This is a two-part trial.

The primary objective of Part A is to estimate the ratio of geometric means of pharmacokinetic (PK) parameters and their within-subject variability for the 300mg quetiapine tablet formulation A and the 300mg quetiapine tablet formulation B compared to 300mg Seroquel.

The primary objective of Part B is to estimate the ratio of geometric means of PK parameters and their within-subject variability for the selected tablet formulation from Part A of 25mg quetiapine compared to 25mg Seroquel.

Inclusion Criteria:

Part A:

• Must have a current diagnosis of schizophrenia or bipolar disorder, as determined by the Diagnostic and Statistical Manual of Mental Disorders Version 5 (DSM-5) criteria.
• Must have a Body Mass Index between 18 and 35 kg/m^2.
• Good physical health as determined by no clinically significant deviation from normal, in the opinion of the investigator, in medical history, clinical laboratory determination, ECGs, or physical examinations.
• Participants must be considered stable, per the investigator's judgment, on one of the following atypical oral antipsychotic medications at an adequate dose (eg, low- to mid-range of the recommended dose range for the treatment of schizophrenia or bipolar disorder, according to the manufacturer labeling) and for an adequate duration (30 days) prior to the administration of IMP: aripiprazole, brexpiprazole, risperidone, olanzapine, quetiapine, ziprasidone, paliperidone, cariprazine, lurasidone, and asenapine. Other oral antipsychotic medications may be allowed if approved by the medical monitor and sponsor; however, clozapine will not be allowed. Per the investigator's judgment, they should be comfortable with the participant discontinuing background antipsychotic therapy during the trial period and then restarting the antipsychotic therapy once trial participation has been completed.

Part B:

• Male or female participants between 18 and 45 years of age, inclusive.
• Must have a Body Mass Index between 18 and 32 kg/m^2.
• Good physical health as determined by no clinically significant deviation from normal, in the opinion of the investigator, in medical history, clinical laboratory determination, ECGs, or physical examinations.
• Able to provide informed consent prior to the initiation of any protocol-related procedures.
• Male and female participants who are surgically sterile, female participants who have been postmenopausal for at least 12 consecutive months prior to the screening visit, or male participants/female participants (of childbearing potential) who agree to practice 2 of the approved birth control methods from the screening visit and for at least 30 days after the last dose of IMP for a female participant or 80 days after the last dose of IMP for a male participant.

Exclusion Criteria:

Part A:

• Participants who are unable to stop receiving varenicline beyond the screening visit. If a participant is receiving varenicline at the screening visit, attempts should be made to discontinue the medication, if clinically feasible, to allow potential participants to enter the trial.
• Participants who have a significant risk of committing suicide based on history, routine psychiatric status examination, investigator's judgment, or who have an answer of "yes" on questions 4 or 5 (current or over the last 3 months) on the Baseline/Screening Version of the C-SSRS or participants with any suicidal behavior during the 6 months prior to the screening visit.
• Participants currently in an acute relapse of schizophrenia as assessed by the investigator. Bipolar participants who currently have an unstable mood (manic, mixed, or depressed) as assessed by the investigator.
• Participants with a current DSM-5 diagnosis other than schizophrenia or bipolar disorder, including schizoaffective disorder, major depressive disorder, delirium, dementia, amnestic, or other cognitive disorders. Also, participants with borderline, paranoid, histrionic, schizotypal, schizoid, or antisocial personality disorder.
• Use of any moderate-strong CYP3A4 inhibitor or inducer within 14 days or 5 plasma half-lives (whichever is longer) prior to the administration of IMP and for the duration of the trial. Weak CYP3A4 inhibitors, including valproic acid, may be allowed based on the investigator's judgment, provided the participant has been on a stable dose for at least 30 days prior to IMP administration and throughout the duration of the trial.
• Participants with a history of neuroleptic malignant syndrome, seizure disorder, or clinically significant tardive dyskinesia as assessed by the investigator.
• Subjects with a history of any significant drug allergy or known or suspected hypersensitivity to antipsychotics, in particular to quetiapine.
• Participants who are maintained on long-acting insulin.
• Any participant who does not successfully tolerate a quetiapine dose of 300 mg BID during the titration period of this trial.

Part B:

• History of any significant drug allergy to quetiapine, prescription drugs, non-prescription drugs, or food.
• Any history of significant bleeding or hemorrhagic tendencies.
• Exposure to any substances known to stimulate hepatic microsomal enzymes within 30 days prior to screening through the end of the trial (eg, occupational exposure to pesticides, organic solvents, etc).
• Participants who have supine blood pressure after resting for ≥ 5 minutes, higher than 130/80 mmHg or lower than 100/50 mmHg (systolic/diastolic). The sponsor may allow exceptions if they are not deemed clinically significant.
• Participants who have a supine pulse rate, after resting for ≥ 5 minutes, outside the range of 60 to 90 beats per minute. The sponsor may allow exceptions if they are not deemed clinically significant (eg, bradycardia attributable to superior physical fitness).
• History of serious mental disorders that, in the opinion of the investigator, would exclude the participant from participating in this trial.
• Use of any CYP3A4 inhibitors or CYP3A4 inducers within 14 days or 5 plasma half-lives (whichever is longer) prior to the administration of IMP and for the duration of the trial.

Anonymized Individual participant data (IPD) that underlie the results of this study will be shared with researchers to achieve aims pre-specified in a methodologically sound research proposal. Small studies with less than 25 participants are excluded from data sharing.

Data will be available after marketing approval in global markets, or beginning 1-3 years following article publication. There is no end date to the availability of the data.

Otsuka will share data on an Otsuka-owned remotely accessible data sharing platform with Python and R analytical software. Research requests should be directed to clinicaltransparency@Otsuka-us.com