The U.K. National Institute for Health and Care Excellence did not recommend Ultragenyx Pharmaceutical Inc.'s Crysvita for use under the National Health Service to treat a rare hereditary skeletal disorder.

Crysvita, or burosumab, is intended to treat X-linked hypophosphatemia, a genetic disorder, with radiographic evidence of bone disease in children aged 1 year and over as well as young people with growing skeletons.

X-linked hypophosphatemia, or XLH, is characterized by low levels of phosphate in the blood. This causes significant skeletal deformities in children from a young age, and lifelong disability and pain.

Crysvita works by inhibiting the activity of Fibroblast growth factor 23 protein to increase reabsorption of phosphate from the kidney and, through vitamin D production, improve intestinal absorption of calcium and phosphate.

The institute, or NICE, said in a draft guidance that it did not recommend the therapy because the evidence from clinical trials of Crysvita's short-term clinical benefits in children ages 1 to 12 is "limited and uncertain" and no evidence exists that Crysvita works in young people ages 13 to 17.

The price watchdog also said there is some lifetime benefit for people treated with Crysvita since it can prevent irreversible bone damage, but there is uncertainty in the long-term consequences of the progressive bone disease and ongoing metabolic symptoms of XLH, which would not be affected by the drug.

NICE added that estimates for the cost-effectiveness of Crysvita are all much higher than the range it normally considers acceptable for highly specialized technologies.

In February, Crysvita was granted conditional approval by the European Commission to treat children at least 1 year old. Crysvita was approved by the U.S. Food and Drug Administration in April to treat XLH among adults and children ages 1 and older.

Ultragenyx is in a collaborative license agreement with Kyowa Hakko Kirin Co. Ltd. for Crysvita.