Phase 3 Study of ALXN1850 in Treatment-Naïve Pediatric Participants With HPP

Inclusion Criteria:

• Diagnosis of HPP documented in the medical records, and the following criteriafulfilled without other probable cause than HPP:

1. Presence of HPP-related rickets on skeletal X-rays during the Screening Period,with a minimum Rickets Severity Score (RSS) of 1.0 AND

2. Serum ALP activity below the age- and sex-adjusted normal range during theScreening Period as measured by the Central Laboratory OR 2 documented serumALP activity results, at least 15 days apart, below the age- and sex-adjustedlocal laboratory normal range during the 24 months before the Day 1 Visit.Note: Local laboratories need to be Clinical Laboratory Improvement Amendments (CLIA) or ISO 15189 certified, or have other local equivalent laboratorycertification with Alexion's approval.

• Must meet 1 of the following criteria:

1. Documented ALPL gene variant (pathogenic, likely pathogenic, or variant ofunknown significance) from a CLIA certified laboratory (Section 8.7)

2. Plasma PLP above the upper limit of normal (ULN) during the Screening Period (central or local laboratory results allowed per local regulations)

• Tanner stage 2 or less during the Screening Period

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 ALXN1850 orthe placebo comparator