Effects of phenylbutazone and oxyphenbutazone on acidic drug detection in high performance thin layer chromatographic systems

Interference or "masking" in thin layer chromatography occurs when the presence of one drug on a thin layer plate physically obscures or interferes with the detection of another drug. We investigated the ability of phenylbutazone and oxyphenbutazone to mask or interfere with the detection of acidic drugs of high performance thin layer chromatography. Of 20 acidic drugs called "positive" since 1981 by laboratories affiliated with the Association of Official Racing Chemists, 16 did not comigrate with phenylbutazone or oxyphenbutazone and could not, therefore, be masked by these agents. Three medications (diclofenac, fenoprofen, ibuprofen) were potentially masked by phenylbutazone and one (sulindac) was potentially masked by oxyphenbutazone. These agents were therefore administered to horses to determine whether or not their metabolites would allow their detection. In each case, metabolites of these agents were detectable for at least 24 hr after drug administration and detection was not interfered with by phenylbutazone or oxyphenbutazone. These results suggest that these 20 acidic drugs should be readily detectable in postrace urines of horses in the presence of phenylbutazone either as the parent drug or by virtue of the easily distinguishable metabolites that each agent possesses. There is, therefore, no reason to believe that the agents tested in this study can be effectively masked or interfered with by phenylbutazone or its metabolites in equine urine.