• Subject is 18 to 75 years of age (inclusive) at the time of consent. The date of signature of the informed consent is defined as the beginning of the Screening Period. This inclusion criterion will only be assessed at the Screening Visit (Visit 1).

• Subject has been diagnosed with AOSD based on classification criteria (according to Yamaguchi et al, 1992) defined as having 5 or more of the following criteria, 2 of which are major:

• a. Major Criteria i. Fever \>39°C, lasting 1 week or longer ii. Arthralgia or arthritis, lasting 2 weeks or longer iii. Typical rash iv. Leukocytes \>10,000 mm3 with \>80% polymorphonuclear cells b. Minor Criteria i. Sore throat ii. Recent development of significant lymphadenopathy iii. Hepatomegaly or splenomegaly iv. Abnormal liver function tests v. Negative tests for antinuclear antibody and rheumatoid factor

• Subject has reported a recurring fever \>38°C, consistent with active disease, within the last 5 days of the Screening and Baseline Visits.

• If undergoing treatment with non-steroidal anti-inflammatory drugs (NSAID), subject is on a stable dose for at least 48 hours prior to the Baseline Visit (Visit 2).

• If undergoing treatment with glucocorticoids, subject is on a stable dose for at least 48 hours prior to the Baseline Visit (Visit 2).

• If undergoing treatment with conventional disease-modifying anti-rheumatic drugs (DMARDs), subject is on a stable dose for at least 4 weeks prior to the Baseline Visit (Visit 2).

• For subjects who have received treatment with biological DMARDs, subject has the required washout (normalization) period prior to the Baseline Visit (Visit 2).

• The washout (normalization) period for biological DMARDs is as follows:

‣ Anakinra - 1 week

‣ Etanercept, rilonacept - 4 weeks

‣ Adalimumab, certolizumab, infliximab, golimumab, abatacept, tocilizumab and canakinumab - 8 weeks

‣ Rituximab - 36 weeks

• Non-pregnant female subjects of childbearing potential who are heterosexually active and non-sterile male subjects with female sexual partners of childbearing potential agree to use a highly effective method of contraception during treatment and for 25 weeks following the last dose of investigational product. A highly effective method of birth control is defined as one that results in a low failure rate (ie, \<1% per year) when used consistently and correctly, such as oral/injectable/inserted/implanted/transdermal contraceptives, intrauterine hormone-releasing system or intrauterine device (IUD), or sexual abstinence. Contraception is not required where at least 6 weeks has passed since sterilization, defined as females having undergone one of the following surgeries: hysterectomy, bilateral tubal ligation or occlusion, bilateral oophorectomy, or bilateral salpingectomy; and males who are vasectomized. Contraception is also not required where females are postmenopausal (defined as 12 consecutive months of spontaneous amenorrhea and age ≥51 years). Females of reproductive potential must have a negative pregnancy test as part of the screening/baseline assessment.

• Subject has provided written informed consent for this study.

⁃ Subject is willing and able to comply with the protocol.