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 …