Switching to burosumab from conventional therapy in siblings with relatively well-controlled X-linked hypophosphatemia.

Burosumab, a fully human monoclonal antibody against fibroblast growth factor 23, is mainly administered to patients with severe X-linked hypophosphatemia (XLH). However, there have been few reports on its use in relatively mild cases. In this report, we administered burosumab to two siblings with XLH who had been effectively treated with oral phosphate and active vitamin D. Both patients showed further improvement in radiographic and laboratory findings with burosumab compared with conventional treatment. Upon switching treatment, popliteal pain was reported in case 1 until her phosphorus levels normalized. This emphasizes the importance of monitoring not only rickets and calcium/phosphate metabolism but all symptoms of XLH after initiating burosumab. Notably, in cases 1 and 2, burosumab sustained catch-up growth, especially in case 1, who had not yet reached puberty. Further clinical studies are needed to determine whether burosumab improves growth and proportional abnormalities in patients with mild XLH.