Relevant Targets:

Primary: Ornithine transcarbamylase (OTC) deficiency and urea cycle disorders (UCDs)

Secondary: Families following gene therapy, mRNA therapy, rare disease clinical trials, liver-directed therapies, and emerging metabolic treatment research

What this means for families

Two recent clinical trial updates are offering encouraging signs for people living with ornithine transcarbamylase (OTC) deficiency, one of the urea cycle disorders.

Researchers are continuing to investigate therapies that aim to address the underlying cause of OTC deficiency rather than only managing symptoms through diet, formula, ammonia-scavenging medications, and emergency interventions.

In a Phase III study, Ultragenyx Pharmaceutical's investigational gene therapy DTX301 demonstrated significant reductions in ammonia levels compared to placebo. Many participants were also able to reduce their reliance on ammonia-scavenging medications and increase dietary protein intake.

Meanwhile, Arcturus Therapeutics reported encouraging interim Phase II data for ARCT-810, an investigational mRNA therapy designed to help restore OTC enzyme activity within liver cells. Together, these studies highlight growing progress in the development of potentially transformative therapies for OTC deficiency.

Article

Gene and mRNA therapies mark progress in OTC care

New clinical trial updates from Ultragenyx Pharmaceutical and Arcturus Therapeutics are showing continued momentum in the development of gene and mRNA therapies for OTC deficiency.

Both therapies aim to target the underlying metabolic cause of OTC deficiency rather than focusing solely on symptom management.

Ultragenyx announced positive Phase III data for DTX301, an investigational one-time gene therapy for OTC deficiency. According to the company, the therapy met a key study endpoint by significantly lowering ammonia levels compared with placebo.

Researchers also reported that many participants receiving DTX301 maintained ammonia levels within normal range while reducing the use of ammonia-scavenging medications and increasing dietary protein intake. These findings may represent an important step toward more stable long-term metabolic management for some individuals living with OTC deficiency.

At the same time, Arcturus Therapeutics shared positive interim Phase II results for ARCT-810, an investigational mRNA therapy intended to deliver functional OTC enzyme instructions directly to liver cells.

Early findings from the ARCT-810 program demonstrated meaningful reductions in ammonia and other biomarkers related to urea cycle function, along with a favourable safety profile. Researchers noted that the therapy continues to be studied in individuals with OTC deficiency.

OTC deficiency is one of the urea cycle disorders, a group of inherited metabolic conditions that impair the body's ability to remove ammonia from the bloodstream. Elevated ammonia levels can rapidly become life-threatening and may lead to neurological injury, coma, or death if not treated promptly.

While both therapies remain investigational and are not currently approved for general use, these clinical updates reflect continued progress in rare disease treatment research and the growing potential of precision therapies for inherited metabolic disorders.

Last updated: May 2026. This summary is provided for informational and educational purposes only and should not be considered medical advice. Investigational therapies discussed in this article may not be approved or available in Canada. Families should continue to speak with their metabolic clinic regarding treatment decisions and clinical trial participation.