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Cysteinyl leukotrienes in asthma: old mediators up to new tricks
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Cysteinyl leukotrienes in asthma: old mediators up to new tricks Douglas W. P. Hay, Theodore Bradley J. Undem The cysteinyl leukotrienes
have long been suspected to
play a role in the pathogenesis speculation
J. Torphy and
of asthma. This
was based largely on their release in
human lung following antigen challenge potent bronchoconstrictor increasing
as well as their
activity. However, there is
evidence that the cysteinyl leukotrienes
produce several pro-inflammatory activity of neuronal pathways
also
effects and alter the
in the airways. Douglas
Hay, Theodore Torphy and Bradley Undem review these recent data and discuss the therapeutic cysteinyl leukotriene 5-lipoxygenase
receptor
antagonists
possibilities of and
inhibitors.
In 1940Kellaway and Trethewie demonstrated that when antigen-sensitized guinea-pig lungs are stimulated they release a substance which contracts smooth muscle’; this material was initially called ‘slow reacting substance’ and later renamed ‘slow reacting substance of anaphylaxis’z. The physicochemical and biological properties of this sub stance were shown subsequently to reside in the cysteinyl leukotrienes (also referred to as the peptidoleukotrienes) LTC, LTD, and LTE, whose structures and synthetic pathways were elucidated in 1983 (Ref. 3).
Formation of the cysteinyl leukotrienes Thecysteinyl leukotrienes are synthesized de novafrom
D. w. P. HIIT.
Head. Depanment of Pulmonary Pharmacology.
T.J. Torphy, Gmup Director. Departments of Pulmonary and Inflammation Pharmacology, SmithKline Beecham Pharmaceuticals. 709 Swedeland Road. King of Prussia, PA 19406. USA, and 6.
J. Undrm.
Assmwe Professor. Division of Allergy and Clinical Immunology. Johns Hopkins Asthma & Allergy Center. 301 Bayview Boulevard. Baltimore. MD 21224. USA.
304
membrane-associated arachidonic acid via the activity of 5-lipoxygenase, and a membrane-bound 5-lipoxygenaseactivating protein (FLAP)4 (Fig. 1). The cysteinyl leukotrienes are produced from a variety of inflammatory cells including mast cells, basophils, eosinophils and macrophages, all of which may contribute to the pathogenesis of asthmas,6.The effects of the cysteinyl leukotrienes are mediated via G protein-coupled receptors, with evidence for receptor subtypes, particularly in guinea-pigsT8. In human lung distinct cysteinyl leukotriene receptors appear to be present in bronchial smooth muscle and pulmonary veins. Four major strategies have been followed in the search for drugs which control the release or effects of the cysteinyl leukotrienes: 5-lipoxygenase inhibitors; FLAP inhibitors; cysteinyl leukotriene receptor antagonists; and phospholipase A, (PLA,) inhibitors (Table 1; Fig. 1). Several potent and selective members of the first three classes of compounds have been identified and many of these have been or are in clinical trials for asthma. Some
TiPS - September
1995 (Vol. 16)
of the cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors have shown relatively impressive activity in clinical trials, which has highlighted the role of the cysteinyl leukotrienes in asthma. Moreover, these results have demonstrated the potential therapeutic utility of representatives of these two classes of compounds as a novel strategy for the treatment of asthma. There has also been appreciable research on the other major product of 5-lipoxygenase activity, LTB, whose main biological activity is chemoattraction of inflammatory cells. However, the evidence for a significant role of LTB, in asthma is not as convincing as that for the cysteinyl 1eukotrienesrJ.
Potential role in asthma: early findings The initial evidence in support of the involvement of the cysteinyl leukotrienes in asthma came from the observation that they are released by antigen challenge in human lungs2. In addition, cysteinyl leukotriene levels in body fluids (including bronchoalveolar lavage and urine) of asthmatics are elevated compared to nonasthmatics9. These findings, together with the demonstration that the cysteinyl leukotrienes are very potent (several orders of magnitude more potent than acetylcholine or histamine) and effective contractile agonists of human airways in vitro and in vivo 510,led to the hypothesis that the cysteinyl leukotrienes are important mediators in asthma. Asthma is now recognized as a serious, chronic inflammatory condition with a number of characteristic features in addition to acute bronchospasm. These include inflammatory cell recruitment and activation, mucus hypersecretion, airways hyperreactivity, and changes in airway morphology (for example, increased airway smooth muscle mass, subepithelial fibrosis, oedema, epithelial cell damage)nu. Historically, the airway contractile activity of the cysteinyl leukotrienes has been the primary focus of research investigating their potential pathophysiological contribution to asthma. However, it is becoming increasingly clear that this view is an oversimplification and the role of the cysteinyl leukotrienes in asthma may be much more multi-dimensional than previously envisaged, encompassing bronchoconstriction, inflammation, neuronal dysfunction and airways remodelling.
Pro-inflammatory actions Microvascular permeability The plasma exudation and resultant oedema in the airway wall and lumen that occur in asthma may be involved in its pathogenesis by contributing to airways hyperresponsiveness, impairment of mucociliary transport, mucus plug formation, shedding of epithelial cells and small airway narrowingi3. Several studies have demonstrated the ability of the cysteinyl leukotrienes to increase microvascular permeability in guinea-pig airwaysiPi6. Electron microscopic analysis indicates that LTE, increases microvascular permeability by inducing gaps in the endothelium of venules by contracting endothelial cellsi7.The mechanism for the
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R effects of the cysteinyl leukotrienes appears to involve both direct and indirect pathways (that is, secondary mediators), and is antagonized by FPL55712 (Ref. 18), the first cysteinyl leukotriene receptor antagonist identified, and pranlukast (ON01078; SB205312)16J9, a more recent, more potent and selective compound. Pranlukast antagonizes antigen-induced microvascular permeability in guinea-pig trachea, main bronchi and intrapulmonary airways20.
