Preparation, gas chromatography and mass spectrometry of methyl and trimethylsilyl esters of indomethacin.

The preparation and gas chromatographic-mass spectrometric behavior of the methyl and trimethylsilyl esters of indomethacin, 1-(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic acid, are described. Reaction of this anti-inflammatory drug with diazomethane or bis(trimethylsilyl)acetamide forms the expected esters. Derivatization with dimethylformamide dimethylacetal yields two compounds, the methyl ester (major product) and a methyl ester-dimethylaminomethylene condensation (at the alpha-carbon of the side chain) product (minor). Experiments with 5-O-desmethyl-indomethacin have demonstrated that using the described diazomethane methylation conditions no alkylation of the phenolic group occurs. Esterification combined with an isolation procedure allows the determination of indomethacin levels in plasma and aqueous humor of rabbits, the 4-fluorobenzoyl analog serving as internal standard. The derivatives exhibit excellent electron capture properties allowing quantitative assay of the drug at the submicrogram level. Precision and accuracy for plasma samples varied from 92 +/- 19% (5 ng/ml) to 96 +/- 1.5% (1000 ng/ml). The analogous values for aqueous humor are superior: 97 +/- 5.6% and 99 +/- 2.2%, resectively.