Burosumab Versus Phosphate/Active Vitamin D in Pediatric X-Linked Hypophosphatemia : A Sub-group Analysis by Dose Level

PURPOSE: In an open label, randomized, controlled, phase 3 trial in 61 children 1 to 12 years old with X-linked hypophosphatemia (XLH), burosumab improved rickets versus continuing conventional therapy with active vitamin D and phosphate. Here, we conducted an analysis to determine whether skeletal responses differed when switching to burosumab versus continuing higher or lower doses of conventional therapy. METHODS: Conventional therapy dose groups were defined as: higher dose phosphate >40 mg/kg [HPi], lower dose phosphate ≤40 mg/kg [LPi], higher dose alfacalcidol >60 ng/kg or calcitriol >30 ng/kg [HD], and lower dose alfacalcidol ≤60 ng/kg or calcitriol ≤30 ng/kg [LD]. RESULTS: At Week 64, the Radiographic Global Impression of Change (RGI-C) for rickets was higher (better) in children randomized to burosumab versus conventional therapy for all pre-baseline dose groups: HPi (+1.72 versus +0.67), LPi (+2.14 versus +1.08), HD (+1.90 versus +0.94), LD (+2.11 versus +1.06). At Week 64, the RGI-C for rickets was also higher in children randomized to burosumab (+2.06) versus conventional therapy for all on-study dose groups: HPi (+1.03), LPi (+1.05), HD (+1.45), LD (+0.72). Serum alkaline phosphatase also decreased in the burosumab treated patients more than in the conventional therapy group, regardless of on-study phosphate and active vitamin D doses. MAIN CONCLUSIONS: Prior phosphate or active vitamin D doses did not influence treatment response after switching to burosumab among children with XLH and active radiographic rickets. Switching from conventional therapy to burosumab improved rickets and serum alkaline phosphatase more than continuing either higher or lower doses of phosphate or active vitamin D.