The Food and Drug Administration (FDA) has expanded the approval of Crysvita (burosumab; Ultragenyx and Kyowa Kirin) to include treatment in pediatric patients as young as 6 months with X-linked hypophosphatemia (XLH). Previously, the treatment had been approved for patients aged ≥1 year.

The approval in patients 6 months of age and older was supported by evidence from studies in pediatric patients 1 year to less than 13 years of age. Safety and effectiveness in patients younger than 6 months have not been established.

The updated labeling also includes new data from 2 trials evaluating the treatment in pediatric and adult populations. The pediatric trial was a 64-week open-label study that included 61 patients with XLH (1 to 12 years old) and compared Crysvita to an active control (oral phosphate and active vitamin D). Results showed that Crysvita was superior to conventional therapy for all efficacy end points, showing a meaningful improvement in the severity of rickets, lower leg extremity abnormalities and growth.

In the adult trial, which evaluated the effect of Crysvita compared with placebo on increasing serum phosphorus levels, results demonstrated serum phosphorus was maintained during continued Crysvita therapy, with no evidence of loss of effect through Week 48. During the open-label treatment period, patients who continued receiving Crysvita showed continued healing of fractures at Week 48. In addition, at 24 weeks, the Crysvita arm showed a mean improvement from baseline in the stiffness severity score.

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X-linked hypophosphatemia is caused by excess fibroblast growth factor 23 (FGF23) which suppresses renal tubular phosphate reabsorption and the renal production of 1,25 dihydroxy vitamin D. Crysvita binds to and inhibits the biological activity of FGF23 restoring renal phosphate reabsorption and increasing the serum concentration of 1,25 dihydroxy vitamin D.

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