Inhaled β2-adrenoceptor agonists are a mainstay of therapy for airways diseases and are almost universally prescribed for patients with asthma or chronic obstructive pulmonary disease (COPD). Very few studies have evaluated the efficacy of these commonly used therapies during acute disease exacerbations which are frequently triggered by viral infection. In this edition of Clinical Science, Donovan et al. assess the ex vivo effects of the most commonly used short-acting β2-agonist salbutamol on small airway reactivity using precision cut lung slices (PCLS) from a mouse model of virus-induced exacerbation of COPD. They demonstrate that combined challenge with cigarette smoke and influenza infection in mice markedly impairs salbutamol-mediated airway relaxation. The findings of the present study suggest that cigarette smoke and respiratory virus infection may intefere with the ability of commonly prescribed therapies to effectively bronchodilate the airways.
Beta-2 agonists are frequently used to treat inflammatory airway diseases such as asthma and COPD.
Few studies have previously examined whether the bronchodilator efficacy of these commonly used therapies is impaired during acute virus-induced exacerbations.
The study by Donovan et al. demonstrates that cigarette smoke and/or influenza infection may impair the ability of salbutamol to effectively bronchodilate the airways.
Asthma and chronic obstructive pulmonary disease (COPD) are chronic inflammatory airway disorders that are responsible for a major burden of respiratory morbidity and mortality worldwide . Both diseases are characterized by the occurrence of acute symptomatic deteriorations or ‘exacerbations’ [2,3]. Acute exacerbations are associated with detrimental effects including reduced quality of life , accelerated lung function decline  and enormous healthcare costs . It is becoming increasingly recognized that respiratory viruses such as rhinovirus and influenza are the most important aetiological triggers for exacerbations. Studies that have employed highly sensitive polymerase chain reaction (PCR) assays have clearly demonstrated that viruses are detected in approximately 80% of naturally occurring exacerbations [7–9]. Human experimental challenge studies have further confirmed a direct causative role for virus infection in precipitation of exacerbation in patients with asthma [10,11] or COPD  with a high proportion of patients experimentally infected developing features of exacerbation. Despite the major burden of disease associated with exacerbations, very few effective therapies are presently available.
β2-Adrenoceptor agonists have been a mainstay of therapy in the management of airways diseases for many years and are almost universally prescribed for patients with asthma and COPD. They are available in twice daily ‘long-acting’ forms (such as salmeterol or formoterol), with once daily ‘ultra long-acting’ forms more recently coming to market. Additionally, they are also frequently used as a short-term reliever of symptoms (most commonly salbutamol) to be taken by patients ‘as required’. β2-agonists mediate their protective effects by inducing raised levels of intracellular cAMP in smooth muscle cells leading to smooth muscle relaxation and thereby, bronchodilatation.
During an acute exacerbation, patients experience heightened respiratory symptoms such as dyspnoea and wheeze and are thus advised to increase usage of their short-acting β2-agonist inhaler to try and stimulate airway relaxation and relieve acute bronchoconstriction. However, this approach is recognized to be relatively ineffective and patients frequently require additional therapy with oral corticosteroids and/or hospitalization for more intensive management of their exacerbation. Upon hospitalization, patients typically have treatment escalated from inhaled to nebulized salbutamol. Several epidemiological studies have linked overuse of short-acting β2-agonists during asthma exacerbations with increased risk of hospitalization and mortality [13–15]. This has led to concerns being raised that overuse of these medications may be detrimental in the context of respiratory viral infections. Our group has shown previously that interleukin (IL)-6, a pro-inflammatory cytokine, is induced in bronchial epithelial cells by β2-agonists alone and importantly in relation to overuse of β2-agonists in asthma and COPD exacerbations, IL-6 induction by rhinovirus infection is further augmented by β2-agonists . Despite this evidence showing that, at least ex vivo, β2-agonists adversely affect inflammatory responses to respiratory viruses, very few studies have previously examined the bronchodilate effects of salbutamol during an acute virus-induced exacerbation and mechanistic insight into any potential impairment of efficacy has been thus far lacking .
