Due to differences in the labelling requirements among regulatory authorities around the world, two different labelling approaches were included in this global study: labelling that includes a pictogram plus standard labelling elements, or a standard labelling without the pictogram. Participants (and/or their caregiver) attended three on treatment visits at Week 0, 4, 8, and End of Study Visit. Training on the study treatment, device handling and administration technique was provided by the investigator or qualified site staff at Week 0 and then first dose was self-administered under observation of investigator/site staff in clinic. Second dose self-administered unobserved, at home (Week 4) and third dose was self-administered under the observation of investigator/site staff in clinic (Week 8). All Subjects (Safety) Population included all enrolled participants attempting at least one self-administration of mepolizumab. Only participants with data available at Week 8 were analyzed.

Due to differences in the labeling requirements among regulatory authorities around the world, two different labeling approaches were included in this global study: labeling that includes a pictogram plus standard labeling elements, or a standard labeling without the pictogram. Participants (and/or their caregiver) attended three on treatment visits at Week 0, Week 4, Week 8, and the End of Study Visit. Training on the study treatment, device handling and administration techniques was provided by the investigator or qualified site staff at Week 0 and then first dose was self-administered under observation of investigator/site staff in clinic. Second dose self-administered unobserved, at home (Week 4) and third dose was self-administered under the observation of investigator/site staff in clinic (Week 8). Only participants with data available at Week 8 were analyzed.

During the clinic visits the Investigator or designee evaluated if the participants were able to self-administer the third dose at Week 8 by visual inspection immediately following injection and by using an 'Observer checklist' based on the safety syringe Instructions for Use (IFU). The 'self-administration' was defined as administration of mepolizumab liquid drug product in safety syringe either by the participants themselves or by their caregiver. Failure to perform one of the critical steps was deemed to be failure to successfully administer the injection. Participants with data available at Week 8 have been analyzed. Analysis was performed on All Subjects (Safety) Population which comprised of all enrolled participants attempting at least one self administration of mepolizumab liquid drug product in a safety syringe.

Blood samples were collected at indicated time points. Cmax following a single dose administration of liquid mepolizumab using a safety syringe and an autoinjector were compared with reconstituted lyophilized drug product from the vial. Pharmacokinetic (PK) Population comprised of all participants receiving study drug for whom a pharmacokinetic sample was obtained and analyzed.

Blood samples were collected at indicated time points. AUC(0-t) and AUC(0-inf) following a single dose administration of liquid mepolizumab using a safety syringe and an autoinjector were compared with lyophilized drug product. Fixed effects analysis of covariance model was used for analysis. Only those participants with data available at the specified data points were analyzed (represented by n=X in category titles).

Clinically significant exacerbation was defined as worsening of asthma which requires use of systemic corticosteroids (e.g., prednisone) for at least 3 days or a single intramuscular (IM) corticosteroid dose and/or hospitalization and/or emergency department (ED) visits. For participants on maintenance systemic corticosteroids, at least double the existing maintenance dose for at least 3 days is required. Percentage of participants with clinically significant exacerbation over time during the on-treatment period of Part C and 95% confidence interval were estimated using Kaplan-Meier estimates. Intent-to-Treat Population includes all randomized participants who received at least one dose of double-blind study medication within Part C.

SGRQ consisted of 50 questions (scored from 0 to 100 where 0 indicates best and 100 indicates worst health) designed to measure Quality of Life in par. with diseases of airway obstruction, measuring symptoms, impact, and activity. Questions were completed by the par. with a recall over the past 4 weeks. SGRQ Total Score was calculated by summing the pre-assigned weights of answers, dividing by the sum of the maximum weights for items in SGRQ and multiplying by 100 to get a %. Change from BL in SGRQ was calculated as value at Week 24 minus value at BL for each par. and was analyzed using mixed model repeated measures allowing for covariates of BL value, region, BL maintenance oral corticosteroid (OCS) therapy, exacerbations in the year prior to the study (as an ordinal variable), BL % predicted FEV1 and visit, plus interaction terms for visit by BL and visit by treatment group. Modified Intent-to-Treat (mITT) Population consisted of all randomized par. who received \>= 1 dose of drug.

Exacerbations are defined as the worsening of asthma which requires use of systemic corticosteroids intravenous (IV) or oral steroid like prednisone, for at least 3 days or a single intramuscular (IM) corticosteroid (CS) dose is required. For maintenance systemic corticosteroids, at least double the existing maintenance dose for at least 3 days was required) and/or hospitalization and/or emergency department (ED) visit. On-Treatment data between first dose date and earliest of Withdrawal date/last dose + 28 days was considered for analysis. Analysis of the number of exacerbations was performed using a negative binomial generalized linear model.

