Disposition of heptabarbitone in the rat: identification of a new metabolite by tandem mass spectrometry.

The purpose of this study was to elucidate the structure of a metabolite of heptabarbitone, the occurrence of which was reported previously in a number of pharmacokinetic and pharmacodynamic modelling studies. By application of thermospray liquid chromatography (tandem) mass spectrometry, the identity of the metabolite was proposed to be 5-ethyl-5-(1',[3' or 6']-cycloheptadienyl)-barbituric acid. By measuring the ratios between the areas under the concentration time curves of the metabolite and heptabarbitone after administration of three different intravenous dosages of heptabarbitone, it was shown that the exposure to the metabolite is directly correlated with the exposure of heptabarbitone.