Phase 3 Study of ALXN1850 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 injectionadministered as either 2mg/kg 3 times per week or 1 mg/kg 6 times per week for ≥ 6months 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 anyother disorders that are capable of significantly altering the absorption,metabolism, or elimination of drugs; constituting a risk when taking the studyintervention; or interfering with the interpretation of data as determined by theInvestigator.

• 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 (inthe opinion of the Investigator) during the Randomized Evaluation Period

• History of allergy or hypersensitivity to any ingredient contained in asfotase alfaor ALXN1850

• Body weight < 10 kg during the Screening Period