Ultragenyx Pharmaceutical Inc. (RARE) reported positive 36-week results from its Phase 3 Enh3ance study of DTX301, an investigational AAV8 gene therapy for ornithine transcarbamylase (OTC) deficiency, a rare metabolic disorder that causes dangerous ammonia buildup in the blood.
The company said the therapy achieved statistically significant improvements in ammonia control compared with placebo.
In the randomized, placebo-controlled portion of the trial, patients treated with DTX301 showed 18% reduction in 24-hour plasma ammonia levels at Week 36, a key primary endpoint, while maintaining average ammonia levels within the normal range. Eight of nine patients who began the study with abnormal ammonia levels reached normal levels rapidly, and most maintained control throughout the treatment period.
Ultragenyx noted that many treated patients were able to reduce their use of alternate pathway (scavenger) medications and liberalize their protein-restricted diets yet still kept ammonia levels stable- an important sign that the therapy may address the underlying cause of the disease rather than just managing symptoms.
Patient-reported outcomes also favored DTX301. At Week 24, 71% of treated patients reported being "much improved" in overall OTC symptoms, compared with none in the placebo group. Measures of daily functioning and symptom burden similarly showed meaningful improvements.
DTX301 was generally well tolerated, with most side effects described as mild to moderate and manageable with steroids. One treatment-related case of acute hepatitis resolved with steroid therapy. In contrast, the placebo group experienced five hyperammonemia crises requiring hospitalization, including one death.
The study will continue to its second primary endpoint, which evaluates reduction in treatment burden-including medication use and dietary restrictions- through 64 weeks of follow-up. These data are expected in the first half of 2027.
DTX301 is designed to deliver a functional copy of the OTC gene through a single intravenous infusion. The therapy has received Orphan Drug and Fast Track designation in the U.S. and E.U.
RARE has traded between $18.41 and $42.37 over the past year. The stock closed Wednesday's trading session at $22.21, down 2.76. In pre-market trading, the stock fell further to $22.05, down 0.72%.
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