Rare Disease Report

FDA Reviewing Burosumab for Rare Form of Rickets

OCTOBER 10, 2017
James Radke
The U.S. Food and Drug Administration (FDA) has accepted the Biologics License Application (BLA) for burosumab to treat pediatric and adult patients with X-linked hypophosphatemia.

The drug was also granted a Priority Review status, so it will be reviewed within 6 months instead of the customary 10 months; its Prescription Drug User Fee Act (PDUFA) action date is April 17, 2018. 

Burosumab was previously designated as a drug for a rare pediatric disease, meaning that if it is approved, the its sponsors – Ultragenyx Pharmaceutical Inc and Kyowa Hakko Kirin International PLC – will be eligible for a Priority Review Voucher. In February, Sarepta Therapeutics sold its Priority Review Voucher for $125 million.

X-linked hypophosphatemia is a heritable form of rickets. Because the disease is X-linked, both males and females can get the condition, however, it is generally more severe in males.
The condition is characterized by inadequate mineralization of bone that leads to a spectrum of abnormalities, including rickets, progressive bowing of the leg, osteomalacia, bone pain, waddling gait, short stature, gross motor impairment, muscle weakness, frequent/poorly healing pseudofractures, spinal stenosis, enthesopathy, and osteoarthritis.
There are currently no approved therapies that act directly on this phosphate wasting condition, and most pediatric patients are managed using oral phosphate replacement and vitamin D (calcitriol) therapy.

Burosumab is a monoclonal IgG1 antibody directed against the phosphaturic hormone fibroblast growth factor 23 (FGF23), a hormone that reduces serum levels of phosphorus and active vitamin D. By inhibiting this hormone, burosumab can increase phosphate and vitamin D.

The impetus to expedite the review of the drug appears to be backed up by some solid data showing the drug to be effective and safe.

At the The American Society for Bone and Mineral Research (ASBMR) 2017 Annual Meeting held September 8-11 in Denver, Karl Insogna, M.D., of the Yale School of Medicine presented data from a phase 3 study in which the drug met its primary endpoint. In the presentation, 94% of patients taking burosumab had serum phosphorous levels above 2.5 mg.dL, compared to only 7.6% in the placebo group after 24 weeks of treatment. Further, 44% of the patients in the burosumab had healed fractures compared to only 18% in the placebo group (see figures below).

In a news release, Mitsuo Satoh, Executive Officer, Vice President, Head of Research and Development Division of Kyowa Hakko Kirin said: “I believe burosumab has the potential to be an effective treatment option for patients with conditions with excess FGF23 such as XLH and tumor induced osteomalacia and we will keep working to provide this advance in therapy for patients as soon as possible.”

Figures obtained from Untragenyx’s poster presentation available at www.ultragenyx.com/pipeline/krn23-xlh/

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