The study will be conducted in 3 parts. Parts 1 and 2 of the study will include a double-blind treatment period of 24 weeks and a safety follow-up period of 48 weeks for participants who do not continue onto Part 3. Patients meeting the eligibility criteria for enrollment in Part 3, will enter Part 3 after completion of the Week 24 visit but prior to completion of the Week 72 visit. In Part 1, participants will be randomized in a 2:1 ratio (active drug:placebo) to receive either rontalizumab 750 mg or matching placebo intravenously every 4 weeks for 24 weeks. Part 2 was will be initiated upon the completion of recruitment for Part 1. In Part 2, participants will be randomized in a 2:1 ratio (active drug:placebo) to receive either rontalizumab 300 mg or matching placebo subcutaneously every 2 weeks for 24 weeks. After Week 24, patients will enter a 48-week safety follow-up period, or, after the open label extension became available via protocol amendment, will have the option of entering Part 3 of the study, if eligible. In Part 3, all participants will receive rontalizumab 750 mg intravenously every 4 weeks for 120 weeks (up to 144 weeks total).
