THE PROBLEM OF DRUG APPLICATION IN COMPET ITION HORSES FROM THE VIEW OF THE LABORATORY

The detection and identification of pharmacologically active substance s is done by chemical analysis methods, where mainly gas chromatograph y and mass spectrometry are used. With exception of a few compounds, o nly qualitative detection is performed and a case is determined positi ve independent of the concentration. From the analytical data usually no conclusion is possible as to when and in which amount a substance h as been applied. How long before a competition should the application be ceased? Indications given from excretion studies can only be used a s a guide line and should be interpreted very carefully. The detection time of a substance is mainly influenced by the elimination kinetics and the sensitivity of the analytical method. There is a lot of indivi dual variation, in which the amount, the kind and the duration of an a pplication are important factors. In addition the elimination is influ enced by condition, health status and age of the horse as well as by t he pH of the urine. The detection period of a substance can be extende d if the method of detection and sample preparation are improved. This is required for compounds which are difficult to detect, as they are are applied in very low concentrations. The two substances phenylbutaz one and isoxsuprine are presented as an example for metabolism and eli mination. Phenybutazone, belonging to the group of nonsteroidal antiin flammatory drugs, is mainly excreted unmetabolized. Main metabolites a re the pharmacologically active oxyphenbutazone and the unactive g-hyd oxy-phenylbutazone metabolite. The halflife time of phenylbutazone was determined as 4.3 hours and increased in correlation to the dose. Iso xsuprin is mainly excreted as its glucuronid conjugate. It could be de tected in plasma 72 hours after a single dose. The detection time in u rine is much longer.