This study aims to measure changes in participants' blood phenylalanine (Phe) levels in participants with PKU when switching from a conventional amino acid based protein substitute to a GMP based protein substitute (PKU Sphere) over a 16 week period.

This is a stepped wedge, open-label study of PKU Sphere for the dietary management of participants with Phenylketonuria. It will be carried out with 33 participants who will be randomised into one of three clusters. The total study length per patient is 16 weeks. Each cluster transitions from taking their standard amino acid product alone to incorporating PKU Sphere (at least 50% of their protein substitue requirement) at different timepoints: week 5, 9 or 13.

Participants will have a face-to-face clinical review at the time of recruitment (week 0) and at 16 weeks (completion of the study). They will also have an additional telephone review when they switch onto PKU Sphere.

Blood samples will be taken at baseline, and then every two weeks for the duration of the study.

In addition to these study visits and procedures, routine clinical care will continue in line with local guidelines.

Inclusion Criteria:

• Diagnosis of PKU made in neonatal period
• Treated by a low-phenylalanine diet and necessity of intake of at least one protein substitute per day
• Aged 10 years and above
• Three blood Phe measurements taken within the preceding six months
• Willing to replace current amino acid based protein substitute with PKU Sphere
• Ability to take at least 50% of protein requirements as PKU Sphere
• Ability to comply with the study protocol, in the opinion of the investigator
• Willingly given, written, informed consent from patient or parent/guardian
• Willingly given, written assent (if appropriate)

Exclusion Criteria:

• Women who are pregnant or planning to become pregnant during the study period
• Participants with atypical PKU (e.g. BH4 deficient)
• Intake of sapropterin dihydrochloride (Kuvan) in the six weeks prior to recruitment in the study
• Patients with soya, milk or fish allergies
• Any other severe disease

At the point of randomisation every effort will be made to ensure that participants are unaware of the cluster they are randomised to; this is to help ensure that compliance is not affected by participants knowing when they will start the new treatment.