Establishing the cut-off concentration for the detection of etorphine in horse urine

An 125I radioimmunoassay to determine the pattern of urinary excretion of etorphine (a semisynthetic opiate agonist) after its administration to horses is described. Three thoroughbred horses were each given 5, 15, 30 and 100 micrograms of etorphine intramuscularly. Urine was collected for up to 72 after administration. The maximum etorphine concentration after administration of a dose of 5 micrograms was 711 pg ml-1 (concentrations were greater than 100 pg ml-1 after 23 h in all three horses); a 15 micrograms gave 2661 pg ml-1 (levels remained above 100 pg ml-1 for more than 44 h in each horse); a 30 micrograms dose gave a maximum of 3344 pg ml-1 (levels were above 100 pg ml-1 for 24, 72 and 72 h); and 100 micrograms gave in excess of 10,000 pg ml-1 (levels were greater than 300 pg ml-1 for up to 70 h). Forty-eight urine samples from horses not given etorphine all had levels of etorphine less than 100 pg ml-1. There was no increase in apparent etorphine concentrations after hydrolysis of samples with beta-glucuronidase and aryl sulfatase. The half-lives of etorphine equivalents (calculated with a mono-exponential equation after the 100 micrograms dose) in the urine of the three horses were 569, 803 and 821 min, respectively. We conclude that radioimmunoassay can provide a useful first line screening procedure for the assessment of etorphine use in racing horses.