The study by Donovan et al. in this edition of Clinical Science assessed the ex vivo effects of the commonly used β2-agonist salbutamol on small airway reactivity using precision cut lung slices (PCLS) from a mouse model of virus-induced exacerbation of COPD induced by exposure to cigarette smoke and challenge with influenza A virus (A/Memphis/1/71 (H3) strain). The authors have previously reported that this mouse model of short-term (four days) exposure to cigarette smoke followed by challenge with influenza reproduces some of the immunopathological features of human COPD exacerbation with increased virus loads and enhanced cellular airways inflammation observed in mice treated with combined cigarette smoke exposure and influenza infection compared with either stimulus alone . The PCLS technique employed in the present study allowed the authors to directly measure airway relaxation and contraction in an ex vivo system where structural interactions between the airways and surrounding parenchyma are preserved.
The authors examined dilator responses to increasing concentrations of salbutamol on airway sections pre-contracted with 300 nM serotonin (5HT; 5-hydroxytryptamine). Firstly, they evaluated the effect of cigarette smoke alone on dilator responses by comparing relaxation with salbutamol in lung sections from mice exposed to four days of cigarette smoke compared with control mice exposed to room air. Salbutamol-mediated relaxation was markedly impaired with approximately 5-fold lower potency observed in PCLS from cigarette smoke-exposed mice compared with controls. Cigarette smoke exposure was also shown to cause a significant increase in lung mRNA expression of pro-inflammatory cytokines IL-1β and tumour necrosis factor (TNF)-α and, additionally, a reduction in lung β2-adrenoceptor mRNA expression, an effect that may explain the reduced efficacy of salbutamol observed in the model. More detailed mechanistic insight into these effects was not fully elucidated in the present study however and requires further characterization. The relevance of a short-term smoke exposure model to human disease also may be questioned, particularly given that detectable structural lung changes and significant lung function abnormalities are not induced . It is notable that impairment of salbutamol efficacy was lost when evaluated by the authors at 7 days following cigarette smoke exposure. Other models of COPD exacerbation employing elastase and/or lipopolysaccharide administration to induce emphysematous lung changes may thus be more relevant [18,19]. Nonetheless, these data suggest that smoking, and thus possibly COPD, may be associated with an impairment of bronchodilate response to salbutamol.
The authors also examined the effects of viral infection and combined cigarette smoke/infection on bronchodilate effects of salbutamol. At day 7 post-infection, salbutamol-mediated relaxation was markedly impaired in lung sections from mice challenged with cigarette smoke and infected with influenza. However, in contrast with the effects of acute cigarette smoke alone, there were no differences in lung β2-adrenoceptor mRNA expression observed in mice treated with cigarette smoke and/or influenza compared with sham-treated controls. This suggests that the reduced efficacy of salbutamol observed in this model is not directly due to a direct effect of cigarette smoke and/or virus infection on β2-adrenoceptor expression, with other, yet to be characterized, mechanisms responsible.
What are the implications of these observations to clinical management of patients with COPD? The findings of the present study suggest that cigarette smoke and respiratory virus infections may impair the ability of therapies used commonly in clinical practice to effectively bronchodilate the airways. However, these findings should be interpreted with caution as the study solely employed animal models and in vitro methods and the relevance to COPD, a complex and clinically heterogeneous disease, may be questionable. Confirmation of these effects in vivo using studies in human experimental viral challenge models of COPD is now needed to determine whether similar effects occur in man. Additionally, it would be interesting to determine whether the other commonly used reliever bronchodilate ipratropium bromide, which is prescribed by inhaler in community based exacerbations and in nebulized form for hospitalized exacerbations and acts via a different mechanism, is similarly impaired by cigarette smoke and/or viral infection.
Ultimately, the present study may pave the way for new avenues of future therapeutic development in asthma and COPD. Development of novel bronchodilates that remain effective, even in the context of the cigarette smoke- and virus exposed- airway, have the potential to be more beneficial for patients, both during clinical stability and upon acute exacerbation. The ex vivo methods employed in the present study could provide a useful preclinical model for early testing of such therapies prior to investment in more complex models or in large-scale clinical trials.