Children and young people with X-linked hypophosphataemia (XLH) are unlikely to get routine access to Kyowa Kirin’s Crysvita on the NHS.

In draft guidelines, the National Institute for Health and Care Excellence has rejected NHS funding for the drug, after concluding that the most likely cost-effectiveness estimates are much higher than would normally be considered value for money for highly specialised technologies.

There are around 250 children and young people in England with XLH, an inherited genetic disorder that causes low levels of phosphate in the blood. This leads to soft, weak bones, and children with the condition usually have bowed or bent legs, short stature, bone pain and delayed walking, and may also have dental problems and hearing loss.

Current treatment mainly consists of vitamin D supplementation and oral phosphate, and is aimed at improving growth, lessening the severity of symptoms and preventing skeletal abnormalities.

Crysvita (burosumab) is an anti-FGF23 fully human monoclonal antibody, and the first treatment to target the underlying pathophysiology of XLH.

An injection given every two weeks from the age of one until the skeleton stops growing, aims to increase reabsorption of phosphate from the kidney and, through vitamin D production, improve intestinal absorption of calcium and phosphate.

However, while evidence suggests the drug is more clinically effective than conventional therapy and provides clinical benefits in children aged between one and 12 years, the data is “limited and uncertain”, NICE said, and also highlighted a lack of data in young people aged between 13 and 17 years.

Also, while the committee recognised that there may be some lifelong benefits from Crysvita because it can prevent irreversible bone damage, the long-term consequences of disease progression - which the drug may not affect - are uncertain, the Institute said.

Final guidance is expected in October.