Safety and Efficacy of Subcutaneous Sarilumab in Improving the Quality of Life in People With Indolent Systemic Mastocytosis

* INCLUSION CRITERIA:

Participants must meet all of the following criteria to be enrolled in this study:

1. Male or female participant greater than or equal to 18 and \< 75 years of age at screening.
2. Enrolled on NIAID protocol 02-I-0277.
3. Documented pathologic diagnosis of ISM.
4. Mc-QoL score of at least 25% (which suggests participant is at least somewhat affected by all Mc-QoL questions).
5. Willing and able to undergo a bone marrow biopsy and aspirate.
6. Absolute neutrophil count (ANC) greater than or equal to 2000/mL.
7. Hemoglobin greater than or equal to 12.0 g/dL (males), greater than or equal to 11 g/dL (females).
8. Platelet count greater than or equal to 150,000/microliters.
9. Alanine transaminase (ALT) and aspartate transaminase (AST) \< 1.5 times the upper limit of normal (ULN).
10. Willing to allow storage of blood and bone marrow for future use in medical research.
11. Willing to allow genetic testing on biospecimens.
12. Able to provide informed consent.
13. Participants who can become pregnant must agree to use adequate contraception when engaging in sexual activities that can result in pregnancy. Adequate contraception must be used consistently, beginning at least 1 month before the beginning of dosing and lasting until 3 months after the final dose of study drug. Acceptable methods of contraception include the following:

    * Hormonal contraception (non-oral only).
    * Male or female condom with spermicide.
    * Diaphragm or cervical cap with a spermicide.
    * Intrauterine device.

EXCLUSION CRITERIA:

Individuals meeting any of the following criteria will be excluded from study participation:

1. Any abnormality that would be scored as a Grade 4 toxicity on the Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. Only clinically significant lab results will deem the subject ineligible
2. Infected with HIV or has other known immunodeficiency.
3. Has an active infection, including localized infection.
4. Active diverticulitis.
5. Active or chronic viral hepatitis.
6. Active or latent tuberculosis.
7. Use of any other anti-IL-6 or anti-IL-6R agent within 1 year prior to the date informed consent was obtained.
8. Use of cytoreductive therapy for mastocytosis within 1 year prior to the date informed consent was obtained.
9. Known lymphoma or advanced and metastatic solid tumors on active therapy (including chemotherapy) within 1 year prior to the date informed consent was obtained
10. Use of chemotherapy within 1 year prior to the date informed consent was obtained.
11. Receipt of any marketed (eg, omalizumab) or investigational biologic or monoclonal antibody reported to affect mast cell activation within 5 half-lives prior to date informed consent was obtained.
12. Receipt of intravenous (IV) immunoglobulin within 30 days prior to the date informed consent was obtained.
13. Receipt of live attenuated vaccines within 30 days prior to the date informed consent was obtained.
14. History of alcohol or drug/abuse within 12 months prior to date informed consent was obtained.
15. Is allergic to any component of the sarilumab formulation.
16. Pregnant or breastfeeding.
17. Any condition that, in the opinion of the investigator, contraindicates participation in this study.