1. Has given consent and signed an Informed Consent Form (ICF).
2. Male or female, of any ethnic origin.
3. 18 to 70 years of age, inclusive.
4. Has had a prior index episode of pericarditis which, in the judgement of the Investigator based on the available data, met the criteria for an acute pericarditis event, using the 2015 ESC Guidelines for the Diagnosis and Management of Pericardial Diseases (Adler et al, 2015) as a frame of reference -- i.e., met at least 2 of the 4 following criteria: pericarditic chest pain, pericardial rub, new widespread ST-segment elevation or PR-segment depression on ECG, and pericardial effusion (new or worsening). Additional supportive findings could include elevations of markers of inflammation (i.e., CRP, erythrocyte sedimentation rate, and white blood cell count) or evidence of pericardial inflammation by an imaging technique (e.g., MRI).
5. Has had at least one prior recurrent episode of pericarditis, in the judgement of the Investigator, based upon the available diagnostic information.
6. Has an ongoing symptomatic episode of pericarditis at the time of study enrollment in the judgement of the Investigator, based upon the available diagnostic information.
7. Has an elevated CRP value (i.e., >1 mg/dL) at the time of study enrollment.
8. If used, has received NSAIDs, and/or colchicine and/or corticosteroids (in any combination) at stable dose levels for at least 7 days (although stable doses for at least 3 days will be acceptable if in the opinion of the Investigator, in consultation with the Sponsor, a shorter period of stability is not anticipated to alter the baseline CRP values) and is anticipated to continue these concomitant medications at these dose levels for the duration of the active Treatment Period.
9. If female, must be nonpregnant and nonlactating and must agree to use an effective method of contraception, e.g., hormonal contraception or double-barrier birth control.
10. Is able to adequately maintain a daily medication diary.
11. Agrees to refrain from making any new, major life-style changes that may affect pericarditis symptoms (e.g., starting a new diet or changing exercise pattern) from the time of signature of the ICF to the End-of-Trial Visit (Week 7).
A patient who meets any of the following criteria will not be eligible to participate in the trial:
1. Has a diagnosis of pericarditis that was secondary to specific etiologies, including tuberculous, neoplastic, or purulent etiologies, post cardiac injury syndromes, myocarditis, or systemic diseases including autoinflammatory diseases, autoimmune diseases (e.g., rheumatoid arthritis, systemic lupus erythematosus, etc.).
2. Has a history of immunodepression, including a positive human immunodeficiency virus test result.
3. Has received treatment with any systemic immunosuppressants (other than, for example, corticosteroids or mycophenolate) which, in the opinion of the Investigator (in consultation with the Sponsor), may interfere with the study endpoints within the 6-month period before dosing.
4. Currently receiving other interleukin (IL)-1 or IL-6 blockers, janus-activating kinase (JAK) or tumor necrosis factor (TNF) inhibitors.
5. Has a history of myeloproliferative disorder, demyelinating disease, or symptoms suggestive of multiple sclerosis.
6. Female patient who is pregnant or lactating or who does not agree to use an effective method of contraception, e.g., hormonal contraception or double-barrier birth control.
7. Has a history of active or latent treated tuberculosis (TB), or had a positive QuantiFERON (QFT-TB G In-Tube) test result, or a chest radiograph during the 3 months prior to Study Drug dosing suggestive of prior TB infection. A patient with a positive purified protein derivative (PPD) test result (≥5-mm induration) after the first attack of pericarditis is excluded unless he/she has had either a negative chest x-ray result or a negative QuantiFERON test result. Signs or symptoms suggestive of active TB (e.g., new cough of >14 days in duration or a change in chronic cough, persistent fever, unintentional weight loss, night sweats) upon review of medical history and/or physical exam. Have recent close contact with a person with active TB.
8. Chest radiograph (or historic results within 3 months of SCV1) that shows evidence of malignancy or any abnormalities suggestive of prior TB infection, including, but not limited to, apical scarring, apical fibrosis, or multiple calcified granulomata. This does not include non-caseating granulomata.
9. Has received immunization with a live (attenuated) vaccine within 12 weeks before the start othe study.
10. Has history of or positive or intermediate results for hepatitis B surface antigen (HBsAg), hepatitis B core antibody (HBcAb), or hepatitis C virus (HCV) antibody at SCV1.
11. Has an estimated glomerular filtration rate (eGFR) <30 mL/min.
12. Has a history of malignancy of any organ system within the past 5 years (other than a successfully treated non-metastatic cutaneous squamous cell or basal cell carcinoma and/or localized carcinoma in situ of the cervix).
13. Has a known or suspected current active infection or a history of chronic or recurrent infectious disease, including, but not limited to, chronic renal infection, chronic chest infection, sinusitis, recurrent urinary tract infection, or an open, draining infected skin wound.
14. Has had a serious infection, has been hospitalized for an infection, has been treated with oral antibiotics within 2 weeks of Study Drug administration, or has been treated with intravenous (IV) antibiotics for an infection within 2 months of first Study Drug administration.
15. Has had an organ transplant.
16. In the Investigator's judgement, has a history of alcoholism or drug/chemical abuse within 2 years prior to Study Drug administration.
17. Has a drug screen positive for amphetamines, cocaine, or phencyclidine or positive alcohol test at SCV1. Exceptions may be made if a patient is on an approved medication for a stable concomitant condition that explains the positive screen.
18. Has taken commercially-available rilonacept (ARCALYST®) or participated in a rilonacept clinical study during the 90 days before SCV1. Has used anakinra within 30 days prior to Study Drug administration.
19. Has a history of hypersensitivity to rilonacept or to any of the excipients contained in the Study Drug.
20. Has received an investigational drug during the 30 days (or 5 half-lives, whichever is longer) before SCV1 or is planning to receive an investigational drug (other than that administered during this trial) or use an investigational device at any time during the trial.
21. In the Investigator's judgement, has any other medical condition that could adversely affect the subject's participation or interfere with study evaluations.
22. Patient who, in the opinion of the Investigator, is not likely to be compliant with the study protocol.
23. Patient who, in the opinion of the Investigator in consultation with the Sponsor, should not participate in this study.