Improvement in Physical Function Associated With Burosumab in a Patient With X-linked Hypophosphatemia: A Case Report

Burosumab, a human immunoglobulin G1 monoclonal antibody that targets fibroblast growth factor 23 (FGF23), was developed to treat X-linked hypophosphatemic rickets/osteomalacia (XLH). By binding to FGF23, burosumab promotes the reabsorption of phosphorus from the kidney, increases the production of active vitamin D, and promotes the intestinal absorption of phosphorus and calcium. Herein, we report a one-year evaluation of physical function in adult patients with XLH who were treated using burosumab. A 40-year-old male patient with a history of lower limb varus deformity was diagnosed with vitamin D-resistant rickets in early childhood. Around October 2021, the patient began experiencing pain in both lower limbs when ascending and descending stairs at his place of work, making it difficult for him to perform his duties. We prescribed subcutaneous burosumab (70 mg) in December 2022. Twelve weeks after starting this treatment, his 10-meter walking speed and Time Up-and-Go Test results showed significant improvement. We therefore concluded that burosumab treatment may improve physical function in adult patients with XLH.