An AE is any untoward medical occurrence in a participant or clinical investigation participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. Any untoward event resulting in death, life threatening, requires hospitalization or prolongation of existing hospitalization, results in disability/incapacity, congenital anomaly/birth defect, any other situation according to medical or scientific judgment that may not be immediately life-threatening or result in death or hospitalization but may jeopardize the participant or may require medical or surgical intervention or event associated with liver injury and impaired liver function were categorized as SAE. On-treatment AEs and on-treatment SAEs are the events occurring on/after the first dose of open-label mepolizumab date and before/on last dose+28 days.

AEs were collected from the Baseline visit until the follow-up visit (approx. 12 weeks post-last dose). Participants were monitored to evaluate the AEs of systemic and local site reaction. AE is defined as any untoward medical occurrence in a patient or clinical investigation subject, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. On treatment AEs were defined as events occurring from the first dose until 28 days after the last dose of mepolizumab.

Clinically significant exacerbations of asthma are defined as worsening of asthma which required use of systemic corticosteroids (IV or oral steroid like prednisone, for at least 3 days or a single intramuscular (IM) corticosteroid (CS) dose is required. For maintenance of systemic corticosteroids, at least double the existing maintenance dose for at least 3 days was required) and/or hospitalization and/or emergency department (ED) visits. The frequency of clinically significant exacerbations of asthma over the 32-week treatment period is expressed as the number of exacerbations per year. Analysis of the number of exacerbations performed using a negative binomial model with covariates of treatment group, baseline maintenance OCS therapy (OCS vs. no OCS), region, exacerbations in the year prior to the study (as an ordinal variable) and baseline % predicted FEV1, and with logarithm of time on treatment as an offset variable.

Baseline (BL) dose was the prescribed optimized prednisone/prednisolone dose following the OCS Optimization Phase. Maintenance (MN) dose was the mean of all daily prednisone/prednisolone doses during the MN Phase (weeks 20 to 24). The percent reduction of OCS dose during weeks 20 to 24 compared to BL dose was calculated as: 100 x (BL dose minus MN dose)/BL dose. Asthma control between weeks 20 and 24 was defined as no clinically significant exacerbation (worsening of asthma that required use of systemic corticosteroids or hospitalization and/or emergency department visits) during this period. The percent reduction of OCS was categorized as: 90 to 100%; 75 to \<90%; 50 to \<75%; \>0 to \<50%; no decrease in prednisone dose, or lack of asthma control, or withdrawal (WD) from treatment. Analysis was performed using a proportional odds model with terms for treatment group, region, duration of OCS use at BL (\<5 years vs. \>=5 years) and BL OCS dose.

AE is any untoward medical occurrence in a participant or clinical investigation participant, temporally associated with use of a medicinal product (MP), whether or not considered related to MP. AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with use of MP. SAE is any untoward medical occurrence that, at any dose results in death, is life-threatening, requires hospitalization or prolongation of existing hospitalization, results in disability/incapacity, or is a congenital anomaly/birth defect or is medically significant or all events of possible drug induced liver injury with hyperbilirubinemia. As Treated (AT) Population consisted of participants who received at least one dose of open label mepolizumab. On-treatment AEs and on-treatment SAEs are the events occurring on/after the first dose of open-label mepolizumab date and before/on last dose of mepolizumab + 28 days.

PK of mepolizumab was evaluated in participants using Cmax. PK samples were collected at pre-dose on Weeks 4 and 8; and at Weeks 9, 12, 16 and 20 post-dose. Cmax was evaluated by population PK methods and mean and standard error from the final model has been tabulated. Estimates have been presented from the final model centered to mean bodyweights of 27 kg, 50 kg and 70 kg. Note the average bodyweight of 70 kg (mean body weight observed in adults) was not investigated in the study. PK Population included all participants receiving at least one dose of mepolizumab beginning at Visit 2 (Week 0) and having at least one blood sample taken at Visit 3 (Week 4) or thereafter with measurable mepolizumab plasma concentration.

PK of mepolizumab was evaluated in participants using AUC (0-inf). PK samples were collected at pre-dose on Weeks 4 and 8; and at Weeks 9, 12, 16 and 20 post-dose. AUC (0-inf) was evaluated by population PK methods and mean and standard error from the final model has been tabulated. Estimates have been presented from the final model centered to mean bodyweights of 27 kg, 50 kg and 70 kg. Note the average bodyweight of 70kg (mean body weight observed in adults) was not investigated in the study.

