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

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.