Ultragenyx Reports Promising Phase 3 Data for DTX301 Gene Therapy in OTC Deficiency
1. Groundbreaking Results from the *Enh3ance* Study
On March 12, 2026, Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE) unveiled encouraging findings from its Phase 3 *Enh3ance* study of DTX301, an investigational AAV8 gene therapy aimed at treating ornithine transcarbamylase (OTC) deficiency. This study marks a significant advancement in the management of a condition that has long posed serious health risks to affected individuals, highlighting the potential of gene therapy in addressing underlying genetic disorders.
2. Significant Reduction in Plasma Ammonia Levels
At the 36-week mark of the randomized, double-blind, placebo-controlled trial, patients treated with DTX301 (n=18) exhibited a statistically significant 18% reduction in 24-hour plasma ammonia levels (AUC0-24) compared to placebo (n=19), with a p-value of 0.018. The results indicate that DTX301 not only reduced ammonia levels but also helped maintain them within the normal range throughout the treatment period. Notably, among the nine patients who initially presented with abnormal ammonia levels despite optimal drug treatment and dietary restrictions, eight achieved normal levels quickly and sustained them.
Dr. Eric Crombez, Chief Medical Officer of Ultragenyx, emphasized the importance of these findings, stating, “The further reduction in ammonia levels in patients treated with DTX301 demonstrates the benefit of this gene therapy and of directly addressing the underlying cause of this disease.” He further noted that some patients were able to reduce their reliance on alternate pathway medications and liberalize their protein-restricted diets, showcasing the therapy’s potential to ease the treatment burden.
3. Patient Outcomes and Quality of Life Improvements
In addition to biochemical improvements, qualitative measures of treatment impact were also assessed. The patient global impression scale (PGIC) evaluated at Week 24 revealed that 71% of treated patients reported significant improvement in OTC symptoms, compared to none in the placebo group. For a broader evaluation of OTC deficiency symptoms and its impact on daily living, 64% of DTX301-treated patients reported being much or moderately improved, while only 19% of the placebo group achieved similar outcomes.
4. Safety Profile and Adverse Events
The safety profile of DTX301 was consistent with previous Phase 1/2 studies, demonstrating that the therapy was well tolerated. The majority of treatment-emergent adverse events were mild to moderate, primarily transient hepatic reactions managed effectively with steroids. Importantly, the study recorded one serious adverse event (SAE) of acute hepatitis, which resolved with treatment, but no other SAEs related to thrombotic microangiopathy or malignancies were reported.
In terms of clinical outcomes, the placebo group faced significant challenges, with five hyperammonemic crises requiring hospitalization, including one fatality. In contrast, only one such event occurred in the DTX301 group, underscoring the therapy's potential to mitigate life-threatening complications associated with OTC deficiency.
5. Next Steps in Clinical Development
The ongoing *Enh3ance* study will continue to its second primary endpoint, which focuses on evaluating the reduction in treatment burden, including the use of ammonia scavengers and dietary management, across both treatment and placebo-crossover groups over a follow-up period of 64 weeks. Results from this phase are anticipated in the first half of 2027.
6. About DTX301 and OTC Deficiency
DTX301, also known as avalotcagene ontaparvovec, is designed to deliver stable expression and activity of OTC following a single intravenous infusion. It has received Orphan Drug Designation in both the United States and the EU, alongside Fast Track Designation in the U.S. OTC deficiency, the most prevalent urea cycle disorder, results from genetic defects affecting ammonia detoxification, leading to severe health consequences.
Ultragenyx Pharmaceutical Inc. remains committed to advancing innovative therapies for rare and ultra-rare genetic diseases. The results from the *Enh3ance* study represent a significant milestone in the quest for effective treatments for patients suffering from OTC deficiency, an area of significant unmet medical need.
