High and Low Dose Carbidopa Treatment of Parkinson's Disease

Hypothesis: We hypothesize that carbidopa in daily doses of 450mg will enter the central nervous system and partially inhibit AAAD, thereby reducing the decarboxylation of exogenous levodopa to dopamine, and thereby blunt the therapeutic effects of levodopa in PD subjects. The purpose of this study is to see how low dose vs. high dose of the study drug, carbidopa effect movement in subjects with Parkinson's disease. The low dose of the study drug is 75 mg and the high dose is 450mg. Subjects will be recruited from the investigators clinic when they are seen for treatment for Parkinson's disease. Subjects will also be recruited through flyers hung at OHSU and at the VA. Subjects will take part in 2 screening visits one week apart to determine eligibility. Subjects will be randomly chosen to start either high or low dose carbidopa and take it for 4 weeks. Subjects will be called 2, 4, and 6 or 7 days after this visit to ask how they are doing after starting this dose of study drug. We will leave them a message if we cannot reach them. If there are any problems, we will schedule them to come to the clinic within the next 2 days. Subjects will have an outpatient visit 2 weeks after screening and a hospital admission 2 weeks after that. At the hospital, subjects will stay for 3 days. They will have blood drawn and their Parkinson's disease assessed by a finger tapping exercise, timing their walking, and looking at their uncontrolled movements. The subject will then receive the opposite dose of carbidopa for 4 weeks. Subjects will be called 2, 4, and 6 or 7 days after this visit to ask how they are doing after starting this dose of study drug. We will leave them a message if we cannot reach them. If there are any problems, we will schedule them to come to the clinic within the next 2 days. The outpatient visit and hospital admission will repeat again. At the end of the second hospital admission, treatment on the study is over and subjects will go back to their origi