Study to Assess the Efficacy and Safety of Rilonacept Treatment in Participants With Recurrent Pericarditis

In the single-blind run-in (RI) period, rilonacept 320 mg (or 4.4 mg/kg in pediatric participants ≥12 and \<18 years old) subcutaneous (SC), followed by 160 mg (or 2.2 mg/kg in pediatric participants ≥12 and \<18 years old) injections once weekly.

During the Randomized-Withdrawal (RW) period, eligible participants are randomized 1:1 to double-blinded administration of study drug:

* Rilonacept 160 mg (or 2.2 mg/kg in pediatric participants ≥12 and \<18 years old) SC injections once weekly * Matching placebo SC injections once weekly.

Participants with pericarditis recurrence who meet the protocol criteria for bailout rilonacept (report at least 1 day with pericarditis pain ≥4 on the 11-point numerical rating scale (NRS) and have 1 C-reactive protein (CRP) value ≥ 1 mg/dL \[either on the same day or separated by no more than 7 days\]) receive bailout rilonacept (2 open-label injections of 160 mg rilonacept \[or 4.4 mg/kg for pediatric subjects\]) irrespective of randomized treatment assignment and as soon as at least 5 days have passed since the last study drug injection.

Upon completion of the RW period (i.e., when the prespecified number of primary efficacy endpoints \[clinical events committee-confirmed pericarditis recurrence\] events have occurred), all participants who did not discontinue study drug have an option to continue treatment with open-label rilonacept in the Long-Term Extension (LTE) or to withdraw from the study. Participants still in the RI period at the time that the RW period has ended and the LTE is opened will have the option to enter the LTE directly when they have completed the RI period and have met the definition of clinical response or to withdraw from the study.