Small Airways Disease (SAD) in Severe Asthma as a Novel Endpoint and Distinct Target for Mepolizumab (SASAM Study)

Rationale Although the majority of asthma patients can be effectively treated with currently available medications, a substantial subset remains severe, causing a considerable proportion of resource expenditure. Severe asthma is now widely accepted to be a heterogeneous syndrome consisting of multiple phenotypes identified by specific biomarkers and targeted by tailored biological therapies. However, much remains unclear regarding the best approaches to manage these patients, or concerning the pathophysiological mechanisms underlying the disease.

Small airways (SA) are defined as those airways with an internal diameter <2 mm. In patients affected by asthma, it has been reported that SA are the predominant site of airflow resistance. Peripheral airways are thickened in asthma due to chronic inflammation in the epithelium, submucosa and muscle area. It has been suggested that the outer wall is more inflamed than the inner wall, with a higher number of lymphocytes, eosinophils, and neutrophils associated to an increased mRNA expression of interleukin-4 (IL-4), IL-5 and eotaxin. Moreover, it is well documented that SA inflammation and dysfunction contributes significantly to the clinical impact of asthma and that 50-60% of asthmatics have a SA involvement across all disease severities. An important question is whether SA disease in asthma is variable among distinct asthma phenotypes and whether it occurs in all patients. Cluster analyses have been recently used to identify specific asthma phenotypes, but markers of SA function have not been investigated. However, evidence is accumulating to support the concept that SA dysfunction and inflammation may contribute to distinct asthma phenotypes. Recent findings indicate that SA are significantly affected in severe asthma and that their involvement is associated with worse disease outcomes. It has been reported that patients with asthma and a history of frequent exacerbations per year had a significant SA involvement Furthermore, peripheral airways significantly contribute not only to the level of asthma control, but also to patients' quality of life and perception of symptoms. At last more thickened SA and higher numbers of eosinophils are detectable in subjects with fatal asthma.

The assessment of SA represents a big challenge and requires qualified expertise and sophisticated techniques including body plethysmography, single and multiple breath nitrogen washout, impulse oscillometry (IOS), fraction exhaled NO at multiflow, sputum induction and high-resolution chest CT (HRCT). Such procedures can either provide functional information on the degree/extent of ventilation heterogeneity and air trapping or facilitate the understanding of the inflammatory and remodeling processes. These measures are not usually part of the evaluation of asthmatic patients and in the monitoring of the effects of drugs recommended for severe asthma.

Mepolizumab represents an innovative weapon for the treatment of severe eosinophilic asthma. In most of these patients the drug controls inflammation, improves lung function, ameliorates clinical symptoms, reduces exacerbations and has a marked steroid-sparing effect. However, there is still a significant proportion of non-responders and a lack of validated predictive biomarkers in such subpopulation. In regard to this, very limited findings are available about the effect of mepolizumab on SA. At the best of our knowledge, the only paper available in literature, addressing the topic, is the study of Farah and co-workers. The authors found that an early improvement in SA function was associated with better asthma control and represented a significant contributor to the therapeutic response. However, the study was conducted in a limited cohort of patients, assessing SA only through multi breath nitrogen washout, and not considering the relationship between SA disease and levels of peripheral/sputum eosinophils. Also, a study was recently initiated at the Hopitaux de Paris to evaluate airway remodelling during mepolizumab treatment (REMOMEPO, NCT03797404).

A better definition of severe asthma phenotypes and endotypes, as well as the identification of novel disease targets and biomarkers to predict treatment response and monitor efficacy and safety of biological drugs over time, would favor a Precision Medicine approach translating in both improved disease management and reduced healthcare costs and social burdens. This is considered a crucial unmet need and further research in the field is strongly recommended by international guidelines, respiratory scientific societies, healthcare systems and regulatory boards.