The Pharmaceutical Journal Vol 263 No 7066 p600-601
October 9, 1999 The Conference

Science symposium

Asthma: the role of leukotrienes

In a symposium on September 15 discussing the role of leukotrienes in asthma, conference participants heard experts discuss some of the latest biological and clinical findings in this field

Clinical efficacy of antileukotrienes

Early studies in mild to moderate asthma with LT receptor antagonists (such as zafirlukast, montelukast, pranlukast) suggested that LTs might mediate asthmatic effects through bronchoconstriction. However, in a longer term study with zafirlukast it was noted that the number of asthma exacerbations was reduced significantly by 50 per cent among patients taking the drug and not in those only receiving inhaled steroids. This was difficult to explain just in terms of a reduction in bronchoconstriction. Furthermore, it was found that in patients needing high doses of inhaled steroids, a gradual improvement in lung function was seen with zafirlukast. It appeared that, as the severity of asthma increased, so the anti-inflammatory action of these drugs became more marked; for mild asthma the effects on bronchoconstriction predominated. Coupled with this spectrum of action, there was also marked variation in the way in which individual patients responded to these drugs.

Neil Barnes: More data needed to define use of drugs

In studies comparing the effects of inhaled steroids and LT receptor antagonists, it was evident that the two types of agents had effects on both the early (EAR) and late (LAR) asthmatic responses. However, differences in eosinophil counts (a marker of inflammatory response) in samples after biopsy and lung lavage suggested that each agent might block the LAR by different mechanisms. Also, where zafirlukast showed a tendency to reduce the number of neutrophils in the airway wall in severe asthma, the number was found to be greatly increased after treatment with inhaled steroids. Could steroids actually be pro-inflammatory in this respect, said Dr Barnes.
Concluding, Dr Barnes reviewed the place of antileukotriene agents in clinical practice according to asthma severity (severity is defined in stages 1-4 by the British Thoracic Society). For most patients with stage two (mild, persistent) asthma, inhaled steroids were used successfully for control, he said. LT antagonists might be indicated at stage two where inhaled steroids failed adequately to control symptoms or for those patients for whom steroids were not suitable.
Most commonly, the LT antagonists were currently used in stages three and four in addition to treatment with inhaled steroids. However, more "real life studies in actual patients" were needed to effectively integrate these drugs into clinical use.
Important mediators

Describing the emerging role of LTs in asthma, Professor Sven-Erik Dahlen (department of pharmacology, Karolinska institute, Stockholm, Sweden) said that it was during the 1980s that the bronchoconstrictive and inflammatory effects of leukotrienes were first appreciated and a potential role in asthma proposed.

Sven-Erik Dahlen: Drugs might benefit several diseases

Leukotrienes (LTs) stood out among many of the mediators studied in animals as being relevant to pathophysiological changes in humans, he said. Several drugs had been developed that either inhibited 5-LO or the cell receptor for the cysteinyl LTs themselves (see Panel). One 5-LO inhibitor (zileuton) was marketed, but its four-times daily dose and potential liver toxicity were reflected in a minor market share. Current receptor antagonist drugs targeted the CysLT1 receptor although, in addition to the characterised CysLT2, it was likely that several other receptor subtypes existed. Three CysLT1 antagonists were available but all had the same mode of action and results in clinical trials had been very similar, he said.

Leukotriene background Leukotrienes were inflammatory mediators that contributed to the pathophysiology of asthma and other inflammatory conditions, said Professor Dahlen. They were generated from arachidonic acid, an essential fatty acid found in all cell membranes. Arachidonic acid breakdown to LTA4 was catalysed by 5-lipoxygenase (5-LO) with the assistance of 5-lipoxygenase activating protein (FLAP). LTA4 was then catalysed by LTC4 synthase to form LTC4, a cysteinyl LT, which was converted to LTD4 and then to LTE4 by other enzymes. The cysteinyl LTs (C4, D4, E4) were the predominant mediators of bronchoconstriction and eosinophil chemoattraction in asthma, he said.

Like Dr Barnes, Professor Dahlen commented that there was good evidence that the antileukotrienes had benefits during the late asthmatic response (LAR). It had been suggested, he said, that their main action was in counteracting the LTs produced in mast cells thought originally to predominate in the early asthmatic response. However, there was now evidence for continued mast cell activation in the LAR. Indeed, the involvement of such mediator-producing cells was a commonality that many asthma triggers shared, he said. For example, other asthmatic responses could also be mediated by LTs, such as those triggered by exercise, aspirin or allergens.
There was also evidence of beneficial effects of antileukotrienes on the upper airways, including rhinorrhea and congestion, said Professor Dahlen. Moreover, there was interest in the idea that LT modulation might be of benefit in the treatment of conditions such as irritable bowel disease, nephritis, atopic eczema and pulmonary hypertension.

