Endocrine Abstracts

Hypophosphatasia (HPP) is an inherited skeletal disease characterized by defective bone mineralization due to a deficiency in tissue-nonspecific alkaline phosphatase (TNALP). Patients with the severe infantile form of HPP have a poor prognosis that often results in high mortality by one year. Asfotase alfa is an approved therapy for HPP, while requires chronic injections to maintain efficacy. To develop a one-time gene therapy for HPP, we examined the safety and efficacy of AAV-TNALP-D10 (adeno-associated viral vector expressing TNALP-D10) in an Akp2^-/- HPP mouse model. Neonatal Akp2^-/ mice were injected with AAV-TNALP-D10 intramuscularly. Wild type mice were injected with AAV-GFP vector (1.0x10¹² vector genome (vg)/body) as a control. Plasma ALP activity was assessed and the organs of the mice were examined for any possible macroscopic lesions. Following treatment of neonatal Akp2^-/- mice with a single local injection of AAV-TNALP-D10-vector (1.0x10¹² vg/body), high plasma ALP levels (19.38 ± 5.02 U/ml) were detected and persisted for up to 18 months. Computed tomography analysis showed mature bone mineralization. 5/7 of the animals survived until the end of the study (18 months). Histochemical staining for ALP activity in the knee joint revealed ALP activity on the surface of the endosteal bone of mice. Throughout their lives, the treated Akp2^-/- mice exhibited normal physical activity and a healthy appearance, whereas untreated controls died within 3 weeks. No ectopic calcification or abnormal calcium metabolism together with unusual cell growth was detected in the treated mice. AAV-TNALP-D10-mediated neonatal gene therapy is both safe and effective. The current study demonstrates durability and survival up to 18 months, the longest ever demonstrated in this animal model. The study supports the development of AAV-TNALP-D10 as one-time treatment of the severe infantile form of HPP. AAV-TNALP-D10 has the potential to shift HPP treatment paradigm from chronic to one-time dose.