Ultragenyx Pharmaceutical (RARE Quick QuoteRARE - Free Report) reported positive results from a late-stage study of its investigational AAV8 gene therapy, DTX301, for the treatment of ornithine transcarbamylase (OTC) deficiency.

The candidate is designed to provide durable expression and activity of OTC following a single intravenous infusion. DTX301 enjoys the Orphan Drug designation in the United States and the EU and the Fast Track designation in the United States for treating OTC deficiency.

Ultragenyx Pharmaceutical’s phase III Enh3ance study enrolled 37 patients who were randomized equally to receive either DTX301 or placebo during the 36-week randomized control period. After this period, the study will be unblinded, and patients in the placebo arm will crossover to receive treatment.

RARE’s Phase III OTC Deficiency Study Data in Detail

Per the data readout, at week 36 of the controlled phase of the Enh3ance study, patients treated with DTX301 achieved a statistically significant and clinically meaningful 18% reduction in 24-hour plasma ammonia compared with placebo, meeting one of the primary endpoints. Average ammonia levels among treated patients remained within the normal range through week 36, highlighting the therapy’s ability to provide sustained metabolic control.

The treatment benefit was particularly evident among patients with elevated ammonia levels at baseline despite standard therapy and dietary restrictions. Eight of nine such patients receiving DTX301 rapidly achieved normal ammonia levels, which were generally maintained throughout the treatment period. Overall, ammonia reductions were observed as early as week six, demonstrating a relatively quick onset of effect following treatment.

In the past six months, Ultragenyx Pharmaceutical shares have lost 25.1% against the industry’s 18.8% growth.

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The improvements were observed even as patients began easing other aspects of disease management. By week 36, patients in the DTX301 arm reduced the use of ammonia-scavenger medications by an average of 27% and increased protein intake by approximately 13% while still maintaining ammonia levels within the normal range. In contrast, placebo patients did not show meaningful changes in medication use or diet.

Patient-reported outcomes also supported the clinical findings. Based on the Patient Global Impression of Change scale at week 24, 71% of treated patients reported being “much improved” in terms of OTC symptoms compared to none in the placebo group. Evaluations of symptom severity and daily-life impact also favored DTX301, with 64% of treated patients reporting moderate or substantial improvement compared with only 19% reporting moderate improvement in the placebo arm.

DTX301 demonstrated an acceptable safety profile that was consistent with earlier phase I/II findings. The most common treatment-emergent adverse events were mild-to-moderate transient liver enzyme elevations, which were successfully managed with steroids. One treatment-related serious adverse event of acute hepatitis occurred but resolved following steroid therapy. No adverse events related to thrombotic microangiopathy, dorsal root ganglion toxicity, malignancies or complex immune reactions were reported. Hyperammonemic crises requiring hospitalization occurred five times in the placebo group, including one death, compared with only one such event in the DTX301 arm and no deaths.

RARE’s Next Steps in the OTC Deficiency Program

Ultragenyx Pharmaceutical is continuing the phase III Enh3ance study of DTX301 to evaluate its second primary endpoint, which focuses on reducing overall treatment burden for patients with OTC deficiency. This analysis will assess changes in the use of ammonia-scavenger medications and dietary restrictions across both the treatment and placebo-crossover groups following treatment with DTX301 through 64 weeks of follow-up.

Data from this portion of the Enh3ance study are expected in the first half of 2027. RARE has indicated that the continued development of DTX301 is already incorporated into its February 2026 financial guidance and will be managed within its broader objective of achieving profitability by 2027.

Ultragenyx Pharmaceutical Inc. Price and Consensus

Ultragenyx Pharmaceutical Inc. price-consensus-chart | Ultragenyx Pharmaceutical Inc. Quote

RARE’s Zacks Rank & Stocks to Consider

Ultragenyx Pharmaceutical currently carries a Zacks Rank #3 (Hold).

Some better-ranked stocks in the biotech sector are USANA Health Sciences (USNA Quick QuoteUSNA - Free Report) , Catalyst Pharmaceuticals (CPRX Quick QuoteCPRX - Free Report) and ALX Oncology Holdings (ALXO Quick QuoteALXO - Free Report) . While USNA and CPRX sport a Zacks Rank #1 (Strong Buy) each, ALXO carries a Zacks Rank #2 (Buy) at present. You can see the complete list of today’s Zacks #1 Rank stocks here.

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