Safety and Tolerability of RTX-134 in Adults With Phenylketonuria

Inclusion Criteria 1. Age 18 years or older with: 1. A clinical diagnosis of PKU, and 2. Average blood phenylalanine level ≥ 600 µmol/L based on 2 assessments up to 3 weeks apart during the 6-month period before Day 0 (per available data) 2. Stable diet, including medical formula 3. Must be a man or a woman not of childbearing potential and agree to use adequate contraception throughout and for one year following study participation. 4. Adequate organ function 5. Negative antibody detection on type and screen and no evidence of clinical hemolysis Exclusion Criteria 1. Known hypersensitivity to any component of study treatment 2. Prior treatment with Pegaliase 3. Start of sapropterin dihydrochloride within 3 weeks of study dosing 4. Use of an investigational agent within 28 days of study dosing 5. Concurrent participation in an interventional trial involving ongoing treatment, including placebo. 6. Infections requiring antimicrobial treatment within 7 days of study dosing 7. Chronic infections, such as HIV, hepatitis B, or untreated hepatitis C 8. Conditions that may alter survival of red blood cells, (e.g., autoimmune diseases, splenectomy, etc) 9. Pregnant or breastfeeding