Inclusion Criteria:  Completed a PTC-sponsored Phase 3 PKU clinical study. Women of childbearing potential must have a negative pregnancy test at screening and agree to abstinence or the use of at least one highly effective form of contraception for the duration of the study, and for up to 30 days after the last dose of the study drug. Males who are sexually active with women of childbearing potential who have not had a vasectomy must agree to use a barrier method of birth control during the study and for up to 90 days after the last dose of study drug. Males must also refrain from sperm donations during this time period. Willing to continue current diet unchanged while participating in the study (unless specifically instructed to change diet during the study by the investigator).  Exclusion Criteria:  Inability to tolerate oral medication. A female who is pregnant or breastfeeding, or considering pregnancy. Serious neuropsychiatric illness (for example, major depression) not currently under medical control, that in the opinion of the investigator or sponsor, would interfere with the participant's ability to participate in the study or increase the risk of participation for that participant. Past medical history and/or evidence of renal impairment and/or condition including moderate/severe renal insufficiency (glomerular filtration rate [GFR] <60 milliliters [mL]/minute [min]) and/or under care of a nephrologist. Any other condition that in the opinion of the investigator or sponsor, would interfere with the participant's ability to participate in the study or increase the risk of participation for that participant. Requirement for concomitant treatment with any drug known to inhibit folate synthesis (for example, methotrexate). Concomitant treatment with tetrahydrobiopterin (BH4) supplementation (for example, sapropterin dihydrochloride, KUVAN) or pegvaliase-pqpz (PALYNZIQ).