The international trial enrolled 61 XLH-affected children ages 1 to 12 years who previously received conventional therapy with oral phosphate and active vitamin D but still had evidence of rickets on X-rays. The patients were randomized to continue this conventional therapy or switch to receive burosumab injections given every two weeks. By 40 weeks of treatment, that improvement was more than two times greater for the burosumab group than the conventional therapy group. Substantial healing of rickets occurred in 72% of the patients receiving burosumab (21 of 29) versus only 6% in the conventional therapy group (two of 32). Burosumab also reportedly led to greater improvements in leg deformities, height and distance walked in a 6-minute test, as well as larger increases in serum phosphorus and active vitamin D levels. Source
About X-linked hypophosphatemia
X-linked hypophosphatemia (XLH) is an inherited disorder characterized by low levels of phosphate in the blood. Phosphate levels are low because phosphate is abnormally processed in the kidneys, which causes a loss of phosphate in the urine (phosphate wasting) and leads to soft, weak bones (osteomalacia). XLH is usually diagnosed in childhood. XLH is caused by mutations in the PHEX gene on the X chromosome, and inheritance is X-linked dominant.
Burosumab is a recombinant fully human monoclonal IgG1 antibody against the phosphaturic hormone Fibroblast Growth Factor 23 (FGF23). Adults and children with XLH have increased FGF23 activity. The kidneys are unable to reabsorb phosphorus, leading to low levels in the blood. Thus, phosphorus is not deposited into the bones, leading to the symptoms of XLH. Burosumab blocks the activity of FGF23 and helps to restore phosphorus absorption in the kidneys so the phosphorous can then be deposited into the bones.
The FDA approved burosumab in April of 2018 for children and adults with x-linked hypophosphatemia. It is currently marketed as Crysvita. The FDA granted Crysvita a Rare Pediatric Disease Priority Review Voucher, Breakthrough Therapy designation and Orphan Drug designation.
Ultragenyx and Kyowa Hakko Kirin entered into a collaboration and license agreement in August 2013 to develop and commercialize burosumab.