The primary purpose of this study is to assess the safety and tolerability of AG-181 in subjects with Phenylketonuria (PKU).
Diagnosis of PKU, defined as documented presence of 2 mutant alleles in the phenylalanine hydroxylase (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.
At least 1 plasma Phe concentration greater than (>) 600 micromoles per liter (μmol/L) in the 52 weeks before providing informed consent.
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.
Body mass index (BMI) greater than or equal to (≥) 18.0 kilograms per meter square (kg/m²) to lesser than or equal to (≤) 35.0 kg/m² and weight ≥ 50 kilograms (kg) at any time during the Screening Period.
Prior exposure to AG-181.
Receiving inhibitors of P-glycoprotein (P-gp) 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.
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.
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.