US regulators have approved Wellstat Therapeutics' uridine triacetate for the treatment of hereditary orotic aciduria (HOA), an ultra-orphan indication that has been reported in only 20 people worldwide.

HOA is an inherited disease that is caused by a defect in the gene that encodes uridine 5′-monophosphate synthase. This leads to an inability to normally synthesize uridine, a necessary component of RNA, and causes haematologic abnormalities including anaemia, leukopenia and neutropenia. In case studies of the disease, oral administration of uridine improved haematologic abnormalities in patients.

Uridine triacetate is an acylated form of uridine that is absorbed into the blood better than is uridine itself. Given the tiny HOA patient population, the US Food and Drug Administration approved the drug on the basis of results from a 4-patient 6-week clinical trial with a 6-month extension phase. Treatment improved the stability of haematologic parameters in all four patients. No side effects were observed with treatment.

Wellstat has not yet disclosed pricing for the ultra-orphan drug.

Wellstat also received a priority review voucher — which allows a sponsor to get a drug reviewed in 8 months rather than the standard 12 months — because the drug was approved for a rare paediatric indication. The company sold the voucher to AstraZeneca for an undisclosed sum, making it the fifth priority review voucher to be sold. Previous vouchers have sold for US$67.5 million–$350 million. The July approval of Sanofi and Regeneron's proprotein convertase subtilisin–kexin type 9 (PCSK9)-specific antibody alirocumab marked the first successful use of a priority review voucher.