US regulators have approved the first drug to treat adults and children with x-linked hypophosphatemia (XLH), a rare, inherited form of rickets.

XLH causes low levels of phosphorus in the blood, leading to impaired bone growth and development in children and adolescents and problems with bone mineralisation throughout a patient’s life.

Ultragenyx Pharmaceutical and Kyowa Hakko Kirin’s Crysvita (burosumab-twza) is an antibody that blocks fibroblast growth factor 23 (FGF23), a hormone that causes phosphate urinary excretion and suppresses active vitamin D production by the kidney.

“The approval of Crysvita is truly a watershed moment for patients with X-linked hypophosphatemia as it is the first therapy directed toward correction of renal phosphate wasting,” said Tom Carpenter, lead study investigator, director of the Yale Center for X-Linked Hypophosphatemia, and Professor of Pediatric Endocrinology at Yale University School of Medicine.

“By targeting this mechanism Crysvita leads to sustained improvements in phosphate metabolism with concurrent repair of the skeleton, even after prior treatment with conventional approaches.

“Most importantly, the dosing regimen for Crysvita is far less burdensome than for currently available therapies and should be readily acceptable by families. I expect it to revolutionise the care of patients with XLH.”