2020-07-03| Trials & Approvals

FDA Approves Crysvita for Victims of Tumor-Induced Osteomalacia

by Judy Ya-Hsuan Lin
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By Judy Ya-Hsuan Lin

A number of patients globally suffer from excessive fibroblast growth factor 23 (FGF23). This rare disease causes tumors and induces a low phosphate concentration in the body and further softens and weakens bones. FGF23 plays a crucial role in regulating phosphate levels for bone maintenance, energy production, and nerve function. Excessive FGF23 diseases include X-linked hypophosphatemia (XLH) and tumor-induced osteomalacia (TIO). XLH affects approximately 3,000 children and 12,000 adults, while TIO inflicts about 500~1000 people in the US. Of the 500~1000 afflicted, “approximately half of all individuals with TIO, surgical removal of the tumors is not possible, leaving these patients with no other treatment options,” according to Camille L. Bedrosian, M.D., Chief Medical Officer of Ultragenyx Pharmaceutical Inc. In other words, such patients are only suitable for non-invasive treatment.

On June 18th, Crysvita (burosumab-twza), co-developed by Ultragenyx Pharmaceutical Inc. and Kyowa Kirin Co., Ltd., was approved by the FDA for treating TIO in adult and pediatric patients two years and older. The company expressed that the approval granted hopes to patients living with the rare TIO disorder, that previously had no other treatment options. Crysvita was also FDA approved to treat XLH in adults and children six months and older.

“Treatment for TIO focuses on identifying and removing the tumor that causes disease. However, when that is not possible, Cysvita can help increase the levels of phosphate in the blood,” said Theresa E. Kehoe, M.D., acting director of the Division of General Endocrinology in the FDA’s Center for Drug Evaluation and Research. Crysvita now has the approval of the FDA in addition to approvals from other countries and regional organizations to treat FGF23-related hypophosphatemia. Health Canada and Brazil’s National Health Surveillance Agency (ANVISA) approved the drug to treat FGF23-related hypophosphatemia in TIO associated with phosphaturic mesenchymal tumors that can hardly be resected or localized in adults and pediatric patients two years of age and older.

The Ministry of Health, Labor and Welfare (MHLW) in Japan approved it for the treatment of FGF23-related hypophosphatemic rickets and osteomalacia. Europe received conditional marketing authorization for the treatment of XLH with radiographic evidence of bone disease in children one year of age and older and adolescents with growing skeletons; currently, Europe reviews the expanded use in adults. Although it has not yet received approval to treat TIO in Japan or Europe, promising clinical results and endorsements demonstrates Crysvita’s potential to win more support.


Mechanism of Action

Crysvita is a recombinant fully human monoclonal IgG1 antibody that reduces the phosphaturic hormone produced by FGF23 protein by blocking FGF23’s biological activity. The excessive presence and activity of FGF23 triggers phosphate wasting in TIO and other hypophosphatemic conditions such as XLH. Crysvita helps increase the reabsorption of phosphate in the kidney and the active production of 1,25-dihydroxyvitamin D to enhance the absorption of phosphate and calcium in the intestine of patients.


Clinical Results & Safety

The FDA approval fundamentally relied upon the data from two single-arm Phase 2 studies, an 88-week study in 13 adult patients conducted by Kyowa Kirin in South Korea and Japan as well as a 144-week study in 14 adult patients conducted by Ultragenyx in the U.S. Both studies presented an increase in serum phosphorus and serum 1,25-dihydroxyvitamin D levels, leading to a direct improvement in osteomalacia.

The uptake of Crysvita also demonstrated the healing of bone lesions based upon the observation of whole-body bone scans. Despite Crysvita’s benefits, its adverse effects include tooth abscess, muscle spasms, dizziness, headache, rash, injection site reaction, and constipation. The data did not include clinical studies for adolescents, children, and infants, and it could be because of the previous, tremendous success of Crysvita in treating XLH. As for XLH, the previous study included a sample size of 65 in two clinical trials and observed that a normal phosphorus level was found in 94% to 100% of children administered with Crysvita compared to 8% with placebo.

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