The purpose of the study is to assess the efficacy and safety of a range of doses of SCH 420814 when used together with a stable dose of L-dopa/dopa decarboxylase inhibitor to treat Parkinson's disease. SCH 420814 is a drug that may prove to be effective in treating Parkinson's disease, and therefore we are currently gathering data to allow us to gain approval from the FDA and/or other Health Authorities, so that SCH 420814 can be used to treat patients in the future. In this study, we will be comparing 3 doses (1 mg, 2mg, and 5 mg taken twice a day) of SCH 420814 with placebo (sugar pill). Following an Interim Analysis (temporary hold for new enrollment-ongoing subjects will continue on treatment) to review drug safety, a new dose group of 10 mg (taken twice a day) may be added.

Whether you will receive SCH 420814 or placebo will be decided by a process called randomization (as in the flip of a coin). Your chance of receiving the investigational drug, SCH 420814, is 75% in the first phase of the study (with a 1mg, 2mg, 5mg dose, or placebo dose arm). After 40 patients complete 12 weeks of treatment there will be an Interim Analysis which will determine the safety of the drug. If the 5 mg arm is found to be safe, then the 10 mg arm will also be added to the study. Your chance of receiving study drug (1mg, 2mg, 5mg, 10 mg, or placebo) with the 10 mg arm is 80%. If the 5 mg arm is found to be not safe, then the 10 mg arm will not be added and the 5 mg arm will also be dropped from the study. Your chance of receiving study drug (1mg, 2 mg, or placebo arm) will be 67%.

Approximately 160 subjects will be randomized in this study in approximately 22 study centers worldwide for the first part of this study. Following the Interim Analysis, we may add 40 new patients, for a total of 200 patients. The study is double blind, which means neither you nor your study doctor will know whether you are receiving the study medication or placebo.

• Parkinson Disease
• Movement Disorders
• Central Nervous System Diseases
• Neurodegenerative Diseases
• Brain Diseases

• Drug: SCH 420814
• Drug: Placebo

Inclusion Criteria:

• Subjects must be 30 years of age, of either sex and of any race, with adiagnosis of moderate to severe idiopathic Parkinson's disease for at least 5 years.
• Women of childbearing potential must have a negative serum pregnancy test at Visit 2. If subject is postmenopausal (not surgically induced), she must be postmenopausal by history for at least 2 years before study entry. If not, proper birth control must be used.

Note: Acceptable methods of birth control include oral or injectable hormonal contraceptive, medically prescribed IUD, and double-barrier method (eg, condom in combination with spermicide). Bilateral tubal ligation is an acceptable method of birth control for this study.

• Subjects' clinical laboratory tests (CBC, blood chemistries, and urinalysis) must be within normal limits or clinically acceptable to the investigator/sponsor.

Exclusion Criteria:

• Subjects with any form of drug-induced or atypical parkinsonism, cognitive impairment (Mini-Mental State Examination [MMSE] score <=23), a history of DSM IV diagnosed major depression, unstable mild depression or psychosis, or subjects taking tolcapone will be excluded. (Subjects with mild depression who are well controlled on a stable dose of an antidepressant medication for at least 4 weeks before screening will be eligible.)
• All subjects with a severe or ongoing unstable medical condition will be excluded including those with a history of poorly controlled diabetes, obesity associated with metabolic syndrome, uncontrolled hypertension, cerebrovascular disease, or any form of clinically significant cardiac disease, symptomatic orthostatic hypotension, renal failure, history of abnormal renal function, seizures, alcohol/drug dependence, or previous surgery for Parkinson's disease.
• Average daily consumption of more than two 4-oz (120 mL) glasses of wine or their equivalent.
• Because it is not known whether SCH 420814 passes into breast milk and because the effects, if any, of SCH 420814 on the developing human are unknown, women who are breastfeeding or who are considering breastfeeding are excluded from this trial.
• Subjects with allergy/sensitivity to study drug or its excipients.
• Subjects with any clinically significant condition or situation, other than the condition being studied that, in the opinion of the investigator, would interfere with the study evaluations or optimal participation in the study.
• Subjects who have used any other investigational drugs within 30 days of Screening.
• Subjects who are participating in any other clinical study.