The primary objective is to evaluate the efficacy of DTX301 on the improvement of ornithine transcarbamylase (OTC) function by maintaining safe plasma ammonia levels.

This study is a Phase 3, randomized, double-blind, placebo-controlled study of DTX301 in patients with late-onset OTC deficiency 12 years of age and older.

Participants will be randomized 1:1 to DTX301 or placebo and followed closely for 36-64 weeks. Between Week 36 and Week 64, eligible participants will cross over and receive DTX301 if they had previously received placebo, and some who received DTX301 may receive placebo.

The planned study duration is up to 324 weeks. Upon completion of this study or early withdrawal, all participants who received DTX301 are invited to enroll in the Disease Monitoring Program (DMP) for follow-up for up to an additional 5 years.

Key Inclusion Criteria:

• Confirmed clinical diagnosis of late-onset OTC deficiency with historical documentation by enzymatic (ie, liver biopsy), biochemical (ie, hyperammonemia in the presence of elevated plasma glutamine, low citrulline, and elevated spot urine orotic acid), or molecular testing (ie, OTC analysis)
• Free from symptomatic hyperammonemia and has not required emergent active intervention for hyperammonemia within 4 weeks before screening/baseline
• If on ongoing daily ammonia scavenger therapy, must be at stable daily dose(s) for ≥ 4 weeks prior to screening
• If on a protein-restricted diet, must be on a stable total daily protein intake that does not vary more than 20% for ≥ 4 weeks prior to screening
• From the time written informed consent through Visit 28, females of childbearing potential and fertile males must consent to use highly effective contraception. If female, agree not to become pregnant. If male, agree not father a child or donate sperm

Key Exclusion Criteria:

• Significant hepatic inflammation or cirrhosis
• Estimated glomerular filtration rate < 60 mL/min/1.73 m2 at screening by the 2021 CKD-EPI creatinine-based formula (Inker et al., 2021) for patients ≥ 18 years of age or the Schwartz bedside formula (Schwartz and Work, 2009) for patients < 18 years of age
• Evidence of active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection, documented by current use of antiviral therapy for HBV or HCV or by hepatitis B surface antigen (HBsAg) or HCV RNA positivity
• Active infection (viral or bacterial)
• Detectable pre-existing antibodies to the AAV8 capsid
• Presence or history of any condition that, in the view of the Investigator, would interfere with participation, pose undue risk, or would confound interpretation of results
• Participation (current or previous) in another gene transfer study

Note: Additional inclusion/exclusion criteria may apply, per protocol