MAIN STUDY: SWITCH SUB-STUDY: SWITCH-JCV

MAIN STUDY: Teriflunomide (TFM) is the primary metabolite of leflunomide, which is marketed worldwide for the treatment of rheumatoid arthritis. Teriflunomide inhibits dihydroorotate dehydrogenase( DHODH), the forth enzyme in the de novo synthesis pathway of pyrimidines. Activated T-lymphocytes utilize both the de novo pyrimidine and salvage pathways of pyrimidines ribonucleotide synthesis.After mitogen stimulation, teriflunomide inhibits in vitro T cell proliferation, DNA and RNA synthesis and expression of cell surface and nuclear antigens that are directly involved in T-cell activation and proliferation. Natalizumab (NTZ) is a FDA approved treatment for relapsing-forms of multiple sclerosis (MS) with pivotal studies showing an annualized relapse-rate (ARR) reduction of 68%, a reduction of new Gadolinium "enhancing" (Gd+) lesions by 92% and a reduction of disability of 42% compared to placebo. NTZ is highly effective in controlling MS but the risk of developing progressive multifocal leukoencephalopathy (PML) increases with the duration of the use of natalizumab. It can have a serious and life threatening complication in about 1 in 500 to 1 in 250 patients who have had more that 18 infusions due PML and who have a detected antibody (Ab) for the JC virus. The risk of PML is much greater in patients who have had prior immunosuppressive (IS) treatment. The combination of detected anti-JCV Ab , duration of NTZ treatment of greater than 24 months and prior IS increases the risk of development PML to an incidence of 11 per 1000 treated patients. So there is a need to have an alternative MS disease modifying treatment (DMT) to use for patients at risk to develop PML from NTZ treatment that might be sufficiently effective for MS so as not to have the patients' MS worsen while lowering or eliminating the risk of PML. SUB-STUDY: John Cunningham virus (JCV) is the responsible organism for the development of progressive multifocal encephalopathy (PML). Multiple sclerosis (MS) patients treated with natalizumab (NTZ), who have evidence of exposure to JCV have a significantly elevated risk, as high as 12/1000 patients, of developing PML, necessitating the elective transition of patients off NTZ. JCV is a member of the polyoma virus family, and closely related to BK virus. BK virus has been found to be a significant threat to destroy transplanted kidneys and there is growing evidence that treatment with leflunomide, the pro-drug of teriflunomide (TFM) clears BKV from the kidney. Main study assessed the clinical efficacy of TFM in MS patients transitioned off NTZ solely because they have evidence of exposure to JCV, as determined by the presence of serum anti-JCV immunoglobulin G (IgG). Previous work by other investigators has demonstrated a close correlation between positive anti-JCV antibody titers and the presence of active JCV infection. There is evidence that JCV is closely related to its fellow polyoma virus BKV, and that BKV is cleared by treatment with the TFM pro-drug leflunomide, we propose to determine if treatment of the JCV antibody positive patients currently enrolled in the approved protocol results in reduction in, or reversion to negative, of the anti-JCV antibody titer as measured by the JCV antibody Index, a commercially available assay which is used internationally as the standard for evidence of JCV exposure in NTZ-treated patients.