Safety, Tolerability, and Pharmacokinetics Study of Elpipodect (MK-8189) in Participants With Schizophrenia and Healthy Participants (MK-8189-011)

Inclusion Criteria

• Has a body mass index (BMI) ≤40 kg/m2
• Has no clinically significant abnormality on 12-lead safety electrocardiogram (ECG) performed at the prestudy (screening) visit and/or prior to randomization
• Has a normal resting blood pressure (BP: systolic BP is ≥90 millimeter of mercury [mmHg] and ≤140 mmHg; diastolic BP is ≥60 mmHg and ≤90 mmHg) and normal resting heart rate (≥45 beats per minute [bpm] and ≤100 bpm) in the semirecumbent position at the prestudy (screening) visit and/or prior to randomization. Repeat evaluations may be done if the values for a participant are, per investigator discretion, minimally outside the designated range. Participants may be included if values are outside the normal range but considered not clinically significant per investigator discretion
• Participants with schizophrenia only: Meets diagnostic criteria for schizophrenia or schizoaffective disorder according to the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) criteria with the onset of the first episode being no less than 2 years prior to screening and monotherapy with antipsychotics for treatment should be indicated
• Participants with schizophrenia only: Has a total Brief Psychiatric Rating Scale (BPRS) score of <48 with a BPRS score <4 for #10 (hostility) and #14 (uncooperativeness) at the screening visit
• Participants with schizophrenia only: Is in the nonacute phase of their illness and clinically stable for 3 months prior to screening as demonstrated by the following: 1) no clinically significant change in dose of prescribed antipsychotic medication, or clinically significant change in antipsychotic medication to treat symptoms of schizophrenia for 2 months prior to screening 2) no increase in level of psychiatric care due to worsening of symptoms of schizophrenia for 3 months prior to screening
• Participants with schizophrenia only: Has a history of receiving and tolerating antipsychotic medication within the usual dose range employed for schizophrenia
• Participants with schizophrenia only: Has a stable living situation
• Participants with hypothyroidism, diabetes, high BP, chronic respiratory conditions or other mild forms of these medical conditions could be considered as candidates for study enrollment if their condition is stable
• Has regular bowel movements
• Participants with schizophrenia only: Is able to discontinue the use of all antipsychotic medication at least 5 days prior to the start of the treatment period and during the study period
• Female participants are not pregnant and not breastfeeding, and are not a woman of childbearing potential (WOCBP) or are a WOCBP who agrees to follow contraceptive guidance during the treatment period and for at least 14 days after the last dose of study intervention

Exclusion Criteria

• Is a WOCBP who has a positive urine pregnancy test within 48 hours before the first dose of study intervention
• Participants with schizophrenia only: Has evidence or history of a primary DSM-5 axis I psychiatric diagnosis other than schizophrenia or schizoaffective disorder per the allowed DSM-5 criteria within 1 month of screening
• Has evidence or history of a primary DSM-5 axis I psychiatric diagnosis other than schizophrenia or schizoaffective disorder per the allowed DSM-5 criteria within 1 month of screening
• Has evidence or history of mental retardation, borderline personality disorder, anxiety disorder, or organic brain syndrome
• Has a history of neuroleptic malignant syndrome or moderate to severe tardive dyskinesia
• Has a substance-induced psychotic disorder or behavioral disturbance thought to be due to substance abuse
• Has a DSM-5 defined substance use disorder within 3 months of screening
• Has a history of seizure disorder beyond childhood or is receiving treatment with any anticonvulsant to prevent seizures
• Has an untreated or uncompensated clinically significant renal, endocrine, hepatic, respiratory, gastrointestinal, psyc