Key Inclusion Criteria:
1. Aged ≥18 years and 2. Diagnosis of PKU, defined as documented presence of 2 mutant alleles in the PAH gene,
of which at least 1 is the R408W mutation, as determined during Screening per the
genotyping performed by the study central genotyping laboratory.
3. At least 1 plasma Phe concentration >600 μmol/L in the 52 weeks before providing
informed consent.
4. Average concentration of plasma Phe >600 μmol/L in Phe samples taken during
Screening, with no individual assessment below 360 μmol/L. Any Phe samples taken
after Day −20 will not be included.
5. BMI ≥18.0 kg/m2 to ≤35.0 kg/m2 and weight ≥50 kg at any time during the Screening
Period.
6. Documented approval from a dietitian confirming that the subject can maintain their diet
consistent in protein and Phe intake throughout the study as outlined in the Diet Manual.
7. If taking an allowed chronic concomitant medication, must have been on a stable dose for
>8 weeks before study Day 1 and have no planned dose adjustments during the study.
8. Agree to refrain from plasma and platelet donation from the time of informed consent and
for the duration of study participation. Blood samples that are medically necessary or
collected as part of this study are allowed.
9. Women of childbearing potential (WOCBP) must be abstinent of sexual activities that
may result in pregnancy as part of their usual lifestyle or agree to use a highly effective
method of contraception from the time of providing informed consent throughout the
study and for 14 days after the last dose of study drug. Due to the unknown potential of
AG-181 to reduce the effectiveness of hormonal contraceptives, if the highly effective
method of contraception is hormonal contraception, then an acceptable barrier method
must also be used; WOCBP using nonhormonal methods of contraception as a highly
effective method do not need to use an additional barrier method (see protocol
Appendix 1). Men with partners who are WOCBP must be abstinent of sexual activities
that may result in pregnancy as part of their usual lifestyle or agree to use a condom from
the time of providing informed consent, throughout the study, and for 14 days after the
last dose of study drug.
10. Written informed consent from the subject before any study-related procedures are
conducted and willing to comply with all study procedures for the duration of the study.

Key Exclusion Criteria
Subjects are excluded from the study if any of the following criteria apply:
1. Prior exposure to AG-181.
2. Receiving inhibitors of P-glycoprotein that have not been stopped for ≥5 days or a
timeframe equivalent to 5 half-lives (whichever is longer) before administration of the
first dose of study drug.
3. Receiving products that are strong inhibitors or strong inducers of cytochrome P450
CYP1A2, CYP2C8, or CYP3A that have not been stopped for ≥28 days before
administration of the first dose of study drug.
4. Receiving treatment with an acid-reducing agent, including but not limited to proton
pump inhibitors and H2 blockers. Short-acting acid-reducing agents such as calcium
carbonate are permitted.
5. Any preexisting condition that could (in the opinion of the Investigator) interfere with
gastrointestinal anatomy or motility that may disrupt the absorption, metabolism, and/or
excretion of the study drug.
6. Any preexisting condition that could (in the opinion of the Investigator) interfere with
hepatic or renal function that may disrupt the absorption, metabolism, and/or excretion of
the study drug.
7. Inability to tolerate oral medication.
8. Unwillingness to washout from BH4 supplementation (eg, sapropterin dihydrochloride,
Kuvan), pegvaliase-pqpz (Palynziq), or any other PKU therapy
9. Hepatobiliary disorders including but not limited to:
a. Liver disease with histopathological evidence of cirrhosis or severe fibrosis
b. History of drug-induced hepatitis
c. Aspartate aminotransferase>2× the upper limit of normal (ULN) and alanine
aminotransferase >2× the ULN
d. Other markers of clinically significant liver dysfunction (eg, elevated bilirubin >ULN
[except for patients with confirmed Gilbert disease] or elevated prothrombin time/
international normalized ratio >ULN)
10. Estimated glomerular filtration rate Epidemiology Collaboration creatinine equation (National Kidney Foundation, 2021).
11. History of any malignancy except for nonmelanomatous skin cancer in situ, cervical
carcinoma in situ, or breast carcinoma in situ. Subjects must not have active disease or
received anticancer treatment ≤5 years before providing informed consent.
12. Heart rate–corrected QT interval using Fridericia’s method ≥450 milliseconds (ms) for
men or ≥470 ms for women, except for right or left bundle branch block.
13. Positive test for hepatitis C virus (HCV) antibody (Ab) with evidence of active HCV
infection, or positive test for hepatitis B surface antigen (HBsAg).
14. Positive test for human immunodeficiency virus (HIV)-1 Ab or HIV-2 Ab.
15. Active infection requiring systemic antimicrobial therapy at the time of providing
informed consent. If systemic antimicrobial therapy is required during the Screening
Period, screening procedures should not be performed while systemic antimicrobial
therapy is being administered, and the last dose of systemic antimicrobial therapy must be
administered ≥5 days or 5 half-lives of the systemic antimicrobial therapy (whichever is
longer) before administration of the first dose of study drug.
16. History of long QT syndrome, torsades de pointes, or any risk factors for torsades de
pointes in the opinion of the Investigator.
17. A history of any family member of any of the following:
a. Sudden cardiac death
b. Unexplained death
c. Long QT syndrome
d. Death from a primary dysrhythmia potentially associated with QT prolongation
18. History of major surgery ≤16 weeks before providing informed consent or planning to
undergo a major surgical procedure during the study.
19. Current enrollment or past participation (within 12 weeks or 5 half-lives, whichever is
longer, before administration of the first dose of study drug) in any other clinical study
involving an investigational treatment or device.
20. Known allergy, or other contraindication to AG-181 or tablet excipients (silicified
microcrystalline cellulose, croscarmellose sodium, sodium stearyl fumarate, and Opadry
II Blue film coat [polyvinyl alcohol, titanium dioxide, polyethylene glycol, talc, and
21. Pregnant or breastfeeding.
22. Any medical, hematologic, psychological, or behavioral condition(s) or prior or current
therapy that, in the opinion of the Investigator, may confer an unacceptable risk to
participating in the study or could confound the interpretation of the study data. Also
excluded are:
a. Diagnosis of BH4 deficiency
b. Subjects with PKU that is not caused by PAH deficiency
c. Subjects who are institutionalized by regulatory or court order
d. Subjects with any condition(s) that could create undue influence (including but not
limited to incarceration, involuntary psychiatric confinement, and financial or familial
affiliation with the Investigator or Sponsor)