GLC determination of ethinamate and its hydroxy derivative in biological fluids

A sensitive, specific GLC assay was developed for the determination of ethinamate in plasma and its major metabolite, trans-4-hydroxyethinamate, in urine. The assay uses a mass internal standard of dimethylethinamate. Ethinamate is extracted from alkalinized plasma with dichloromethane. Urine samples require beta-glucuronidase hydrolysis prior to extraction of hydroxyethinamate. The dichloromethane is removed by evaporation, and the compounds are measured by GLC using a flame-ionization detector. By using GLC-chemical-ionization mass spectrometry, the compounds measured were identified as the intact ethinamates. Plasma and urine data are presented from a bioavailability study to demonstrate the utility of this method. From these data, the ethinamate plasma half-life was calculated as 1.9 +/- 0.3 hr.