Burosumab use in fibroblast growth factor-23-mediated hypophosphatemia in McCune-Albright syndrome/fibrous dysplasia.

McCune-Albright syndrome (MAS) is a rare disorder characterized by fibrous dysplasia (FD), café-au-lait spots, and hyperfunctioning endocrinopathies. FD lesions can overproduce fibroblast growth factor-23 (FGF-23), leading to renal phosphate wasting, hypophosphatemia, and impaired bone mineralization. Conventional treatment with oral phosphate and calcitriol is limited by gastrointestinal intolerance. Burosumab, a monoclonal antibody against FGF-23, is approved for X-linked hypophosphatemia and tumor-induced osteomalacia and has shown promise in case reports of pediatric and adult patients with MAS-related FGF-23-mediated hypophosphatemia. We describe a 46-year-old woman with MAS and extensive FD who presented with worsening bone pain and FGF-23-mediated hypophosphatemia. She was started on phosphate and calcitriol with improvement in pain. Due to failure to achieve treatment targets, she was transitioned to burosumab 0.50 mg/kg injections every 4 weeks with normalization of serum phosphorus but persistent elevation of alkaline phosphatase. This case represents the second reported adult and oldest patient treated with burosumab for MAS-related FGF-23-mediated hypophosphatemia after failure of conventional therapy. Although its use in MAS remains off-label, burosumab may provide a more targeted and better-tolerated therapeutic option.