Novel assays based on human growth hormone receptor as alternatives to the rat weight gain bioassay for recombinant human growth hormone

Abstract

Two methods, High-Performance Receptor Binding Chromatography (HPRBC) and Cell Proliferation (CP), have been developed as alternatives to the classical hypophysectomized rat weight gain bioassay for the determination of potency for recombinant human growth hormone (rhGH). In the HPRBC assay, rhGH is combined with an excess of the soluble extracellular domain of the recombinant human growth hormone receptor (referred to as 'receptor' in the discussion of the HPRBC assay). Nondenaturing size-exclusion chromatography is used to analyzed the resulting complex, which forms in a 2:1 receptor to rhGH ratio. The 2:1 complex is assayed at a concentration near the Kd (approximately 0.4 nM), providing high specificity for rhGH and detection of rhGH variants with reduced activity. In the CP assay, a mouse myeloid leukaemia cell line (FDC-P1) transfected with the full-length receptor is exposed to varying levels of rhGH for 16-20 h. The incorporation of 3H-thymidine into DNA is used as an index of cell proliferation. The results show that the HPRBC assay provides significantly improved precision with a relative standard deviation (RSD) of < or = 5% vs. an RSD of 23% for the rat bioassay. The CP assay has RSDs of 4-16%. Analysis of rhGH variants and mutants shows that the potencies measured by both the HPRBC and CP assays are in general agreement with the rat weight gain bioassay. Both of the HPRBC and CP assays are sufficiently rugged for operating in a Good Manufacturing Practices (GMP) routine batch release testing environment. In vitro alternatives such as the HPRBC and CP assays build a foundation for replacing the hypophysectomized rat weight gain bioassay by correlating receptor dimerization, binding specificity and signal transduction with the biological activity of rhGH.