Manisha C. Yadav, Isabelle Lemire, Pierre Leonard, Guy Boileau, Laurent Blond, Martin Beliveau, Esther Cory, Robert L. Sah, Michael P. Whyte, Philippe Crine and José Luis Millán
Hypophosphatasia (HPP) features rickets or osteomalacia from tissue-nonspecific alkaline phosphatase (TNSALP) deficiency due to deactivating mutations within the ALPL gene. Enzyme replacement therapy with a bone-targeted, recombinant TNSALP (sALP-FcD10, renamed ENB-0040) prevents manifestations of HPP when initiated at birth in TNSALP knockout (Akp2−/−) mice. Here, we evaluated the dose–response relationship of ENB-0040 to various phenotypic traits of Akp2−/− mice receiving daily subcutaneous (SC) injections of ENB-0040 from birth at 0.5, 2.0, or 8.2 mg/kg for 43 days. Radiographs, μCT, and histomorphometric analyses documented better bone mineralization with increasing doses of ENB-0040. We found a clear, positive correlation between ENB-0040 dose and prevention of mineralization defects of the feet, rib cage, lower limbs, and jaw bones. According to a dose–response model, the ED80 (the dose that prevents bone defects in 80% of mice) was 3.2, 2.8 and 2.9 mg/kg/day for these sites, respectively. Long bones seemed to respond to lower daily doses of ENB-0040. There was also a positive relationship between ENB-0040 dose and survival. Median survival, body weight, and bone length all improved with increasing doses of ENB-0040. Urinary PPi concentrations remained elevated in all treatment groups, indicating that while this parameter is a good biochemical marker for diagnosing HPP, it may not be a good follow up marker for evaluating response to treatment when administering bone-targeted TNSALP. These dose–response relationships strongly support the pharmacological efficacy of ENB-0040 for HPP, and provide the experimental basis for the therapeutic range of ENB-0040 chosen for clinical trials. We find a positive correlation between ENB-0040 dose and prevention of mineralization defects in a murine model of hypophosphatasia. We found a positive relationship between ENB-0040 dose and survival. Urinary pyrophosphate is not a good follow up marker for evaluating response when using bone-targeted TNSALP treatment. These studies help set the therapeutic range of ENB-0040 to be chosen for clinical trials.