The US Food and Drug Administration (FDA) has approved Crysvita (burosumab) as a treatment for a rare form of inherited rickets. It is the first drug therapy approved as a treatment for x-linked hypophosphatemia (XLH) with evidence of bone disease in children over one year of age and adolescents with growing skeletons.

XLH is a hereditary disorder characterized by low phosphorus (phosphate) levels in the blood (hypophosphatemia). It is the most common genetic cause of rickets, affecting around 3,000 children and 12,000 adults in the United States. Phosphate is essential for bone and teeth building and for the maintenance of strength and low phosphate levels can lead to rickets, growth problems, and bone deformities. 

Current treatment of XLH usually involves phosphate supplements, such as Phos-nak, and active forms of vitamin D, for example, Rocaltrol (calcitriol). “XLH differs from other forms of rickets in that vitamin D therapy is not effective… This is the first FDA-approved medication for the treatment of XLH and a real breakthrough for those living with this serious disease,” stated Julie Beitz, MD, director of the Office of Drug Evaluation III in the FDA’s Center for Drug Evaluation and Research.

Patients with XLH have abnormally high levels of a certain protein, which hampers the kidney’s ability to reabsorb phosphate into the bloodstream. Crysvita is a monoclonal antibody designed to recognize and attach to this protein and block its activities. This enables the kidneys to reabsorb phosphates and stabilize phosphate levels in the blood.

Common Adverse Reactions in Adults

• Back pain
• Headache
• Restless leg syndrome
• Decreased vitamin D
• Dizziness and constipation

Common Adverse Reactions in Children

• Headache
• Injection site reaction
• Vomiting
• Decreased vitamin D
• Pyrexia (fever)

Efficacy of the drug was shown in four clinical trials, including a placebo-controlled trial, which demonstrated that 94 percent of adults receiving Crysvita once a month achieved normal phosphorus levels, compared to just eight percent of those on the placebo. Among pediatric patients, 94 to 100 percent of patients treated with Crysvita every two weeks managed to achieve normal phosphorus levels. The Ultragenyx Pharmaceutical Inc. drug was granted Breakthrough Therapy and Orphan Drug designations by the FDA.