MON-537 Fusion of Parathyroid Hormone to Growth Hormone Binding Protein Delays Clearance Whilst Retaining Biological Activity

Parathyroid hormone (PTH) is an 84 aa peptide and biological activity resides in residues 1-34. PTH 1-84 (Natpara) is licensed for treatment of hypoparathyroidism replacement but requires daily injections and is complicated by fluctuating calcium levels. There is an unmet need for a long acting PTH that provides constant physiological levels of PTH bioactivity. We have tested whether fusions of human PTH 1-34 to growth hormone binding protein (GHBP) could generate long acting PTH molecules. Two PTH fusion molecules were constructed: Fusion-1 consists of PTH (1-34) linked to GHBP (residues 1-238), and Fusion-2 consists of human PTH (1-34) linked to the N-terminal PTH receptor domain (PTHrExt, residues 29-187) and GHBP (residues 1-238). Fusions were expressed as secreted products from a CHO cell line and purified using a combination of ion exchange and affinity chromatography. In vitro bioactivity was measured using a Dual Luciferase Reporter Assay (DLRA), using the rat osteoblast-like cell line, UMR-106. Both fusions showed biological activity when compared to PTH 1-34 with an EC(50) nM (mean ± SD) for PTH 1-34 of 40 ± 15.66, Fusion-1, 151 ± 5.85, & Fusion-2, 721 ± 307. PTH was 3.8-fold more active than Fusion-1 and 18-fold more active than Fusion-2; although, Fusion-2 showed a consistently higher maximal fold induction compared to both PTH and Fusion-1. A pharmacokinetic (PK) study for Fusion-1 was conducted in Crl:WI(Han) male rats (n = 6) by administering a single subcutaneous bolus injection at 10 nmol/kg. Serum samples were analysed for the presence of Fusion-1 using an in-house Elisa. A Cmax (mean ± SD) of 1.16 ± 0.36 nM was observed at 8 hrs with a serum half-life of 36 hrs. In conclusion, a PTH fusion molecule with GHBP showed bioactivity and delayed clearance. The reduced bioactivity of Fusion-2 compared to PTH and Fusion-1 is compatible with our hypothesis that there is intra-molecular binding to the extracellular domain of the PTH receptor which may result in a store of bioactivity and may explain the greater maximal fold induction in the bioassay despite a reduced EC(50). The published serum half-life of PTH 1-34 in rats is 59 minutes(1) and so Fusion-1 had greatly delayed clearance. These results validate the approach of generating a long-acting PTH through protein fusion with GHBP. 1. Kostenuik, PJ et al. Journal of Bone and Mineral Research, Volume 22, Number 10, 2007 Sources of Research Support: MRC P2D