aorta.

Therapeutic potential of thromboxane inhibitors in asthma

This paper reviews the role of thromboxane A(2) (TXA(2)) in the pathogenesis of pulmonary allergies, particularly asthma. The potential of TXA(2) modifiers in the prevention and/or treatment of pulmonary allergies is also discussed. Bronchial asthma is characterised by reversible airway obstruction, bronchial hyperresponsiveness and inflammation. Several studies have elucidated the role of arachidonic acid metabolites (leukotrienes, prostaglandins and TXA(2)) in the pathogenesis of asthma. Among those mediators, TXA(2) has attracted attention due to its strong physiological activity. Indeed, TXA(2) demonstrates not only potent bronchoconstrictive activity but is also believed to be involved both in late asthmatic responses and in bronchial hyperresponsiveness, a typical feature of this disease. Several thromboxane receptor antagonists (TXRAs) and thromboxane synthase inhibitors (TXSIs) have been studied with the aim of reducing or preventing asthma. As double-blind, placebo-controlled clinical trials have proven the efficiency of some TXA(2) modifiers in treating asthma, the TP receptor antagonist seratrodast (AA-2414) and the thromboxane synthase inhibitor ozagrel hydrochloride (OKY-046) are now available as anti-asthmatic agents in Japan. Moreover, seratrodast and ramatroban (BAY-U-3405), another thromboxane receptor antagonist, are currently under Phase III clinical evaluation in the US for the treatment of asthma.

Thromboxane A2 inhibition: therapeutic potential in bronchial asthma

Bronchial asthma is a disease defined by reversible airway obstruction, bronchial hyperresponsiveness and inflammation. In addition to histamine and acetylcholine, recent studies have emphasized the role of arachidonic acid metabolites (leukotrienes, prostaglandins and thromboxane A(2)) in the pathogenesis of asthma. Among these mediators, thromboxane A(2) (TXA(2)) has attracted attention as an important mediator in the pathophysiology of asthma because of its potent bronchoconstrictive activity. Thromboxane A(2) is believed to be involved not only in late asthmatic responses but also in bronchial hyperresponsiveness, a typical feature of asthma. Strategies for inhibition of TXA(2) include TXA(2) receptor antagonism and thromboxane synthase inhibition. Results of double-blind, placebo-controlled clinical trials have proven the efficacies of the thromboxane receptor antagonist seratrodast and the thromboxane synthase inhibitor ozagrel in the treatment of patients with asthma. Seratrodast and ozagrel are available in Japan for the treatment of asthma. Ramatroban, another thromboxane receptor antagonist, is currently under phase III clinical evaluation in Europe and Japan for the treatment of asthma. The pharmacological profiles of the thromboxane modulators may be improved by combination with leukotriene D(4) receptor antagonists. A multi-pathway inhibitory agent such as YM 158, which is a novel orally active dual antagonist for leukotriene D(4) and thromboxane A(2 )receptors, may have potent therapeutic effects in the treatment of bronchial asthma. Large scale clinical trials are necessary to further define the role of thromboxane modulators in the treatment of patients with asthma.

Effect of dietary supplementation with polyunsaturated fatty acids on bronchial hyperreactivity in subjects with seasonal asthma

Dietary supplementation with omega-3 essential fatty acids results in the production of uniqe 5-lipoxygenase and cyclooxygenase products which are biologically less active and may inhibit the production, or actions, of the eicosanoids produced when arachidonic acid is the substrate for 5-lipoxygenase and cyclooxygenase, rather than omega-3 essential fatty acids. Since airway inflammation may play a central role in the pathophysiology of asthma, we studied the effect of omega-3 essential fatty acids on bronchial responsiveness in 7 atopic patients suffering from seasonal asthma due to airborne allergens, and positive to intracutaneous skin reaction to two or more allergens. Bronchial responsiveness to ultrasonically nebulized distilled water (UNDW) was determined 30 days from the initial ingestion of 3 g/day of omega-3 essential fatty acids and 30 days after stopping dietary supplementation. Flow volume curves and Raw were recorded before the provocation test, at the end of inhalation, and at 10-, 20-, 30- and 60-min intervals. The maximum fall in forced expiratory volume in 1 s (FEV1) and the maximum increase in airway resistance (Raw) were chosen as the main outcome parameters. After 30 days of dietary supplementation, bronchial responsiveness to UNDW was significantly improved (in fact maximum fall in FEV1 was -11% vs. -28% before treatment, and maximum increase in Raw was +37% vs. +265% before treatment). The challenge test repeated 30 days after stopping dietary supplementation was the same as that recorded before treatment. The present data strongly suggest the hypothesis that dietary supplementation with omega-3 essential fatty acids could decrease bronchial hyperreactivity in atopic patients.

