The purpose of this study is to evaluate the safety, tolerability, and efficacy of MZE782 in adults with Phenylketonuria (PKU).

Key Inclusion Criteria:

• Males and females 18 to 75 years old
• Clinical diagnosis of PKU
• Willing and able to keep diet and protein intake consistent during the study
• For some study groups, is willing and able to stop certain PKU medications before starting study treatment
• For some study groups, may need to meet additional blood phenylalanine level or background-treatment requirements
• Meets minimum blood phenylalanine level requirements for study group

Key Exclusion Criteria:

• Is pregnant, breastfeeding, or planning to become pregnant or conceive a child during the study or within 90 days after the last dose
• Medical conditions or abnormal test results that could make participation unsafe or make it difficult to complete study procedures or interpret study results
• Has recently received another investigational drug
• Is unable to swallow tablets or tolerate oral medication

Other inclusion/exclusion criteria defined in protocol apply.