Study of the Efficacy, Safety, Pharmacokinetics, Pharmacodynamics, and Immunogenicity of BCD-261 in Subjects With Moderate to Severe Active Ulcerative Colitis

Inclusion Criteria: 1\. Diagnosis of ulcerative colitis with involvement of the colon proximal to the rectum (≥15 cm from the distal edge of the anal canal), established ≥3 months before signing the ICF and confirmed by endoscopic examination data. 2\. Moderate to severe active ulcerative colitis with a modified Mayo score (mMS) of ≥4 and ≤9 points, which includes an endoscopic component of ≥2 points (according to a central independent review) and a stool blood score of ≥1 point. 3\. Inadequate response to therapy according to the investigator's assessment, manifested by at least one of the following signs: 1. Persistent symptoms of disease activity despite treatment with at least one course of glucocorticoids including prednisolone at a dose of ≥40 mg/day or equivalent or budesonide ≥9 mg/day or equivalent for at least 2 weeks with oral administration (at least 1 week with intravenous administration at a dose equivalent to oral prednisolone ≥40 mg/day). 2. Steroid dependence manifested by an increase in disease activity after initial improvement, with a decrease in the dose of glucocorticoids below the dose equivalent to 10 mg of oral prednisolone per day, within 3 months from the beginning of treatment, or a relapse of the disease within 3 months after the end of glucocorticoid use. 3. Persistent symptoms of disease activity despite treatment with at least one course of immunosuppressants (azathioprine at a dose of ≥2.0 mg/kg and/or 6-mercaptopurine at a dose of ≥1.0 mg/kg) for ≥12 weeks, or in response to another treatment regimen with these drugs according to a regional standard of care. 4. Primary lack of response to therapy with TNFa inhibitors and/or anti-integrins, and/or IL-12/23 inhibitors, and/or Janus kinase inhibitors, and/or sphingosine-1-phosphate receptor modulators, defined as the persistence of symptoms of disease activity despite at least one course of induction of remission according to a treatment scheme approved by the regional standard. 5. Loss of response to therapy with TNFa inhibitors and/or anti-integrins, and/or IL-12/23 inhibitors, and/or Janus kinase inhibitors, and/or sphingosine-1-phosphate receptor modulators, defined as the appearance of symptoms of disease activity after initial improvement as a result of treatment with at least one course of induction of remission and at least one course of maintenance of remission according to a treatment scheme approved by the regional standard. 6. A history of intolerance to glucocorticoids and/or immunosuppressants (azathioprine, 6-mercaptopurine) and/or biological therapy/targeted immunosuppressants (TNFa inhibitors, anti-integrins, anti-IL-12/23 monoclonal antibodies, Janus kinase inhibitors, sphingosine-1-phosphate receptor modulators) established by the treating physician. 4\. Maintaining a stable dose of concomitant medications for ≥2 weeks prior to signing the ICF and in the screening period for glucocorticoids and 5-ASCs and for ≥4 weeks prior to signing the ICF and in the screening period for immunosuppressants (azathioprine, 6-mercaptopurine). Exclusion Criteria: 1. A history of or current at the time of signing the ICF Crohn's disease, unspecified colitis, ischemic colitis, radiation colitis, microscopic colitis, complicated form of diverticular disease. 2. A history of primary sclerosing cholangitis. 3. A history of fulminant colitis, toxic dilation of the colon, intestinal obstruction, intestinal perforation (except for those caused by injury or appendicitis). 4. A history of dysplasia of any grade in any part of the gastrointestinal tract at the time of signing the ICF. 5. Presence of intestinal stoma or artificial rectum or the need for them. 6. Failure of ≥3 classes of biologics/targeted immunosuppressors (according to INN) with different mechanisms of action (TNFa inhibitors, anti-integrins, IL-12/23 inhibitors, Janus kinase inhibitors, sphingosine-1-phosphate receptor modulators) or ≥4 biologics/targeted immunosuppressants, regardless of the mechanism of action. 7. Use of any of the indicated therapies within the specified time frame or need for therapy with these drugs during the study period: (1) Use of Janus kinase inhibitors within 2 weeks prior to signing the ICF or during the screening period. (2) Use of TNFa inhibitors within 8 weeks prior to signing the ICF or during the screening period. (3) Using modulators of sphingosine-1-phosphate receptors within 10 weeks prior to signing the ICF or during the screening period. (4) Use of anti-integrins, IL-12/23 inhibitors within 12 weeks before signing the ICF or during the screening period. (5) Use of oral glucocorticoids at a dose equivalent to prednisone \>20 mg/day or budesonide \>9 mg/day or rectal administration of glucocorticoids at any dose within 2 weeks prior to signing the ICF or during the screening period or parenteral administration of glucocorticoids at any dose within 4 weeks prior to signing the ICF or during the screening period. (6) Rectal administration of 5-ASCs within 2 weeks prior to signing the ICF or during the screening period. (7) Use of immunosuppressants not included in the approved therapy (tacrolimus, cyclosporine, mycophenolate mofetil, rapamycin, leflunomide, penicillamine, etc.) within 4 weeks before signing the ICF or during the screening period. (8) Long-term regular use of non-steroidal anti-inflammatory drugs (≥3 times a week for ≥6 weeks) for 2 weeks prior to signing the ICF. (9) Use of any other investigational drugs in other clinical trials at the time of signing the ICF or less than 8 weeks or 5 half-lives (whichever is longer) before the date of randomization.