The main study investigating Palynziq in patients with PKU consisted of different parts. Throughout the study, patients were required to maintain a constant level of dietary protein intake, to ensure that changes in blood phenylalanine levels could be attributed to treatment rather than to changes in protein intake.

During the first part, all patients were given Palynziq at a dose of 20 or 40 mg for up to 13 weeks. Eighty-six patients who responded to treatment (i.e. whose blood phenylalanine levels were reduced by at least 20%) where then either kept on the same dose of Palynziq or were given placebo (a dummy treatment). After 8 weeks of treatment, blood phenylalanine levels remained under control in patients taking Palynziq but they returned to pre-treatment levels in patients on placebo, showing that Palynziq was more effective than placebo in reducing blood phenylalanine levels and keeping them within an acceptable range.

In the extension phase of the study patients received an individual optimized dose of Palynziq. It was shown that for the majority of patients continued treatment with Palynziq for 18 months was effective at keeping blood phenylalanine levels under control (below 600 micromoles per litre).