Detection of Eltenac in the Horse: Identification of Phase I Metabolites in Urine by Capillary Gas Chromatography-Mass Spectrometry and the Determination of Excretion Profile by Liquid Chromatography-Mass Spectrometry

Telzenac® (Eltenac; 0.5 mg kg−1) was administered intravenously to two thoroughbred horses. After initial alkaline saponification followed by enzymolysis of the urinary phase II conjugates, the combined unconjugated compounds and aglycones were isolated by mixed mode solid phase extraction (SPE). The acidic isolate was either methylated or silylated (trimethylsilyl ether, TMS) and analysed by positive ion electron ionisation gas chromatography-mass spectrometry (GC-EI+-MS). Eltenac and two isobaric metabolites, hydroxyeltenac (aromatic oxidation) and eltenac sulfoxide were tentatively identified. Base peaks in the EI+ spectra of underivatised, methylated and TMS derivatised eltenac are formed by an initial loss of H2O, CH3OH or (CH3)3-Si-OH respectively, followed by successive losses of a chlorine atom and a carbonyl group. Similar fragmentation patterns were observed for the methyl and TMS derivatives of the two metabolites. Triamcinolone acetonide was used as the internal marker for the semi-quantification of eltenac. Selected samples were base-hydrolysed and extracted on-line on a C2 SPE column using a Prospekt sample handler. The retained analytes were eluted directly on to an analytical LC column and analysed by high performance liquid chromatography positive ion atmospheric pressure chemical ionisation MS in the selective ion recording mode. Most of the drug was excreted in less than 24 h. However it could still be detected in urine by full-scan GC-EI+-MS for over 96 h.