Pharmacokinetics and Pharmacodynamics of Mepolizumab Administered Subcutaneously in Children

Inclusion Criteria:

- Between 6 and 11 years of age inclusive, at the time of screening.

- Diagnosis of severe asthma, defined by the regional asthma guidelines (i.e., National
Institute of Health (NIH), Global Initiative for Asthma (GINA), etc.), for at least 12
months prior to Visit 1. If the participant is naïve to the study site, the
participant/guardian must self-report a physician diagnosis of asthma and the
investigator must confirm by review of medical history with the participant/guardian.

- Eosinophilic airway inflammation that is related to asthma characterized as
eosinophilic in nature as indicated by: elevated peripheral blood eosinophil count of
>=300 cells per microliter (cells/μL) demonstrated in the past 12 months OR elevated
peripheral blood eosinophil count of >=150/μL at visit 1.

- A well-documented requirement for regular treatment with inhaled corticosteroid (>200
μg/day fluticasone propionate drug powder inhaler [DPI] or equivalent daily) in the 12
months prior to Visit 1 with or without maintenance oral corticosteroids (OCS). The
ICS dose should represent medium or high dose in children aged 6-11 years of age
[GINA, 2015].

- Current treatment with an additional controller medication for at least 3 months or a
documented failure in the past 12 months of an additional controller medication for at
least 3 successive months. [e.g., long-acting beta-2-agonist (LABA), leukotriene
receptor antagonist (LTRA), or theophylline.]

- Forced expiratory volume in one second (FEV1): Persistent airflow obstruction at
either Visit 1 or Visit 2 (FEV1 performed prior to first dose of study medication) as
indicated by: A pre-bronchodilator FEV1 <110% predicted (Quanjer, 2012) OR FEV1:
Forced vital capacity (FVC) ratio <0.8.

- Previously confirmed history of two or more exacerbations requiring treatment with
systemic corticosteroids (CS) (intramuscular [IM], intravenous, or oral), in the 12
months prior to visit 1, despite the use of high-dose inhaled corticosteroids (ICS).
For participants receiving maintenance CS, the CS treatment for the exacerbations must
have been a two-fold increase or greater in the dose.

- No changes in the dose or regimen of baseline ICS and/or additional controller
medication during the run-in period.

- Male or female: Females of childbearing potential must commit to consistent and
correct use of an acceptable method of contraception for the duration of the trial and
for 4 months after the last dose of investigational product. A urine pregnancy test is
required of girls of childbearing potential. This test will be performed at the
initial screening visit (visit 1) and will be performed at each scheduled study visit
prior to the administration of investigational product, and during the early
withdrawal and follow-up visits.

- Parent(s)/guardian able to give written informed consent prior to participation in the
study, which will include the ability to comply with the requirements and restrictions
listed in the consent form. If applicable, the participant must be able and willing to
give assent to take part in the study according to the local requirement.

- For Part B: The subject has completed all study assessments up-to and including Visit
8 and received all 3 doses of investigational product (IP) in Part A

- For Part B: The Principal Investigator (PI) has performed a benefit/risk assessment
and this assessment supports continued therapy with mepolizumab.

- The subject's parents (or guardian) have given consent and the subject has given
assent for continued treatment

Exclusion Criteria:

- Participants with any history of life threatening asthma (e.g. requiring intubation),
immunosuppressive medications intake or immunodeficiency disorder.

- Participants with any medical condition or circumstance making the volunteer
unsuitable for participation in the study.

- Significant abnormality of rate, interval, conduction or rhythm in the 12-lead
electrocardiogram (ECG), determined by the investigator in conjunction with the age
and gender of the child at Visit 1.

- Alanine aminotransferase (ALT), and bilirubin >2x upper limit of normal (ULN)
(isolated bilirubin >1.5xULN is acceptable if bilirubin is fractionated and direct
bilirubin <35%) at Visit 1.

- Positive Hepatitis B Surface Antigen or positive Hepatitis C antibody at Visit 1.

- Parent/guardian has a history of psychiatric disease, intellectual deficiency,
substance abuse, or other condition (e.g. inability to read, comprehend and write)
which will limit the validity of consent to participate in this study.

- Unwillingness or inability of the participant or parent/guardian to follow the
procedures outlined in the protocol.

- Participant who is mentally or legally incapacitated.

- Children who are wards of the state or government.

- A participant will not be eligible for this study if he/she is an immediate family
member of the participating investigator, sub-investigator, study coordinator, or
employee of the participating investigator.

- Omalizumab: Participants who have received omalizumab within 130 days of Visit 1.

- Other Biologics: Participants who have received any biological (other than omalizumab)
to treat inflammatory disease within 5 half-lives of visit 1.

- 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 Medical
Monitor, contraindicates their participation.

- Hypersensitivity: Participants with allergy/intolerance to a monoclonal antibody or
biologic.

- The participant has participated in a clinical trial and has received an
investigational product within the following time period prior to the first dosing day
in the current study: 30 days, 5 half-lives or twice the duration of the biological
effect of the investigational product (whichever is longer).