4) Receiving stable doses of L-Dopa/carbidopa (immediate or sustained release) administered at least 3 times per day OR Rytary™ administered 3 times per day, for at least 4 weeks before the initial Screening Visit (SV1). Subjects receiving L-Dopa/carbidopa 3 times a day must also be on stable treatment with adjunctive PD medication regimens. These regimens bust me maintained at a stable dose for at least 4 weeks prior to the initial Screening Visit (SV1) with the exception that MAO-B inhibitors must be maintained at a stable level for at least 8 weeks prior to the initial Screening Visit (SV1).