PK of mepolizumab was evaluated in participants using t1/2. PK samples were collected at pre-dose on Weeks 4 and 8; and at Weeks 9, 12, 16 and 20 post-dose. T1/2 was evaluated by population PK methods and mean and standard error from the final model has been tabulated. Estimates have been presented from the final model centered to mean bodyweights of 27 kg, 50 kg and 70 kg. Note the average bodyweight of 70kg (mean body weight observed in adults) was not investigated in the study.

PK of mepolizumab was evaluated in participants using CL/F. PK samples were collected at pre-dose on Weeks 4 and 8; and at Weeks 9, 12, 16 and 20 post-dose. CL was evaluated by population PK methods and mean and standard error from the final model has been tabulated. Estimates have been presented from the final model centered to mean bodyweights of 27 kg, 50 kg and 70 kg. Note the average bodyweight of 70kg (mean body weight observed in adults) was not investigated in the study.

PD of mepolizumab was evaluated in participants using ratio to Baseline in absolute blood eosinophil count. Blood samples were collected at indicated time points. Baseline was defined as the latest value recorded prior to the first dose of mepolizumab. Ratio to Baseline was calculated as post-dose visit value/Baseline value. It was evaluated by Pharmacodynamic Eosinophils (PDe) Population which included all participants receiving at least one dose of mepolizumab beginning at Visit 2 (Week 0) and having at least one Part A blood sample evaluable for blood eosinophil count.

An AE is any untoward medical occurrence in a participant or clinical investigation participant, temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. Any untoward event resulting in death, life threatening, requires hospitalization or prolongation of existing hospitalization, results in disability/incapacity, congenital anomaly/birth defect, any other situation according to medical or scientific judgment or all events of possible drug-induced liver injury with hyperbilirubinemia are to be categorized as SAE. On-treatment SAEs and non-SAEs are defined as events occurring from the first Part B dose until 28 days following the last Part B dose. Safety Population includes all participants who received at least one dose of mepolizumab beginning at Visit 9.

Blood sample were collected for the determination of anti-mepolizumab binding antibodies and neutralizing antibodies response in Part B at Weeks 44, 68 and 80 prior to study treatment administration. Participant was considered 'Positive' if they had at least one positive post-Baseline anti-drug antibody assay result. All Part B visits (including scheduled and unscheduled) post-Baseline were considered for Any-time Post-Baseline visit derivation. The number of participants with positive anti-mepolizumab binding antibodies and neutralizing antibodies response at Any Time Post Baseline has been presented. The neutralizing antibodies response results only presented for participants with positive anti-drug antibody assay.

Sitting blood pressure measurements included SBP and DBP. Measurements were done pre-infusion/injection with the participant sitting, having rested in this position for at least 5 minutes before each reading. The Baseline was defined as the latest value recorded prior to the first dose of mepolizumab. Change from Baseline was defined as value at indicated time point minus Baseline value. Only those participants with data available at the specified time points were analyzed (represented by n=X in the category titles).

Sitting pulse rate measurements were performed pre-infusion/injection with the participant sitting, having rested in this position for at least 5 minutes before each reading. The Baseline was defined as the latest value recorded prior to the first dose of mepolizumab in Part A. Change from Baseline was defined as value at indicated time point minus Baseline value. Only those participants with data available at the specified time points were analyzed (represented by n=X in the category titles).

Blood samples were collected for analysis of alanine aminotransferase (ALT), alkaline phosphatase (ALP), aspartate aminotransferase (AST), gamma glutamyl transferase (GGT), albumin, protein, bilirubin, creatinine, urate, direct bilirubin, calcium, carbon dioxide (CO2), chloride, glucose, potassium, sodium and urea. Baseline was defined as the latest value recorded prior to the first dose of mepolizumab in Part A. Change from Baseline was defined as value at indicated time point minus Baseline value. All Part B visits (scheduled and unscheduled) post-Baseline were considered for Any-time Post-Baseline visit derivation. If participant had at least one value for categories "To Low" and/or "To High" along with "To Normal or No Change" then participant was counted under "To Low" and/or "To High". If participant had values which belong only to "To Normal or No Change" then participant was counted under "To Normal or No Change" only. Any Time Post-Baseline values have been presented.

Blood samples were collected for analysis of basophils, eosinophils, leukocyte, monocyte, neutrophils, lymphocyte, platelets, mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), hemoglobin (Hgb), mean corpuscular volume (MCV), erythrocytes, hematocrit, and reticulocytes/erythrocytes (Ret/Ery). Baseline was defined as the latest value recorded prior to first dose of mepolizumab in Part A. Change from Baseline was defined as value at indicated time point minus Baseline value. All Part B visits (scheduled and unscheduled) post-Baseline were considered for Any-time Post-Baseline visit derivation. If participant had at least one value for categories "To Low" and/or "To High" along with "To Normal or No Change" then participant was counted under "To Low" and/or "To High". If participant had values which belong only to "To Normal or No Change" then participant was counted under "To Normal or No Change" only. Any Time Post-Baseline values have been presented.

