Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE) announced the U.S. Food and Drug Administration has cleared its Investigational New Drug application for UX016, a sialic acid prodrug being developed to treat GNE myopathy.

The company plans to begin a Phase 1/2 study in the second half of 2026, which will be funded externally through a venture philanthropy agreement with a patient group. The study will enroll approximately 24 adults ages 18 to 55 years with GNE myopathy in the United States.

UX016 is designed as a substrate replacement therapy for the rare neuromuscular disorder caused by mutations in the GNE gene that lead to deficient sialic acid production. The drug consists of sialic acid combined with a fatty acid tail intended to improve delivery to muscle tissue compared to naturally occurring sialic acid.

GNE myopathy affects an estimated 10,000 people in commercially accessible regions and currently has no approved treatment in the United States. The progressive muscle-wasting disease typically leads to loss of mobility and dependence on caregivers for daily activities.

The planned study will evaluate safety and efficacy of UX016 over 48 weeks of treatment. The trial will assess pharmacokinetics and muscle delivery at two doses compared to placebo during the first 12 weeks, then measure muscle strength and functional outcomes through the remainder of the study period.

In preclinical studies using a mouse model of GNE myopathy, repeated subcutaneous administration of UX016 increased sialic acid concentrations across multiple skeletal muscles and restored levels toward normal ranges, according to the company's press release statement.