Reduced corticosteroid sensitivity in smokers with asthma : potential mechanisms and treatment

Smokers with asthma display reduced responses to both inhaled and oral corticosteroids with associated increased symptoms, accelerated decline in lung function and increased use of health care services. Little work has been undertaken to address the possible causes of this reduced response and to find effective replacement therapies. Therefore this thesis was carried out with the aim of identifying potential mechanisms and new therapies for this group. The oral bronchodilator theophylline has been suggested as a treatment for corticosteroid insensitivity due to its ability to increase HDAC activity in-vitro. I undertook an exploratory proof of concept clinical trial based on the hypothesis that low dose theophylline would restore corticosteroid sensitivity in smokers with asthma through theophylline induced recovery of HDAC activity. Low dose oral theophylline added to inhaled corticosteroid increased pre-bronchodilator lung function and reduced symptoms of asthma whilst low dose theophylline given alone reduced symptoms but had no effect on pre-bronchodilator lung function. This research provides a foundation for future studies designed to examine the efficacy of theophylline in smokers with asthma. Agonists of the nuclear hormone receptor peroxisome proliferator activated receptor-γ (PPARγ) have been demonstrated to be effective at reducing inflammation in both in-vitro and animal models of asthma. Therefore to examine the hypothesis that PPARγ stimulation would reduce the inflammation present in smokers with asthma I undertook an exploratory, proof of concept clinical trial using the PPARγ agonist rosiglitazone. Treatment with rosiglitazone was associated with a trend to improvement in FEV1 and improvement in a marker of small airway lung function and as such may provide an alternative treatment for small airways obstruction in conditions such as asthma and chronic obstructive airways disease. This trial will enable powering of future confirmatory studies. Altered cytokine profiles, specifically the combination of increased interleukin (IL)-2 and 4, are observed in asthmatic subjects with corticosteroid insensitivity. Based on this work I examined the hypothesis that the altered response to corticosteroids in smokers with asthma was associated with an altered cytokine milieu including raised levels of IL-2 and 4. Smokers with asthma, characterised as corticosteroid resistant by oral corticosteroid trial, demonstrated significantly raised sputum supernatant IL-6 levels and raised levels of a number of other sputum cytokines compared to non smokers with asthma. This altered phenotype suggests cigarette smoking in asthma may be associated with a deviation to Th1 mediated inflammation and could provide an explanation for the reduced corticosteroid response of smokers with asthma. The cell type/s responsible for both this shift in immunological phenotype and production of increased levels of sputum cytokines is unclear and will require further study. Previous in-vitro and in-vivo research has identified altered histone acetylation patterns in subjects with relative corticosteroid resistance. Therefore I examined the hypothesis that smokers with asthma displayed reduced responses to corticosteroids as a result of a cigarette smoke induced reduction in histone de-acetylase (HDAC) activity. Smokers with asthma provided sputum macrophages and blood for peripheral blood borne monocytes to examine total HDAC activity. Sputum and blood macrophage total HDAC activity was equivalent in smokers and non-smokers with asthma. Therefore reduced blood total HDAC activity does not appear to explain the altered corticosteroid response in this group. However the number of sputum macrophages obtained may have been too low to allow conclusive examination of this endpoint. Another consideration is that contamination of the sample due to the technique used may be altering the signal obtained. Further work either through modification of sputum induction techniques to increase macrophage number or bronchoscopic sampling is required to conclusively address the role of alveolar macrophage HDAC activity in the reduced corticosteroid response displayed by smokers with asthma. Exhaled nitric oxide has been exploited as a useful exploratory and confirmatory endpoint in asthma. However exhaled nitric oxide, measured using standard flow rates and methodology, is unhelpful in smokers with asthma as cigarette smoking is associated with a marked reduction in exhaled nitric oxide levels in the majority of subjects. Recent research has demonstrated that measurement of exhaled nitric oxide at multiple flow rates followed by mathematical modelling reveals increased levels of alveolar nitric oxide that were unaltered by current smoking. Therefore to examine the hypothesis that smokers with asthma display altered levels of alveolar nitric oxide and flow independent parameters compared to non-smokers with asthma I carried out a cross-sectional study. Alveolar nitric oxide, determined by linear modelling, was significantly reduced in smokers with asthma compared to non smokers with asthma. The concentrations observed were within the range for normal subjects and therefore this method does not overcome the problems inherent in measuring exhaled nitric oxide at standard flows. The use of non-linear modelling did demonstrate parity between smokers and non-smokers with asthma for alveolar nitric oxide. Nitric oxide flux was lower in smokers with asthma when derived by both linear and non-linear modelling and displayed sensitivity to oral corticosteroids. Therefore nitric oxide flux is worthy of further investigation as an exploratory endpoint in smokers with asthma. In conclusion treatment of smokers with asthma with low dose theophylline alone, the combination of low dose theophylline and inhaled corticosteroid and the PPARγ agonist rosiglitazone was associated with clinical improvements and further clinical trials to assess the role for these treatments in the management of smokers with asthma are justified. Smokers with asthma display an altered sputum cytokine profile with raised levels of the proinflammatory cytokine IL-6, equivalent blood total HDAC activity and reduced alveolar nitric oxide compared to non-smokers with asthma. Sputum HDAC activity requires further development before it can be confidently employed as a method of assessing total pulmonary HDAC activity.