Urine samples were collected from participants at indicated time points for analysis of urinalysis parameters including Specific gravity and potential of hydrogen (pH) of urine, presence of glucose, protein, blood and ketones in urine by dipstick test. Microscopic examination was performed if blood or protein was abnormal. Only those participants with data available at the specified time points were analyzed.

Change from Baseline in blood eosinophils was calculated as the post-Baseline value minus the Baseline value. The change from Baseline in log-transformed blood eosinophil levels at Week 12 was analyzed using both a linear and non-linear (Imax) dose response models. The dose response was found to be non-linear and hence only the results of the non-linear model are presented. Mepolizumab 75mg IV assumed to equate to 100 mg SC within model. Prior to log10-transformation, zero values were imputed with half the minimum value across all dose groups and time points. An adjustment for Baseline eosinophil count was also incorporated into the model.

Area under the absolute blood eosinophil time curve to Day 84 (AUECeos\[0-day 84\]) determined using the linear trapezoidal rule for subset of participants with blood eosinophil data to Day 84. Blood samples for the analyses of AUEC(eos) (0-day 84) were collected at Days 1, 3, 7, 28, 56, 70 and 84.

Blood samples were collected at Days 1, 3, 7, 28, 56, 70, 84, 112 and 140 to assess the maximum reduction from Baseline in blood eosinophils between Day 1 pre-dose and last quantifiable study measurement. Change from Baseline was calculated as the ratio of the post-Baseline value divided by the Baseline value. The maximum reduction from Baseline in eosinophils is represented by the minimum ratio to Baseline.

Blood samples were collected at Days 1, 3, 7, 28, 56, 70, 84, 112 and 140 to assess the time to first occurrence of maximum reduction from baseline in blood eosinophil levels between Day 1 pre-dose and last quantifiable study measurement.

This summarizes the number of participants who returned to at least 50% of their Baseline blood eosinophil levels after maximum inhibition had been achieved and without any subsequent decrease in blood eosinophil levels. Blood samples were collected at Days 1, 3, 7, 28, 56, 70, 84, 112 and 140.

AUC of mepolizumab was estimated by population modeling techniques using non-linear mixed effect methods for the individual and population pharmacokinetic parameters from the sparse sampling. Individual cumulative plasma of mepolizumab AUC to Day 84 (cumAUC(0-day 84)), is the sum of the AUCs over each dosing interval after each of the three doses administered, for those participants with data up to Day 84. Individual cumulative plasma of mepolizumab AUC to Day 140 (cumAUC(0-day 140) is the sum of the AUCs over each dosing interval after each of the three doses administered plus the AUC post the last dose interval up to Day 140 (i.e. from Day 84 to Day 140). Blood samples for PK analyses were collected on dosing days (Days 1, 28 and 56) at pre-dose and 0.5 hour (h), 1 h and 2 h post-dose (time was relative to the end of infusion in the IV cohort) as well as on Days 3, 7, 70, 84, 112 and 140 (follow-up visit).

Maximum plasma concentration was estimated by population modelling techniques using non-linear mixed effect methods for the individual and population pharmacokinetic parameters from the sparse sampling. Blood samples for PK analyses of mepolizumab were collected on dosing days (Days 1, 28 and 56) at pre-dose and 0.5 h, 1 h and 2 h post-dose (time was relative to the end of infusion in the IV cohort) as well as on Days 3, 7, 70, 84, 112 and 140 (follow-up visit).

Time to maximum plasma concentration was estimated by population modelling techniques using non-linear mixed effect methods for the individual and population pharmacokinetic parameters from the sparse sampling. Blood samples for PK analyses of mepolizumab were collected on dosing days (Days 1, 28 and 56) at pre-dose and 0.5 h, 1 h and 2 h post-dose (time was relative to the end of infusion in the IV cohort) as well as on Days 3, 7, 70, 84, 112 and 140 (follow-up visit).

Terminal half-life (t1/2) was estimated by modelling techniques using non-linear mixed effect methods for the individual and population pharmacokinetic parameters for mepolizumab from the sparse sampling. Blood samples for PK analyses of mepolizumab were collected on dosing days (Days 1, 28 and 56) at pre-dose and 0.5 h, 1 h and 2 h post-dose (time was relative to the end of infusion in the IV cohort) as well as on Days 3, 7, 70, 84, 112 and 140 (follow-up visit).