This study will consist of Single Ascending Dose (SAD) and Multiple Ascending Dose (MAD) cohorts that will be randomized, double-blind, and placebo-controlled to assess the safety, tolerability, and pharmacokinetics of CAD-9303. The first SAD cohort will be in healthy volunteer subjects. The remaining cohorts will be in participants with schizophrenia.

This study will be conducted in two Parts. Part 1 will consist of Single Ascending Dose (SAD) cohorts that will be randomized, double-blind, and placebo-controlled to assess the safety, tolerability, and pharmacokinetics of CAD-9303. Part 2 will consist of Multiple Ascending Dose (MAD) cohorts that will be randomized, double blind, and placebo-controlled to assess the safety, tolerability, and pharmacokinetics of CAD-9303. The effects of CAD-9303 will be explored on event-related potential (ERP), and on sensory and cognitive function. The first SAD cohort will be in healthy volunteer subjects. The remaining cohorts will be in participants with schizophrenia. Participants will meet specified eligibility criteria.

SAD Cohorts will be comprised of 8 subjects; 6 subjects will be administered CAD-9303, and 2 subjects will be administered matching placebo. MAD Cohorts will be comprised of 12 subjects; 9 subjects will be administered CAD-9303, and 3 subjects will be administered matching placebo.

Potential subjects will undergo a screening period (up to 28 days), baseline assessments on Day -3, -2 and -1, and dosing on Day 1 for SAD and Days 1 - 14 for MAD. A follow-up visit will occur 7 days after the last dose. The total duration of individual subject participation may be up to 5 weeks for SAD and 7 weeks for MAD, depending on the duration of the screening period.

The study will assess safety by adverse events, vital signs, laboratory parameters (including chemistry, hematology and urinalysis) and electroencephalogram (EEG); pharmacokinetics of CAD-9303; and exploratory efficacy measures effects on neurophysiological biomarkers, cognitive and negative symptoms.

Key Inclusion Criteria:

• For SAD Cohorts: able to tolerate washout from antipsychotic medications for the duration of the planned cohort.
• For MAD Cohorts: not taking an antipsychotic medication at screening and throughout participation in the study or taking one antipsychotic medication at least 6 weeks prior to screening and as a concomitant antipsychotic medication throughout participation in the study.
• Diagnosis of schizophrenia by Diagnostic and Statistical Manual of Mental Disorders (DSM-V) criteria with duration of illness > 1 year since diagnosis.
• Clinical history of minimal, stable positive symptoms while under treatment with a stable psychotropic regimen.

Key Exclusion Criteria:

• Abnormal findings on screening safety EEG or lifetime history of seizures or stroke.
• Meets current criteria of any psychiatric diagnosis other than schizophrenia, or lifetime history of any psychiatric diagnosis other than schizophrenia based upon the Mini International Neuropsychiatric Interview (MINI).
• Participants with moderate to severe extrapyramidal symptoms including tardive dyskinesia (Simpson-Angus Scale>6, any Abnormal Involuntary Movement Scale (AIMS) Item 1-7 >2), and akathisia (Barnes Akathisia Rating Scale [BARS] Item 4 ≥1).