Validation of serum IGF-I as a biomarker to monitor exogenous growth hormone agonist and antagonist bioactivity in rabbits

Abstract

Abstract Development of new growth hormone (GH) agonists and antagonists (GHA) requires animal models for pre-clinical testing. Ideally, effects of treatment can be monitored using the same pharmacodynamic marker later used in clinical practice. However, intact rodents are of limited value for this purpose because serum IGF-I - the most sensitive pharmacodynamic marker for GH-action in humans - shows no response to recombinant human GH (rhGH) treatment and there is little evidence for effects of GHA except when administered at very high doses or overexpressed. As an alternative, more suitable model we explored pharmacodynamic markers of GH action in intact rabbits. We performed the first validation of an IGF-I assay for rabbit serum and tested precision, sensitivity, linearity, and recovery using an automated human IGF-I assay (IDS-iSYS). Furthermore, IGF-I was measured in rabbits of different strains, age groups and sexes, and we monitored IGF-I response to treatment with rhGH or GHA. In a subset of samples we used LC-MS/MS to measure IGF-I and quantitative Western-ligand blot to analyze IGF-binding proteins. Results: Although recovery of recombinant rabbit IGF-I was only 50% in the human IGF-I assay, sensitivity, precision (1.7-3.3%CV) and linearity (90.4-105.6%) were excellent in rabbit samples. As expected, sex, age and genetic background were major determinants of IGF-I in rabbits. IGF-I and IGFBP-2 levels increased after single and multiple rhGH injections (IGF-I: 286±22 vs. 434±26ng/ml; p<0.01) and were highly correlated (p<0.0001). GHA treatment lowered IGF-I from the fourth injection onwards (p<0.01). In summary, we demonstrated that the IDS-iSYS IGF-I immunoassay can be used in rabbits. Similar to rodents, rabbits display variations in IGF-I depending on sex, age and genetic background. Unlike in rodents, the IGF-I response to rhGH or GHA treatment closely mimics the pharmacodynamics seen in humans suggesting rabbits as a suitable new model to test human GH agonists and antagonists.