Asfotase alfa therapy for children with hypophosphatasia

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Abstract

Background. Hypophosphatasia (HPP) is caused by loss-of-function mutation(s) of the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). Consequently, cell-surface deficiency of TNSALP phosphohydrolase activity leads to extracellular accumulation of inorganic pyrophosphate, a natural substrate of TNSALP and inhibitor of mineralization. Children with HPP can manifest rickets, skeletal pain, deformity, fracture, muscle weakness, and premature deciduous tooth loss. Asfotase alfa is a recombinant, bone-targeted, human TNSALP injected s.c. to treat HPP. In 2012, we detailed the 1-year efficacy of asfotase alfa therapy for the life-threatening perinatal and infantile forms of HPP.

Methods. Here, we evaluated the efficacy and safety of asfotase alfa treatment administered to children 6–12 years of age at baseline who were substantially impaired by HPP. Two radiographic scales quantitated HPP skeletal disease, including comparisons to serial radiographs from similarly affected historical control patients.

Results. Twelve children receiving treatment were studied for 5 years. The 6-month primary endpoint was met, showing significant radiographic improvement. Additional significant improvements included patient growth, strength, motor function, agility, and quality of life, which for most patients meant achieving normal values for age- and sex-matched peers that were sustained at 5 years of treatment. For most, pain and disability resolved. Mild to moderate injection-site reactions were common and were sometimes associated with lipohypertrophy. Low anti–asfotase alfa antibody titers were noted in all patients. No evidence emerged for clinically important ectopic calcification or treatment resistance.

Conclusions. Asfotase alfa enzyme replacement therapy has substantial and sustained efficacy with a good safety profile for children suffering from HPP.

Trial Registration. ClinicalTrials.gov NCT00952484 (https://clinicaltrials.gov/ct2/show/NCT00952484) and NCT01203826 (https://clinicaltrials.gov/ct2/show/NCT01203826).

Funding. Alexion Pharmaceuticals Inc. and Shriners Hospitals for Children.

Authors

Michael P. Whyte, Katherine L. Madson, Dawn Phillips, Amy L. Reeves, William H. McAlister, Amy Yakimoski, Karen E. Mack, Kim Hamilton, Kori Kagan, Kenji P. Fujita, David D. Thompson, Scott Moseley, Tatjana Odrljin, Cheryl Rockman-Greenberg

NFAT restricts osteochondroma formation from entheseal progenitors

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Abstract

Osteochondromas are common benign osteocartilaginous tumors in children and adolescents characterized by cartilage-capped bony projections on the surface of bones. These tumors often cause pain, deformity, fracture, and musculoskeletal dysfunction, and they occasionally undergo malignant transformation. The pathogenesis of osteochondromas remains poorly understood. Here, we demonstrate that nuclear factor of activated T cells c1 and c2 (NFATc1 and NFATc2) suppress osteochondromagenesis through individual and combinatorial mechanisms. In mice, conditional deletion of NFATc1 in mesenchymal limb progenitors, Scleraxis-expressing (Scx-expressing) tendoligamentous cells, or postnatally in Aggrecan-expressing cells resulted in osteochondroma formation at entheses, the insertion sites of ligaments and tendons onto bone. Combinatorial deletion of NFATc1 and NFATc2 gave rise to larger and more numerous osteochondromas in inverse proportion to gene dosage. A population of entheseal NFATc1- and Aggrecan-expressing cells was identified as the osteochondroma precursor, previously believed to be growth plate derived or perichondrium derived. Mechanistically, we show that NFATc1 restricts the proliferation and chondrogenesis of osteochondroma precursors. In contrast, NFATc2 preferentially inhibits chondrocyte hypertrophy and osteogenesis. Together, our findings identify and characterize a mechanism of osteochondroma formation and suggest that regulating NFAT activity is a new therapeutic approach for skeletal diseases characterized by defective or exaggerated osteochondral growth.

Authors

Xianpeng Ge, Kelly Tsang, Lizhi He, Roberto A. Garcia, Joerg Ermann, Fumitaka Mizoguchi, Minjie Zhang, Bin Zhou, Bin Zhou, Antonios O. Aliprantis