Phase 1b Study to Assess Safety, Tolerability, and Pharmacokinetics of ARCT-810 in Stable Adult Subjects With Ornithine Transcarbamylase Deficiency

Inclusion Criteria: 1. Adequate cognitive ability to consent and recall symptoms over a 1-week time period 2. Males and females ≥18 years of age with documented diagnosis of ornithine transcarbamylase deficiency (OTCD) confirmed with genetic testing, or willing to consent to OTC gene sequencing and deletion/duplication testing 3. Subject's ornithine transcarbamylase deficiency (OTCD) is stable as evidenced by 1. no clinical symptoms of hyperammonemia AND 2. an ammonia level \<100 µmol/L (170 µg/dL) at the screening evaluation Subjects must remain free from symptoms of hyperammonemia throughout the screening period. 4. If using nitrogen ammonia scavenger therapy, must be on a stable regimen (no change in dose or frequency) for ≥ 28 days prior to providing informed consent and throughout the screening period 5. Must have maintained a stable protein-restricted diet (+/- amino acid supplementation) for at least 28 days prior to providing informed consent and continue to maintain a stable diet for the duration of the study 6. Good general health other than OTCD, in the opinion of the Investigator 7. Willing to refrain from strenuous exercise/activity and alcohol for 72 hours before study visits 8. Willingness to comply with procedures and visits 9. Willingness to follow contraception guidelines Exclusion Criteria: 1. History of clinically significant disease(s), in the opinion of the Investigator 2. Clinically significant screening laboratory values 3. Uncontrolled diabetes 4. Clinically significant anemia 5. Subjects who develop infection during screening must be asymptomatic for at least 7 days prior to dosing 6. Unwillingness to comply with study requirements 7. History of positive HIV, hepatitis C, or chronic hepatitis B 8. Uncontrolled hypertension 9. Malignancy within 5 years prior to study 10. Treatment with another investigational drug, biological agent, or device within 30 days of screening, or 5 half-lives of investigational drug 11. Treatment with any oligonucleotide or mRNA within 6 months of screening, with exceptions for some vaccinations and investigational treatments 12. History of gene therapy, hepatocyte or mesenchymal stem cell transplantation 13. Prior organ transplant 14. History of severe allergic reaction to a liposomal product 15. Recent history of, or current, drug or alcohol abuse 16. Dependence on inhaled (smoked or vaped) or oral cannabis products 17. Systemic corticosteroids within 6 weeks prior to screening 18. Blood donation of 50 to 499 mL within 30 days of screening or of .499 mL within 60 days of screening 19. Other conditions, in the opinion of the Investigator, that would make the subject unsuitable for participation