Study of the Safety and Tolerability of HPN-100 Compared to Sodium Phenylbutyrate in Children With Urea Cycle Disorders

Inclusion Criteria: * Male and female subjects 6-17 years old. * Signed informed consent by subject's legally acceptable representative and assent by subject, as applicable. * Diagnosis of urea cycle disorder (enzyme or transporter deficiency) confirmed via enzymatic, biochemical, or genetic testing. * On a stable dose of NaPBA for a diagnosis of UCD for at least 1 week prior to the Day 1 visit. \*Subjects who are not on a stable dose of NaPBA at the initial screening visit may be converted to a stable dose of NaPBA during the screening period and enrolled as long as they are on a stable dose of NaPBA at least 1 week prior to Day 1 * Able to perform and comply with study activities, including blood draws and urine collections. * Negative pregnancy test for all females of childbearing potential. * All females of childbearing age and all sexually active males must agree to use an acceptable method of contraception throughout the study. Exclusion Criteria: * Screening ammonia level of ≥100 μmol/L or signs and symptoms indicative of hyperammonemia; subjects may be re-screened after their ammonia is controlled, at the discretion of the investigator. * History of 4 or more hyperammonemic events as defined in Section 3.5.1 in the preceding 12 months. * Use of any investigational drug within 30 days of Day 1. * Active infection (viral or bacterial) or any other condition that may increase ammonia levels. * Any clinical or laboratory abnormality of Grade 3 or greater severity ac…