Patient description: The patient is a 20 year old Caucasian female presents to the clinic with one-sided vision lost and facial weakness, dysarthria and numbness lasting for 1 day. She visited her PCP and underwent brain MRI which revealed a single plaque in the brainstem. After few months, she experienced lower extremities weakness led to balance problem. The second MRI revealed another lesion in the left cerebral hemisphere. The diagnosis of Relapsing Remitting Multiple Sclerosis (RRMS) is confirmed after the second MRI. Her past surgical history is significant for a C-section 2 years ago and she has a one child. She is divorced and is not currently in any sexual relationship. She smokes 10 cigarettes per day and drinks alcohol occasionally. She is working as an editor in a publisher company and she is happy that she can keep working from home most of the time. She is under the treatment of RRMS from 7 months ago.

Clinical trial eligibility criteria: eligible ages (years): 18.0-70.0,  Multiple Sclerosis, Remittent Progressive Multiple Sclerosis, Primary Progressive Relapsing-Remitting Multiple Sclerosis Secondary Progressive Multiple Sclerosis Multiple Sclerosis Key Part A To be eligible to participate in Part A, candidates must meet the following at screening or at the timepoint specified in the individual criterion listed (potential subjects who fail screening may be rescreened 1 time): 1. Must have a diagnosis of primary-progressive, secondary progressive, progressive relapsing, or relapsing-remitting MS as defined by the revised McDonald Committee ([Lublin and Reingold 1996; McDonald 2001; Polman 2005]) of at least 2 months duration. 2. Must be able to complete the T25FW with or without a walking aid in 8 to 45 seconds at the screening visit. Part B To be eligible to participate in Part B, candidates must meet the following at the Week 21 visit in Part A, which is the first visit for Part B: 1. Completed all visits in Part A of the study. Part C To be eligible to participate in Part C, candidates must meet the following at the Week 52 visit in Part B, which is the first visit for Part C: 1. Completed all visits in Part B of the study. Key  Known allergy to pyridine-containing substances, or any of the inactive ingredients of the prolonged-release fampridine tablet 2. Any prior history of seizures, epilepsy, or other convulsive disorder, with the exception of febrile seizures in childhood, or prior history of epileptiform activity on electroencephalogram. 3. Any form of renal impairment as defined by a creatinine clearance (CrCl) of <80 mL/min (estimated by the central laboratory). 4. Known history of cardiac arrhythmia or cardiac conduction disorders requiring medical or surgical intervention, or any clinically significant ECG abnormality (as determined by the Investigator) at the screening visit or Day 1. 5. Any prior treatment with fampridine (4 AP) or 3,4 diaminopyridine in any formulation. 6. Treatment with an investigational drug or approved therapy for investigational use within 30 days (or 5 half lives, whichever is longer) prior to the screening visit. 7. Participation in an investigational study (with the exception of observational studies) within 30 days prior to the screening visit or plans to enroll in another interventional investigational study at any time during this study. NOTE: Other protocol defined Inclusion/