GSK-239512

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.