potent and selective
phospholipase A,
!N!x!)(!x!X!)~~~ 54ipoxygenase or FLAP inhibitors
*
arachidkic
acid bMc!x!x!)bbbb
54ipoxygenase or FLAP I -
5-HPETE LTA synthetase J
LTB, synthetase
r
LTA4 3
LTB,
LTC,,
1 i3LT receptor antagonists
.++
glutathione LTD,,
S-transferase
LTE,,
i
BLT receptor
CysLT, and CysLT, receptors
+
CysLT receptor antagonists
Fig.1.Formation of cysteinyl leukotrienes and potential therapeutic strategies to inhibit their release or effects. FLAP, !i-lipoxygenase-activating protein; 5-HETE, 5-hydroxyeicosatetraenoic acid; 5-HPETE, 5hydroperoxyeicosatetraenoic acid
lar effects are observed with LTD, (10 kg ml-l), whereas LTE4(10 kg ml-l) or LTB, (10pg ml-l) are without influence. More recently, single exposure (1min) of guinea-pigs to aerosol administration of LTD, (0.3-30 pgml-1) has been shown to elicit a significant accumulation of eosinophils into the lung (assessed histologically and in bronchoalveolar lavage fluid) which persists for at least four weeks after challenge24.Consistent with the results of previous studies, this infiltration is abolished by the cysteinyl leukotriene receptor antagonist pranlukast. In cynomolgus monkeys another potent and selective cysteinyl leukotriene receptor antagonist, IC1198615, attenuates antigen-induced increase in bronchoalveolar lavage eosinophils (and also peripheral blood lymphocytes)
compounds that control the release or actions of the cysteinyl leukotrienes
Compound
Company
Class
Administration route
Development asthma
ABT761
Abbott Bayer
5-lipoxygenase inhibitor CyslTI receptor antagonlst
lralukast (CGP45715A) MK886
Ciba-Geigy
CyslT, receptor antagonist
Oral Oral Inhalation Inhalation
Phase Phase Phase Phase
Merck
Oral
Discontinued
Montelukast Pobilukast Pranlukast Zafirlukast 202138 Zileuton
Merck SmithKline Beecham SmithKline Beecham Zeneca Zeneca Abbott
5-lipoxygenase-activating protein inhibitor CyslT, receptor antagonist CyslT, receptor antagonist Receptor antagonist CyslT, receptor antagonist 5-lipoxygenase inhibitor Nipoxygenase inhibitor
Oral Inhalation Oral Oral Oral Oral
Phase Ill Discontinued Phase Ill Phase Ill Phase II Filed in USA
Bayx7195
W
cell
Eosinophil injlux
Table 1. Representative
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5-HETE
The eosinophil has been identified as a key inflammatory cell in the pathophysiology of asthma*l. Results from recent studies have provided compelling evidence that the cysteinyl letikotrienes promote eosinophil migration (this is in contrast to the prevailing dogma that the cysteinyl leukotrienes do not influence inflammatory cell recruitment). The most important observation supporting this contention is the relatively selective increase in the number of eosinophils in the lamina propria of mucosal biopsies from four asthmatic subjects challenged with aerosol administration of LTE, but not methacholine; LTE, also produces a much smaller increase in the number of neutrophils, but does not influence the number of mast cells, lymphocytes, macrophages or plasma cell.+. Experiments in animals strongly support a role for cysteinyl leukotrienes in recruiting eosinophils into the airways. For example, aerosol administration of ovalbumin to ovalbumin-sensitized guinea-pigs increases eosinophil recruitment (assessed histologically) into the airway submticosa (maximum increase was approximately fourfold, 12h after challengey. This influx is antagonized by the CysLT, receptor antagonist, MK571 (1 mg kg-1 p.o.), but not by the histamine H, receptor antagonist mepyramine or the H, receptor antagonist cimetidine, or the cyclooxygenase inhibitor indomethacir+. Furthermore, aerosol administration of LTC, (10 and 30 FLgml-l) produces a dose-related stimulation of eosinophil accumulation into guinea-pig lungs, which is antagonized by MK571. Simi-
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direct
indirect
indirect
effect on eosinophils
effect on endothelium (for example, adhesion molecules)
release of chemotactic mediators (for example, IL-5)
recruitment feedback cysteinyl leukotrienes
cysteinyl i leukotrienes
Fig. 2. Possible direct and indirect mechanisms underlying cysteinyl leukotriene-induced
3 0 6
enhanced release of cysteinyl leukotrienes
recruitment of eosinophils. 11-5, interleukin 5.
concomitant with the associated increase in airway responsiveness25. The precise mechanism(s) by which cysteinyl leukotrienes recruit eosinophils into the airway is unclear although several are possible (Fig. 2). Spada and colleagues demonstrated that LTD, potently stimulates chemotaxis of peripheral blood eosinophils isolated from nonasthmatic individuals (threshold concentration was lO-‘o~1)26. This direct stimulatory effect is abolished by the potent and selective CysLT, receptor antagonist, pobilukast (SK&F104353). A potential link has emerged between the actions of the cysteinyl leukotrienes and interleukin 5 (IL-5), a key cytokine responsible for the differentiation, trafficking and activation of eosinophils27. Recent studies indicate that LlD&duced airway eosinophilia in guinea-pigs is antagonized by the rat anti-mouse IL-5 monoclonal antibody derived from the TRFK-5hybridoma cell line (Ref.24). This suggests an indirect mechanism, involving IL-5, underlies the effects of LTD, on eosinophil recruitment. Moreover, the cysteinyl leukotrienes are produced by eosinophils5 and IL-5 potentiates the release of the cysteinyl leukotrienes induced by various stimuli8,29.Thus, it is possible that there is a positive feedback amplification pathway whereby IL-5 enhances the release of cysteinyl leukotrienes which, in turn, contribute to eosinophil recruitment. This mechanism may underlie the persistent airway eosinophilia observed after a single LTD, challenge in guinea-pigs”. However, the evidence that, unlike human eosinophils, guinea-pig eosinophils do not appear to synthesize the cysteinyl leukotrienes (due to the lack of LTA, glutathione S-transferasero opposes this theory.
mucosal explants 31132. This effect is blocked by FPL55712. Histochemical analysis reveals that aerosol administration of LTD, increases epithelial mucus secretion in guinea-pig airways via a receptor-mediated mechanism that is antagonized by pobilukast33.LTC, increases mucin release from cat trachea (an effect that is antagonized by FPL55712) in viva but not in uifr’034.In addition to these effects of the cysteinyl leukotrienes on mucus release, aerosol administration of LTD, slows mucus transport in sheep airways% and decreases the activity of human respiratory cilia%.The potential pathophysiological importance of this observation is demonstrated in a study of six patients with asthma, in which the decrease in tracheal mucus velocity induced by aerosol administration of ragweed antigen is prevented by FPL55712 (Ref. 37).
Effects on mucus secretion and mucociliary transport Hypertrophy of mucosal glands and enhanced secretion of mucus are features of asthma which, along with cellular debris, may be responsible for the formation of mucus plugs that are found in the airways of individuals of who have died after an acute asthma attack”. Both LTC, and LTD, potently (EC,= 1 no and 10 no, respectively) stimulate mucus release, but are without effect on lysozyme secretion, from cultured human airway
Interactions with nerves Evidence is mounting that the cysteinyl leukotrienes may influence pulmonary function by modulating the activity of the afferent nervous system. Stewart and colleagues noted that LTD, significantly potentiates the bronchoconstrictor response to histamine by a mechanism that is inhibited either by capsaicin-induced denervation or by hexamethonium treatment39.Cysteinyl leukotrienes did not potentiate exogenously added acetylcholine or stimulation of the vagus nerve. These
TiPS - September
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Airway smooth muscle proliferation Airway smooth muscle cell hyperplasia is another feature of chronic severe asthma”. MK571 (2 mg kg-1 i.p., 1h before and 3 h after challenge) inhibits the increase in airway smooth muscle mass in large airways elicited by aerosol administration of ovalbumin (six challenges at five day intervals) in ovalbumin-sensitized Brown Norway rats, concomitant with antagonism of ovalbumin-induced bronchospasm and the airway hyperresponsiveness to methacholine3. These data suggest that the cysteinyl leukotrienes play a role in the regulation of airway smooth muscle proliferation. However, it is not known if they are acting as direct mitogens or as co-mitogens, potentiating the airway smooth muscle proliferative effects induced by other substances (for example, growth factors).
x data indicate that LTD, may enhance the responsiveness of capsaicin-sensitive afferent fibres in the guinea-pig airway. This idea is supported by electrophysiological studies, in which LTC, inhibits an apamin-insensitive, Ca2+-activated K+ current and depolarizes the resting membrane potential of C fibres in guinea-pig vagal sensory ganglia. Inhibition of this current leads to inhibition of the hyperpolarization and a substantial increase in the frequency at which the afferent fibre can elicit action potentials40-42. In the airways a subset of afferent C fibres stores and, upon electrical stimulation, releases substance P and related tachykinins locally. The effects induced by the tachykinins include airway smooth muscle constriction and pro-inflammatory activity. Exogenously added LTD, causes the release of substance I’ from guinea-pig isolated trachea43. Concentrations of LTD, that are threshold for smooth muscle contraction may also potentiate the tachykinin-mediated response in the guinea-pig isolated airway evoked by threshold electrical stimulation of the vagus nerve or electrical-field stimulations; this appears to be a result of an enhancement of the action potentialstimulated release of tachykinins from the afferent neurones. It is interesting to note that vagally mediated smooth muscle contraction and plasma extravasation, induced by tachykinin release in the guinea-pig isolated airways, are inhibited by selective cysteinyl leukotriene receptor antagonists, suggesting that endogenous cysteinyl leukotrienes prejunctionally modulate responses to tachykinins in the airway@. The cysteinyl leukotrienes or cysteinyl leukotriene receptor antagonists have no effect on the contraction of airway smooth muscle elicited by exogenously applied tachykinins~. Also supporting a role for endogenous cysteinyl leukotrienes in modulating tachykinin-mediated innervation in the airways are data from experiments on hyperpnoea-induced bronchoconstriction in guinea-pigs, a model of exercise-induced asthma45. The evidence suggests that the bronchoconstriction in this model is secondary to the release of tachykinins from afferent fibres. Inhibitors of 5-lipoxygenase and cysteinyl leukotriene receptor antagonists decrease the magnitude of the bronchoconstriction in this model+ Considered together, these data suggest that in guinea-pigs cysteinyl leukotrienes, synthesized by airway mast cells (or other cell types), interact with sensory fibres leading to changes in their excitability as well as enhance ment of the release of tachykinins from their terminals. It remains to be determined if cysteinyl leukotrienes have a similar neuromodulatory function in human asthmatic airways.
