Study of the Safety, Pharmacokinetics and Efficacy of HPN-100, in Pediatric Subjects With Urea Cycle Disorders (UCDs)

Inclusion Criteria: * Male and female subjects 29 days to \< 6 years old. If the subject is born prematurely, calculation of the lower age limit begins at the corrected gestational age of 40 weeks. * Signed informed consent by the subject's legally acceptable representative * Suspected or confirmed UCD diagnosis of any subtype, except NAGS deficiency * On stable dose of NaPBA powder for at least 5 days before Day 1 * Not receiving sodium benzoate for at least 5 days before Day 1 * No concomitant illness which would preclude safe participation as judged by the investigator * Able to receive medication orally * Has not undergone liver transplantation, including hepatocellular transplantation * Judged sufficiently stable and compliant with diet and treatment to be suitable for enrollment Exclusion Criteria: * Screening ammonia level \> 100 μmol/L and signs and symptoms indicative of hyperammonemia; subjects may be rescreened after their ammonia is controlled and they are clinically stable, at the discretion of the investigator * 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 according to the Common Terminology Criteria for Adverse Events (CTCAE) v4.03, except Grade 3 elevations in ammonia and liver enzymes, defined as levels 5-20 times ULN (upper limit of normal)in alanine aminotransferase (ALT), aspartate aminotransferase (AST), or gamma glutamyl transpeptidase (GGT) in a clinically stable subject * Any clinical or laboratory abnormality or medical condition that, at the discretion of the investigator, may put the subject at increased risk by participating in this study * Known hypersensitivity to PAA or PBA * Liver transplant, including hepatocellular transplant * Currently treated with Carbaglu® (carglumic acid)