The purpose of this randomized, double-blind, placebo-controlled, parallel group study is to determine the efficacy of frexalimab in delaying the disability progression and the safety up to 36 months double-blind administration of study intervention compared to placebo in male and female participants with nrSPMS (aged 18 to 60 years at the time of enrollment). People diagnosed with nrSPMS are eligible for enrollment as long as they meet all the inclusion criteria and none of the exclusion criteria. Study details include:

• This event-driven study will end when the target number of 6-month cCDP events is achieved, and the study is expected to last 43 months from randomization of the first participant to the common study end.
• The number of scheduled visits will be up to 25 (including 3 follow-up visits) with a visit frequency of every month for the first 6 months and then every 3 months.
• If the prespecified number of events for 6-month cCDP is not reached by V21/W180, scheduled visits will continue every 3 months.

Inclusion Criteria:

• Participant must have a previous diagnosis of RRMS in accordance with the 2017 revised McDonald criteria.
• Participant must have a current diagnosis of SPMS in accordance with the clinical course criteria revised in 2013 endorsed by an Adjudication Committee.
• Participant must have documented evidence of disability progression observed during the 12 months before screening. Eligibility will be analyzed by an Adjudication Committee.
• Absence of clinical relapses for at least 24 months.
• The participant must have an EDSS score at screening from 3.0 to 6.5 points, inclusive.
• Contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies.
• For patients eligible to be treated with siponimod: 1) does not tolerate it due to side effects or safety reasons, or 2) has failed siponimod treatment due to perceived lack of efficacy, or 3) has declined siponimod treatment.

Exclusion Criteria:

• The participant has a history of infection or may be at risk for infection.
• The presence of psychiatric disturbance or substance abuse.
• 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.
• Current hypogammaglobulinemia defined by immunoglobulin levels (IgG and/or IgM) below the lower limits of normal (LLN) at Screening or a history of primary hypogammaglobulinemia. Patients with a history of secondary hypogammaglobulinemia induced by anti-C20 monoclonal antibodies (eg, ocrelizumab, ofatumumab, ublituximab, rituximab) may be considered for study inclusion provided their immunoglobulin levels are within the normal limits (WNL) at time of Screening.
• A history or presence of disease that can mimic MS symptoms, such as, but not limited to neuromyelitis optica spectrum disorder, systemic lupus erythematosus, Sjogren's syndrome, acute disseminated encephalomyelitis, and myasthenia gravis.
• The participant has sensitivity to any of the study interventions, or components thereof, or has a drug or other allergy that, in the opinion of the Investigator, contraindicates participation in the study.
• The participant was previously exposed to frexalimab.

The above information is not intended to contain all considerations relevant to a patient'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