Rapid Drug Desensitization Study in Adults Experiencing Hypersensitivity Reactions to Palynziq

The purpose of this study is to determine if rapid drug desensitization (RDD) to Palynziq will improve drug tolerability and treatment persistence in adult patients on commercial Palynziq experiencing hypersensitivity reactions (HSRs) leading to treatment interruption or reduction of dose or dosing frequency. See Section 10.8 for full list of HSR preferred terms. Study details include: * Study duration: Up to 30 weeks (up to 6 weeks for Screening, then RDD, and 24 weeks of follow-up) * RDD duration: 1 day * Palynziq dosing/follow-up duration: 24 weeks * Palynziq dosing frequency: Individualized

This is a Phase 4 study to evaluate the impact of RDD on adult (≥18 years old) participants with PKU who have experienced HSRs leading to treatment interruption or reduction of dose or dosing frequency while receiving commercial Palynziq. This is not a blinded study. All participants will undergo the same assessments: Screening, the RDD in clinic on Day 1, in clinic dosing on Day 2, followed by remote visits in weeks 2-24 post-RDD to monitor for HSRs and changes in Palynziq dosing. Remote visits will occur every other week from Week 2 through Week 12, then monthly until Week 24. The total duration of participation in the trial will be approximately 30 weeks: up to 6 weeks for screening, followed by RDD and 24 weeks of post-RDD follow-up. No Data Monitoring Committee (DMC) will be used for this trial.