Unravelling the genetics of asthma

Certain genetic factors may predispose patients to asthma, contribute to its severity, and govern patient responses to antileukotriene agents, said Dr Anthony Sampson (immuno- pharmacology group, Southampton General hospital) discussing the role of genetic factors in asthma. He said that it had been found that the over-production of cysteinyl leukotrienes (LTs) was associated with both acute and chronic asthma, and that LT levels were particularly marked in aspirin-intolerant asthma (AIA). Patients with AIA had baseline LT levels three to seven fold greater than normal. Furthermore, there was a "remarkable" 19-fold overexpression of LTC4 synthase in AIA, underpinning a greater capacity to trigger LT production. Indeed, of 17 clinical or enzymatic markers, the only parameter found to correlate with clinical outcome (ie, decline in forced-expiratory volume, FEV1) was the number of LTC4 synthase containing cells. But what were the underlying mechanisms of these findings?

Anthony Sampson: Could genes explain variable responses to leukotriene antagonists?

Dr Sampson proceeded to review how variants of the genes coding for LTC4 synthase and 5-lipoxygenase (5-LO) might explain asthma severity and drug response.
In studies of the gene coding for LTC4 synthase, AIA patients were found to be more likely than non-asthmatics to have a single point (adenine to cytosine) mutation in the promoter region of the gene. This mutation created an extra "AP-1" binding site that acted as an on-switch for the gene, thus leading to constitutive overexpression of LTC4 synthase. The majority of non-asthmatics or aspirin-tolerant asthmatics either carried two copies of the normal "A" gene (ie, the AA genotype, 57 per cent) or had one variant allele (ie, the AC genotype, 40 per cent). In AIA, however, the AC and CC (both alleles variant) genotypes were found in 62 and 13 per cent of patients, respectively. It seemed that this variant allele conferred a strong tendency towards AIA and presented "a gene" for AIA, but it was probably not the only one.
To investigate whether the genotype (ie, AA, AC or CC) was clinically relevant to LT levels, a sample of 30 people without asthma had been studied. Among the 25 per cent found to carry the AC genotype, only a slight tendency to increased urinary LTE4 was detected. However, commented Dr Sampson, no stimulus for LT release had been provided in this case. Further comparison of stimulated eosinophil samples showed that cells from subjects carrying one or more variant (C) alleles released significantly more LTs. Thus, even in non-asthmatics, the variant allele conferred a trend towards an increased capacity for LT production.
The variant C allele also appeared to be more common among patients with severe, chronic asthma (56 per cent of 23 patients) than in non-asthmatics (32 per cent, P=0.04), said Dr Sampson. Furthermore, patients with the variant allele tended to show better lung function following treatment with zafirlukast than those with the normal phenotype (P=0.1) suggesting that their asthma was more dependent on LT over-production. Thus, in addition to mediating AIA, this gene could also be linked to asthma severity and might explain the variability of response among patients taking LT receptor antagonists.
Variants of the gene coding for 5-LO also appeared to have some influence on LT production and sensitivity to antagonists, said Dr Sampson.
He commented that it was important to recognise that most treatments, such as those mediating the LT pathway, did not have the same effect on every patient. This was a critical time where it was becoming possible to dissect away interactions between pharmacological and genetic effects.

New targets for asthma treatment?

More than 50 inflammatory mediators had now been recognised that might mimic many of the features of asthma, said Dr Domenico Spina (Sackler institute of pulmonary pharmacology, King's College London). These included the classical mediators, such as histamine, prostanoids and leukotrienes, and the "mediators of the '90s" including nitric oxide (NO), growth factors and endothelin. Endothelin was a potent constrictor of bronchial smooth muscle in asthmatics and there was evidence to suggest that endothelin pathways might be augmented in this group. It was also possible that endothelin might increase the number of fibroblasts in airway smooth muscle and play a role in airway remodelling. No endothelin antagonists were yet available, he commented.

Domenico Spina: Over 50 mediators identified

Chemokines, small molecular weight peptides, were another of the asthma mediators described by Dr Spina. Of these, eotaxin and Rantes were found to be potent chemoattractants working predominantly through CCR3 receptors localised on eosinophils, basophils and Th2 lymphocytes. Currently, there were only monoclonal antibodies to the CCR3 receptor and no antagonists of which he was aware. The role of NO in causing airway inflammation was also under investigation. One isoform of nitric oxide synthase (NOS), inducible NOS (iNOS), was the form predominantly expressed in the airway epithelium and macrophages. However, the benefit of highly selective iNOS inhibitors in asthma was yet to be demonstrated.
On a cellular level, Dr Spina described the roles suggested for Th2 lymphocytes in asthma and other inflammatory diseases. These cells secreted a range of pro-inflammatory cytokines and were critical for the recruitment of eosinophils and for bronchial hyperresponsiveness in murine models. However, because of the range of activities of this cell type, it might be more appropriate to target the cell itself rather than the individual cytokines, he said.
Dr Spina concluded that, while drugs targeting single mediators might have minimal side effects, it would be the agents that reduced the activation of inflammatory cells, and subsequent release of inflammatory mediators, that would be more likely to succeed in the future treatment of asthma.