A Efficacy, Safety and Pharmacokinetic Study of XP21279 and Sinemet® in Parkinson's Disease Subjects

Inclusion Criteria: 1. Subjects must have predictable motor fluctuations of the wearing off type, defined by meeting the following criteria based on the on/off diaries recorded over 3 days in the Screening Period: * Wearing-off in at least half (50%) of inter-dose intervals between the first and the last daily doses averaged over the 3 diary days, and * An average daily "off" time of 2 hours after the first "on" of the day through awake time up to midnight. 2. Subjects must be on one of the following stable QID or 5 times daily regimens for at least 4 weeks prior to Screening: Sinemet® or carbidopa-levodopa, with a total daily dose ranging from 400 mg to 1000 mg of levodopa Exclusion Criteria: 1. History, signs, or symptoms suggesting the diagnosis of secondary or atypical Parkinsonism. 2. Subject has moderately or severely disabling dyskinesias for greater than 25% of the waking day 3. Subjects who have significant neurological symptoms not accounted for by Parkinson's disease 4. Subjects who are taking Sinemet® CR, Parcopa®, concomitant COMT inhibitors (i.e., entacapone or tolcapone), Stalevo®, or benserazide containing levodopa preparations Madopar® or Prolopa®.