Study to Assess the Safety, Tolerability, Pharmacokinetics and Efficacy of Burosumab in Patients Less Than 1 Year of Age

BUR-CL207 is a multicenter, open-label, non-randomized Phase 1/2 study in pediatric patients with XLH initiating treatment with burosumab at \<12 months of age. The study includes a total treatment period of up to 48 weeks across 3 cohorts. Subjects will be enrolled in 2 age subgroups: (1) ≥6 months to \<12 months of age, and (2) \<6 months of age at initiation of burosumab treatment. Cohorts will aim to include 3 subjects per cohort depending on the starting dose of burosumab and relative response of patients to treatment as assessed by serum phosphate levels and by the treating physician (Investigator). Cohorts will commence in a staggered manner starting with Cohort 1, followed by Cohorts 2 and 3, which may start in parallel after an adequate observation period (4 weeks) in Cohort 1 and with approval from the Sponsor's Medical Monitor and the DSMB. The cohorts are defined as follows:

* Cohort 1: pediatric subjects with XLH and hypophosphatemia (serum phosphate below the age-adjusted lower limit of normal \[LLN\]), and age from ≥6 months to \<12 months of age at initiating treatment with burosumab with starting dose of 0.4 mg/kg. * Cohort 2: pediatric subjects with XLH and hypophosphatemia (serum phosphate below the age-adjusted LLN), and age from ≥6 months to \<12 months of age at initiating treatment with burosumab with a starting dose of 0.8 mg/kg (upon Data Safety Monitoring Board \[DSMB\] confirmation). * Cohort 3: pediatric subjects with XLH and hypophosphatemia (serum phosphate below the age-adjusted LLN), and \<6 months of age at initiating treatment with burosumab with a starting dose of 0.4 mg/kg (upon DSMB confirmation). Following enrollment of the first 3 or 4 subjects in Cohort 3, an interim population PK/PD evaluation may be performed to determine whether the starting dose for the cohort can be increased to 0.8 mg/kg.