Postprandial serum phosphorus and calcium concentrations in adults and children with X-linked hypophosphatemia during burosumab treatment

X-linked hypophosphatemia (XLH) is caused by <i>PHEX</i> gene variants that result in increased circulating levels of fibroblast growth factor 23 (FGF23). FGF23 in turn decreases renal reabsorption of phosphate and suppresses renal production of 1,25(OH)₂D, leading to rickets and growth impairment in children and osteomalacia in children and adults. Burosumab is a fully human FGF23-blocking monoclonal antibody approved for treating XLH. Limited data are available on the impact of phosphorus-containing meals or supplements or diurnal variation on serum phosphorus levels, with increases observed in some, but not all studies. It is recommended that serum phosphorus be measured in the morning fasted state when monitoring treatment in patients with XLH. The present substudy of the pivotal pediatric and adult phase 3 clinical trials of burosumab examined the impact of meal consumption and timing around meals on serum phosphorus and calcium levels in children and adults with XLH during burosumab treatment. Thirty-nine participants (pediatric, <i>n</i> = 13; adult, <i>n</i> = 26) were included. The mean (SD) duration of burosumab treatment prior to the substudy was 15.4 (6.6) mo for pediatric and 24.2 (3.7) mo for adult participants. Serum phosphorus and calcium levels were measured before and after breakfast in children, and before and after both breakfast and lunch in adults. In both age groups, there was no clinically meaningful difference in mean levels of serum phosphorus measured at 1 and 2 h after meals compared to fasted levels, and serum calcium levels remained within the normal range for all pediatric participants and most adults, although interpatient variation was observed. These results suggest that, when fasting is not possible, nonfasting serum phosphorus levels may be a suitable alternative in patients with XLH receiving a stable dose of burosumab.