A Placebo- and Active-Controlled Study of Preladenant in Early Parkinson's Disease (PD) (P05664)

Inclusion Criteria: * Has a diagnosis of idiopathic PD for \< 5 years. * If receiving amantadine and/or anticholinergics, must have been on a stable regimen of treatment for at least the 5 weeks immediately before Screening. (Note: Participants who are not taking any medications for PD are permitted to enroll in this trial.) * Must have a UPDRS Part 3 score of ≥10, a Hoehn and Yahr Stage ≤3, be ≥30 to ≤85 years of age, and have results of Screening clinical laboratory tests drawn within 5 weeks prior to randomization, clinically acceptable to the investigator, and not within the parameters specified for exclusion. * If sexually active or plan to be sexually active, must agree to use a highly effective method of birth control while the participant is in the study and for 2 weeks after the last dose of study drug. A male participant must also not donate sperm during the trial and within 2 weeks after the last dose of study drug. Exclusion Criteria: * Must not have a form of drug-induced or atypical Parkinsonism, cognitive impairment (ie, Montreal Cognitive Assessment \[MoCA\] score \<22), bipolar disorder, untreated major depressive disorder, schizophrenia, or other psychotic disorder; history of exposure to a known neurotoxin, or any neurological features not consistent with the diagnosis of PD as assessed by the investigator. * Must not have had surgery for PD. * Must not have a history of repeated strokes with stepwise progression of Parkinsonism or head injuries, or a stroke within 6 months of screening; poorly controlled diabetes; abnormal renal function; or a severe or ongoing unstable medical condition. * Must not have failed to show a therapeutic response if a diagnostic levodopa (L dopa) challenge had been done with a large test dose (\>500 mg) of L dopa (if malabsorption excluded), or failed to respond to an adequate previous treatment with dopaminergic therapy. * Must not have been treated with L dopa or dopamine agonists for 30 days or more. A participant who has been treated with L-dopa or dopamine agonists for \<30 days will be allowed to enter the study. These participants must stop taking dopaminergic medication 30 days prior to Randomization. * Must not be at imminent risk of self-harm or harm to others. * Must not have elevated blood pressure (BP) (systolic BP ≥150 mm Hg or diastolic BP ≥95 mm Hg) that cannot be adequately controlled with antihypertensive medication, as demonstrated by 2 BP measurements meeting acceptable BP criterion at consecutive scheduled or unscheduled visits between Screening and Randomization (a 5-6 week period), one of which must be the Randomization visit. * Must not have had any clinically significant cardiovascular event or procedure for 6 months prior to Randomization, including, but not limited to, myocardial infarction, angioplasty, unstable angina, or heart failure; and a participant must not have heart failure staged New York Heart Association Class III or IV. * Must not have an alanine aminotransferase (ALT) or aspartate amino transferase (AST) ≥ 3 x the upper limit of normal (ULN) or total bilirubin (T BIL) ≥ 1.5 x ULN. * Must not have active serologically-confirmed hepatic dysfunction (defined as viral infection \[Hepatitis B, C, or E; Epstein-Barr virus (EBV)\]; cytomegalovirus \[CMV\] or a history of diagnosis of drug- or alcohol-induced hepatic toxicity or frank hepatitis, or a history of diagnosis of drug- or alcohol-induced hepatic toxicity or frank hepatitis.) * Must not have a history within the past 5 years of a primary or recurrent malignant disease with the exception of adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, or in situ prostate cancer with a normal prostate-specific antigen (PSA) post resection. * Must not have received certain prespecified medications or ingested high tyramine-containing aged cheeses (eg, Stilton) for a prespecified time window before the trial, during the trial, and for 2 weeks after the trial. * Must not have an average daily consumption of more than three 4 ounce glasses (118 mL) of wine or the equivalent. * Must not have a severe or ongoing unstable medical condition (eg, any form of clinically significant cardiac disease, symptomatic orthostatic hypotension, seizures, or alcohol/drug dependence.) * Must not have allergy/sensitivity to investigational product(s) or its/their excipients. * Must not be breast-feeding, considering breast-feeding, pregnant or intending to become pregnant. * Must not have used preladenant ever, or any investigational drugs within 90 days immediately before screening.