This study is being done to demonstrate whether KPL-387 is an effective and safe treatment for recurrent pericarditis.

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 regimens 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 regimen 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.

Key Inclusion Criteria:

• Phase 2 and 3: Has a diagnosis of recurrent pericarditis
• Phase 2 and 3: Has signs and symptoms of recurrent pericarditis despite treatment with standard therapies
• Phase 2 and 3: Weighs at least 40 kg
• Phase 2: Taking NSAIDS and/or colchicine (in any combination)
• Phase 3: Taking NSAIDs and/or colchicine and/or glucocorticoids (in any combination)

Key Exclusion Criteria:

• Phase 2 and 3: Has a diagnosis of pericarditis that is secondary to specific prohibited etiologies.
• Phase 2 and 3: Has received an investigational drug during the 4 weeks before screening or is planning to receive an investigational drug at any time during the study.
• Phase 2 and 3: Has a history of active or untreated, latent tuberculosis (TB) prior to screening.
• Phase 2 and 3: Has a history of immunodeficiency.
• Phase 2 and 3: Has a history of immunosuppression, including positive human immunodeficiency virus (HIV) test results.
• Phase 2 and 3: Has chest x-ray at Screening or within 12 weeks before first study drug administration, with evidence of malignancy, abnormality consistent with prior or active TB infection or active infection.
• Phase 2 and 3: Has a history of malignancy of any organ system within the past 5 years before Screening (other than a successfully treated non metastatic cutaneous squamous cell carcinoma or basal cell carcinoma and/or localized carcinoma in situ of the cervix).
• Phase 2 and 3: Has a known or suspected current active infection or a history of chronic or recurrent infectious disease (> 3 episodes in prior 12 months), including but not limited to, genitourinary infection, chest infection, sinusitis, or skin/soft tissue infection.
• Phase 2 and 3: Has had a serious infection, has been admitted to the hospital for an infection, or has been treated for a documented infection requiring antibiotics for a documented infection within 2 weeks prior to first study drug administration.
• Phase 2 and 3: Has had an organ transplant (except corneal transplant performed more than 3 months prior to first study drug administration).
• Phase 2 and 3: In the Investigator's opinion, has any other medical condition that could adversely affect the subject's participation or interfere with study evaluations.