Placebo Controlled Study of Preladenant in Participants With Moderate to Severe Parkinson's Disease (P07037)

Inclusion Criteria: * Each participant must have a diagnosis of idiopathic Parkinson's disease. * Each participant must have received prior therapy with L-dopa for approximately 1 or more years immediately before Screening and must continue to have a beneficial clinical response to L-dopa. * Each participant must have been on a stable dopaminergic treatment regimen for at least the 5 weeks immediately before Randomization. Participants receiving other adjunctive treatments (eg, dopamine agonist, anticholinergics) are permitted to enroll in this trial. Participants taking only L-dopa are permitted to enroll in this trial. * Each participant must be experiencing motor fluctuations with or without dyskinesias within the 4 weeks immediately before Screening, must be experiencing a minimum of 2 hours/day of "off" time, and have a Hoehn \& Yahr stage between 2.5 and 4 when in the "on" state. * Each participant, with or without the help of a caregiver, must be capable of maintaining an accurate and complete symptom diary and to adhere to dose and visit schedules. * Each participant must 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 (below). * All participants who are sexually active or plan to be sexually active agree to use a highly effective method of birth control while in the study and for 2 weeks after the last dose of study drug. A male participant must also not donate sperm within 2 weeks after the last dose of study drug. Exclusion Criteria: * A participant must not have a form of drug induced or atypical parkinsonism, a cognitive impairment, bipolar disorder, untreated major depressive disorder, schizophrenia, or other psychotic disorder; history exposure to a known neurotoxin, or any neurological features not consistent with the diagnosis of PD as assessed by the investigator. * A participant must not have a history of repeated strokes 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. * A participant must not have had surgery for their PD. * A participant must not be at imminent risk of self-harm or harm to others. * A participant must not have a systolic blood pressure (BP) ≥150 mm Hg OR diastolic BP ≥95 mm Hg at Screening and at 2 BP rechecks prior to study start. * A participant must not have had any clinically significant cardiovascular event or procedure for 6 months prior to study start, 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. * A participant must not have an alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≥3 x the upper limit of normal (ULN) or total bilirubin (T BIL) ≥1.5 x ULN. * A participant must not have a history of 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. * A participant 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. * A participant must not have received certain prespecified medications for a prespecified time window before the trial. * A participant must not have an average daily consumption of more than three 4 ounce glasses (118 mL) of wine or the equivalent. * A participant 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). * A participant must not have allergy/sensitivity to investigational product(s) or its/their excipients. * A participant must not be breast-feeding, considering breast-feeding, pregnant, or intending to become pregnant. * A participant must not have used preladenant ever, or any investigational drugs within 90 days immediately before Screening.