This is a Phase 2, multinational, multicenter, randomized, double-blind, placebo-controlled, dose ranging study to evaluate the efficacy and safety of SAR441566 in adults with moderate-to-severe UC. The primary objective of this study is to assess efficacy of different doses of SAR441566 on clinical remission in participants with moderate-to-severe ulcerative colitis.

This study will include a screening period of up to 28 days (+ 7 calendar days if needed) followed by the main study treatment period of 52 weeks which will be comprised of a double blind (DB) treatment period with 12 weeks of induction period followed by a maintenance period of 40 weeks and 2-week follow-up after end of treatment.

Additionally, an Open Label (OL) period of up to 40 weeks will be offered to eligible participants (for participants not enrolling in the LTS study).

• The study duration will be up to 59 weeks.
• The treatment duration will be up to 52 weeks in the DB arm and up to 40 weeks in the OL arm.
• The number of visits will be 12 for the main study treatment period and 8 for the OL treatment period.

Inclusion Criteria:

Participants are eligible to be included in the study only if all of the following criteria apply:

• Male or female participants aged 18 to 75 years inclusive, at the time of signing the informed consent

• Participants who have clinical evidence of active UC for ≥3 months before screening and confirmed by endoscopy during the screening period

• Active moderate-to-severe UC at screening as defined by a modified Mayo Score (mMS) of 5 to 9 (without the Physician global Assessment (PGA), with a minimum rectal bleeding (RB) subscore ≥1, a minimum stool frequency (SF) subscore ≥1, a mMES ≥2 confirmed by central reader, a minimum sum of all subscores of 5, and a disease extent >15 cm from the anal verge

• Must have received prior treatment for UC (either "a" or "b" below or a combination of both "a" and "b"):

  1. History of no prior exposure to approved Advanced Therapy (AT), but having inadequate response to, loss of response to or intolerance to standard treatment with any of the following : 5-ASA, thiopurines (eg.6-MP, AZA), MTX, oral or intravenous (IV) corticosteroids or history of corticosteroid dependence (defined an inability to successfully taper corticosteroids without recurrence of UC) OR
  2. History of inadequate response to, loss of response to or intolerance to treatment with ≥1 approved AT such as a biologic agent (eg. TNF antagonists, anti-integrin other than natalizumab, anti-IL-12/23, anti-IL-23, or a small molecule (such as a JAKi or S1PRm) for UC

• Contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

• Participants with active CD, indeterminate colitis, ischemic colitis, microscopic colitis
• Participants with the following ongoing known complications of UC: fulminant colitis, toxic megacolon, or any other manifestation that might require bowel surgery while enrolled in the study
• Participant with prior colectomy, ostomy or ileoanal pouch, or anticipated colectomy during their participation in the study
• Participants with fecal sample positive for ova or parasites, bacterial pathogens, or positive for Clostridium difficile B toxin in stools
• Participants with active tuberculosis (TB) or a history of incompletely treated active TB or latent TB infection per local guidelines
• Participants with Positive Hepatitis B surface antigen (HBsAg), or hepatitis B core antibody (HBcAb) and/or hepatitis C virus antibody (HCVAb) at the screening visit
• Participants with any other active, chronic or recurrent infection, including recurrent or disseminated herpes zoster or disseminated herpes simplex
• Participants with a known history of human immunodeficiency virus (HIV) infection or positive HIV-1 or HIV-2 serology at screening
• Participants presenting with active malignancies, lymphoproliferative disease, or recurrence of either, within the 5 years before screening
• History of gastro-intestinal dysplasia other than completely removed low grade dysplasia OR presence of colonic mucosal dysplasia or adenomatous colonic polyps not removed during endoscopy by the time of the screening visit.
• If the participant has extensive colitis for ≥8 years or disease limited to left side of colon (ie, distal to splenic flexure) for >10 years, regardless of age, a colonoscopy within 1 year of the screening visit is required to survey for dysplasia. Participants with dysplasia or cancer identified on biopsies will be excluded.
• Female participants who is pregnant, breastfeeding, or is considering becoming pregnant during the study or within 3 months after the last dose of study drug
• Infection(s) requiring treatment with IV anti-infectives within 30 days prior to the screening visit or oral/intramuscular anti-infectives within 14 days prior to the screening visit
• Participants requiring or receiving any parental nutrition and/or exclusive enteral nutrition
• Participants who received cyclosporine, tacrolimus, mycophenolate mofetil, or thalidomide within 30 days prior to screening
• Participants who received fecal microbial transplantation within 30 days prior to screening
• Participants who have ever been exposed to natalizumab (Tysabri®) or oral carotegrast methyl (Carogra®)
• Participants who received IV corticosteroids within 14 days prior to screening or during screening period
• Screening laboratory and other analyses show abnormal results.

The above information is not intended to contain all considerations relevant to a participant's potential participation in a clinical trial.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org