Study of KRN23 (Burosumab), a Recombinant Fully Human Monoclonal Antibody Against Fibroblast Growth Factor 23 (FGF23), in Pediatric Subjects With X-linked Hypophosphatemia (XLH)

The objectives of the study are to: * Identify a dose and dosing regimen of burosumab, based on safety and pharmacodynamic (PD) effect, in pediatric XLH participants * Establish the safety profile of burosumab for the treatment of children with XLH including ectopic mineralization risk, cardiovascular effects, and immunogenicity profile * Characterize the pharmacokinetic (PK)/PD profile of the KRN23 doses tested in the monthly (Q4) and biweekly (Q2) dose regimens in pediatric XLH patients * Determine the PD effects of burosumab treatment on markers of bone health in pediatric XLH patients * Obtain a preliminary assessment of the clinical effects of burosumab on bone health and deformity, muscle strength, and motor function * Obtain a preliminary assessment of the effects of burosumab on participant-reported outcomes, including pain, disability, and quality of life in pediatric XLH patients * Evaluate the long-term safety and efficacy of burosumab