Results of clinical trials in asthma The clinical trials using the first developed cysteinyl leukotriene receptor antagonists, such as FPL55712,were disappointing, probably due to the inherent poor potency and pharmacokinetics of these compoundsza. Much more encouraging and impressive data have been obtained with the more recent potent and selective compounds
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from diverse structural classes, in particular zafirlukast (ICI204219), pranlukast, montelukast (MK476) and pobilukast (Table 1). These, and other compounds with similar pharmacological profiles that are generally spe cific for the CysLT, receptor, are effective against bronchospasm induced by antigen, aspirin, exercise, and cold air in asthmatic individuals7. In some studies there is an improvement in baseline pulmonary function in asthmatics7,47,@. Additive bronchodilator effects are observed following pretreatment of asthmatic individuals with the cysteinyl leukotriene receptor antagonists verlukast (MK679)@or MK571 (Ref. 47), followed by a p,-adrenoceptor agonist. Interestingly, in several studies of antigeninduced bronchospasm, cysteinyl leukotriene receptor antagonists and FLAP inhibitors attenuate not only the early-phase response (thought to be the result of acute airway smooth muscle contraction) but also the late-phase response that occurs approximately four to eight hours after antigen challenge and is thought to involve mechanisms other than airway smooth muscle contraction (for example, oedema, inflammatory cell recruitment and activation)7,50,51. There is recent clinical information of a significant improvement in several objective and subjective measures of asthma [including asthma symptom severity scores, forced expiratory volume in 1 s values, airway hyperresponsiveness or p,-adrenoceptor agonist usage (or both)] with pranlukastsz or zafirlukasts3. In addition, zileuton (1.6 or 2.4 g day-i for 4 weeks), a 5-lipoxygenase inhibitor, improved pulmonary function and decreased symptoms and the frequency of /3,-adrenoceptor agonist use in clinical trials using mild-to-moderate asthmatic subjects%.
Unanswered questions There is increasing evidence from in vitro and in vim studies that the cysteinyl leukotrienes exert a variety of effects with relevance to the aetiology of asthma (Fig. 3). The recent clinical studies with potent and selective cysteinyl leukotriene receptor antagonists or synthesis inhibitors suggest a greater magnitude and broader spectrum of therapeutic efficacy of this class of compounds than would be anticipated, based solely on antagonism of cysteinyl leukotriene-induced bronchospasm. Furthermore, the effectiveness of these compounds suggests that a strategy to modify only one mediator may, in fact, offer significant therapeutic benefit even in a complex disease such as asthma whose overall pathogenesis is thought to involve multiple cells, chemical mediators and cytokines and several underlying mechanisms. Despite the growing optimism that these agents will fulfil their promise of being a truly novel and effective class of antiasthmatic drugs, several important questions remain. For example, how do the efficacies of cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors compare to each other (is there a significant role for LTB,?) and to current therapies? Where will the cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors be positioned and utilized according to the
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cationic proteins
(epithelial cell damage) decreased mucus transport ---+ airway epithelium
increased release
inflammatory cells (e.g. mast cells, eosinophils)
l
a ~-+
cysteinyl
leukotrienes
e /
smooth muscle
.
contraction and proliferation (?) Fig.3. Potential sites and effects of cysteinyl leukotrienes relevant to a pathophysiological
present International Guidelines for asthma treatment? Will these agents be effective in all asthmatics or only in a specific subset? If only a subset of patients respond, can this selectivity be explained by heterogeneity in environmental or genetic factors? Will these agents prove efficacious in treating other atopic disorders (for example, allergic rhinitis, atopic dermatitis)? Finally, how important are the pro-inflammatory, smooth muscle proliferating and neuronal effects of the cysteinyl leukotrienes in the pathophysiology of asthma and, accordingly, how much of the clinical efficacy of cysteinyl leukotriene receptor antagonists and 54ipoxygenase inhibitors results from countering these activities? Answers to these and other questions should be forthcoming as more clinical studies with these agents are carried out. Selected references 1 Kellaway, C. H. and 2 3 4 5 6 7
8 9 10 11 12
3 0
8
Trethewie, E. R. (1940) Q. J. Exp. Physiol. Cogn. Med. Sci. 30,121-145 Brocklehurst, W. E. (1960) J. PhysioI. 151,416-435 Samuelsson, B. (1983) Science 220,568-575 Musser, J. H. and Kreft, A. F. (1992) J. Med. Chem. 35,2501-2524 Drazen, J. M and Austen, K. F. (1987) Am. Rev. Respir. Dis. 136, 985-988 Busse, W. (1992) Ann. AUkrgy 69,261-266 Hay, D. W. P. and Griswold, D. E. (1994) in Handbook ofImmunopharmucology: LipidMediators (Vol. 10) (Cunningham, F., ed.),pp. 117-179, Academic Press Hay, D. W. P. and Raebum, D. in Ainoays Smooth Muscle (Vol. 5) (Raebum, D. and Giembicz, M. A., eds), Birkhluser (in press) A~I, J. P. and Lee, T. H. (1993) Clin. Sci. 84,501-510 Hay, D. W. P. et al. (1987) J Phnrmacol. Exp. 7kr. 243,474-481 Dunrxill, M. S. (1987) in Pulmonary Pathology (Dunnill, M. S., ed.), pp. 61-80, Churchill Livingstone Beasley, R., Burgess, C., Crane, J. and Roche, W. (1993) J. Allergy Clin. Immunol. 92,1&l%
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role in asthma
