Safety and Efficacy Study of Asfotase Alfa in Severely Affected Infants With Hypophosphatasia (HPP)

Inclusion Criteria: * Legal guardian(s) must provide informed consent prior to any study procedures * Documented diagnosis of severe HPP as indicated by: * Total serum alkaline phosphatase at least 3 standard deviations (SD) below the mean for age * Plasma pyridoxal 5'-phosphate (PLP) at least 4 times the upper limit of normal * Radiographic evidence of HPP (hypophosphatasia), characterized by: * Flared and frayed metaphyses * Severe, generalized osteopenia * Widened growth plates * One or more HPP-related findings: * History or presence of: * Non-traumatic post-natal fracture * Delayed fracture healing * History of elevated serum calcium * Functional craniosynostosis with decreased head circumference growth * Nephrocalcinosis * Respiratory compromise * Rachitic chest deformity and/or vitamin B6 dependent seizures * Failure to thrive * Onset of symptoms prior to 6 months of age * Age ≤ 36 months * Otherwise medically stable (patient may be on ventilatory support) * Legal guardian(s) must be willing to comply with the study Exclusion Criteria: * History of sensitivity to any of the constituents of the study drug * Current or prior clinically significant cardiovascular, endocrinologic, hematologic, hepatic, immunologic, metabolic, infectious, urologic, pulmonary, neurologic, dermatologic, renal condition and/or other major disease which, in the opinion of the investigator, precludes study participation * Treatment with an investigational drug within 1 month prior to the start of study drug administration * Current enrollment in any other study involving an investigational new drug, device or treatment for HPP (e.g., bone marrow transplantation) * Low serum calcium, phosphate or 25(OH) vitamin D * Current evidence of a treatable form of rickets * Prior treatment with bisphosphonate