Effect of Burosumab Compared With Conventional Therapy on Younger vs Older Children With X-linked Hypophosphatemia

Context: Younger age at treatment onset with conventional therapy (phosphate salts and active vitamin D; Pi/D) is associated with improved growth and skeletal outcomes in children with X-linked hypophosphatemia (XLH). The effect of age on burosumab efficacy and safety in XLH is unknown. Objective: This work aimed to explore the efficacy and safety of burosumab vs Pi/D in younger (< 5 years) and older (5-12 years) children with XLH. Methods: This post hoc analysis of a 64-week, open-label, randomized controlled study took place at 16 academic centers. Sixty-one children aged 1 to 12 years with XLH (younger, n = 26; older, n = 35) participated. Children received burosumab starting at 0.8 mg/kg every 2 weeks (younger, n = 14; older, n = 15) or continued Pi/D individually titrated per recommended guidelines (younger, n = 12; older, n = 20). The main outcome measure included the least squares means difference (LSMD) in Radiographic Global Impression of Change (RGI-C) rickets total score from baseline to week 64. Results: The LSMD in outcomes through 64 weeks on burosumab vs conventional therapy by age group were as follows: RGI-C rickets total score (younger, +0.90; older, +1.07), total Rickets Severity Score (younger, -0.86; older, -1.44), RGI-C lower limb deformity score (younger, +1.02; older, +0.91), recumbent length or standing height Z-score (younger, +0.20; older, +0.09), and serum alkaline phosphatase (ALP) (younger, -31.15% of upper normal limit [ULN]; older, -52.11% of ULN). On burosumab, dental abscesses were not reported in younger children but were in 53% of older children. Conclusion: Burosumab appears to improve outcomes both in younger and older children with XLH, including rickets, lower limb deformities, growth, and ALP, compared with Pi/D. Key Words: burosumab, fibroblast growth factor 23, X-linked hypophosphatemia, rickets, children Abbreviations: 1,25[(OH).sub.2]D, 1,25-dihydroxyvitamin D; 25(OH)D, 25-hydroxyvitamin D; AE, adverse event; ALP, alkaline phosphatase; FGF23, fibroblast growth factor 23; GEE, generalized estimating equation; iPTH, intact parathyroid hormone; LSMD, least squares means difference; Pi/D, phosphate salts and active vitamin D; RGI-C, Radiographic Global Impression of Change; RSS, Rickets Severity Score; TEAE, treatment-emergent adverse event; TmP/GFR, tubular maximum for phosphate reabsorption per glomerular filtration rate; ULN, upper limit of normal; XLH, X-linked hypophosphatemia.