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 criteria fulfilled 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 the Screening Period as measured by the Central Laboratory OR 2 documented serum ALP activity results, at least 15 days apart, below the age- and sex-adjusted local 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 laboratory certification with Alexion's approval. * Must meet 1 of the following criteria: 1. Documented ALPL gene variant (pathogenic, likely pathogenic, or variant of unknown 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 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 ALXN1850 or the placebo comparator