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

Alexion has a public commitment to allow requests for access to study data and will be supplying a protocol, CSR, and plain language summaries.