This study is looking at how well and how safely a medicine called remibrutinib works for people with Secondary Progressive Multiple Sclerosis, or SPMS. SPMS is a stage of multiple sclerosis (MS), a condition where the body's immune system attacks the brain and spinal cord. This study will include around 1,275 adults aged 18 to 65. Participants will get either remibrutinib or a placebo (a pill with no medicine inside) without knowing which one they are taking.

In the beginning, the study is "double-blind," meaning neither the doctors nor the participants know who is getting remibrutinib or the placebo. Later, everyone will get remibrutinib. Participants need to have SPMS and meet certain health conditions to join. They must be comfortable with having MRI scans, which are detailed pictures of the inside of the body.

• The study is event-driven, meaning it will last until enough data is collected.
• Participants must visit the study center for assessments and tests.
• There are specific health requirements to join the study, and certain medications are not allowed.