13 Persson, C. G. A. (1988) Lung 166,1-23 14 Woodward, D. F., We&man, B. M., Gill, C. A. and Wasserman, M. A. (1983) Prostuglandins 25,131-142 15 Evans, T. W., Rogers, D. F., Aursudkij, B., Chung, K. F. and Barnes, P. J. (1989) Clin. Sci. 76,479-485 16 Bochnowicz, S. and Underwood, D. C. (1995) Prostuglandins Leukotrienes Essential Fatty Acids 52,403411 17 Joris, I., Majno, G., Corey, E. J. and Lewis, R. A. (1987) Am. J. Puthol. 126,19-24 18 Woodward, D. F., Wasserman, M. A. and Weichman, B. M. (1983) Eur. J. Pharmucol. 93,9-19 19 Nakagawa, N. et al. (1992) Jpn. J. Pharmucol. 60,217-225 20 Obata, T. et al. (1992) Life Sci. 51,157~1583 21 Gleich, G. J. (1990) J. Allergy Clin. Immunol. 85,422-436 22 Laitinen, L. A. et al. (1993) Lancet 341,989-990 23 Foster, A. and Chan, C. C. (1991) Int. Arch. AllergyAppl. Immunol. 96, 279-284 24 Underwood, D. C., Osbom, R. R., Newsholme, S. J., Torphy, T. J. and Hay, D. W. P. (1995) Abstract, at ‘Asthma 1995: Theory to Treatment’, 15-17 July 1995, Chicago, USA 25 Turner, C. R., Smith, W. B., Andersen, C. J., Swindell, A. C. and Watson, J. W. (1994) Pulm. Phurmucol. 7,49-58 26 Spada, C. S., Nieves, A. L., Krauss, A. H-P. and Woodward, D. F. (1994) J. Leukocyte Biol. 55,18>191 27 McKenzie, A. N. J. and Sanderson, C. J. (1992) in Chemical Immunology: Interleukins: Molecular Biology and Immunology (Vol. 51) (Kishimoto, T., ed.), pp. 135-152, Karger 28 Van Oosterhout, A. J. M., Van Der Poel, A., Koenderman, L., Roos, D. and Nijkamp, F. P. (1994) Med. Inflam. 3,53-55 29 Takafuji, S., Bischoff, S. C., De Week, A. L. and Dahinden, C. A. (1991) J. Immunol. 147,385!%3861 30 Sun, F. F. et al. (1991) J. Leukocyte Biol. 50,140-150 31 Marom, Z., Shelhamer, J. H., Bach, M. K., Morton, D. R. and Kaliner, M. (1982) Am. Rev. Respir. Dis. 126,449-451 32 Coles, S. J. et al. (1983) Prostuglundins 25,155-170 33 Hoffstein, S. T., Malo, P. E., Bugelski, P. and Wheeldon, E. B. (1990) Exp. Lung Res. 16,711-725 34 Peatfield, A. C., Piper, P. J. and Richardson, P. S. (1982) Br. J. Phwmucol. 77,391-393 35 Russi, E. W. etal.(1985) J. A@. Physiol. 59,1416-1422 36 Bisgaard, H. and Pedersen, M. (1987) Clin. Allergy 17,95-103
R 37 Ahmed, T., Greenblatt, D. W., Birch,S., Marchette, B. and Wanner, A. (1981)Am. Rev. Respir. Dis. 124,110-114
38 Wang, C. G., Du, T., Xu, L. J. and Martin, J. G. (1993)Am. Rev.Respir. Dis. 148,413-417 39 Stewart, A. G., Thompson, D. C. and Fennessy, M. R. (1984)Agents Actions 15,sSO8 40 Undem, B. J., Hubbard, W. and Weinreich, D. (1993)J. Auton. Net-v. Syst. 44,35-I4
41 42 43 44 45 46 47 48 49 50 51
52 53 54
Undem, B.J. and Weinreich, D. (1993)J.Auton. Nerv. Syst. 44,17-34 Weinreich, D. and Wonderlin, W. F. (1987)J. Physiol. 394,41%27 Bloomquist, E. I. and Kream, R. M. (1990)Exp. Lung Res. 16,645-659 Ellis,J. E. and Undem,B. J. (1991)J Physiol.436,46%484 Ray, D. W., Hernandez, C., Munoz, N., Leff, A. R. and Solway, J. (1988)J. AppI. Physiol. 65,934-939 Garland,A. et al. (1993)I. Appl. Physiol. 75,2797-2804 Gaddy,J. N., Margolskee,D. J., Bush, R. K., Williams, V. C. and Busse, W. W. (1992)Am. Rev.Respir. Dis. 146,358-363 Irnpens, N. et al. (1993)Am. Rev. Respir. Dis. 147,1442-1446 Lammers, J-W. J. et al. ‘(1992)Pulm. Phmmacol.5,121-125 Taylor, I. K., O’Shaughnessy, K. M., Fuller, R. W. and Dollery, C. T. (1991)Luncet 337,690-694 Friedman, B. S. et al. (1993)Am. Rev. Respir. Dis. 147,839-844 Taki, F. et al. (1991)Antim.-Forsch. /Drug Res. 44,330-333 Spector, S. L., Smith, L. J., Glass, M. and the Accolate Asthma Trial& Group (1994)Am. J Respir. Crit. Cure Med. 150,618-623 Israel, E. et al. (1993)Ann. Intern. Med. 119,1059-1066
Na+channels as targets for neuroprotectivedrugs Charles P. Taylor and Brian S. Meldrum Drugs that block voltage-dependent well known as local anaesthetics, anticonvulsants. compounds
Na+ channels are antiarrhythmics
also provide a powerful mechanism
cytoprotection
and
Recent studies show that these in animal models of cerebral
of
ischaemia,
hypoxia or head trauma. In this article Charles Taylor and Brian Meldrum
review evidence
Na+ channel modulators
indicating that
are neuroprotective
describe recent ideas for the molecular of voltage-dependent
and
sites of action
Na+ channel blockers. Clinical
trials with several compounds stroke and traumatic
are now in progress for
head injury, and the therapeutic
potential for this group of compounds
is discussed.
Na+ channels have long been known to underlie axonal and somatic action potentials; more recently it has been shown that Na+ channels actively propagate information within the dendritic tree of pyramidal neuronesi. Tetrodotoxin (TTX)-sensitive Na+ channels also underlie small inward currents that do not inactivate*. These persistent Na+ currents amplify synaptic potentials and other subthreshold depolarizations and serve as a pacemaker for action potential firing; they also may underlie a detrimental Na+ influx in neurones during seizures or ischaemia.
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Chemical names ABT761: (R)-N-(3-[5-(4fluorobenzyl)thien-Z-yl]-lmethyl-2-propyl]-l\r hydroxyurea ZD2138: 6-( [3-fluoro-5-(4methoxy-3,4,5,6-tetrahydro-W-pyran-4yl)phenoxy]methyl]-lmethylquinol-2-one Bayx7195: (4s)-(4-carboxyphenylthio)-7-[4(phenoxybutoxy)phenyl]-hept-5(Z)-enoic
acid
MK866: 3-[I-(p-chlorophenyl)&sopropyl-3-tbutylthio-lff-indol-2-yl]-22_dimethylpropanoic acid MK57l: 3-((3-[-2(7-chloro-2-quinolinyl> ethenyl]phenyl}( [3-(dimethylamino-3oxopropyl)thio]methyl]thio)propanoic
acid
FPL55712: 7-[3-(4-acetyl-3-hydroxy-2-propylphenoxy)-2-hydroxypropoxy]-4-oxo-8-propyl-4?fl-benzopyran-2-carboxylic acid
Distinct types of Na+ channels have been identified in rat brain, heart and skeletal muscle. In rat brain, the differential distibution of Na+ channels has been studied using immunohistochemical techniques with sitedirected antibodies to unique regions of the three channel types, and also by immunoprecipitation (for review see Ref. 3) (see Box 1). Type II Na+ channels comprise more than half the channels in the neocortex and forebrain, with type I channels making up a large proportion of the remainder. These channels are predominantly located in cell bodies and dendrites (type I) or in axons (type II), although type II channels are in higher overall abundance in both areas. Type III channels are expressed significantly only at early stages of development. So far, no major differences in physiology or drug sensitivity have been described between type I and II channels. However, cloned type III Na+ channels have slower activation and particularly slower inactivation than type I or II channels. Recently, a novel type of Na+ channel has been cloned and is expressed widely throughout the brain both in neurones and in glia4.
Drug-induced block of Na+ channels
C. P. Taylor.
Within the physiological range of membrane voltages, ITX blocks the same percentage of channels whether membranes are depolarized or not, so its action is voltage independent. This contrasts with voltage-dependent channel blockers such as phenytoin, carbamazepine and a family of other drugs that are similar to lidocaine in several functional respect@. Channel block is enhanced by sustained depolarization, for example, for phenytoin IC, = 120 PM at - 85 mV, but IC, = 10 FM at -60 mV, and is thus voltage dependent. In addition, block with these agents is enhanced by brief preceding voltage steps that momentarily open Na+ channels; this is called
Research Fellow.
TiPS - September
1995
(Vol.
