A Study of PTC923 in Participants With Phenylketonuria

The study includes 2 parts: Part 1 and 2. Part 1 of the study tests for responsiveness to PTC923, with 14 days of open-label treatment with PTC923. At the end of treatment in Part 1, the mean change in blood Phe levels over the 14-day treatment period for all participants will be assessed against their pretreatment (baseline) blood Phe level. Participants ≥2 years of age who experience a \<15% reduction in blood Phe levels will be classified as non-responsive and participation in the study will be terminated. Participants (≥2 years of age) who experience a ≥15% reduction in blood Phe levels will continue into Part 2. Participants \<2 years of age who experience ≥15% reduction in blood Phe levels will be offered the option to enroll directly into an open-label extension Study PTC923-MD-004-PKU. Participants \<2 years of age who experience a \<15% reduction in blood Phe levels will be classified as nonresponsive, and participation in the study will be terminated. Following the minimum 14-day PTC923 washout period, all eligible participants will be randomized in Part 2 to receive either PTC923 or placebo. After 6 weeks of treatment with either PTC923 or placebo, participants will be offered the option to enter an open-label extension Study PTC923-MD-004-PKU (NCT05166161).