RARE's DTX301 Meets First Key Goal in Phase III OTC Deficiency Study

Core Insights - Ultragenyx Pharmaceutical (RARE) reported positive results from a late-stage study of its investigational AAV8 gene therapy, DTX301, for treating ornithine transcarbamylase (OTC) deficiency, demonstrating its potential for durable expression and activity following a single intravenous infusion [2] Study Results - The phase III Enh3ance study enrolled 37 patients, randomized to receive either DTX301 or placebo over a 36-week period, with patients in the placebo group crossing over to treatment after the controlled phase [3] - At week 36, DTX301-treated patients showed an 18% reduction in 24-hour plasma ammonia levels compared to placebo, achieving one of the primary endpoints, with average ammonia levels remaining within the normal range [4] - Among patients with elevated ammonia levels at baseline, 8 of 9 treated with DTX301 achieved normal ammonia levels, indicating a quick onset of effect as reductions were observed as early as week six [5] Treatment Benefits - DTX301-treated patients reduced the use of ammonia-scavenger drugs by 27% and increased protein intake by 13% by week 36, while maintaining normal ammonia levels [7][8] - Patient-reported outcomes indicated that 71% of treated patients reported being "much improved" in OTC symptoms compared to none in the placebo group, with 64% reporting moderate or substantial improvement [9] Safety Profile - DTX301 demonstrated an acceptable safety profile consistent with earlier phase I/II findings, with mild-to-moderate transient liver enzyme elevations being the most common treatment-emergent adverse events [10] - One serious adverse event of acute hepatitis occurred but resolved with steroid therapy, and no adverse events related to thrombotic microangiopathy or malignancies were reported [10] Future Steps - Ultragenyx is continuing the phase III Enh3ance study to evaluate a second primary endpoint focused on reducing overall treatment burden, with data expected in the first half of 2027 [11][13]