Joseph H. Butterfield, M.D.

This study is being done to store blood cells, genetic material, blood serum and tissue biopsies so that they can be used in laboratory studies now and in the future to find causes of the rare disorder of mast cells and/or eosinophils and factors that may contribute to disease progression and treatment response.

The purpose of this study is to attempt to establish new mast cell lines from patients with systemic mastocytosis using peripheral blood and/or bone marrow.

This is an open-label extension study to Study 200622.In this study subjects from Study 200622 will be continued on 4-weekly dosing with open-label mepolizumab 300 milligram (mg) subcutaneously (SC) for an additional 20 Weeks after completing the 32 Week study assessments post-randomization, while they continue with their background HES therapy per standard of care (SoC). Subjects from study 200622 will participate in this extension study if they had completed the 32-Week treatment period in study 200622 or if they were withdrawn from the study pre-maturely, but were continued in the study per protocol until 32 Weeks from randomization. Data from this study (205203) and 200622 will be combined to provide up to 52-Week exposure data to further characterize the long-term safety profile of mepolizumab and provide additional data on the clinical benefit in HES subjects beyond 32 Weeks. The duration of the study participation will be 20 Weeks for subjects who continue with mepolizumab treatment via MHE104317/MHE112562 after this open-label extension study; and 28 Weeks for subjects who do not continue with MHE104317/MHE112562.

The purpose of this study is to determine the effectiveness and tolerability of a novel topical preparation for the treatment of cutaneous lesions of mastocytosis.

Study drug and placebo will be supplied in Teva multidose dry powder inhaler (MDPI) devices and provided for participants to use at home. Participants will perform spirometry at every visit. Each participant will be given a diary at each visit for use until the next visit. Rescue medication (albuterol/salbutamol) will be dispensed at each visit, if needed, as determined by the investigational center personnel.

Mepolizumab is a humanized monoclonal antibody. In conditions where eosinophilia is considered to play an important part in the pathology, including eosinophilic asthma, HES, and eosinophilic granulomatosis with polyangiitis, a consistent reduction in blood eosinophil counts is observed in association with mepolizumab administration, with concomitant clinical improvement. This is a 32-week treatment period, randomized, double-blind, placebo-controlled, parallel group, multicentre study of mepolizumab in adolescent and adult subjects with severe HES receiving standard of care (SoC) therapy. This study will demonstrate the efficacy of mepolizumab compared with placebo based on maintenance of control of HES symptoms during the treatment period. The study will comprise of a screening period of up to approximately 4 weeks followed by a 32-Week study treatment period (subjects will be randomized 1:1 to placebo or mepolizumab) and up to 8-week additional follow-up period (12 weeks after the last dose of study treatment).

In this study, the investigators will determine the utility of Tamoxifen, a non-cytotoxic agent, to improve quality of life, biochemical parameters, and bone marrow involvement in systemic mastocytosis patients having 1) up to 20% bone marrow infiltration by mast cells and/or 2) mediator-release symptoms which are not controlled by tolerated doses of standard "non-cytotoxic" medications regardless of the percentage bone marrow involvement by mastocytosis. The dose of Tamoxifen will be 20 mg/day and the duration of treatment will be for one year. Patients currently taking interferon alfa, imatinib mesylate, or cladribine will be excluded until these medications have been stopped.