Placebo Controlled Study of Preladenant in Participants With Moderate to Severe Parkinson's Disease (P07037 AM3)

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 4 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 amino
transferase (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 or C; 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.