16)
Department of Neurolqcal
and
Neurodegeneratlve O~seases. Parke-Davts Pharmaceutxal Research Drasion. Warner-Lambert, 2BW Plymouth Road, Ann Arbor. MI 48105. USA, and B. S. Meldrum, Professor. Department of Neurology. Institute of Psychlatw. De Cresplgny Park. Denmark Hill. London, UK SE5 8AF
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Cysteinyl leukotrienes in asthma: old mediators up to new tricks Douglas W. P. Hay, Theodore Bradley J. Undem The cysteinyl leukotrienes
have long been suspected to
play a role in the pathogenesis speculation
J. Torphy and
of asthma. This
was based largely on their release in
human lung following antigen challenge potent bronchoconstrictor increasing
as well as their
activity. However, there is
evidence that the cysteinyl leukotrienes
produce several pro-inflammatory activity of neuronal pathways
also
effects and alter the
in the airways. Douglas
Hay, Theodore Torphy and Bradley Undem review these recent data and discuss the therapeutic cysteinyl leukotriene 5-lipoxygenase
receptor
antagonists
possibilities of and
inhibitors.
In 1940Kellaway and Trethewie demonstrated that when antigen-sensitized guinea-pig lungs are stimulated they release a substance which contracts smooth muscle’; this material was initially called ‘slow reacting substance’ and later renamed ‘slow reacting substance of anaphylaxis’z. The physicochemical and biological properties of this sub stance were shown subsequently to reside in the cysteinyl leukotrienes (also referred to as the peptidoleukotrienes) LTC, LTD, and LTE, whose structures and synthetic pathways were elucidated in 1983 (Ref. 3).
Formation of the cysteinyl leukotrienes Thecysteinyl leukotrienes are synthesized de novafrom
D. w. P. HIIT.
Head. Depanment of Pulmonary Pharmacology.
T.J. Torphy, Gmup Director. Departments of Pulmonary and Inflammation Pharmacology, SmithKline Beecham Pharmaceuticals. 709 Swedeland Road. King of Prussia, PA 19406. USA, and 6.
J. Undrm.
Assmwe Professor. Division of Allergy and Clinical Immunology. Johns Hopkins Asthma & Allergy Center. 301 Bayview Boulevard. Baltimore. MD 21224. USA.
304
membrane-associated arachidonic acid via the activity of 5-lipoxygenase, and a membrane-bound 5-lipoxygenaseactivating protein (FLAP)4 (Fig. 1). The cysteinyl leukotrienes are produced from a variety of inflammatory cells including mast cells, basophils, eosinophils and macrophages, all of which may contribute to the pathogenesis of asthmas,6.The effects of the cysteinyl leukotrienes are mediated via G protein-coupled receptors, with evidence for receptor subtypes, particularly in guinea-pigsT8. In human lung distinct cysteinyl leukotriene receptors appear to be present in bronchial smooth muscle and pulmonary veins. Four major strategies have been followed in the search for drugs which control the release or effects of the cysteinyl leukotrienes: 5-lipoxygenase inhibitors; FLAP inhibitors; cysteinyl leukotriene receptor antagonists; and phospholipase A, (PLA,) inhibitors (Table 1; Fig. 1). Several potent and selective members of the first three classes of compounds have been identified and many of these have been or are in clinical trials for asthma. Some
TiPS - September
1995 (Vol. 16)
of the cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors have shown relatively impressive activity in clinical trials, which has highlighted the role of the cysteinyl leukotrienes in asthma. Moreover, these results have demonstrated the potential therapeutic utility of representatives of these two classes of compounds as a novel strategy for the treatment of asthma. There has also been appreciable research on the other major product of 5-lipoxygenase activity, LTB, whose main biological activity is chemoattraction of inflammatory cells. However, the evidence for a significant role of LTB, in asthma is not as convincing as that for the cysteinyl 1eukotrienesrJ.
Potential role in asthma: early findings The initial evidence in support of the involvement of the cysteinyl leukotrienes in asthma came from the observation that they are released by antigen challenge in human lungs2. In addition, cysteinyl leukotriene levels in body fluids (including bronchoalveolar lavage and urine) of asthmatics are elevated compared to nonasthmatics9. These findings, together with the demonstration that the cysteinyl leukotrienes are very potent (several orders of magnitude more potent than acetylcholine or histamine) and effective contractile agonists of human airways in vitro and in vivo 510,led to the hypothesis that the cysteinyl leukotrienes are important mediators in asthma. Asthma is now recognized as a serious, chronic inflammatory condition with a number of characteristic features in addition to acute bronchospasm. These include inflammatory cell recruitment and activation, mucus hypersecretion, airways hyperreactivity, and changes in airway morphology (for example, increased airway smooth muscle mass, subepithelial fibrosis, oedema, epithelial cell damage)nu. Historically, the airway contractile activity of the cysteinyl leukotrienes has been the primary focus of research investigating their potential pathophysiological contribution to asthma. However, it is becoming increasingly clear that this view is an oversimplification and the role of the cysteinyl leukotrienes in asthma may be much more multi-dimensional than previously envisaged, encompassing bronchoconstriction, inflammation, neuronal dysfunction and airways remodelling.
Pro-inflammatory actions Microvascular permeability The plasma exudation and resultant oedema in the airway wall and lumen that occur in asthma may be involved in its pathogenesis by contributing to airways hyperresponsiveness, impairment of mucociliary transport, mucus plug formation, shedding of epithelial cells and small airway narrowingi3. Several studies have demonstrated the ability of the cysteinyl leukotrienes to increase microvascular permeability in guinea-pig airwaysiPi6. Electron microscopic analysis indicates that LTE, increases microvascular permeability by inducing gaps in the endothelium of venules by contracting endothelial cellsi7.The mechanism for the
0 1995,
Elsevier Science Ltd
R effects of the cysteinyl leukotrienes appears to involve both direct and indirect pathways (that is, secondary mediators), and is antagonized by FPL55712 (Ref. 18), the first cysteinyl leukotriene receptor antagonist identified, and pranlukast (ON01078; SB205312)16J9, a more recent, more potent and selective compound. Pranlukast antagonizes antigen-induced microvascular permeability in guinea-pig trachea, main bronchi and intrapulmonary airways20.
