Effects of a Prolonged-Release Protein Substitute on 24-h Phenylalanine and Tyrosine Profiles in Phenylketonuria: A Randomized Crossover Study

Background/Objectives: Achieving optimal metabolic control in phenylketonuria (PKU) remains challenging, particularly in adolescents and adults. Conventional free amino acid–based protein substitutes are absorbed rapidly, whereas prolonged-release formulations are designed to provide a more gradual amino acid release profile. Whether this translates into more favorable 24 h Phe and Tyr profiles and improved short-term biochemical parameters of metabolic control remains uncertain. This study evaluated whether a prolonged-release amino acid–based protein substitute (PR-AA) could improve 24 h metabolic control compared with standard protein substitutes. Methods: This randomized, controlled, open-label crossover study included adolescents and adults with PKU aged ≥16 years. Participants received either conventional protein substitutes (standard of care; SC) or PR-AA three times daily for two consecutive days, followed by crossover to the alternate treatment. Blood spot samples were collected at five time points over 24 h on the second day of each treatment period. The primary outcome was the 24 h blood Phe profile. Secondary outcomes included blood Tyr, Phe/Tyr ratio, and exploratory assessment of branched-chain amino acids (BCAAs). Results: Twenty-one patients were enrolled, of whom 16 were included in the intention-to-treat analysis and 12 in the per-protocol analysis. PR-AA treatment resulted in significantly lower mean blood Phe levels over 24 h compared with SC (522 ± 194 vs. 561 ± 165 µmol/L; p = 0.0009), corresponding to a reduction of approximately 6–7%. The effect was consistently observed throughout the 24-h assessment period, particularly during the early morning, and became detectable after only two days of treatment. PR-AA was also associated with significantly higher blood Tyr levels (approximately +18–20%; p < 0.05) and a lower Phe/Tyr ratio (approximately −20%; p < 0.001) compared with SC. Exploratory analyses demonstrated modestly lower and more stable BCAA profiles with PR-AA. No significant carryover effects or treatment-related safety concerns were observed. Conclusions: Short-term treatment with a prolonged-release protein substitute was associated with modest but consistent improvements in short-term biochemical parameters of metabolic control in adolescents and adults with PKU, including lower blood Phe and higher Tyr levels, resulting in a reduced Phe/Tyr ratio. These findings suggest that optimizing amino acid delivery kinetics may represent a promising strategy to improve short-term metabolic control in PKU. Given the preliminary nature of these findings and the limited number of participants who completed the study protocol, larger studies with longer follow-up, including direct clinical and neurocognitive outcomes and formal variability metrics, are needed to determine the clinical relevance of these biochemical effects.