Multi-center, double blind, placebo-controlled study with parallel groups of outpatients with early, symptomatic Huntington's disease. Participants will be randomized to receive 1 gram twice daily (2 gram/day total daily dose) of active study drug or placebo. The 6-month placebo-controlled phased will be followed by a subsequent 6-month open-label extension phase with all subjects receiving 1 gram twice daily (2 grams/day total daily dose) of active study drug.