Confirmatory analysis of etanercept in equine plasma by LC-MS for doping control

Etanercept is a protein-based medication for the treatment of human patients with rheumatoid arthritis and other autoimmune-based diseases; its pharmacological action is to inhibit and antagonize tumour necrosis factor alpha. Etanercept was rumoured to be used in horse racing in North America. To detect such use, the aim of this study was to develop a liquid chromatography-mass spectrometry (LC-MS) method for confirmation of etanercept in equine plasma. Etanercept was extracted from plasma by anti-human IgG antibody linked to magnetic beads. The analyte was reduced and alkylated, and then digested by trypsin. Tryptic peptides (T1 from human tumour necrosis factor receptor 2 of etanercept, T15 and T27 from human IgG1 of the protein) were employed for detection and confirmation of the analyte. The limit of detection was 5 ng/mL, and the limit of confirmation 10 ng/mL. This method is specific for confirmation of etanercept, as assessed using the results from BLAST and SEQUEST searches. The results from SEQUEST searches also revealed an unexpected unique specificity of product ion spectrum of IgG1 T27 with only a single product ion for identification of etanercept. It is the first report for such a finding, to the authors' knowledge. The method was successful in analyses of the plasma samples collected post administration of etanercept to horses. Etanercept was detected up to 11 days post administration. This method will be helpful for confirmation of etanercept or other protein-based drugs consisting of human IgG1, in equine drug testing. Copyright © 2016 John Wiley & Sons, Ltd.