Evaluation of Safety, Tolerability, and PK of VX15/2503 In Patients With MS

Inclusion Criteria: * Patients 18-60 years of age who have been diagnosed with MS for at least 1 year as defined by the 2010 revisions to the McDonald criteria * Has an EDSS score of 0 to 6.5 inclusive at screening * Has a body mass index of 18 to 32 kg/m2 * Is willing to undergo and has no contraindications to brain MRI * Willing to use a medically acceptable method of contraception throughout the study period and for 6 months after the dose of VX15/2503, unless patient is surgically sterile or postmenopausal. Women of childbearing potential must have started using adequate contraception at least 2 months before the Day 1 visit. * Male patients must agree to defer from donating sperm for 6 months after VX15/2503 administration * Women of childbearing potential must have a negative serum pregnancy test at screening, which will be confirmed at baseline using a urine test before administration of VX15/2503 * Is willing to forego other forms of experimental treatment during the study Exclusion Criteria: * Had an MS relapse that did not stabilize within the 30 days before the start of screening. * Has any clinically significant cardiac, endocrine, hematologic, hepatic, immunologic, metabolic, urologic/gynecologic, pulmonary, neurologic, psychiatric, or renal conditions; has a history of relevant clinically significant allergic or anaphylactic reactions; or has any other clinically significant major disease that, as assessed by the investigator, would pose a risk to patient safety or interfere with the study evaluations, procedures, or completion * Has any clinically significant laboratory value outside the normal range for MS patients at screening, or has abnormal hematologic, renal, or hepatic function based on laboratory tests * Is a pregnant or breastfeeding woman * Has received treatment with any MS disease-modifying therapy other than interferon beta or glatiramer acetate within 3 months prior to dosing * Has been treated with natalizumab, daclizumab, or fingolimod for any indication within 6 months prior to dosing * Has had any prior treatment with alemtuzumab, rituximab, mitoxantrone, total lymphoid irradiation, bone marrow transplantation, or T cell or T cell receptor vaccination * Has received any experimental agent within 6 months prior to dosing, or within a period equivalent to 5 half-lives of the agent (whichever is longer); or is currently involved in any other research study * Has undergone any major surgical procedure within the 4 weeks prior to dosing * Has a history of congestive heart failure (New York Heart Association Class III to IV), symptomatic ischemia, conduction abnormalities uncontrolled by conventional intervention, or myocardial infarction within 6 months prior to dosing * Has a clinically significant ECG finding at screening * Has a known or suspected human immunodeficiency virus (HIV) or hepatitis B or hepatitis C infection * Has a known or suspected allergy to Gd or other contraindication to brain MRI * Has a history of an opportunistic infection or a history of acute infection requiring systemic antibiotics, antivirals, or antifungals within 6 weeks prior to dosing (antiinfective therapy must have been completed at least 4 weeks prior to dosing) * Has any other intercurrent illness or condition, including alcohol or drug dependence as determined by the investigator, which could impact the patient's compliance with or ability to complete the study * History of seizure disorder or unexplained blackouts or history of seizure within 3 months of screening * History of suicidal ideation within 3 months prior to screening, episode of severe depression within 3 months prior to screening * Has a sensitivity to VX15/2503 or the ingredients or excipients of VX15/2503, or known or suspected sensitivity to mammalian cell-derived products * Has donated or lost more than 1 unit of blood in the 60 days prior to screening