A Phase 1b, Open-label, Multicenter, Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics Study of AG-181 in Subjects With Phenylketonuria

Key Inclusion Criteria: * 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…