potent and selective
phospholipase A,
!N!x!)(!x!X!)~~~ 54ipoxygenase or FLAP inhibitors
*
arachidkic
acid bMc!x!x!)bbbb
54ipoxygenase or FLAP I -
5-HPETE LTA synthetase J
LTB, synthetase
r
LTA4 3
LTB,
LTC,,
1 i3LT receptor antagonists
.++
glutathione LTD,,
S-transferase
LTE,,
i
BLT receptor
CysLT, and CysLT, receptors
+
CysLT receptor antagonists
Fig.1.Formation of cysteinyl leukotrienes and potential therapeutic strategies to inhibit their release or effects. FLAP, !i-lipoxygenase-activating protein; 5-HETE, 5-hydroxyeicosatetraenoic acid; 5-HPETE, 5hydroperoxyeicosatetraenoic acid
lar effects are observed with LTD, (10 kg ml-l), whereas LTE4(10 kg ml-l) or LTB, (10pg ml-l) are without influence. More recently, single exposure (1min) of guinea-pigs to aerosol administration of LTD, (0.3-30 pgml-1) has been shown to elicit a significant accumulation of eosinophils into the lung (assessed histologically and in bronchoalveolar lavage fluid) which persists for at least four weeks after challenge24.Consistent with the results of previous studies, this infiltration is abolished by the cysteinyl leukotriene receptor antagonist pranlukast. In cynomolgus monkeys another potent and selective cysteinyl leukotriene receptor antagonist, IC1198615, attenuates antigen-induced increase in bronchoalveolar lavage eosinophils (and also peripheral blood lymphocytes)
compounds that control the release or actions of the cysteinyl leukotrienes
Compound
Company
Class
Administration route
Development asthma
ABT761
Abbott Bayer
5-lipoxygenase inhibitor CyslTI receptor antagonlst
lralukast (CGP45715A) MK886
Ciba-Geigy
CyslT, receptor antagonist
Oral Oral Inhalation Inhalation
Phase Phase Phase Phase
Merck
Oral
Discontinued
Montelukast Pobilukast Pranlukast Zafirlukast 202138 Zileuton
Merck SmithKline Beecham SmithKline Beecham Zeneca Zeneca Abbott
5-lipoxygenase-activating protein inhibitor CyslT, receptor antagonist CyslT, receptor antagonist Receptor antagonist CyslT, receptor antagonist 5-lipoxygenase inhibitor Nipoxygenase inhibitor
Oral Inhalation Oral Oral Oral Oral
Phase Ill Discontinued Phase Ill Phase Ill Phase II Filed in USA
Bayx7195
W
cell
Eosinophil injlux
Table 1. Representative
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5-HETE
The eosinophil has been identified as a key inflammatory cell in the pathophysiology of asthma*l. Results from recent studies have provided compelling evidence that the cysteinyl letikotrienes promote eosinophil migration (this is in contrast to the prevailing dogma that the cysteinyl leukotrienes do not influence inflammatory cell recruitment). The most important observation supporting this contention is the relatively selective increase in the number of eosinophils in the lamina propria of mucosal biopsies from four asthmatic subjects challenged with aerosol administration of LTE, but not methacholine; LTE, also produces a much smaller increase in the number of neutrophils, but does not influence the number of mast cells, lymphocytes, macrophages or plasma cell.+. Experiments in animals strongly support a role for cysteinyl leukotrienes in recruiting eosinophils into the airways. For example, aerosol administration of ovalbumin to ovalbumin-sensitized guinea-pigs increases eosinophil recruitment (assessed histologically) into the airway submticosa (maximum increase was approximately fourfold, 12h after challengey. This influx is antagonized by the CysLT, receptor antagonist, MK571 (1 mg kg-1 p.o.), but not by the histamine H, receptor antagonist mepyramine or the H, receptor antagonist cimetidine, or the cyclooxygenase inhibitor indomethacir+. Furthermore, aerosol administration of LTC, (10 and 30 FLgml-l) produces a dose-related stimulation of eosinophil accumulation into guinea-pig lungs, which is antagonized by MK571. Simi-
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direct
indirect
indirect
effect on eosinophils
effect on endothelium (for example, adhesion molecules)
release of chemotactic mediators (for example, IL-5)
recruitment feedback cysteinyl leukotrienes
cysteinyl i leukotrienes
Fig. 2. Possible direct and indirect mechanisms underlying cysteinyl leukotriene-induced
3 0 6
enhanced release of cysteinyl leukotrienes
recruitment of eosinophils. 11-5, interleukin 5.
concomitant with the associated increase in airway responsiveness25. The precise mechanism(s) by which cysteinyl leukotrienes recruit eosinophils into the airway is unclear although several are possible (Fig. 2). Spada and colleagues demonstrated that LTD, potently stimulates chemotaxis of peripheral blood eosinophils isolated from nonasthmatic individuals (threshold concentration was lO-‘o~1)26. This direct stimulatory effect is abolished by the potent and selective CysLT, receptor antagonist, pobilukast (SK&F104353). A potential link has emerged between the actions of the cysteinyl leukotrienes and interleukin 5 (IL-5), a key cytokine responsible for the differentiation, trafficking and activation of eosinophils27. Recent studies indicate that LlD&duced airway eosinophilia in guinea-pigs is antagonized by the rat anti-mouse IL-5 monoclonal antibody derived from the TRFK-5hybridoma cell line (Ref.24). This suggests an indirect mechanism, involving IL-5, underlies the effects of LTD, on eosinophil recruitment. Moreover, the cysteinyl leukotrienes are produced by eosinophils5 and IL-5 potentiates the release of the cysteinyl leukotrienes induced by various stimuli8,29.Thus, it is possible that there is a positive feedback amplification pathway whereby IL-5 enhances the release of cysteinyl leukotrienes which, in turn, contribute to eosinophil recruitment. This mechanism may underlie the persistent airway eosinophilia observed after a single LTD, challenge in guinea-pigs”. However, the evidence that, unlike human eosinophils, guinea-pig eosinophils do not appear to synthesize the cysteinyl leukotrienes (due to the lack of LTA, glutathione S-transferasero opposes this theory.
mucosal explants 31132. This effect is blocked by FPL55712. Histochemical analysis reveals that aerosol administration of LTD, increases epithelial mucus secretion in guinea-pig airways via a receptor-mediated mechanism that is antagonized by pobilukast33.LTC, increases mucin release from cat trachea (an effect that is antagonized by FPL55712) in viva but not in uifr’034.In addition to these effects of the cysteinyl leukotrienes on mucus release, aerosol administration of LTD, slows mucus transport in sheep airways% and decreases the activity of human respiratory cilia%.The potential pathophysiological importance of this observation is demonstrated in a study of six patients with asthma, in which the decrease in tracheal mucus velocity induced by aerosol administration of ragweed antigen is prevented by FPL55712 (Ref. 37).
Effects on mucus secretion and mucociliary transport Hypertrophy of mucosal glands and enhanced secretion of mucus are features of asthma which, along with cellular debris, may be responsible for the formation of mucus plugs that are found in the airways of individuals of who have died after an acute asthma attack”. Both LTC, and LTD, potently (EC,= 1 no and 10 no, respectively) stimulate mucus release, but are without effect on lysozyme secretion, from cultured human airway
Interactions with nerves Evidence is mounting that the cysteinyl leukotrienes may influence pulmonary function by modulating the activity of the afferent nervous system. Stewart and colleagues noted that LTD, significantly potentiates the bronchoconstrictor response to histamine by a mechanism that is inhibited either by capsaicin-induced denervation or by hexamethonium treatment39.Cysteinyl leukotrienes did not potentiate exogenously added acetylcholine or stimulation of the vagus nerve. These
TiPS - September
1995 (Vol. 16)
Airway smooth muscle proliferation Airway smooth muscle cell hyperplasia is another feature of chronic severe asthma”. MK571 (2 mg kg-1 i.p., 1h before and 3 h after challenge) inhibits the increase in airway smooth muscle mass in large airways elicited by aerosol administration of ovalbumin (six challenges at five day intervals) in ovalbumin-sensitized Brown Norway rats, concomitant with antagonism of ovalbumin-induced bronchospasm and the airway hyperresponsiveness to methacholine3. These data suggest that the cysteinyl leukotrienes play a role in the regulation of airway smooth muscle proliferation. However, it is not known if they are acting as direct mitogens or as co-mitogens, potentiating the airway smooth muscle proliferative effects induced by other substances (for example, growth factors).
x data indicate that LTD, may enhance the responsiveness of capsaicin-sensitive afferent fibres in the guinea-pig airway. This idea is supported by electrophysiological studies, in which LTC, inhibits an apamin-insensitive, Ca2+-activated K+ current and depolarizes the resting membrane potential of C fibres in guinea-pig vagal sensory ganglia. Inhibition of this current leads to inhibition of the hyperpolarization and a substantial increase in the frequency at which the afferent fibre can elicit action potentials40-42. In the airways a subset of afferent C fibres stores and, upon electrical stimulation, releases substance P and related tachykinins locally. The effects induced by the tachykinins include airway smooth muscle constriction and pro-inflammatory activity. Exogenously added LTD, causes the release of substance I’ from guinea-pig isolated trachea43. Concentrations of LTD, that are threshold for smooth muscle contraction may also potentiate the tachykinin-mediated response in the guinea-pig isolated airway evoked by threshold electrical stimulation of the vagus nerve or electrical-field stimulations; this appears to be a result of an enhancement of the action potentialstimulated release of tachykinins from the afferent neurones. It is interesting to note that vagally mediated smooth muscle contraction and plasma extravasation, induced by tachykinin release in the guinea-pig isolated airways, are inhibited by selective cysteinyl leukotriene receptor antagonists, suggesting that endogenous cysteinyl leukotrienes prejunctionally modulate responses to tachykinins in the airway@. The cysteinyl leukotrienes or cysteinyl leukotriene receptor antagonists have no effect on the contraction of airway smooth muscle elicited by exogenously applied tachykinins~. Also supporting a role for endogenous cysteinyl leukotrienes in modulating tachykinin-mediated innervation in the airways are data from experiments on hyperpnoea-induced bronchoconstriction in guinea-pigs, a model of exercise-induced asthma45. The evidence suggests that the bronchoconstriction in this model is secondary to the release of tachykinins from afferent fibres. Inhibitors of 5-lipoxygenase and cysteinyl leukotriene receptor antagonists decrease the magnitude of the bronchoconstriction in this model+ Considered together, these data suggest that in guinea-pigs cysteinyl leukotrienes, synthesized by airway mast cells (or other cell types), interact with sensory fibres leading to changes in their excitability as well as enhance ment of the release of tachykinins from their terminals. It remains to be determined if cysteinyl leukotrienes have a similar neuromodulatory function in human asthmatic airways.
