Established and Emerging Asthma Biomarkers with a Focus on Biologic Trials: A Narrative Review

Chronic airway inflammation with variable airflow obstruction is clinical asthma, and it arises from distinct molecular and pathological mechanisms called endotypes. Biomarkers allow for precise endotype characterization and have been used in clinical trials to design, monitor, and evaluate outcomes for asthma biologic therapies. This review will highlight the central and evolving role of biomarkers for past, present, and future asthma, with a focus on regulatory-approved biologic therapies and emerging biomarkers. Established biomarkers, including serum immunoglobulin E (IgE), blood eosinophils, the fraction of exhaled nitric oxide (FeNO), and serum periostin, helped elucidate the complex pathophysiology of the eosinophilic type 2 (T2) asthma endotype. Emerging biomarkers, or older biomarkers with emerging utility, include sputum inflammatory cells (eosinophils, neutrophils, interleukins), thymus and activation-regulated chemokine (TARC), plasma eotaxin-3, eosinophil peroxidase (EPX), Clara/club cell secretory protein (CC16), and quantitative computerized tomography (QCT) imaging biomarkers (evaluating mucus plugging, air trapping, airway wall thickness, small airway remolding) and are increasingly used in clinical trials as secondary endpoints in evaluating efficacy, as well as in the clinical setting at specialized centers. The rapid advances in asthma research, due in part to biomarkers and biologic therapies, may soon standardize an end goal: symptom-free asthma remission without exacerbations.