The study was terminated after the completion of Cohort 1 (0.3 mg/kg) due to slow enrollment in this placebo-controlled study.

The primary objective is to evaluate the safety and tolerability of repeated doses of intravenously administered ARCT-810.

This study is a Phase 2, randomized, placebo-controlled study of ARCT-810 in people living with OTC deficiency 12 years of age and older. After an at least 4 week screening and diet stabilization period, participants will be randomized 3:1 to receive ARCT-810 or placebo. Following the first dose and safety evaluation, participants will receive up to an additional 5 doses of ARCT-810 or placebo, each separated by 14 days. The treatment period is followed by a 12-week observation period.

Key Inclusion Criteria:

1. Adequate cognitive ability to understand study requirements and give informed consent
2. Males and females aged 12 to 65 years inclusive, at Screening
3. Documented diagnosis of OTC deficiency
4. Clinical stability (no clinical symptoms of hyperammonemia within 1 month, no hospitalizations for metabolic decompensation within 3 months, ≤ 2 hospitalizations within 1 year)
5. Stable protein-restricted diet, dietary supplements, and ammonia scavenger regimen (if applicable) for at least 28 days.
6. BMI = 18.0 - 32.0 kg/m2, inclusive for adults, and >5th percentile for adolescents ≥12 to 17 years
7. Must be willing to adhere to contraception guidelines

Key Exclusion Criteria:

1. History of any OTC gene therapy, or history of liver-derived stem cell therapy in the past 3 years
2. History of other medical conditions that may make the participant unsuitable for inclusion or could interfere with study participation (e.g., uncontrolled hypertension or diabetes, malignancy, HIV, hepatitis B or C)
3. History of severe allergic reaction to liposomal or PEG-containing products
4. Abuse of illicit drugs, medications or alcohol
5. Clinically significant laboratory abnormalities on screening labs

Double-Blinded