Crysvita (burosumab-twza), the first drug approved in the US for X-linked hypophosphatemia (XLH) for both adult and pediatric patients, has been made available by Ultragenyx Pharmaceutical and Kyowa Kirin International.

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. 

In clinical trials, the most common adverse reactions associated with treatment in adults included back pain, headache, tooth infection, restless leg syndrome, reduced vitamin D, dizziness, constipation, and increased blood phosphorus. In pediatric patients, the most common side effects included headache, injection site reaction, vomiting, pyrexia, pain in extremity and reduced vitamin D. 

Crysvita should not be used with oral phosphate and active vitamin D analogs. It should also not be initiated if serum phosphorus is within or above the normal range for age. The treatment is contraindicated in patients with severe renal impairment and end stage renal disease.

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Crysvita is available in 10mg/mL, 20mg/mL, and 30mg/mL strength solutions for subcutaneous (SC) injection in single-dose vials. Administration of Crysvita may result in local injection site reactions; treatment should be discontinued if severe injection site reactions occur.

“With this commercial launch of Crysvita, adults and children living with XLH now have access to the only treatment that targets the underlying cause of this debilitating disorder,” said Emil D. Kakkis, MD, PhD, Chief Executive Officer and President of Ultragenyx. “Through the UltraCare™ program, our next job is helping everyone who can benefit from Crysvita to navigate the health-care system and gain access to this new treatment.”

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