Overview

The primary objective of this study is to evaluate the pharmacokinetics (PK) of olokizumab (OKZ) in patients with polyarticular juvenile idiopathic arthritis aged \>2 and \<18 years in two doses (64 mg or 48 mg every 4 weeks) depending on patient's weight. Secondary objectives are to evaluate the pharmacodynamic (PD) profile, the long-term efficacy and safety of olokizumab in patients with polyarticular juvenile idiopathic arthritis aged \>2 and \<18 years.

Eligibility

Inclusion Criteria:

1. Study informed consent form voluntarily and independently signed by patient legal representative
2. Study assent form voluntarily and independently signed by minor study subject (patient)
3. Male or female patients aged ≥12 and \<18 years (cohort 1 - subgroup A) or \>2 and \<12 years (cohort 1 - subgroup B) or \>2 and \<18 years (cohort 2) at the time of screening initiation and on Day 0
4. Body weight at the start of screening and on Day 0 ≥45 kg (cohort 1 - subgroup A) or ≥30 and \<45 kg (cohort 1 - subgroup B) or ≥18 and \<30 kg (cohort 2)
5. A reliable diagnosis of juvenile idiopathic arthritis (JIA) according to the JIA International League of Associations for Rheumatology (ILAR) 1 criteria with onset before the age of 16 years:
   1. Seropositive or seronegative polyarthritis (pJIA) ≥3 months before screening, or
   2. Systemic JIA (sJIA) for ≥3 months before screening, provided that joint symptoms persist without active systemic manifestations for ≥3 months before screening, or
   3. Extended oligoarticular JIA (оJIA) ≥3 months before screening
6. American College of Radiology (ACR) criteria of active polyarthritis are met: 5 or more active joints at screening and on Day 0
7. C-reactive protein (CRP) level on screening or in anamnesis, not associated with alternative causes of increase other than the activity of the underlying disease, ≥6 mg/l
8. Intolerance or failure of methotrexate in the dose of ≥15 mg/m\^2/week (or less, in a case of documented intolerance of higher doses) for ≥3 months in medical history

Exclusion Criteria:

1. Prior use of any drug that acts directly on IL-6 or IL-6R
2. If methotrexate is administered - any change in dose or in a formulation within 6 weeks prior to Day 0
3. Previous therapy with marketed or experimental conventional synthetic disease-modifying antirheumatic drugs (csDMARDs) or biologic disease-modifying anti-rheumatic drugs (bDMARDs) within less than 5 elimination half-lives
4. Use of oral steroids in the doses above 0.2 mg/kg or 10 mg/day of prednisolone daily, whatever is lower, or a change in dose within 2 weeks prior to Day 0, or use of parenteral or topical steroids within 4 weeks prior to Day 0
5. Change in dose of a non-steroidal anti-inflammatory drug (NSAID) within ≤2 weeks prior to Day 0
6. Vaccination with live vaccines within 6 weeks before baseline, or planned vaccination with live vaccines during the study and/or within 6 weeks after the last olokizumab administration
7. Active uveitis at screening or uveitis exacerbation within 24 weeks before screening
8. Laboratory abnormalities (creatinine ≥1 mg/dL (88 mM) for children aged 12 or ≥1.2 mg/dL (106 mM) for children aged 13 and older; alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≥1.5 х upper limit normal (ULN); platelets \<180,000/mm\^3; white blood count (WBC) \<4000/mm\^3; neutrophils \<2000/mm\^3; hemoglobin ≤80 g/L
9. Exclusion criteria related to past or current infection other than tuberculosis
10. Suspected or confirmed current tuberculosis (TB) infection, history of an active or latent TB infection
11. Active course of a disease associated with formation of intestinal diverticula, or any other symptomatic gastrointestinal disease that may increase risk of perforation; or a history of diverticulitis or perforation; or concurrent Crohn's disease or ulcerative colitis
12. Concurrent heart failure New York Heart Association (NYHA) III or IV functional class
13. In patients with diabetes mellitus - HbA1c \> 7% within the last 3 months (non-controlled diabetes mellitus)
14. Patients with Steinbrocker class IV functional impairment
15. Presence of systemic autoimmune or autoinflammatory disease, except JIA, or chronic autoimmune hepatitis or diseases of the primary immunodeficiencies group
16. Patients with history of macrophage activation syndrome episodes
17. Exclusion criteria related to concurrent diseases and conditions that may increase potential risk related to participation in the study and study drug exposure
18. Known hypersensitivity to any component of the study drug
19. Pregnant or breast-feeding female participants or planned pregnancy
20. Other protocol-defined non-inclusion criteria apply