This study is testing a medicine called frexalimab to see if it can help people with a condition known as **nonrelapsing secondary progressive multiple sclerosis (nrSPMS)**. In nrSPMS, the disease progresses without new relapses or attacks. Frexalimab will be compared to a placebo, which is a substance with no active medicine, to check its safety and effectiveness over **36 months**. People aged **18 to 60** with this condition, who meet certain health criteria, can join. This study involves **up to 25 visits** where participants will have check-ups. For the first **6 months**, visits are every month, then every **3 months**. Participants must have had no attacks for at least **24 months** and must use birth control if required. Some people cannot join, such as those with a history of certain infections or other medical conditions.

• Study lasts up to **43 months** and ends when enough events are recorded.
• Participants will have **up to 25 visits** during the study.
• Eligibility is strict; certain medical histories may prevent participation.