Abstract
The development of new growth hormone (GH) agonists and growth hormone antagonists (GHAs) requires animal models for pre-clinical testing. Ideally, the effects of treatment are monitored using the same pharmacodynamic marker that is 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 the action of GH in humans, shows no response to treatment with recombinant human GH and there is little evidence for the effects of GHAs, except when administered at very high doses or when 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 the analysis of 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 recombinant human GH or the GHA Pegvisomant. For a subset of samples, we used LC-MS/MS to measure IGF-I, and quantitative western ligand blot to analyze IGF-binding proteins (IGFBPs). Although recovery of recombinant rabbit IGF-I was only 50% in the human IGF-I assay, our results show that the sensitivity, precision (1.7–3.3% coefficient of variation) and linearity (90.4–105.6%) were excellent in rabbit samples. As expected, sex, age and genetic background were major determinants of IGF-I concentration in rabbits. IGF-I and IGFBP-2 levels increased after single and multiple injections of recombinant human GH (IGF-I: 286±22 versus 434±26 ng/ml; P<0.01) and were highly correlated (P<0.0001). Treatment with the GHA lowered IGF-I levels 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 treatment with recombinant human GH or a GHA closely mimics the pharmacodynamics seen in humans, suggesting that rabbits are a suitable new model to test human GH agonists and antagonists.
Highlights
The development of pharmacological therapies to treat disorders of the growth hormone and insulin-like growth factor (GH-IGF) system allows better disease control and improves quality of life for the affected individuals
Single and multiple injections of recombinant human GH increased IGF-I concentrations, whereas treatment with a growth hormone antagonists (GHAs) lowered IGF-I in rabbits. In these animals, unlike in rodents, the IGF-I response to treatment with recombinant human GH or a GHA closely mimics the pharmacodynamics seen in humans
The possibility to measure IGF-I in rabbits, together with the human-like behavior of IGF-I that was detected in response to recombinant human GH and GHA, will aid the development of novel therapeutics to treat pathological conditions of the GH-IGF system
Summary
The development of pharmacological therapies to treat disorders of the growth hormone and insulin-like growth factor (GH-IGF) system allows better disease control and improves quality of life for the affected individuals. In GH excess and GH insufficiency, the circulating concentration of IGF-I is considered to be the most important biomarker for diagnosis and monitoring of the disease (Clemmons, 2007; Keller et al, 2007). GH replacement therapy aims to normalize endogenous IGF-I concentrations, whereas effective treatment in individuals with acromegaly is reflected by decreasing IGF-I concentrations (Kargi and Merriam, 2013; Katznelson et al, 2011). Some studies suggest that circulating IGF-binding proteins (IGFBPs) are sensitive markers for monitoring the disease and the treatment (Blum et al, 1993; Zapf et al, 1990). Because IGFBPs are determinants of serum IGF-I half-life, their measurement has been proposed to provide additional information in intervention studies (Metzger et al, 2011)
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