Successful treatment using asfotase alfa for delayed healing of a metatarsal stress fracture in an adolescent girl with hypophosphatasia

BACKGROUND: Hypophosphatasia (HPP) is a rare metabolic bone disorder that can present with a wide spectrum of skeletal and extraskeletal signs and symptoms. Biochemically, HPP is characterized by a decreased activity of the tissue nonspecific alkaline phosphatase (TNSALP) and subsequent accumulation of inorganic pyrophosphate (PPi). Fractures in patients with HPP can show delayed healing or even progress to nonunion. In pediatric patients, fractures can be particularly debilitating, as children usually engage in high levels of physical activity. CASE PRESENTATION: The 16-year-old girl, carrying a pathogenic variant in the ALPL gene, presented 17 weeks (4 months) after the initial diagnosis of a fracture at the base of the fifth metatarsal (MT-V). The patient was still symptomatic, and magnetic resonance imaging and cone beam computed tomography (CBCT) showed no radiological signs of healing despite immobilization. Given the prolonged absence of fracture consolidation, TNSALP enzyme replacement therapy with asfotase alfa (AA) was initiated and dosed according to body weight, in line with approved pediatric regimens. Thirteen weeks after initiation of AA, CBCT demonstrated full radiological consolidation. Furthermore, bone mineral density (BMD) in the fracture gap increased by 36%, returning to the average BMD level of the MT-V. CONCLUSION: Here we present, to the best of our knowledge, the first pediatric case in which treatment with AA supported healing of a delayed union stress fracture in the context of clinically diagnosed and genetically supported HPP. Our findings support considering AA initiation in pediatric patients diagnosed with HPP who present delayed fracture healing despite standard conservative management.