Inclusion Criteria

Male and female patients of any ethnic group and of any age are eligible for participation in this study, providing they meet all the following criteria:

• Subjects with a clinical diagnosis of idiopathic Parkinson's disease, defined as the presence of at least three of the cardinal features of the disease including: rest tremor, rigidity, bradykinesia and/or akinesia, postural balance abnormalities, in the absence of alternative explanations or atypical features.
• Psychosis, defined by the presence of visual and/or auditory hallucinations, with or without delusions, of at least four weeks duration.
• Psychosis, assessed by items A and B of the NPI, and defined as Hallucinations (Frequency x Severity) and Delusions (Frequency x Severity) = a total score of 4 or greater.
• Stable anti-Parkinsonian medication(s) use for at least one week prior to study entry.
• A reliable caretaker who will accompany the subject to each visit, and who can reliably report on the subject's daily level of function.

Exclusion Criteria

Patients who meet any of the following conditions are excluded from the clinical study:

• Inability of subject or caretaker to provide informed consent.
• Pregnant or breastfeeding. Female subjects of child-bearing potential must have a negative urine pregnancy test at screening.
• Female subjects must be of non-childbearing potential or must comply with double-barrier protection methods against conception during the study and for at least one month prior to randomization and one month following completion of the study.
• Presence of any systemic factor contributing to the psychosis such as urinary infection, liver disease, renal failure, anemia, infection, etc. as defined by a comprehensive medical evaluation.
• History of a significant pre-morbid psychiatric condition before the diagnosis of Parkinson's disease, including major depression, mania, or psychotic depression.
• Dementia precluding accurate assessment on psychiatric assessment battery and defined as a score on the MMSE < 21.
• Use of depot neuroleptic within the past year.
• Prior exposure to non-depot neuroleptics within the past 90 days, except for quetiapine or clozapine. Quetiapine and clozapine-treated patients may be enrolled if these agents were discontinued due to drug intolerability. Such patients must not have taken these drugs within the past two weeks.
• Use of the following drugs within the past two weeks: benztropine, biperiden, trihexylphenidyl, amitriptyline, clomipramine, desipramine, nortriptyline, imipramine, doxepin, fluvoxamine, mirtazepine, nefazodone and trazodone.
• Change of anti-depressant, anxiolytic, anticholinergic (specifically oxybutynin, tolterodine), or cognitive enhancer (specifically rivastigmine, tacrine, donepezil, galantamine) dose within the past 30 days and during the 28-day duration of the trial.
• Use of any investigational product within the past 30 days.
• Inability to tolerate a stable level of anti-parkinsonian medications for one week.
• Uncontrolled angina or history of a myocardial infarction within the past three months.
• Concurrent illness that would make use of ACP-103 potentially hazardous.