The primary objective of this study is to characterize the efficacy and safety of dosing regimens used to transition from prior pericarditis therapies to KPL-387 monotherapy in participants with well-controlled recurrent pericarditis on standard therapies.

During the Posology (Dosing and Administration) Study, participants on oral pericarditis therapies alone (i.e., non-steroidal anti-inflammatory drugs [NSAIDs], colchicine, and/or glucocorticoids) and participants on Interleukin-1 (IL-1) pathway inhibition drugs will transition to KPL-387 per protocol guidelines. Participants will receive KPL-387 for up to 16 weeks in the Posology Study.

At study visits throughout the Posology Study, participants will be queried for symptoms of chest pain (and participants will fill out pericarditis pain numerical rating scale [NRS] scores) and will be assessed for physical signs (e.g., pericardial friction rub) and laboratory evidence of pericarditis activity (e.g., C-reactive protein [CRP]) as well as treatment tolerability and pharmacokinetics (PK). Participants experiencing new or worsening symptoms consistent with pericarditis will be managed per study protocol.

All active, enrolled participants in the Posology Study who have not completed an End of Treatment (EOT) Visit may be eligible to transition into the Long-Term Extension (LTE) following completion of the Week 16 Visit. The Week 16 Visit will serve as both a Posology Study End of Study Visit and a Baseline Visit for the LTE. Participants who choose not to participate in the LTE will complete an EOT Visit followed by a Safety Follow-Up until 8 weeks after the last KPL-387 dose administration.

The open-label LTE is designed to evaluate the long-term efficacy and safety of KPL-387 in recurrent pericarditis (RP). Treatment in the LTE may continue until the participant has received up to 24 months of treatment in the LTE or until KPL-387 is approved for commercial use in that region to treat pericarditis, whichever occurs first.

At study visits throughout the LTE, participants will be queried for symptoms of chest pain (and participants will fill out pericarditis pain NRS scores) and will be assessed for physical signs (e.g., pericardial friction rub) and laboratory evidence of pericarditis activity (e.g., CRP) as well as treatment tolerability and PK. Participants experiencing new or worsening symptoms consistent with pericarditis will be managed as described in the protocol.

Key Inclusion Criteria:

• Has well-controlled recurrent pericarditis (i.e., including having CRP < 0.5 mg/dL within 14 days of Baseline and a pericarditis pain NRS score ≤ 3 at Baseline)
• Has a documented history of CRP elevation (> 1 mg/dL) associated with at least one prior acute pericarditis episode, whether the incident event or any pericarditis recurrence
• Has received treatment for RP for at least 3 months prior to Baseline with standard therapy(ies) and is currently on a stable dosing regimen including NSAIDs and/or colchicine, and/or glucocorticoids or an IL-1 pathway inhibitor (anakinra or rilonacept).

Key Exclusion Criteria:

• Has a diagnosis of pericarditis that is secondary to specific prohibited etiologies
• Has had a pericarditis recurrence in the last 3 months prior to Baseline
• Has received an investigational drug during the 4 weeks before study drug administration or is planning to receive an investigational drug at any time during the study.
• Has a history of active or untreated, latent tuberculosis (TB) prior to screening.
• Has a history of immunodeficiency.
• Has a history of immunosuppression, including positive human immunodeficiency virus (HIV) test results.
• 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.
• 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).
• 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.
• 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.
• Has had an organ transplant (except corneal transplant performed more than 3 months prior to first study drug administration).
• In the Investigator's opinion, has any other medical condition that could adversely affect the subject's participation or interfere with study evaluations.