Results of clinical trials in asthma The clinical trials using the first developed cysteinyl leukotriene receptor antagonists, such as FPL55712,were disappointing, probably due to the inherent poor potency and pharmacokinetics of these compoundsza. Much more encouraging and impressive data have been obtained with the more recent potent and selective compounds
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from diverse structural classes, in particular zafirlukast (ICI204219), pranlukast, montelukast (MK476) and pobilukast (Table 1). These, and other compounds with similar pharmacological profiles that are generally spe cific for the CysLT, receptor, are effective against bronchospasm induced by antigen, aspirin, exercise, and cold air in asthmatic individuals7. In some studies there is an improvement in baseline pulmonary function in asthmatics7,47,@. Additive bronchodilator effects are observed following pretreatment of asthmatic individuals with the cysteinyl leukotriene receptor antagonists verlukast (MK679)@or MK571 (Ref. 47), followed by a p,-adrenoceptor agonist. Interestingly, in several studies of antigeninduced bronchospasm, cysteinyl leukotriene receptor antagonists and FLAP inhibitors attenuate not only the early-phase response (thought to be the result of acute airway smooth muscle contraction) but also the late-phase response that occurs approximately four to eight hours after antigen challenge and is thought to involve mechanisms other than airway smooth muscle contraction (for example, oedema, inflammatory cell recruitment and activation)7,50,51. There is recent clinical information of a significant improvement in several objective and subjective measures of asthma [including asthma symptom severity scores, forced expiratory volume in 1 s values, airway hyperresponsiveness or p,-adrenoceptor agonist usage (or both)] with pranlukastsz or zafirlukasts3. In addition, zileuton (1.6 or 2.4 g day-i for 4 weeks), a 5-lipoxygenase inhibitor, improved pulmonary function and decreased symptoms and the frequency of /3,-adrenoceptor agonist use in clinical trials using mild-to-moderate asthmatic subjects%.
Unanswered questions There is increasing evidence from in vitro and in vim studies that the cysteinyl leukotrienes exert a variety of effects with relevance to the aetiology of asthma (Fig. 3). The recent clinical studies with potent and selective cysteinyl leukotriene receptor antagonists or synthesis inhibitors suggest a greater magnitude and broader spectrum of therapeutic efficacy of this class of compounds than would be anticipated, based solely on antagonism of cysteinyl leukotriene-induced bronchospasm. Furthermore, the effectiveness of these compounds suggests that a strategy to modify only one mediator may, in fact, offer significant therapeutic benefit even in a complex disease such as asthma whose overall pathogenesis is thought to involve multiple cells, chemical mediators and cytokines and several underlying mechanisms. Despite the growing optimism that these agents will fulfil their promise of being a truly novel and effective class of antiasthmatic drugs, several important questions remain. For example, how do the efficacies of cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors compare to each other (is there a significant role for LTB,?) and to current therapies? Where will the cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors be positioned and utilized according to the
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cationic proteins
(epithelial cell damage) decreased mucus transport ---+ airway epithelium
increased release
inflammatory cells (e.g. mast cells, eosinophils)
l
a ~-+
cysteinyl
leukotrienes
e /
smooth muscle
.
contraction and proliferation (?) Fig.3. Potential sites and effects of cysteinyl leukotrienes relevant to a pathophysiological
present International Guidelines for asthma treatment? Will these agents be effective in all asthmatics or only in a specific subset? If only a subset of patients respond, can this selectivity be explained by heterogeneity in environmental or genetic factors? Will these agents prove efficacious in treating other atopic disorders (for example, allergic rhinitis, atopic dermatitis)? Finally, how important are the pro-inflammatory, smooth muscle proliferating and neuronal effects of the cysteinyl leukotrienes in the pathophysiology of asthma and, accordingly, how much of the clinical efficacy of cysteinyl leukotriene receptor antagonists and 54ipoxygenase inhibitors results from countering these activities? Answers to these and other questions should be forthcoming as more clinical studies with these agents are carried out. Selected references 1 Kellaway, C. H. and 2 3 4 5 6 7
8 9 10 11 12
3 0
8
Trethewie, E. R. (1940) Q. J. Exp. Physiol. Cogn. Med. Sci. 30,121-145 Brocklehurst, W. E. (1960) J. PhysioI. 151,416-435 Samuelsson, B. (1983) Science 220,568-575 Musser, J. H. and Kreft, A. F. (1992) J. Med. Chem. 35,2501-2524 Drazen, J. M and Austen, K. F. (1987) Am. Rev. Respir. Dis. 136, 985-988 Busse, W. (1992) Ann. AUkrgy 69,261-266 Hay, D. W. P. and Griswold, D. E. (1994) in Handbook ofImmunopharmucology: LipidMediators (Vol. 10) (Cunningham, F., ed.),pp. 117-179, Academic Press Hay, D. W. P. and Raebum, D. in Ainoays Smooth Muscle (Vol. 5) (Raebum, D. and Giembicz, M. A., eds), Birkhluser (in press) A~I, J. P. and Lee, T. H. (1993) Clin. Sci. 84,501-510 Hay, D. W. P. et al. (1987) J Phnrmacol. Exp. 7kr. 243,474-481 Dunrxill, M. S. (1987) in Pulmonary Pathology (Dunnill, M. S., ed.), pp. 61-80, Churchill Livingstone Beasley, R., Burgess, C., Crane, J. and Roche, W. (1993) J. Allergy Clin. Immunol. 92,1&l%
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role in asthma
13 Persson, C. G. A. (1988) Lung 166,1-23 14 Woodward, D. F., We&man, B. M., Gill, C. A. and Wasserman, M. A. (1983) Prostuglandins 25,131-142 15 Evans, T. W., Rogers, D. F., Aursudkij, B., Chung, K. F. and Barnes, P. J. (1989) Clin. Sci. 76,479-485 16 Bochnowicz, S. and Underwood, D. C. (1995) Prostuglandins Leukotrienes Essential Fatty Acids 52,403411 17 Joris, I., Majno, G., Corey, E. J. and Lewis, R. A. (1987) Am. J. Puthol. 126,19-24 18 Woodward, D. F., Wasserman, M. A. and Weichman, B. M. (1983) Eur. J. Pharmucol. 93,9-19 19 Nakagawa, N. et al. (1992) Jpn. J. Pharmucol. 60,217-225 20 Obata, T. et al. (1992) Life Sci. 51,157~1583 21 Gleich, G. J. (1990) J. Allergy Clin. Immunol. 85,422-436 22 Laitinen, L. A. et al. (1993) Lancet 341,989-990 23 Foster, A. and Chan, C. C. (1991) Int. Arch. AllergyAppl. Immunol. 96, 279-284 24 Underwood, D. C., Osbom, R. R., Newsholme, S. J., Torphy, T. J. and Hay, D. W. P. (1995) Abstract, at ‘Asthma 1995: Theory to Treatment’, 15-17 July 1995, Chicago, USA 25 Turner, C. R., Smith, W. B., Andersen, C. J., Swindell, A. C. and Watson, J. W. (1994) Pulm. Phurmucol. 