Efficacy of Burosumab Every 2 Weeks in an Adult with X-Linked Hypophosphatemia: Should We Learn from Children?

X-linked hypophosphatemia is a genetic condition that leads to fibroblast-growth-factor 23 (FGF23) increase, causing phosphate renal wasting. Since 2018, burosumab, an anti-FGF23 antibody, has been used for this disease with different dosage in children and adults. We report the case of burosumab administration every 2 weeks, as usually done in children. We retrospectively evaluated parathormone (PTH), alkaline phosphatase, serum phosphate, tubular reabsorption of phosphate (TRP), and 25OH vitamin D every 2 weeks in a 29-year-old man with nephrocalcinosis and tertiary hyperparathyroidism who did not respond to standard treatment with burosumab nor to maximum dosage and was treated with burosumab 90 mg every 2 weeks. His serum phosphate and TRP increased with this regimen compared with 4 weeks frequency (respectively 1.74 ± 0.26 mg/dL vs. 2.3 ± 0.19 mg/dL [p 0.0004] and 71.3% ± 4.8% vs. 83.9% ± 7.9% [p 0.01]) with decrease in PTH (183 ± 24.7 pg/mL vs. 109 ± 12.2 pg/mL [p 0.04]). Burosumab may be a good choice in adult patients with X-linked hypophosphatemia; new data are needed regarding the increase in dosage and/or frequency of administration as usually done in children, to achieve disease control.