The purpose of this study is to explore whether DMF (Dimethyl Fumarate) or MMF (monomethyl fumarate) its main bioactive metabolite, is capable of entering the central nervous system in SPMS patients that are being treated with Tecfidera®. PK samples (pharmacokinetics - or the amount of study drug in blood) will be tested to compare with PK samples, the amount of study drug, in spinal fluid (CSF).