7,49-58 26 Spada, C. S., Nieves, A. L., Krauss, A. H-P. and Woodward, D. F. (1994) J. Leukocyte Biol. 55,18>191 27 McKenzie, A. N. J. and Sanderson, C. J. (1992) in Chemical Immunology: Interleukins: Molecular Biology and Immunology (Vol. 51) (Kishimoto, T., ed.), pp. 135-152, Karger 28 Van Oosterhout, A. J. M., Van Der Poel, A., Koenderman, L., Roos, D. and Nijkamp, F. P. (1994) Med. Inflam. 3,53-55 29 Takafuji, S., Bischoff, S. C., De Week, A. L. and Dahinden, C. A. (1991) J. Immunol. 147,385!%3861 30 Sun, F. F. et al. (1991) J. Leukocyte Biol. 50,140-150 31 Marom, Z., Shelhamer, J. H., Bach, M. K., Morton, D. R. and Kaliner, M. (1982) Am. Rev. Respir. Dis. 126,449-451 32 Coles, S. J. et al. (1983) Prostuglundins 25,155-170 33 Hoffstein, S. T., Malo, P. E., Bugelski, P. and Wheeldon, E. B. (1990) Exp. Lung Res. 16,711-725 34 Peatfield, A. C., Piper, P. J. and Richardson, P. S. (1982) Br. J. Phwmucol. 77,391-393 35 Russi, E. W. etal.(1985) J. A@. Physiol. 59,1416-1422 36 Bisgaard, H. and Pedersen, M. (1987) Clin. Allergy 17,95-103
R 37 Ahmed, T., Greenblatt, D. W., Birch,S., Marchette, B. and Wanner, A. (1981)Am. Rev. Respir. Dis. 124,110-114
38 Wang, C. G., Du, T., Xu, L. J. and Martin, J. G. (1993)Am. Rev.Respir. Dis. 148,413-417 39 Stewart, A. G., Thompson, D. C. and Fennessy, M. R. (1984)Agents Actions 15,sSO8 40 Undem, B. J., Hubbard, W. and Weinreich, D. (1993)J. Auton. Net-v. Syst. 44,35-I4
41 42 43 44 45 46 47 48 49 50 51
52 53 54
Undem, B.J. and Weinreich, D. (1993)J.Auton. Nerv. Syst. 44,17-34 Weinreich, D. and Wonderlin, W. F. (1987)J. Physiol. 394,41%27 Bloomquist, E. I. and Kream, R. M. (1990)Exp. Lung Res. 16,645-659 Ellis,J. E. and Undem,B. J. (1991)J Physiol.436,46%484 Ray, D. W., Hernandez, C., Munoz, N., Leff, A. R. and Solway, J. (1988)J. AppI. Physiol. 65,934-939 Garland,A. et al. (1993)I. Appl. Physiol. 75,2797-2804 Gaddy,J. N., Margolskee,D. J., Bush, R. K., Williams, V. C. and Busse, W. W. (1992)Am. Rev.Respir. Dis. 146,358-363 Irnpens, N. et al. (1993)Am. Rev. Respir. Dis. 147,1442-1446 Lammers, J-W. J. et al. ‘(1992)Pulm. Phmmacol.5,121-125 Taylor, I. K., O’Shaughnessy, K. M., Fuller, R. W. and Dollery, C. T. (1991)Luncet 337,690-694 Friedman, B. S. et al. (1993)Am. Rev. Respir. Dis. 147,839-844 Taki, F. et al. (1991)Antim.-Forsch. /Drug Res. 44,330-333 Spector, S. L., Smith, L. J., Glass, M. and the Accolate Asthma Trial& Group (1994)Am. J Respir. Crit. Cure Med. 150,618-623 Israel, E. et al. (1993)Ann. Intern. Med. 119,1059-1066
Na+channels as targets for neuroprotectivedrugs Charles P. Taylor and Brian S. Meldrum Drugs that block voltage-dependent well known as local anaesthetics, anticonvulsants. compounds
Na+ channels are antiarrhythmics
also provide a powerful mechanism
cytoprotection
and
Recent studies show that these in animal models of cerebral
of
ischaemia,
hypoxia or head trauma. In this article Charles Taylor and Brian Meldrum
review evidence
Na+ channel modulators
indicating that
are neuroprotective
describe recent ideas for the molecular of voltage-dependent
and
sites of action
Na+ channel blockers. Clinical
trials with several compounds stroke and traumatic
are now in progress for
head injury, and the therapeutic
potential for this group of compounds
is discussed.
Na+ channels have long been known to underlie axonal and somatic action potentials; more recently it has been shown that Na+ channels actively propagate information within the dendritic tree of pyramidal neuronesi. Tetrodotoxin (TTX)-sensitive Na+ channels also underlie small inward currents that do not inactivate*. These persistent Na+ currents amplify synaptic potentials and other subthreshold depolarizations and serve as a pacemaker for action potential firing; they also may underlie a detrimental Na+ influx in neurones during seizures or ischaemia.
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Chemical names ABT761: (R)-N-(3-[5-(4fluorobenzyl)thien-Z-yl]-lmethyl-2-propyl]-l\r hydroxyurea ZD2138: 6-( [3-fluoro-5-(4methoxy-3,4,5,6-tetrahydro-W-pyran-4yl)phenoxy]methyl]-lmethylquinol-2-one Bayx7195: (4s)-(4-carboxyphenylthio)-7-[4(phenoxybutoxy)phenyl]-hept-5(Z)-enoic
acid
MK866: 3-[I-(p-chlorophenyl)&sopropyl-3-tbutylthio-lff-indol-2-yl]-22_dimethylpropanoic acid MK57l: 3-((3-[-2(7-chloro-2-quinolinyl> ethenyl]phenyl}( [3-(dimethylamino-3oxopropyl)thio]methyl]thio)propanoic
acid
FPL55712: 7-[3-(4-acetyl-3-hydroxy-2-propylphenoxy)-2-hydroxypropoxy]-4-oxo-8-propyl-4?fl-benzopyran-2-carboxylic acid
Distinct types of Na+ channels have been identified in rat brain, heart and skeletal muscle. In rat brain, the differential distibution of Na+ channels has been studied using immunohistochemical techniques with sitedirected antibodies to unique regions of the three channel types, and also by immunoprecipitation (for review see Ref. 3) (see Box 1). Type II Na+ channels comprise more than half the channels in the neocortex and forebrain, with type I channels making up a large proportion of the remainder. These channels are predominantly located in cell bodies and dendrites (type I) or in axons (type II), although type II channels are in higher overall abundance in both areas. Type III channels are expressed significantly only at early stages of development. So far, no major differences in physiology or drug sensitivity have been described between type I and II channels. However, cloned type III Na+ channels have slower activation and particularly slower inactivation than type I or II channels. Recently, a novel type of Na+ channel has been cloned and is expressed widely throughout the brain both in neurones and in glia4.
Drug-induced block of Na+ channels
C. P. Taylor.
Within the physiological range of membrane voltages, ITX blocks the same percentage of channels whether membranes are depolarized or not, so its action is voltage independent. This contrasts with voltage-dependent channel blockers such as phenytoin, carbamazepine and a family of other drugs that are similar to lidocaine in several functional respect@. Channel block is enhanced by sustained depolarization, for example, for phenytoin IC, = 120 PM at - 85 mV, but IC, = 10 FM at -60 mV, and is thus voltage dependent. In addition, block with these agents is enhanced by brief preceding voltage steps that momentarily open Na+ channels; this is called
Research Fellow.
TiPS - September
1995
(Vol.
16)
Department of Neurolqcal
and
Neurodegeneratlve O~seases. Parke-Davts Pharmaceutxal Research Drasion. Warner-Lambert, 2BW Plymouth Road, Ann Arbor. MI 48105. USA, and B. S. Meldrum, Professor. Department of Neurology. Institute of Psychlatw. De Cresplgny Park. Denmark Hill. London, UK SE5 8AF
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