Churg-Strauss syndrome is a rare type of systemic vasculitis which occurs almost exclusively in patients with asthma and which is characterized by prominent blood and tissue eosinophilia. The disease has a chronic smoldering course with a permanent need for medium to high corticosteroid doses. Available unselective immunosuppressive agents are often insufficient to reduce corticosteroid doses, to induce complete remission and to protect patients from disease flares which occur in more than 50 % of cases.

Interleukin-5 is the most potent cytokine regulating the production of eosinophil granulocytes which are the major effector cells in Churg-Strauss syndrome. Recently, an increased production of interleukin-5 was demonstrated in Churg-Strauss syndrome. Mepolizumab is a monoclonal IgG antibody targeting interleukin-5 and is effective in the treatment of the HES. The hypothesis of this study is, that mepolizumab will induce remission and allow for steroid reduction.

Inclusion Criteria:

• informed consent
• documented history of Churg Strauss Syndrome
• active disease
• subjects must complete screening and baseline assessment
• stable corticosteroid dose of > 12.5 mg prednisolone for at least one week
• treatment with cyclophosphamide (pulse or daily oral) or methotrexate or azathioprin or leflunomide in a stable dose for at least 4 weeks
• not pregnant or nursing
• negative pregnancy test and agree to practice birth control

Exclusion Criteria:

• life threatening disease or other critical illness deemed inappropriate for inclusion in the study by the principal investigator
• treatment with other immunosuppressive agents within 4 weeks prior to randomisation
• corticosteroid pulse of > 60 mg within the last three weeks prior to randomisation
• known secondary cause of eosinophilia
• no history or clinical features of vasculitis
• diagnosis of other primary systemic vasculitis
• currently active malignant disease
• abnormal laboratory values
• impaired cardiac function
• history of allergic reaction due to monoclonal antibodies
• prior treatment with anti-hIL-5 monoclonal antibody
• exposure to investigational drug within 30 days prior to randomisation
• positive pregnancy test