A Study to Evaluate the Pharmacodynamics and Safety of ARCT-810 in Participants With OTCD

Inclusion Criteria:

1. Willingness and ability to comply with all the protocol requirements, complete allstudy visits and sign informed consent.

2. Males and Females aged ≥12 years, at Screening.

3. Documented clinical diagnosis of OTC deficiency.

4. History of symptomatic hyperammonemia or elevated plasma ammonia or glutamine withclinical stability for at least 1 month prior to Screening.

5. Medically managed for OTC deficiency and receiving a stable protein-restricted diet,dietary supplements, and/or ammonia scavenger regimen (if applicable) for at least 28 days.

6. Good general health with no clinically significant abnormal findings that wouldinterfere with study procedures (including plasma ammonia within participant'shistorical range).

7. Must be willing to adhere to contraception guidelines.

Exclusion Criteria:

1. Uncontrolled hypertension.

2. Symptoms of infection for at least 7 days prior to dosing.

3. Malignancy within 5 years, except for basal or squamous cell carcinoma of the skinor carcinoma in situ of the cervix that has been successfully treated.

4. History of any OTC gene therapy, or history of liver-derived stem cell therapy inthe past 2 years.

5. History of any organ transplant.

6. History of severe allergic reaction to a liposomal or PEG-containing product.

7. History of congenital or acquired cardiac disorders.

8. Abuse of medications, illicit drugs or alcohol.

9. Blood donation of 50 to 499 mL within 30 days of screening or of >499 mL within 60days of screening.

10. Clinically significant laboratory abnormalities on screening labs including INR >1.5, eGFR< 60 mL/min/1.73m2 or positive test results for HIV, HBV, or HCV.

11. Inadequately controlled diabetes.

12. Clinically significant anemia.

13. Changes in maintenance therapies for OTC deficiency with 28 days prior to dosing.

14. Medical history requiring continuous or intermittent systemic corticosteroidadministration.

15. Receipt of inhibitors of urea synthesis or drugs that significantly affect renalclearance.

16. Recent treatment with another investigational drug, biological agent, or device.

17. Treatment with any oligonucleotide (siRNA or mRNA) within 6 months prior toscreening. COVID-19 vaccines are not exclusionary.

18. Involved in study conduct or an immediate family member of an individual involved inthe study.

19. Participated in another dosing cohort of the study.

20. Any other conditions, in the opinion of the investigator, that would interfere withparticipation.