The Food and Drug Administration (FDA) approved Crysvita (burosumab-twza) as the first treatment for patients with x-linked hypophosphatemia (XLH). Ultragenyx Pharmaceutical and Kyowa Hakko Kirin developed Crysvita, which 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.

XLH is a rare, inherited form of rickets that causes low levels of serum phosphorous. Crysvita, a recombinant fully human monoclonal IgG1 antibody, works by blocking fibroblast growth factor 23 (FGF23), a hormone that causes phosphate urinary excretion and suppresses vitamin D production by the kidney. This results in increased phosphate reabsorption from the kidney and increased vitamin D production, enhancing intestinal absorption of phosphate and calcium.

The FDA approval was supported by data from two pediatric studies (Study CL201 [N=52] and Study CL205 [N=13]), two adult studies (Study CL303 [N=134] and one open-label bone biopsy study [N=14]). In the placebo-controlled trial, 94% of adults who received Crysvita once monthly achieved normal phosphorous levels vs 8% of adults who received placebo. Normal phosphorous levels were achieved in 94–100% of pediatric patients who received Crysvita every 2 weeks. X-ray findings associated with XLH improved with Crysvita in both adults and children. These studies confirmed the efficacy of Crysvita.

“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, M.D., the 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 revolutionize the care of patients with XLH.”

The FDA previously granted Crysvita a Breakthrough Therapy Designation for the treatment of XLH in pediatric patients one year of age and older, and evaluated Crysvita with Priority Review, which is reserved for drugs that treat a serious condition and, if approved, would provide a significant improvement in safety or effectiveness.

With this approval, the FDA issued a Rare Pediatric Disease Priority Review Voucher, which confers priority review to a subsequent drug application that would not otherwise qualify for Priority Review. The Rare Pediatric Disease Priority Review Voucher program is designed to encourage development of new drugs and biologics for the prevention or treatment of rare pediatric diseases.

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