Hyperion Therapeutics today announced that the first patient has been enrolled in its pivotal phase III clinical trial of investigational compound HPN-100 (glycerol phenylbutyrate). The 4-week, multi-center, randomized, double-blind, cross-over study is designed to evaluate the non-inferiority of glycerol phenylbutyrate to BUPHENYL® (sodium phenylbutyrate) Tablets and Powder in adults with urea cycle disorders. The primary efficacy measure is blood ammonia, assessed as 24-hour area under the curve on Days 14 and 28 (last day of each treatment period). The study will enroll approximately 44 adults, and all subjects completing the study will be eligible to enter a 12-month, open label safety study. The study is being conducted under a Special Protocol Assessment (“SPA") that was granted earlier this year by the U.S. Food and Drug Administration ("FDA").