Phase 3 Study of ALXN1850 Versus Placebo in Adolescent and Adult Participants With HPP Who Have Not Previously Been Treated With Asfotase Alfa

Inclusion Criteria: * Diagnosis of HPP documented in the medical records * Must meet 1 of the following criteria: 1. Documented ALPL gene variant (pathogenic, likely pathogenic, or variant of unknown significance) from a Clinical Laboratory Improvement Amendments (CLIA) or ISO 15189 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) * Must meet 1 of the following criteria without a probably cause other than HPP: 1. Serum ALP activity below the age- and sex-adjusted normal range during the screening period as measured by the Central Laboratory 2. Two 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 CLIA or ISO 15189 certified, or have other local equivalent laboratory certification with Alexion's approval. * Two separate 6MWTs at below 85% of the predicted distance (for age, sex, weight, and height) during the Screening Period without a probable cause other than HPP 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 (Table 9)