Efficacy and Safety of Mepolizumab in Adults With Chronic Rhinosinusitis With Nasal Polyps (CRSwNP)/ Eosinophilic Chronic Rhinosinusitis (ECRS)

Inclusion Criteria

• Participants of 18 years of age and older inclusive, at the time of signing the informed consent.
• Body weight greater than or equal to 40 kilograms (kg).
• Male or female participants (with appropriate contraceptive methods) to be eligible for entry into the study.
• Female participant is eligible to participate if she is not pregnant or breastfeeding, one of the following conditions applies:
• Is a woman of non- childbearing potential (WONCBP) : or
• Is a woman of child bearing potential (WOCBP) and using a contraceptive method that is highly effective [with a failure rate of less than ( )5.
• Presence of symptoms of CRS as described by at least two different symptoms for at least 12 weeks prior to Visit 1, one of which should be either nasal blockage/obstruction/congestion or nasal discharge (anterior/posterior nasal drip), plus facial pain/pressure, and/or reduction or loss of smell
• Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and study protocol.

Exclusion Criteria

• As a result of medical interview, physical examination, or screening investigation the physician responsible considers the participant unfit for the study. (e.g. symptomatic herpes zoster within 3 months prior to screening, evidence of tuberculosis [TB] active or latent).
• Cystic fibrosis
• Eosinophilic granulomatosis with polyangiitis (also known as Churg Strauss syndrome), Young's, Kartagener's or dyskinetic ciliary syndromes.
• Antrochoanal polyps.
• Severe nasal septal deviation preventing full assesment of nasal polyps in both nostrils.
• Acute sinusitis or upper respiratory tract infection (URTI) at screening or in 2 weeks prior to screening.
• Ongoing rhinitis medicamentosa (rebound or chemical induced rhinitis).
• Participants who have had an asthma exacerbation requiring admission to hospital within 4 weeks of screening.
• Participants who have undergone any intranasal and/or sinus surgery (for example polypectomy, balloon dilatation or nasal stent insertion) within 6 months prior to Visit 1.
• Participants where NP surgery is contraindicated in the opinion of the Investigator.
• Participants with a known medical history of Human Immunodeficiency Virus (HIV) infection.
• Participants with a known, pre-existing parasitic infestation within 6 months prior to Visit 1.
• Participants who are currently receiving or have received within 3 months (or 5 half-lives - whatever is the longest) prior to first mepolizumab dose, chemotherapy, radiotherapy or investigational medications/therapies.
• Participants with a history of sensitivity to any of the study medications, or components thereof or a history of drug or other allergy that, in the opinion of the investigator or GlaxoSmithKline (GSK) Medical Monitor, contraindicates their participation. Aspirin sensitive participants are acceptable.
• Participants with a history of allergic reaction to anti-IL-5 or other monoclonal antibody therapy.
• Participants that have taken part in previous mepolizumab clinical studies.
• Participants currently using intranasal corticosteroids (INCS) and inhaled corticosteroids exhalation through nose (ICS/ETN) for the management of their ECRS who are not willing to maintain using this method of administration throughout the study.
• Use of systemic corticosteroids (including oral corticosteroids) or corticosteroid nasal solution (intranasal corticosteroid is excepted) within 4 weeks prior to screening or planned use of such medications during the double-blind period.
• INCS and/or inhaled corticosteroids exhalation through nose (ICS/ETN) dose changes within 1 month prior to Visit 1 (if applicable).
• Treatments with biological or immunosuppressive treatment (other than Xolair) treatment within 5 terminal phase half-lives of Visit 1.
• Omalizumab (Xolair) treatment in the 130 days prior to Visit 1.
• Commencement or change of dose of leukotriene an