Phase 2/3 Study of Participants With Recurrent Pericarditis Evaluating the Efficacy and Safety of KPL-387

This clinical trial is comprised of 3 separate study parts: Phase 2, Phase 3, and Long-Term Extension. The primary objective of Phase 2 is to evaluate how well different dose levels of KPL-387 control the pain and inflammation of pericarditis in a group of participants experiencing an acute episode of recurrent pericarditis. This part of the study will confirm the KPL-387 dose level to be further tested in Phase 3 and the Long-Term Extension. The primary objective of Phase 3 is to confirm the efficacy of KPL-387 for the treatment of recurrent pericarditis and reduction in risk of recurrence in an additional group of participants experiencing an acute episode of recurrent pericarditis. Participants who complete Phase 2 or Phase 3 may be eligible to participate in the Long-Term Extension. The primary objective of the Long-Term Extension is to assess the long-term efficacy of KPL-387 while maintaining long-term disease control. All of the study parts also have other objectives to learn about the safety, tolerability, concentration of KPL-387 in blood, and effects of KPL-387 on blood test markers of immune activity in recurrent pericarditis.