Mepolizumab and In-office Nasal Polypectomy in Patients With Chronic Rhinosinusitis (CRS). A Three Arm Study.

Inclusion Criteria: * Participant 18 years of age or older * Patients with a diagnosis of CRS with polyposis according to the criteria of the EPOS2020 guideline: Chronic rhinosinusitis with nasal polyps in adults is defined as presence of two or more symptoms, one of which should be either nasal blockage / obstruction / congestion or nasal discharge (anterior / posterior nasal drip); ± facial pain/pressure; ± reduction or loss of smell; for ≥12 weeks; and either endoscopic signs of nasal polyps, and/or CT changes: mucosal changes within the ostiomeatal complex and/or sinuses. * Patients with indication for biologic treatment according to the criteria of the EPOS2020 guideline. Presence of bilateral polyps in a patient who had ESS (except in cases where the patient is not fit for surgery, see bellow) and three of the following criteria are required: * Evidence of type 2 inflammation (tissue eos\> 10/hpf\* or blood eos\>250, OR total IgE\> 100). * Need for systemic corticosteroids or contraindication to systemic steroids (≥ 2 courses per yr, OR long term (\>3 months) * Significantly impaired quality of life (SNOT ≥ 40) * Significant loss of smell (≥7 VAS) * Diagnosis of comorbid asthma * Basal eosinophil count (BEC) ≥300 cells/μL for surgical naïve patients- only those not fit for surgery (major criteria) or for those who have had surgery and have recurrence (minor criteria). * Patients have been informed and signed consent to participate in the study. * For those patients not suitable for surgery, the criteria of the EUFOREA guidelines for indication of biological treatment will be used (4 criteria, one of them being ≥300 cells/μL BEC). Exclusion Criteria: * Any serious or uncontrolled systemic or psychiatric illness that prevents a correct cognitive or endoscopic assessment or does not advise participation in the study. * Unilateral localized secondary CRS (odontogenic, fungal ball, tumor). * Bilateral diffuse secondary CRS (cystic fibrosis; eosinophilic granulomatosis with polyangiitis-S. de Churg-Strauss-; granulomatosis with polyangiitis-S de Wegener-; primary ciliary dyskinesia; selective immunodeficiencies. * Pregnancy/ breast feeding * Hypersensitivity to mepolizumab or it's excipients * Patients unable to complete the questionnaire or follow prescribed treatment. * Patients who are participating in another clinical trial related to polyposis or asthma. * Previously documented failure with IL-5/IL-5 receptor biologics