Skeletal Effects of Bone-Targeted TGFbeta Inhibition in a Mouse Model of Duchenne Muscular Dystrophy.

Duchenne muscular dystrophy (DMD) is a severe progressive muscle disease that is frequently associated with secondary osteoporosis. Previous studies have shown that TGFbeta inactivating antibody improves the muscle phenotype in mdx mice, a model of DMD. In the present study, we assessed the skeletal effects of treatment with a bone-targeted TGFbeta antibody (PCT-011) in mdx mice. Micro-computed tomography showed that 8 weeks of intraperitoneal administration of PCT-011 (10 mg per kg body mass, 3 times per week) was associated with more than twofold higher trabecular bone volume at the distal femur, which was explained by a higher trabecular number. At the femoral midshaft, PCT-011 exposure increased cortical thickness but did not significantly affect the results of three-point bending tests. Histomorphometric analyses of the lumbar vertebra 4 showed that PCT-011 treatment led to a lower bone formation rate. In conclusion, treatment with the TGFbeta antibody PCT-011 had a positive effect on bone development in mdx mice. Inhibiting TGFbeta activity thus appears to be a promising approach to treat bone fragility in the context of DMD.

Enzyme-replacement therapy in life-threatening hypophosphatasia.

BACKGROUND: Hypophosphatasia results from mutations in the gene for the tissue-nonspecific isozyme of alkaline phosphatase (TNSALP). Inorganic pyrophosphate accumulates extracellularly, leading to rickets or osteomalacia. Severely affected babies often die from respiratory insufficiency due to progressive chest deformity or have persistent bone disease. There is no approved medical therapy. ENB-0040 is a bone-targeted, recombinant human TNSALP that prevents the manifestations of hypophosphatasia in Tnsalp knockout mice. METHODS: We enrolled infants and young children with life-threatening or debilitating perinatal or infantile hypophosphatasia in a multinational, open-label study of treatment with ENB-0040. The primary objective was the healing of rickets, as assessed by means of radiographic scales. Motor and cognitive development, respiratory function, and safety were evaluated, as well as the pharmacokinetics and pharmacodynamics of ENB-0040. RESULTS: Of the 11 patients recruited, 10 completed 6 months of therapy; 9 completed 1 year. Healing of rickets at 6 months in 9 patients was accompanied by improvement in developmental milestones and pulmonary function. Elevated plasma levels of the TNSALP substrates inorganic pyrophosphate and pyridoxal 5'-phosphate diminished. Increases in serum parathyroid hormone accompanied skeletal healing, often necessitating dietary calcium supplementation. There was no evidence of hypocalcemia, ectopic calcification, or definite drug-related serious adverse events. Low titers of anti-ENB-0040 antibodies developed in four patients, with no evident clinical, biochemical, or autoimmune abnormalities at 48 weeks of treatment. CONCLUSIONS: ENB-0040, an enzyme-replacement therapy, was associated with improved findings on skeletal radiographs and improved pulmonary and physical function in infants and young children with life-threatening hypophosphatasia. (Funded by Enobia Pharma and Shriners Hospitals for Children; ClinicalTrials.gov number, NCT00744042.).

Dose response of bone-targeted enzyme replacement for murine hypophosphatasia.

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-FcD(10), 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 8.2mg/kg for 43days. 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 ED(80) (the dose that prevents bone defects in 80% of mice) was 3.2, 2.8 and 2.9mg/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 PP(i) concentrations remained elevated in all treatment groups, indicating that while this parameter is a good biochemical marker for diagnosing HPP in patients, it may not be a good follow up marker for evaluating response to treatment when administering bone-targeted TNSALP to mice. 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.

Enzyme replacement therapy for murine hypophosphatasia.

INTRODUCTION: Hypophosphatasia (HPP) is the inborn error of metabolism that features rickets or osteomalacia caused by loss-of-function mutation(s) within the gene that encodes the tissue-nonspecific isozyme of alkaline phosphatase (TNALP). Consequently, natural substrates for this ectoenzyme accumulate extracellulary including inorganic pyrophosphate (PPi), an inhibitor of mineralization, and pyridoxal 5'-phosphate (PLP), a co-factor form of vitamin B6. Babies with the infantile form of HPP often die with severe rickets and sometimes hypercalcemia and vitamin B6-dependent seizures. There is no established medical treatment. MATERIALS AND METHODS: Human TNALP was bioengineered with the C terminus extended by the Fc region of human IgG for one-step purification and a deca-aspartate sequence (D10) for targeting to mineralizing tissue (sALP-FcD10). TNALP-null mice (Akp2-/-), an excellent model for infantile HPP, were treated from birth using sALP-FcD10. Short-term and long-term efficacy studies consisted of once daily subcutaneous injections of 1, 2, or 8.2 mg/kg sALP-FcD10 for 15, 19, and 15 or 52 days, respectively. We assessed survival and growth rates, circulating levels of sALP-FcD10 activity, calcium, PPi, and pyridoxal, as well as skeletal and dental manifestations using radiography, microCT, and histomorphometry. RESULTS: Akp2-/- mice receiving high-dose sALP-FcD10 grew normally and appeared well without skeletal or dental disease or epilepsy. Plasma calcium, PPi, and pyridoxal concentrations remained in their normal ranges. We found no evidence of significant skeletal or dental disease. CONCLUSIONS: Enzyme replacement using a bone-targeted, recombinant form of human TNALP prevents infantile HPP in Akp2-/- mice.

Human recombinant endopeptidase PHEX has a strict S1' specificity for acidic residues and cleaves peptides derived from fibroblast growth factor-23 and matrix extracellular phosphoglycoprotein.

The PHEX gene (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) encodes a protein (PHEX) with structural homologies to members of the M13 family of zinc metallo-endopeptidases. Mutations in the PHEX gene are responsible for X-linked hypophosphataemia in humans. However, the mechanism by which loss of PHEX function results in the disease phenotype, and the endogenous PHEX substrate(s) remain unknown. In order to study PHEX substrate specificity, combinatorial fluorescent-quenched peptide libraries containing o -aminobenzoic acid (Abz) and 2,4-dinitrophenyl (Dnp) as the donor-acceptor pair were synthesized and tested as PHEX substrates. PHEX showed a strict requirement for acidic amino acid residues (aspartate or glutamate) in S(1)' subsite, with a strong preference for aspartate. Subsites S(2)', S(1) and S(2) exhibited less defined specificity requirements, but the presence of leucine, proline or glycine in P(2)', or valine, isoleucine or histidine in P(1) precluded hydrolysis of the substrate by the enzyme. The peptide Abz-GFSDYK(Dnp)-OH, which contains the most favourable residues in the P(2) to P(2)' positions, was hydrolysed by PHEX at the N-terminus of aspartate with a k(cat)/ K(m) of 167 mM(-1) x s(-1). In addition, using quenched fluorescence peptides derived from fibroblast growth factor-23 and matrix extracellular phosphoglycoprotein sequences flanked by Abz and N -(2,4-dinitrophenyl)ethylenediamine, we showed that these physiologically relevant proteins are potential PHEX substrates. Finally, our results clearly indicate that PHEX does not have neprilysin-like substrate specificity.