I. Lemire, L. Blond, T. Loisel, G. Boileau, J. Millan, P. Léonard, P. Crine

Hypophosphatasia (HPP) is a heritable form of rickets or osteomalacia caused by deficient activity of the tissue nonspecific isozyme of alkaline phosphatase (TNSALP). Severity is inversely related to the age at symptom onset. Since no established medical therapy for HPP is presently available, we have designed a soluble recombinant form of TNSALP (ENB-0040) as a potential drug for enzyme replacement therapy. ENB-0040 comprises the catalytic domain of the human TNSALP, human immunoglobulin Fc region and a deca-aspartate peptide to target the enzyme directly to bone. We have used the Akp2^-/- mouse model to evaluate the effect of systemic administration of ENB-0040 on skeletal defects and survival of HPP mice. We have previously shown that daily subcutaneous (SC) administration of ENB-0040 prevents bone mineralization defects of the feet in Akp2^-/- mice¹. We have now evaluated the dose response relationship of ENB-0040 SC administration on survival and bone mineralization defects of the feet, rib cages and pelvic limbs. Newborn Akp2^-/- mice received daily SC injection of ENB-0040 at 0.5, 2.0 and 8.2 mg/kg for 43 days. Primary endpoint was presence of mineralization defects; secondary endpoints included mortality, body weight and femur and tibia length. Bone mineralization defects were evaluated by radiographs and classified in a blinded fashion by a qualified veterinarian radiologist. Animals were classified as Abnormal if at least one bone structure including secondary ossification center was absent. There was a clear relationship between administered daily dose of ENB-0040 and mineralization defects of feet, rib cages and pelvic limbs with r²=0.993, 0.986 and 0.999, respectively. According to the dose response model, the ED₅₀, the dose effective in normalizing the bone defects in 50% of mice, was 0.9, 0.8 and 0.5 mg/kg/day for feet, rib cages and pelvic limbs. Long bones seem to respond to lower daily doses of ENB-0040. There was also a clear relationship between dose and survival with r²=0.989. Median survival was improved from 19 days in Vehicle-treated Akp2^-/- mice to 24, 31 and >44 days with increasing doses of ENB-0040. Body weight and bone length were also improved in a dose dependent manner. These dose response relationships strongly support the pharmacological activity of ENB-0040 and provide support for the estimation of the range of human effective doses.
1. Millán et al. (2008) J Bone Miner Res 23 :777-787.

Disclosures: P. Crine, Enobia Pharma: Employment (full or part-time). J. Millan, Enobia Pharma: Consulting Fees or Other Remuneration. T. Loisel, Enobia Pharma: Employment (full or part-time). P. Léonard, Enobia Pharma: Employment (full or part-time). I. Lemire, Enobia Pharma: Employment (full or part-time). L. Blond, Enobia Pharma: Consulting Fees or Other Remuneration. G. Boileau, Enobia Pharma: Consulting Fees or Other Remuneration.