- Secondary-progressive multiple sclerosis (SP-MS) is the chronic phase of multiple sclerosis (MS). The majority of people who have relapsing-remitting MS eventually develop SP-MS. There are currently no effective treatments for SP-MS. Researchers are interested in determining whether the drug rituximab, which is used to treat rheumatoid arthritis and some types of cancer, is able to target certain white blood cells that are thought to play a role in the progression of SP-MS. To ensure that the rituximab will reach the brain and spinal cord, participants will receive it by intravenous drip and by intrathecal injection (through a lumbar puncture into the cerebrospinal fluid).