Burosumab for Fibroblast Growth Factor-23 Mediated Hypophosphatemia in Fibrous Dysplasia

Background: Fibrous dysplasia (FD) is a disorder that affects bone growth. Affected bone tissue is weakened, and people with FD are prone to deformities, fractures, and other problems. People with FD may also have low blood phosphate levels. This can make bones even weaker. Better treatments are needed. Objective: To test a study drug (burosumab) in people with FD who have low blood phosphate levels. Eligibility: People aged 1 year or older who have FD and low blood phosphate levels. Design: Participants will visit the NIH 3 times in 48 weeks. Each visit will last 5 to 7 days. Participants will self-inject burosumab under the skin in their belly, upper arm, or thigh. They (or a caregiver) will do this at home 1 or 2 times a month. They will be trained in person on how to inject the drug. Home injections will be guided via telehealth. During NIH visits, participants will have a physical exam with blood and urine tests. They will have x-rays of different parts of their body. They will have a radioactive tracer injected into their vein; then they will have a bone scan. They will have tests to assess their strength, walking, and movement. They will complete questionnaires about their pain, mobility, and fatigue levels. Adult participants may have bone biopsies. These will be done under anesthesia with sedation. Small samples of FD-affected bone will be removed for study. Between NIH visits, participants will go to a local laboratory for blood and urine tests. Child participants will have an additional follow-up visit 2 weeks after the final NIH visit.