Preclinical evaluation of the efficacy and safety of AAV8-TNAP-D10 in Alpl-/- and AlplPrx1/Prx1 mouse models for the treatment of early and late-onset hypophosphatasia.

We previously documented successful resolution of skeletal and dental disease in the infantile and late-onset murine models of hypophosphatasia (HPP), with a single injection of an adeno-associated serotype 8 vector encoding mineral-targeted TNAP (AAV8-TNAP-D10). Here, we conducted dosing studies in both HPP mouse models. A single escalating dose from 4x108 up to 4x1010 (vg/b) was intramuscularly injected into 4-day-old Alpl-/- mice (an infantile HPP model) and a single dose from 4x106 up to 4x109 (vg/b) was administered to 8-week-old AlplPrx1/Prx1 mice (a late-onset HPP model). Wild-type littermates were used as controls. Serum alkaline phosphatase activity was increased, and PPi levels were decreased in a dose-dependent manner in both the Alpl-/- and AlplPrx1/Prx1 models. Radiographic and μCT analysis of long bones of female and male Alpl-/- mice showed full correction of skeletal phenotype at 4x1010 vg/b. We observed full correction of the bone phenotype at 4x108 and 4x109 in female AlplPrx1/Prx1 mice, but bones remained hypomineralized with the 4x106 and 4x107 (vg/b) doses after 70 days of treatment. We observed skeletal improvements using the 4x109 (vg/b) dose, but the phenotype was not fully corrected in male AlplPrx1/Prx1. Immunohistochemistry using anti-TNAP and anti-D10 antibodies showed high immunolocalization in the femurs of female AlplPrx1/Prx1 mice, while D10 immunolocalization was high in the liver of male AlplPrx1/Prx1 mice at a dose of 4x109 (vg/b). This sex-dependent difference was not seen in the infantile HPP model. A serum proteome analysis showed enhanced inflammatory pathways in treated AlplPrx1/Prx1 males compared to treated female mice. We also found a few areas of ectopic calcification in soft organs at the highest tested dose of 4x1010 (vg/b) in Alpl-/- or 4x109 (vg/b) in the AlplPrx1/Prx1 model. This pre-clinical study will inform the design of clinical trials to develop gene therapy in early-onset and late-onset HPP patients.