Ultragenyx disclosed the 36‑week results of its Phase 3 study of DTX301, an AAV8‑based gene therapy for ornithine transcarbamylase (OTC) deficiency, on March 12, 2026. The study enrolled 18 treated patients and 19 placebo controls and showed an 18 % reduction in 24‑hour plasma ammonia at week 36 compared with placebo. Eight of nine patients who had abnormal baseline ammonia achieved normal levels, and treated patients maintained normal ammonia area‑under‑the‑curve values through week 36 while reducing their use of ammonia‑scavenger medications by 27 % and allowing a 13 % increase in protein intake relative to baseline.
The data provide the first late‑stage evidence that DTX301 can sustainably lower ammonia levels and reduce treatment burden in OTC deficiency, a rare inherited disorder that currently has no curative therapy. The safety profile was consistent with earlier phases; most treatment‑emergent adverse events were mild to moderate transient hepatic reactions managed with steroids, and a single serious adverse event of acute hepatitis resolved with steroids.
Chief Medical Officer Eric Crombez said, "Given the importance of and effort made to keep ammonia levels under control in patients with OTC deficiency, 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 added, "Importantly, the improvement in ammonia control was maintained as some patients began reducing use of alternate pathway medications and liberalizing their protein restricted diet. We are extremely encouraged by these findings given the significant medical needs faced by patients with OTC deficiency, who remain at risk for unpredictable and potentially life‑threatening hyperammonemic crises."
Ultragenyx’s broader financial picture shows that the company reported revenue growth in Q4 2025 but still posted net losses, and it has set a 2026 revenue guidance of $730‑$760 million, below analyst consensus. The company is pursuing a strategic restructuring plan that includes a 10 % workforce reduction to streamline operating expenses and is targeting profitability by 2027. Its key revenue generators remain Crysvita, Dojolvi, and Evkeeza, with Crysvita being the largest contributor. The company also faces headwinds from class‑action lawsuits related to a failed trial of setrusumab.
Investors have focused on the 2026 revenue guidance miss and the announced restructuring plan as key drivers of market sentiment, even as the positive Phase 3 data for DTX301 may support future regulatory and commercial prospects. The company’s CEO, Emil D. Kakkis, noted, "The year ahead marks an important turning point for the company, as we approach two potential product launches and a pivotal data readout that, together, could significantly accelerate our commercial revenue trajectory. We are implementing a strategic restructuring plan to reduce our operating expenses and ensure our resources are squarely aligned with our highest‑impact opportunities, while leading the future of rare disease with multiple first‑ever treatments."
The DTX301 data strengthen Ultragenyx’s gene‑therapy pipeline and reinforce its strategy to bring curative treatments to rare disease patients. However, the company’s financial headwinds and ongoing legal challenges underscore the need for continued focus on cost control and strategic execution as it moves toward regulatory review and potential market entry for DTX301.
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