The purpose of the study is to assess the pharmacokinetics, pharmacodynamics, and safety of XP21279 sustained release formulation [administered with Lodosyn® (carbidopa)] and Sinemet® tablets in subjects with Parkinson's disease with Motor Fluctuations.

This is a Phase 1/Phase 2 multiple-dose, multi-center, open-label, two period sequential-treatment study in subjects with Parkinson's disease to assess the pharmacokinetics, pharmacodynamics, and safety of XP21279 sustained release formulation [administered with Lodosyn® (carbidopa)] and Sinemet® tablets in subjects with Parkinson's disease and to explore dose correspondence between XP21279 and Sinemet® to guide dose selection for future studies in the target population (subjects with Parkinson's disease with motor fluctuations on Sinemet®).

Inclusion Criteria:

1. Subjects with a clinical diagnosis of idiopathic Parkinson's disease, confirmed by the presence of at least two cardinal signs of the disease (resting tremor, bradykinesia, rigidity).
2. Subjects must have predictable motor fluctuations of the wearing off type, defined by wearing off in at least 50% of inter-dose intervals between the first and the last daily doses as recorded on the on/off diary over 3 days (Days -4 to -2) in the Screening Period.
3. Subjects must be on stable TID or QID Sinemet® or carbidopa/levodopa regimens from morning through early evening, with a total daily dose ranging from 400 mg to 1000 mg of levodopa, for at least 1 week prior to Screening.

Exclusion Criteria:

1. History, signs, or symptoms suggesting the diagnosis of secondary or atypical Parkinsonism.
2. Subject has greater than or equal to moderately disabling dyskinesias for greater than 25% of the waking day as assessed by a score of 2 or more on item 32 and a score of 2 or more on item 33 on the UPDRS at Screening.
3. Subjects who are dosing with Sinemet® or carbidopa/levodopa during the night time.
4. Subjects who have significant neurological symptoms not accounted for by Parkinson's disease.
5. Subjects who are taking concomitantly COMT inhibitors (i.e., entacapone or tolcapone) or treated with Stalevo®, Sinemet® CR, or Madopar®/Prolopa® (levodopa/benserazide).