Safety and Effectiveness of Rilonacept for Treating Systemic Juvenile Idiopathic Arthritis in Children and Young Adults

The current standard treatment for SJIA includes nonsteroidal anti-inflammatory drugs (NSAIDS) and corticosteroids. However, in most people, NSAIDS do not completely control the disease. Also, no studies have been done to prove which medication or combination of medications is best to treat children and adolescents with SJIA. Interleukin-1 (IL-1), a protein secreted by certain cells in the body, assists in regulating immune and inflammatory responses. Too much IL-1 can be harmful and has been shown to play a role in the inflammation associated with a variety of diseases, including SJIA. Rilonacept is a drug that inhibits IL-1 activity. The purpose of this study is to determine whether a rilonacept drug regimen initiated early is more effective than a similar rilonacept drug regimen initiated 4 weeks later when treating children and young adults with SJIA. This study will also evaluate the safety of rilonacept, and various tissue samples will be collected from participants for future genetic studies. This study will last 6 months. Participants will be randomly assigned to one of two groups: * Group 1 participants will receive rilonacept injections at a dose of 4.4mg/kg at study entry (loading dose), then 2.2 mg/kg weekly until Week 4. At Week 4, they will receive a loading dose of placebo, followed by weekly rilonacept injections at 2.2 mg/kg for the duration of the study. * Group 2 participants will receive placebo at study entry and then during the first 4 weeks of treatment. At Week 4, they will receive a loading dose of rilonacept injections of 4.4 mg/kg, followed by weekly rilonacept injections at a dose of 2.2 mg/kg for the duration of the study. Participants will continue any previous corticosteroid therapy, but in tapering doses. All participants will attend study visits at Weeks 0, 2, 4, 6, 8, 10, 12, 14 and 24. Study visits will include a physical exam, joint exam, blood collection, interview, and questionnaires. Urine collection may occur for some female participants. Other evaluations may be performed by the participant's regular doctor. Throughout the study, participants will maintain at-home diaries to record fever, morning stiffness and pain, when rilonacept or placebo was taken, any side effects experienced from treatment, and any additional medications that were taken.