Characterization of airway and vascular responses in murine lungs

1. We characterized the responses of murine airways and pulmonary vessels to a variety of endogenous mediators in the isolated perfused and ventilated mouse lung (IPL) and compared them with those in precision-cut lung slices. 2. Airways: The EC50 (microM) for contractions of airways in IPL/slices was methacholine (Mch), 6.1/1.5>serotonin, 0.7/2.0>U46619 (TP-receptor agonist), 0.1/0.06>endothelin-1, 0.1/0.05. In the IPL, maximum increase in airway resistance (RL) was 0.6, 0.4, 0.8 and 11 cmH2O s ml(-1), respectively. Adenosine (< or =1 mM), bombesin (< or =100 microM), histamine (< or =10 mM), LTC4 (< or =1 microM), PAF (0.25 microM) and substance P (< or =100 microM) had only weak effects (<5% of Mch) on RL. 3. Vessels: The EC50 (microM) for vasoconstriction in the IPL was LTC4, 0.06>U46619, 0.05<endothelin-1, 0.02. The maximum increase in pulmonary artery pressure (PAP) was 11, 41 and 48 cmH2O, respectively. At 250 nM, the activity of PAF was comparable to that of LTC4. At 100 microM only, substance P caused a largely variable increase in PAP. Serotonin, adenosine, bombesin, histamine and Mch had no or only very small effects on PAP. 4. Hyperresponsiveness: In both the IPL and slices, U46619 in subthreshold concentrations (10 nM) reduced the EC50 to 0.6 microM. In the IPL, U46619 raised the maximum airway response to Mch 5 fold and the maximum PAF-induced vasoconstriction 4 fold. 5.

CONCLUSION

Murine precision-cut lung slices maintain important characteristics of the whole organ. The maximum reagibility of murine airways to endogenous mediators is serotonin<Mch<U46619<ET-1. The reagibility of the murine pulmonary vasculature is serotonin<LTC4 approximately = to PAF<U46619<ET-1. The airway and vessel hyperreactivity induced by U46619 raises the possibility that thromboxane contributes directly to airway hyperresponsiveness in various experimental and clinical settings.

Effect of DP-1904, a thromboxane synthetase inhibitor, on antigen- and spasmogen-induced bronchoconstriction in rodents

The effect of DP-1904 [6-(1-imidazolylmethyl)-5,6,7,8-tetra-hydronaphthalene-2-car boxylic acid hydrochloride], a selective thromboxane synthetase inhibitor, was examined on antigen- and spasmogen-induced bronchoconstriction in rodents. Oral administration of DP-1904 (1, 3, 10 mg/kg) as well as OKY-046 (sodium (E)-3[4-(1-imidazolylmethyl)-phenyl]-2-propanoate, 100 mg/kg), significantly inhibited immunoglobulin G-mediated bronchoconstriction in actively sensitized guinea pigs. Immunoglobulin E-mediated bronchoconstriction in actively sensitized rats was also inhibited by both DP-1904 (1, 10 mg/kg) and OKY-046 (100 mg/kg). DP-1904 (3-30 mg/kg) and OKY-046 (30 mg/kg) suppressed leukotriene D4-induced bronchoconstriction in guinea pigs. In these models, the endogenous levels of thromboxanes significantly increased following the stimulus (antigen and leukotriene D4). DP-1904 (10 mg/kg) inhibited the increase in thromboxane level in both plasma and bronchial alveolar lavage fluid. These actions of DP-1904 persisted for more than 12 h, indicating a long-lasting effect of DP-1904 on bronchoconstriction. The results showed that the biological activity of DP-1904 in our rodents models is more potent than that of OKY-046 (Ozagrel), which is available as an anti-asthma agent in Japan.

