Study to Evaluate the Effect on Nitrogen Retention of Two Different Posology Schemes of PKU GOLIKE PLUS 3-16 and Free AAs in Patients With Phenylketonuria

This is an open-label, randomized, 4-way crossover, monocentric, controlled study in patients (3 ≤ years of age ≤ 20 )) with phenylketonuria (PKU). The primary comparison will be between the test product (PKU GOLIKE PLUS 3-16, a prolonged-release amino-acids (AAs) mixture) and the reference product (an immediate-release free AAs mixture with the same qualitative and quantitative composition of PKU GOLIKE PLUS 3-16). PKU GOLIKE PLUS 3-16 is a food for special medicinal purposes (FSMP) for the dietary management of PKU. The study will consist of a screening visit (T0), four test days taking place on Sunday (T1-T4), each separated by a washout period (a period of time when a participant is taken off a treatment in order to eliminate its effects) and with the study products being self-administered at home, and a final visit (T5). Following informed consent and verification of eligibility criteria, 24 patients with PKU will be randomized in a 1:1:1:1 ratio to one of the four randomization sequences ABCD, BDAC, DCBA or CADB. A, B, C and D will be: A. One day treatment with 3 self-administrations of GOLIKE PLUS 3-16 (1.2 g/kg total daily dose of AA) B. One day treatment with 2 self-administrations of GOLIKE PLUS 3-16 (1.2 g/kg total daily dose of AA) C. One day treatment with 3 self-administrations of free AAs (1.2 g/kg total daily dose of AA) D. One day treatment with 2 self-administrations of free AAs (1.2 g/kg total daily dose of AA) GOLIKE PLUS 3-16 and the free AAs mixture will be the only protein substitute allowed on each test day, and no sports activities will be allowed on the test days. 24-hour urines and five blood spots will be collected on each test day. A patient's diary will be used to collect information on patient compliance, 24-hour urines and blood spot collections, diet, daily activities, adverse events and concomitant medications. Patients completing all four test days will be asked to go to the center for a final visit (T5) taking place within