Inclusion Criteria:

• Must have a diagnosis of moderate to severe idiopathic Parkinson's disease.
• 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
• 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 agonists, anticholinergics, entacapone) or are taking only L dopa are permitted to enroll in this trial
• 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
• Must be capable of maintaining an accurate and complete symptom diary and to adhere to dose and visit schedules with or without the help of a caregiver
• Must have results of a physical examination and screening clinical laboratory tests clinically acceptable to the investigator
• If 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. Males must also not donate sperm during the trial within 2 weeks after the last dose of study drug

Exclusion Criteria:

• 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 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 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
• Must not have had surgery for their PD
• Must not be at imminent risk of self-harm or harm to others
• Must not have sleep attacks or compulsive behavior that would interfere with the integrity of the trial or would pose a risk to the subject in participating in the trial
• Must not have a systolic blood pressure (BP) ≥150 mm Hg OR diastolic BP ≥95 mm Hg at Screening
• 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 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 a history of serologically confirmed hepatic dysfunction (defined as viral infection [Hepatitis B or C; Epstein Barr virus {EBV}; cytomegalovirus {CMV}]) 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
• A female subject 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