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# FDA, Ultragenyx Settle OTC Phase 3 Study Design
- URL: https://www.fdaweb.com/fda-ultragenyx-settle-otc-phase-3-study-design/
- Published: 2021-04-22T12:00:00.000Z
- Updated: 2026-09-14T16:56:00.000Z
- Author: David McFarland
- Tags: Drugs, #legacy-id-D5149164

At an FDA/Untragenyx end-of-Phase-2 meeting for the DTX301 ornithine transcarbamylase (OTC) deficiency gene therapy program, the parties reached an agreement on the design and endpoints for a Phase 3 study. The company [says](https://www.marketwatch.com/press-release/ultragenyx-completes-successful-end-of-phase-2-meeting-with-fda-and-finalizes-phase-3-study-design-for-dtx301-ornithine-transcarbamylase-otc-gene-therapy-program-2021-04-22?tesla=y&ref=fdaweb.com) the study will include a 64-week primary efficacy analysis period and enroll about 50 patients 12 years of age and older. The co-primary endpoints are a change in 24-hour plasma ammonia levels and the percent of patients who achieve a response as measured by discontinuation or reduction in baseline disease management.

The Phase 3 study is expected to begin dosing in the second half of 2021, Ultragenyx says.