AIM: AMPLIPHY is the first Phase 3 study comparing sepiapterin versus sapropterin in children and adults with phenylketonuria (PKU). METHODS: AMPLIPHY was an international, Phase 3, two-part, open-label study in participants with PKU aged ≥2 years. Participants responsive to sepiapterin (60 mg/kg/day) in Part 1 (≥20% reduction in blood phenylalanine [Phe]) entered Part 2, a crossover treatment period, and were randomized 1:1 to alternative treatment sequences of sepiapterin (60 mg/kg/day, licensed dosage) and sapropterin (20 mg/kg/day, maximum licensed dosage) for 4 weeks each, with a 14-day washout between treatments. The primary endpoint was mean change in blood Phe from baseline to Weeks 3-4 of each treatment period (Part 2). RESULTS: Of 82 participants enrolled, 67 (81.7%) and 62 (75.6%) had reductions in blood Phe ≥20% and ≥30%, respectively, in Part 1. Sixty-two participants were randomized in Part 2 (mean [SD] age, 15.8 [10.8] years). In the primary analysis set (≥30% reduction in blood Phe in Part 1, n = 58), mean (SD) baseline blood Phe before sepiapterin and sapropterin treatment was 725.8 (302.1) and 790.4 (370.0) μmol/L, respectively. Least-squares mean (SE) reduction in blood Phe from baseline was -437.0 (28.0) and -256.6 (28.2) μmol/L, respectively, representing a least-squares mean difference of -180.4 μmol/L (95% CI: -229.5, -131.4; p < 0.0001) and a relative 70% greater reduction with sepiapterin versus sapropterin. Both treatments were well tolerated, with safety profiles consistent with previous reports. CONCLUSIONS: Sepiapterin was superior to the highest approved dose of sapropterin in lowering blood Phe. No new safety signals were observed. The trial was registered in the UK Clinical Study Registry, ISRCTN, on January 29, 2024 (ID number, ISRCTN79102999; https://www.isrctn.com/ISRCTN79102999).