FDA Approves Therapy for Disease that Causes Low Phosphate Blood Levels, Bone Softening

The U.S. Food and Drug Administration (FDA) has approved Crysvita (burosumab-twza) injection to treat patients age two and older with tumor-induced osteomalacia (TIO), a rare disease that is characterized by the development of tumors that cause weakened and softened bones. The tumors associated with TIO release a peptide hormone-like substance known as fibroblast growth factor 23 (FGF23) that lowers phosphate levels.

“Treatment for TIO focuses on identifying and removing the tumor that causes the disease. However, when that is not possible, Crysvita 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. “As the first FDA-approved therapy to treat this debilitating disease, today’s action is an important step in finding treatment options for patients living with TIO whose tumor cannot be found or removed.”

FGF23 regulates levels of phosphate, an electrolyte that plays important roles in bone maintenance, energy production by cells and nerve function. When there is not enough phosphate in the body, bones begin to soften and weaken, causing osteomalacia (marked softening of bones).

The safety and efficacy of Crysvita were evaluated in two studies that together enrolled 27 adults with TIO. In both studies, patients received Crysvita every four weeks. For the first study, half of patients achieved normal phosphate levels through week 24 and maintained normal or near normal phosphate levels through week 144. In the second study, 69% of participants achieved normal phosphate levels through week 24 and maintained normal or near normal phosphate levels through week 88. The results of bone scans for patients in the first study also suggested healing of the bone lesions related to osteomalacia (softening of the bones).

Hypersensitivity reactions such as rash and hives have been reported in patients who took Crysvita. If serious hypersensitivity reactions occur, patients should stop taking Crysvita and talk with their health care provider about further medical treatment. Additionally, higher than normal levels of phosphorus may be associated with an increased risk of nephrocalcinosis (a disorder that occurs when too much calcium is deposited in the kidneys).

The most common side effects reported in adults with TIO taking Crysvita were tooth abscess (infection), muscle spasms, dizziness, constipation, injection site reaction, rash and headaches.

Patients taking oral phosphate or active vitamin D, those who have serum phosphate levels within or above the normal range for their age, and patients with severe kidney impairment or end stage renal disease should not take Crysvita.

Crysvita is also FDA-approved to treat adults and children six months and older with X-linked hypophosphatemia, which causes low levels of phosphate in the blood and leads to impaired bone growth and development in children and teenagers. The FDA granted approval of Crysvita to Ultragenyx Pharmaceutical Inc.