A Study of the Feasibility, Safety and Tolerability of Aticaprant as Adjunctive Treatment in Participants With Schizophrenia

The purpose of this study is to see how feasible it is to enroll participants with schizophrenia and for them to complete the study/assessments. It will also assess how safe and tolerable aticaprant is when compared with placebo in participants with schizophrenia.

Inclusion criteria: * Clinically stable with a diagnosis of schizophrenia confirmed by the mini international neuropsychiatric interview \[MINI\] for psychotic disorders * The participant must be on a stable dose of only one atypical antipyschotic medication * Must be receiving outpatient treatment for schizophrenia from a psychiatric provider at the time of screening * At the Baseline visit, must have a presence of permitted background antipsychotic medication based on blood samples drawn at the screening visit * If taking an antidepressant or anxiolytic, no dose changes are allowed to have occurred within 8 weeks prior to screening or throughout the double blind treatment phase Exclusion criteria: * Has one or more of the current or prior (lifetime) diagnostic and statistical manual of mental disorders (5th edition) (DSM-5) diagnoses (based on the MINI for psychotic disorders): intellectual disability, schizoaffective disorder, schizophreniform disorder, brief psychotic disorder, delusional disorder, psychotic disorder not otherwise specified (NOS), substance-induced psychotic disorder, bipolar disorder, major depressive disorder (recurrent or current episode) * Has a history of moderate-to-severe substance use disorder, including alcohol use disorder, according to DSM-5 criteria within 6 months before screening except for nicotine or caffeine (based on the MINI and clinical judgment) * Current cannabis (marijuana, pot, grass, hash, etcetera) use exceeds 3 to 5 times over the past 30 days as measured by items from the national survey on drug use and health (NSDUH) questionnaire * Has a history in the past 6 months of a peptic ulcer, or lifetime history of upper gastrointestinal bleeding, or known untreated helicobacter pylori infection, or has a diagnosis of zollinger-ellison syndrome (ZES) * Has current homicidal ideation/intent, per the investigator's clinical judgment