Patients with MS receiving NTZ for at least 18 months and without any disease activity during the previous year will be eligible. Included patients will receive either methylprednisolone (1 gramme, 1 day every 4 weeks for a total of 3 grammes) or undistinguishable capsules of placebo.
Every patient will initiate a treatment with FTY 7 weeks after the last NTZ perfusion. Methylprednisolone or placebo will be taken 4, 8 and 12 weeks (W4, W8 and W12) after NTZ discontinuation.
A spinal and brain MRI will be performed at baseline (last NTZ perfusion, noted W0) and 16 to 18 weeks after. The last clinical follow-up will be made after 24 weeks.
