An AE is any untoward medical occurrence in clinical investigation participants temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. AEs are summarized by Treatment phase. Safety and tolerability of the study drug was assessed by number of participants with any AE

Intent-to-Treat (ITT) Population: all enrolled participants who received at least one dose of mepolizumab in this study.

An AE is any untoward medical occurrence in clinical investigation participants temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. AEs are summarized by Follow-up phase. Safety and tolerability of the study drug was assessed by number of participants with any AE

Follow-up Population: subset of the modified ITT Population who had evidence of being in the study > length of dosing cycle + 7 days after the date of their last dose of study medication and up to and including 97 days after their last dose date.

Participants who were receiving a prednisone dose level of =<10 mg as their sole background therapy at the end of the study were included for the analysis.

ITT Population

The criteria for eosinophil count was achieved if the participant's eosinophil count remained below <600 cell/uL for the last observation on study i.e. within length of dosing cycle + 7 days of last dose of study drug. For participants who entered in Stage 1, HES medications taking prior to the first infusion date in Stage 2 were considered as the lowest background therapy. For participants who entered in Stage 2, HES medications taken on the date that immediately preceded the first infusion date of study drug and had not been discontinued was regarded as the lowest background therapy. If the dose of the lowest background therapy had increased or the medication had changed or the participant had not reached their lowest background therapy, the participant was regarded as not achieving this endpoint.

ITT Population

Participants from study MHE100185 who completed the 9 months treatment period and achieved a level of <=10 mg of prednisone at study end were analyzed. Duration of doses <=10 mg was determined by examining changes in the prednisone dosing or allowable alternative corticosteroid medication (prednisone equivalents). Start and stop dates of other HES medications were checked to ensure that they did not overlap with the steroid dosing dates. If it did, then the number of days <=10 mg during this overlap was considered as zero and the cumulative days reset to zero, as the endpoint was assessing prednisone dose as sole background therapy. The dosing criteria for this endpoint was considered as achieved if the duration of dosing was minimum of 84 days (3months).

ITT Population. Only those participants who completed 9 months of dosing in study MHE100185 and achieved a prednisone level <=10 mg were analyzed.

Participants from study MHE100185 who completed the 9 months treatment period and achieved a level of prednisone of >10 mg at the end of the study were analyzed. Duration of doses <= 10 mg were determined by examining changes in the prednisone dosing or allowable alternative corticosteroid medication (prednisone equivalents). Start and stop dates of other HES medications were checked to ensure that they did not overlap with the steroid dosing dates. If overap occurred, then the number of days <=10 mg during this overlap was considered as zero and the cumulative days reset to zero, as the endpoint was assessing prednisone dose as sole background therapy. The dosing criteria for this endpoint was considered as achieved if the duration of dosing was minimum of 53 days (8 weeks).

ITT Population. Only those participants who completed 9 months of dosing in study MHE100185 and achieved a prednisone level >10 mg were analyzed.

Participants from study MHE100185 who completed the 9 months treatment period and achieved a level of <=10 mg of prednisone at study end and participants who withdrew from the study early who were at a prednisone dose level <=10 mg were analyzed. Duration of doses <= 10 mg were determined by examining changes in the prednisone dosing or allowable alternative corticosteroid medication (prednisone equivalents). Start and stop dates of other HES medications were checked to ensure that they did not overlap with the steroid dosing dates. If overlap occurred, then the number of days <=10 mg during this overlap was considered as zero and the cumulative days reset to zero, as the endpoint was assessing prednisone dose as sole background therapy. The dosing criteria for this endpoint was considered as achieved if the duration of dosing was minimum of 84 days (3months).

ITT Population. Only those participants who entered Stage 2 from study MHE100185 with a prednisone level of <=10 mg prednisone were analyzed.

Participants from study MHE100185 who completed the 9 months treatment period and achieved a level of >10 mg prednisone at the end of the study and participants who withdrew from the study early who were at a prednisone dose level >10 mg were analyzed. Duration of doses <= 10 mg was determined by examining changes in the prednisone dosing or allowable alternative corticosteroid medication (prednisone equivalents). Start and stop dates of other HES medications were checked to ensure that they do not overlap with the steroid dosing dates. If it did, then the number of days <=10 mg during this overlap was considered as zero and the cumulative days reset to zero, as the endpoint was assessing prednisone dose as sole background therapy. The dosing criteria for this endpoint was considered as achieved if the duration of dosing was minimum of 84 days (3months)

ITT Population. Only those participants who entered Stage 1 from study MHE100185 with >10 mg prednisone were analyzed.

Mean blood eosinophil counts were summarized over time taking into account the effect of HES background therapy. Eosinophil count observations for only those participants taking mepolizumab in conjunction with prednisone or as monotherapy were included.

ITT Population. Only those participants available at the specified time points were analyzed (represented by n=X in the category titles).

The number of participants at the end of Stage 2 with study medication dosing frequencies of 4 weeks, 5-6 weeks, 7-8 weeks, 9-10 weeks, 11-12 weeks, 13-16 weeks, 17-20 weeks, 21-24 weeks and >24 weeks were summarized. The first infusion date in Stage 3 and the last infusion date in Stage 2 were used to calculate the dosing frequency.

ITT Population. Only those participants available at end of Stage 2 were included.

The pruritus visual analogue scale asks participants to rate the status of their Pruritus based on the severity of their itch. Scores range from 0-100 with 0 = No itch and 100 = Worst imaginable itch. Change from Baseline in pVAS score is the difference between the pVAS score at the time point being considered to the MHE100901 Baseline score.

ITT Population. Only those participants available at the specified time points were analyzed (represented by n=X in the category titles).

The erythema subscale score and edema subscale score are each graded on a 0-3 scale, with 0 = absent , 1 = mild, 2 = moderate, 3 = severe. The total score ranged from 0 to 6, with higher scores indicative of more severe Erythema/Edema. The total score was obtained by summing together the responses for each of the two subscale items. Change from Baseline in erythema/edema total score is the difference between erythema/edema total score at the time point being analyzed to the MHE100901 Baseline score..

ITT Population. Only those participants available at the specified time points were analyzed (represented by n=X in the category titles).

The SF-12v2 is the 12 item abbreviated form of SF-36v2 survey . It provides information about how participants feel, and how well they have been able to perform their usual activities, over the past 4 weeks. SF-12v2 questions make up 8 scales: Physical Functioning, Role Physical, Bodily Pain, General Health, Vitality, Social Functioning, Role Emotional, Mental Health . Transformed physical component summary score (PCS-12) is derived using all the 12 items and scored onto a 0-100 scale such that a higher score indicates a better health state and better functioning. Change from Baseline in scale or summary measure score is the difference between the score at the time point being analyzed to Baseline.

ITT Population. Only those participants available at the specified time points were analyzed (represented by n=X in the category titles).

The SF-12v2 is the 12 item abbreviated form of SF-36v2 survey developed by the Medical Outcomes Trust and QualityMetric Incorporated. It provides information about how participants feel, and how well they have been able to perform their usual activities, over the past 4 weeks. SF-12v2 scale questions make up 8 scales: Physical Functioning, Role Physical, Bodily Pain, General Health, Vitality, Social Functioning, Role Emotional, Mental Health are for mental component summary. Transformed mental component summary score (MCS-12) is derived using all the 12 items and scored onto a 0-100 scale such that a higher score indicates a better health state and better functioning. Change from Baseline in scale or summary measure score is the difference between the score at the time point being analyzed to Baseline.

ITT Population. Only those participants available at the specified time points were analyzed (represented by n=X in the category titles).