Phase 3 Study of ALXN1850 in Pediatric Participants With HPP Previously Treated With Asfotase Alfa

The primary purpose of this study is to assess the safety and tolerability of ALXN1850 versus asfotase alfa in pediatric participants with HPP previously treated with asfotase alfa.

Inclusion Criteria: * Diagnosis of HPP documented in the medical records * Presence of open growth plates by X-ray during Screening Period * Tanner stage 2 or less during the Screening Period * Must have been treated with 6 mg/kg/ week of asfotase alfa via SC injection administered as either 2mg/kg 3 times per week or 1 mg/kg 6 times per week for ≥ 6 months before Day 1. Note: participants currently treated with 9 mg/kg (eg, 3 mg/kg 3 times per week) will not be allowed in the study. Exclusion Criteria: * History or presence of cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrinological, hematological, neurological disorders, or any other disorders that are capable of significantly altering the absorption, metabolism, or elimination of drugs; constituting a risk when taking the study intervention; or interfering with the interpretation of data as determined by the Investigator. * Diagnosis of primary or secondary hyperparathyroidism * Hypoparathyroidism, unless secondary to HPP * Any new fracture within 12 weeks before Day 1 (excluding pseudofractures) * Planned surgical intervention which may impact the results of study assessments (in the opinion of the Investigator) during the Randomized Evaluation Period * History of allergy or hypersensitivity to any ingredient contained in asfotase alfa or ALXN1850 * Body weight \< 10 kg during the Screening Period