Validation of the IDS-iSYS IGF-I automated immunoassay for measurement of serum IGF-I in rabbits

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: SAT 109-133-GHRH, GH & IGF Biology & Signaling
Bench to Bedside
Saturday, June 15, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board SAT-130
Maximilian Bielohuby*1, Jenny Manolopoulou2, Amon Horngacher3, Sarina Meurer4 and Martin Bidlingmaier1
1Ludwig-Maximilians University, Munich, Germany, 2Immunodiagnostic Systems Ltd., Boldon, United Kingdom, 3Ludwig-Maximilians University, Munich, 4Ludwig-Maximilians University
Rabbits have been proposed as a potential animal model for research of novel Growth hormone (GH) agonists and antagonists. Measurement of circulating IGF-I, an important pharmacodynamic marker of GH action, has not been validated for rabbit serum on an automated system. We investigated if the automated iSYS IGF-I immunoassay (IDS, , Boldon, UK), widely used in routine diagnostics can also be used to reliably measure rabbit IGF-I.

Methods: 30 native rabbit serum samples of both sexes, 13-weeks old, with different genetic backgrounds (New Zealand White (NZW) and Chinchilla Bastard (CB)) were purchased (Charles-River, Germany). Custom-made, recombinant rabbit IGF-I (rrIGF-I, analytical purity >95%; monomer content >90%) was obtained from PLR (Protein Laboratories Rehovot, Israel). A validation of precision, sensitivity, linearity, and recovery in rabbit serum samples was carried out.

Results: The intra-assay variability of low, medium and high IGF-I rabbit samples (2 samples each) measured in 10 replicates was 2.55, 2.25 and 1.49% respectively. The inter-assay variability from those 6 samples measured in 5 different assay runs was found to be 2.4%. A functional sensitivity of 10ng/ml was confirmed in rabbit samples. Linearity of the assay in native rabbit serum across the range 504-32ng/ml was excellent with a mean observed/expected of 102.9%. Recovery of IGF-I in rabbit samples was assessed by spiking either recombinant human IGF-I (Increlex®, Ipsen) or rrIGF-I. Spiking of rabbit serum with Increlex showed a mean recovery rate of 110.4% (range: 103.7-121.3%). The recovery of rrIGF-I was only 33.9% (range: 29.7-41%), and did not differ when spiking either human or rabbit serum with the rrIGF-I.

Analysis of samples from different rabbit strains showed significantly higher IGF-I concentrations in CB vs. NZW rabbits (p<0.01 in males and p<0.05 in females). In both strains, female rabbits displayed significantly higher IGF-I concentrations (male NZW: 200.6±9ng/ml; female NZW: 285.8±25.9ng/ml; male CB: 265.7±16.3ng/ml; female CB: 361.2±20.8ng/ml).

In summary, our preliminary data indicate that the IDS-iSYS automated IGF-I assay can also be used to measure circulating IGF-I in rabbit serum. The lower recovery for rrIGF is most likely due to a reduced affinity of the antibodies used in the iSYS assay which were raised against human IGF-I. Furthermore, similar to rodents, rabbits also display variations in IGF-I concentrations depending on sex and genetic background.

Disclosure: JM: Employee, Immunodiagnostic Systems. MB: Consultant, Immunodiagnostic Systems. Nothing to Disclose: MB, AH, SM

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm

Sources of Research Support: This study received support from IDS, Boldon, UK