M R Ambrosio

Summary Tumour-induced osteomalacia (TIO) is due to an overproduction of fibroblast growth factor 23 (FGF23) by mesenchymal tumours, causing hypophosphatemia, osteomalacia and muscle weakness. TIO is usually cured by tumour resection, but neoplasms may be unidentifiable and unresectable or the patient may refuse surgery. In these cases, medical treatment with oral phosphate and calcitriol is mandatory, but it is not fully effective and it is associated with low compliance. Burosumab, a human MAB against FGF23 employed to treat X-linked hypophosphatemia (XLH), has recently been approved for TIO in the USA. Maximum burosumab dose in XLH is 90 mg administered for 2 weeks; there are no data on clinical efficacy and safety of this dose in TIO. We reported the case of a 73 years old male with multiple non-traumatic fractures, low bone mineral density, pain and reduced independence of activities of daily living. Biochemical evaluation showed hypophosphatemia, high alkaline phosphatase (ALP) and C-terminal telopeptide (CTX) and normal albumin-corrected total calcium and parathyroid hormone. Tubular phosphate reabsorption was low (80%), whereas C-terminal tail of FGF23 (cFGF23) was elevated. A 68 Ga-DOTATOC PET was performed, identifying a lesion in the first left rib. The patient refused surgery; therefore, burosumab therapy was started. After 18 months of treatment (maximum dose: 60 mg administered for 2 weeks), plasma phosphate normalized and ALP levels improved (138 U/L). Patient clinical symptoms as well as pain severity and fatigue improved. Neither adverse events nor tumour progression was reported during follow-up except for a painless fracture of the second right rib. Learning points Our case shows efficacy and safety of burosumab treatment administered every 2 weeks in a tumour-induced osteomalacia (TIO) patient. After 18 months of treatment at a maximum dose of 60 mg every 2 weeks, we found plasma phosphate normalization and ALP reduction as well as improvement in clinical symptoms and fatigue. Neither adverse events nor tumour progression was reported during follow-up, except for a painless fracture of the second right rib.