This is a randomized, parallel group, placebo-controlled study designed to assess whether GSK239512 can enhance lesion remyelination in subjects with Relapsing Remitting Multiple Sclerosis (RRMS). Subjects with RRMS on stable background treatment with either Avonex (Interferon-beta1a) or Copaxone (Glatiramer Acetate) are eligible to participate. Subjects will be randomized in a 1:1 ratio between placebo and GSK239512, and will continue to be managed with their current standard of care therapy (Copaxone or Avonex). The total treatment period is 48 weeks, including a standard 4 week titration period and 44 week maintenance treatment period (which could be adapted to a 5-week titration and 43 week maintenance period, if needed). Titration doses start at 10 micrograms (mcg) and increase up to 80 mcg (10 mcg first week, 20 mcg second week, 40 mcg third week, 80 mcg fourth week). Subjects will be titrated to the maximum tolerated dose with the objective of titrating to the highest dose (80 mcg GSK239512), whenever possible, based on investigator judgement of tolerability. The post-treatment follow-up period will be a minimum of 2 weeks in duration following the end of treatment at Week 48 or early withdrawal, as appropriate.

Inclusion Criteria:

• 18 to 50 years of age
• Diagnosed with a relapsing-remitting course of multiple sclerosis as defined by the appropriate McDonald criteria at the time of diagnosis.
• Diagnosis of RRMS made within approximately 10 years prior to the screening visit (as documented by year of diagnosis or duration of disease), and No physical manifestations of other forms of Multiple Sclerosis (MS) including signs of progression to secondary progressive MS (SPMS)
• Currently compliant with a stable dose regimen of Avonex (Interferon Beta1a) or Copaxone (Glatiramer Acetate) for management of MS for >= 1 year prior to the screening visit
• The occurrence of at least one of the following (within the year preceding the screen visit AND after >=2 months of stable treatment with Avonex OR Copaxone): a) 1 reported and/or documented relapse, OR b) 1 Gadolinium Enhanced (GdE) lesion on MRI
• Currently neurologically stable, in the investigator's judgment, and not actively experiencing or recovering from a recent relapse at the screening visit
• A Kurtzke Expanded Disability Status Scale (EDSS) score of 1 to 4.5 (inclusive) at the screening visit.
• Must agree not to participate in a clinical study involving another investigational drug or device throughout their participation in this study. Non-interventional study participation is allowed if the time involvement and scheduling will not interfere with compliance in this study in the opinion of the investigator
• A female subject is eligible to enter the study if she is a) Not pregnant or nursing, b) Of non-childbearing potential (i.e. women who have had a hysterectomy, are postmenopausal, which is defined as >2 years without menses [female subjects who have been post-menopausal for <2 years must be confirmed with Follicle Stimulating Hormone and estradiol levels], have both ovaries surgically removed or have current documented tubal ligation); or, c) Of childbearing potential and agrees to use the acceptable methods of birth control, for one month prior to the start of investigational product to 1 month after the last dose of investigational product.
• Informed Consent: Must be competent to understand the information given in the Institutional Review Board (IRB) or Independent Ethics Committee (IEC) approved Informed Consent Form (ICF) and must sign the form prior to the initiation of any study procedures

Exclusion Criteria:

