ACT16877

Treatment tested

• Frexalimab
• MRI contrast-enhancing preparations

Placebo or sham

• placebo IV
• placebo SC
• Matched Placebo for test product Frexalimab

Other measures (14)

Inclusion criteria: * Participant must be 18 to 55 years of age inclusive, at the time of signing the informed consent. * The participant must have been diagnosed with RMS (relapsing-remitting MS and secondary progressive MS participants with relapses) according to the 2017 revision of the McDonald diagnostic criteria. * The participant must have at least 1 documented relapse within the previous year, or ≥2 documented relapses within the previous 2 years, or ≥1 active Gd-enhancing brain lesion on an MRI scan in the past 6 months and prior to screening. * Body weight within 45 to 120 kg (inclusive) and body mass index (BMI) within the range 18.0 to 35.0 kg/m2 (inclusive) at Screening. * Contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. * Capable of giving signed informed consent. Exclusion criteria: * The participant was diagnosed with PPMS according to the 2017 revision of the McDonald diagnostic criteria or with non-relapsing SPMS. * The participant had conditions or situations that would adversely affect participation in this study. * The participant had a history of or currently has concomitant medical or clinical conditions that would adversely affect participation in this study. * History, clinical evidence, suspicion or significant risk for thromboembolic events, as well as myocardial infarction, stroke and/or antiphosholipid syndrome and any participants requiring antithrombotic treatment. * Allergies to humanized monoclonal antibodies or severe post-treatment hypersensitivity reactions other than localized injection site reaction, to any biological molecule. * The participant had received any of the forbidden medications/treatments within the specified time frame before any baseline assessment. * The participant had taken other investigational drug within 3 months or 5-half-live, whichever is longer, before the screening visit. * The participant had an EDSS score >5.5 at the first screening visit. * The participant had a relapse in the 30 days prior to randomization. * Positive human immunodeficiency virus (HIV) serology (anti HIV1 and anti HIV2 antibodies) or a known history of HIV infection, active or in remission. * Abnormal laboratory test(s) at Screening. * Presence of Hepatitis B surface antigen (HBsAg) or anti-Hepatitis B core antibodies (anti-HBc Ab) at screening or within 3 months prior to first dose of study intervention. * Positive Hepatitis C antibody test result at screening or within 3 months prior to starting study intervention. The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.

Primary Objective: To determine the efficacy of SAR441344 as measured by reduction of the number of new active brain lesions Secondary Objective: * To evaluate efficacy of SAR441344 on disease activity as assessed by other MRI measures * To evaluate the safety and tolerability of SAR441344 * To evaluate pharmacokinetics of SAR441344