Is FeNO a reliable marker for follow-up of patients on anti-IL5 therapy?

<bold>Introduction:</bold> Exhaled nitric oxide (FeNO) is an important surrogate marker of Th2 inflammation and seems promising for monitoring severe eosinophilic asthma (SEA). FeNO values can be altered in smokers, obese, atopic, exacerbated and corticosteroid-dependent patients. <bold>Objectives:</bold> Identify the variation of FeNO after initiating anti-IL5 therapy and its association with population variables, asthma exacerbations, blood eosinophil count (BEC) and drug effectivity. <bold>Methods:</bold> Prospective, observational, longitudinal study. A cohort of adult patients with SEA treated with anti-IL5 therapy (mepolizumab, benralizumab) were followed from 11/2021 to 09/2023. BEC and FeNO were assessed before and after initiating anti-IL5. <bold>Results:</bold> 18 patients were enrolled, 56% (n=10) treated with mepolizumab. Most were female (n=14;78%), atopic (n= 11;61%), average age 59 y.o., diagnosed in adult age (n=13;72%), 16% had chronic rhinosinusitis, 22% were smokers, 39% corticosteroid-dependent. Average FeNO pre-therapy was 22 ppb (SD 28), range 7-113 ppb. Mean BEC was 340 (SD 468). Patients were reassessed after 5 months (SD 4). 39% (n=7) had at least 1 exacerbation. There was a slight increase in average FeNO (29 ppb). Only 3 patients had a decrease of FeNO and 1 maintained the same value. Most patients with increased FeNO had <20% variation (n= 9;64%). However, mean BEC decreased to 20 (SD 305). <bold>Conclusion:</bold> In this real life study, there was an unexpected average increase of FeNO after 5 months. However, BEC lowered significantly which contradicts the variation of FeNO. FeNO does not seem to be a reliable marker in SEA under anti-IL5. More studies are needed in SEA concerning lack of anti-IL5 response, risk and phenotyping of exacerbations.