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