Background: Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare mosaic disorder of Gαs activation. Fibroblast Growth Factor 23 (FGF23)-mediated hypophosphatemia is a feature of FD/MAS that has been associated with poor skeletal outcomes. Standard therapy includes oral phosphorus and vitamin D analogs; however, treatment is limited by potential adverse renal and gastrointestinal effects. Burosumab is a monoclonal antibody to FGF23 approved to treat patients with X-linked hypophosphatemia and tumor-induced osteomalacia. There is currently no safety or efficacy data to support burosumab use in patients with FD/MAS.
Case description: A 7-year-old boy with severe FD/MAS presented with persistent hypophosphatemia and skeletal complications despite conventional treatment with oral phosphate and calcitriol. He was started on burosumab and achieved sustained normalization of serum phosphorus and marked improvement in alkaline phosphatase levels. This was accompanied by an encouraging clinical response, including decreased bone pain, improved muscle strength, and improved ambulation. No adverse effects of burosumab therapy were observed.
Conclusions: This is the first reported case of burosumab treatment in a patient with FD/MAS. The encouraging biochemical and clinical response in this patient highlights the need for future studies to explore the safety and efficacy of burosumab in the FD/MAS pediatric population.