• MRI: Unable to undergo MRI scans (e.g. due to pacemaker, severe claustrophobia, hypersensitivity to contrast media), Lacks adequate venous access for administration of Gd-enhancing agent, or Findings on brain MRI scan indicating any clinically significant brain abnormality other than MS (e.g. damage associated with prior traumatic brain injury)
• Past and Concurrent Medical Conditions: a) History of severe and clinically significant CNS trauma with current sequelae (e.g. traumatic brain injury, spinal cord compression), b) Significant concurrent, uncontrolled medical condition or disease which in the opinion of the investigator could (e.g. significant psychiatric disorder, etc), affect the subjects' safety, impair the subject's reliable participation in the trial, impair the evaluation of the endpoints, or necessitate the use of medication not allowed by this protocol, c) History or presence of myelopathy due to spinal cord compression by disk or vertebral disease or chronic progressive myelopathy, d) Diagnosis of any type epilepsy, e) At risk of suicide, as indicated by: A documented history of attempted suicide or significant suicidal ideation during the 6 months preceding the screening visit, OR If in the investigator's judgment the subject is at risk of a suicide attempt based on the screen visit assessment, including the eC-SSRS, f) Presence of significant and routine sleep disturbance (severe insomnia, nocturnal wandering, confusion, disorientation, agitation, or vivid dreams) that has a negative impact on quality of life that, in the judgement of the investigator, may increase the risk of tolerability issues during dose escalation, g) Presence or history of hallucinations that, in the judgement of the investigator, may increase the safety risk to the subject, h) Known diagnosis or history consistent with positive human immunodeficiency virus (HIV)
• Past and Current Medications and Therapies: Have had treatment with the following to manage their MS: a) Within 1 year: fingolimod (e.g. Gilenya), Rebif (interferon beta1a), interferon beta1b (e.g. Betaseron), mycophenolate mofetil (e.g. CellCept), or Recently approved medication or formulation indicated for the management of MS. Consult with GlaxoSmithKline (GSK) Medical Monitor if there are specific questions regarding eligible treatments b) Within 2 years: natalizumab (e.g. Tysabri), alemtuzumab (e.g. Campath), daclizumab (e.g. Zenapax), rituximab (e.g. Rituxan), mitoxantrone (e.g. Novantrone), cladribine (e.g. Leustatin), or azathioprine (e.g. Imuran)
• Have used corticotropin to manage a relapse within 6 months prior to Screen Visit.
• Have had treatment with the following and were unable to discontinue and refrain from treatment for the specified time period and are not able to discontinue use throughout participation in the clinical trial: dalfampridine /fampridine (e.g. Ampyra) - 1 month prior to Screen Visit, nabiximols (e.g. Sativex) - 1 month prior to Screen Visit, amantadine (e.g. Symmetrel) - 3 months prior to Screen Visit, or leflunomide (e.g. Arava) - 1 year prior to Screen Visit
• Have used the following medications within the last 30 days or 5 half-lives (whichever is longer) prior to screening and are not able to discontinue use throughout participation in the clinical trial: Any CNS stimulants (e.g., modafinil, dexamphetamine, methylphenidate), Known potent P-glycoprotein inhibitors (e.g. itraconazole, ketoconazole, cyclosporin, loperamide, diltiazem, verapamil, spironolactone, quinidine, bepridil, quinine, carvedilol), Known potent inhibitors or inducers of the CYP3A4 enzyme (e.g., verapamil, ketoconazole, cimetidine, rifampin, modafinil), CNS-penetrant antihistamines (e.g. bromopheniramine, chlorpheniramine, clemastine, diphenhydramine, hyrdoxyzine),
• History of medically significant adverse effects (including allergic reactions and hypersensitivities) following treatment with: Histamine H3 receptor antagonist or inverse agonist, Gadolinium enhancing agent, Relapse medication: glucocorticoids (e.g., methylprednisolone) OR A known hypersensitivity to components of the investigational product, relapse medication, or Gadolinium enhancing agent.
• Electrocardiogram (ECG) showing a clinically significant abnormality at screening. Including a QT interval corrected using Bazett's and Fridericia's formulas (QTcB or QTcF) interval of >=450 msec or >=480 msec for patients with a Bundle Branch Block.
• Infectious Disease Status at Screening a) Subjects with no documented record of vaccination against Hepatitis B (primary and secondary immunization and booster) AND a positive test for hepatitis B surface antigen (HBsAg and Hepatitis B Core Antibody [IgM anti-HBc]), b) Subjects with serologic evidence of active Hepatitis C, as indicated by a positive Hepatitis C Virus(HCV) RNA test and anti-HCV antibody test
• Laboratory Values at Screening: Hematology: Total white cell count <2.0 x 109/L, Neutrophils <1.0 x 109/L, Platelets <75 x 109/L (If out of range, platelet count can be repeated to exclude platelet clumping), or Haemoglobin < 80 g/L.
• Screening clinical chemistry liver function test: Alanine aminotransferase (ALT) >2.0 x upper limit of normal (ULN), Aspartate aminotransferase (AST) >2.0 x ULN, Alkaline phosphatise (ALP) >1.5 x ULN, or Bilirubin >1.5 x ULN.
• Documented renal insufficiency or laboratory results indicative of renal insufficiency (due to risks associated with administration of Gadolinium based contrast agents to subjects with moderate to severe kidney disease): Estimated Creatinine Clearance (Cockroft-Gault) <60 mL/minute
• If known, or according to investigator judgement, subject is suspected of not being able to comply with the study protocol requirements. Contributing factors to this assessment by the investigator could be, but not limited to: job demands, substance abuse, alcoholism, drug dependency or psychological disorder
• Prior participation in a clinical trial or use of an investigational product for a non approved intervention: Prior use of an investigational drug for MS or a condition other than MS within 4 weeks or 5 half-lives (whichever is longer) prior to screening. The GSK Medical Monitor should be consulted to confirm eligibility

Patient-level data for this study will be made available through www.clinicalstudydatarequest.com following the timelines and process described on this site.