This is a 12-week study for the treatment of chronic persistent symptoms in participants with schizophrenia. Participants on a stable, optimal dose of up to two atypical antipsychotics who fulfill the screening criteria will be randomized to receive either an FDA-approved drug or placebo in addition to the current treatment. Safety will be closely monitored through vital signs, various tests, and blood and urine samples.

Inclusion criteria:

• Diagnosed with schizophrenia.
• Exhibits persistent positive symptoms that have persisted for a minimum of 3 months prior to screening.
• Continuously taking 1-2 specific atypical antipsychotics for a minimum of 3 months and must be on a stable dose for at least 1 month prior to entering the study.
• Participant or a legal guardian is able to understand and sign the consent form.

Exclusion criteria:

• PANSS (Positive and Negative Syndrome Score) total score increases or decreases by more than 20% between the Screening and Baseline visits.
• Predominant Axis I disorder other than schizophrenia within 6 months prior to screening.
• History of clinically significant or unstable medical disorder or treatment that would interfere with the study.
• History of autistic disorder or another pervasive developmental disorder, organic brain disease, dementia, stroke, epilepsy or a history of seizures requiring treatment (this does not include febrile seizures as a child), or those who have suffered a traumatic head injury.
• Taking psychotropic or primarily centrally active medication at screening.
• Use of antidepressant medications or mood stabilizers within 1 month of screening.