The purpose of this study is to establish the potential benefits, safety, and tolerability of AB-2004 in participants with irritability associated with autism spectrum disorder.

Key Inclusion Criteria:

• Clinically diagnosed, documented ASD (Diagnostic and Statistical Manual of Mental Disorders [DSM-5] criteria)
• Aberrant Behavior Checklist - Irritability (ABC-I) score ≥18 at the Screening Period
• Clinical Global Impression - Severity (CGI-S) scale score ≥4 at the Screening Period

Key Exclusion Criteria:

• Use of an oral, injected, or inhaled antibiotic within 30 days prior to screening. Prophylactic oral antibiotic use of no more than 1 dose will be permitted
• Current use of an oral controlled or extended-release medication
• Have a comorbid major psychiatric condition (eg, schizophrenia or bipolar disorder) at screening that in the opinion of the Investigator may interfere with the subject's ability to complete study procedures/comply with study requirements
• Current use of antipsychotics (eg, aripiprazole or risperidone)