DETERMINATION OF AN ENDOTHELIN RECEPTOR ANTAGONIST IN HUMAN PLASMA BYNARROW-BORE LIQUID-CHROMATOGRAPHY AND IONSPRAY TANDEM MASS-SPECTROMETRY

A method is described for the determination of a new endothelin recept or antagonist, Bosentan Ro 47-0203, in human plasma using narrow-bore liquid chromatography and ionspray tandem mass spectrometry. After pro tein precipitation with acetonitrile, the compounds were extracted wit h dichloromethane at pH 11. The compounds were chromatographed on a 2 mm I.D. reversed-phase column and introduced into the mass spectromete r with an ionspray (pneumatically assisted electrospray) interface at a Sow-rate of 170 mu l/min without postcolumn splitting. Two different internal standards were used for the assay: either a structural analo gue or a deuterated analogue. The limit of quantification was 0.5.ng/m l using a 0.5-ml aliquot of plasma. Concentrations of the drug were de termined in the range 0.5-200 ng/ml. The recovery from human plasma wa s 87%. The new API IIIplus collision cell was about five times more se nsitive than the original API III cell. The assay was demonstrated to be sensitive, selective and robust for the analysis of over 1500 sampl es.