Decrease in emergency room or urgent care visits due to management of bronchial asthma inpatients and outpatients with pharmaceutical services

OBJECTIVE

To evaluate pharmaceutical care of bronchial asthma over a 1-year period and to investigate whether such care is effective.

METHODS

79 male and 97 female asthma patients who were treated with theophylline and other drugs between June 1994 and September 1996 were studied. The 15 subjects received pharmaceutical care (group 1) and were all inpatient asthmatics who had been admitted to the hospital for treatment. The other patients acted as controls.

RESULTS

In 1994, the average medication cost of asthma patients without pharmaceutical care was $199/month and $295/month when such care was provided in the outpatient clinic. The drug treatments which patients received generally included inhaled anti-inflammatory agents and theophylline, but the 161 non-pharmaceutical services patients (group 2) received fewer inhaled anti-inflammatory agents than group 1 patients. The number of visits to the emergency room and hospital admissions were significantly lower in patients after receiving pharmaceutical services than inpatients before receiving pharmaceutical services during our observation period. The serum theophylline concentrations of group 1 patients generally attained levels sufficient for favourable therapeutic effect after patients received pharmaceutical care.

CONCLUSION

Pharmaceutical care offered at the hospital is effective in the management of asthma patients.

The Pathophysiology of Asthma

Atopy predisposes an individual to the development of asthma, whereby specific triggers may repeatedly cause acute exacerbations and contribute to chronic inflammation. This IgE-mediated response to common allergens is the strongest predisposing factor for developing asthma. Airway inflammation involves release of immunological mediators via T lymphocyte dependent, and IgE dependent and independent mechanisms which attract inflammatory cells from the circulation. The role of the eicosanoids as pivotal mediators in promoting some of the changes in asthma has only recently been fully explored. Inflammatory reactions result when mediators and cytokines released from resident and infiltrating cells interact. This interaction also contributes to the bronchoconstriction, hypersecretion, and mucosal edema in the airways.

Eosinophils and neutrophils modify arachidonic acid-induced relaxation of guinea-pig trachea

The influence of inflammatory cells on airway reactivity was investigated on arachidonic acid-induced relaxations of guinea-pig trachea and on arachidonic acid metabolism in guinea-pig tracheal epithelial cells. The presence of either eosinophils or neutrophils (1.0 x 10(7) cells/ml), from bronchoalveolar lavage, decreased the tracheal relaxations induced by arachidonic acid (1.0-30 microM). The basal synthesis of prostaglandin E2 was increased in epithelial cells (from 176 +/- 36 to 7920 +/- 898 pg/ml), eosinophils (from 360 +/- 56 to 2693 +/- 686 pg/ml) and neutrophils (from 352 +/- 81 to 4400 +/- 272 pg/ml) following incubation with arachidonic acid (10 microM). The co-incubation of either eosinophils or neutrophils with epithelial cells, in the presence of arachidonic acid, decreased the synthesis of prostaglandin E2 (2600 +/- 686 and 4400 +/- 272 pg/ml respectively) but increased the synthesis of thromboxane B2 (from 60 +/- 6 to 11634 +/- 840 and 9282 +/- 485 pg/ml respectively). Similarly, when major basic protein-treated (100 micrograms/ml) epithelial cells were incubated with arachidonic acid, the prostaglandin E2 synthesis decreased (75%) but thromboxane B2 synthesis was unaffected. The results suggest that eosinophils and neutrophils may impair arachidonic acid metabolism in guinea-pig epithelium in favor of production of bronchoconstrictor prostanoids.