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Crysvita, the first therapy for rare inherited form of rickets, x-linked hypophosphatemia: approved by FDA

The U.S. Food and Drug Administration ( FDA ) has approved Crysvita ( Burosumab-twza ), the first drug approved to treat adults and children ages 1 year and older with x-linked hypophosphatemia ( XLH ), a rare, inherited form of rickets.
X-linked hypophosphatemia causes low levels of phosphorus in the blood. It leads to impaired bone growth and development in children and adolescents and problems with bone mineralization throughout a patient’s life.

X-linked hypophosphatemia differs from other forms of rickets in that Vitamin-D therapy is not effective.

X-linked hypophosphatemia is a serious disease affecting approximately 3,000 children and 12,000 adults in the United States.
Most children with x-linked hypophosphatemia experience bowed or bent legs, short stature, bone pain and severe dental pain.
Some adults with x-linked hypophosphatemia experience persistent discomfort or complications, such as joint pain, impaired mobility, tooth abscesses and hearing loss.

The safety and efficacy of Crysvita were studied in four clinical trials. In the placebo-controlled trial, 94% of adults receiving Crysvita once a month achieved normal phosphorus levels compared to 8% of those receiving placebo.
In children, 94 to 100% of patients treated with Crysvita every two weeks achieved normal phosphorus levels.
In both children and adults, X-ray findings associated with x-linked hypophosphatemia improved with Crysvita therapy.
Comparison of the results to a natural history cohort also provided support for the effectiveness of Crysvita.

The most common adverse reactions in adults taking Crysvita were back pain, headache, restless leg syndrome, decreased vitamin D, dizziness and constipation.
The most common adverse reactions in children were headache, injection site reaction, vomiting, decreased vitamin D and pyrexia ( fever ).

Crysvita was granted Breakthrough Therapy designation, under which the FDA provides intensive guidance to the company on efficient drug development, and expedites its review of drugs that are intended to treat serious conditions where clinical evidence shows the drug may represent a substantial improvement over other available therapies.
Crysvita also received Orphan Drug designation, which provides incentives to assist and encourage the development of drugs for rare diseases. ( Xagena )

Source: FDA, 2018