A Long-Term Safety Study of PTC923 in Participants With Phenylketonuria

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 PTC, 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 \[min\] min as estimated most recently during qualifying participation in a feeder study) and/or under care of a nephrologist.
• •Any other condition that in the opinion of the investigator or PTC, 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).
• •Additional criteria for non-feeder participants who did not participate in a feeder study:
• •Gastrointestinal disease (such as irritable bowel syndrome, inflammatory bowel disease, chronic gastritis, and peptic ulcer disease, etc) that could affect the absorption of study drug.
• •History of gastric surgery, including Roux-en-Y gastric bypass surgery or an antrectomy with vagotomy, or gastrectomy.
• •History of allergies or adverse reactions to synthetic BH4 or sepiapterin.
• •Any clinically significant laboratory abnormality as determined by the investigator.
• •Any abnormal physical examination and/or laboratory findings indicative of signs or symptoms of renal disease, including calculated GFR \<60 milliliters (mL)/minute/1.73 square meter (m\^2).
• •Confirmed diagnosis of a primary BH4 deficiency as evidenced by biallelic pathogenic mutations in 6-pyruvoyltetrahydropterin synthase, recessive GTP cyclohydrolase I, sepiapterin reductase, quinoid dihydropteridine reductase, or pterin-4-alphacarbinolamine dehydratase genes.