AUTOMATED GAS-CHROMATOGRAPHY TANDEM MASS-SPECTROMETRY ASSAY FOR TEBUFELONE AND A C-13, O-18-LABELED ANALOG IN PLASMA - APPLICABILITY TO ABSOLUTE BIOAVAILABILITY DETERMINATION

An automated capillary gas chromatography/tandem mass spectrometry (GC /MS/MS) assay for the simultaneous quantitation of tebufelone (TE) and C-13,O-18-labeled TE (TE-CO) in plasma was developed. This method per mits the use of stable isotope coadministration (TE and TE-CO dosed co ncurrently via peroral and intravenous routes, respectively) for the d etermination of TE absolute bioavailability. The selectivity of MS/MS conducted on a triple-quadrupole instrument allowed minimal sample pre paration and rapid analysis. Electron ionization produced molecular io ns (M(+.)) for TE, TE-CO, and the 3-methyl-TE internal standard, which were selected in Q1 to undergo collisionally activated dissociation i n Q2. Quantitation was achieved through monitoring product ions at m/z 248, 251, and 248, respectively, in Q3. A 2-1000 ng per sample (40 pg to 20 ng injected) quantitation range provided access to an effective 1-5000 p.p.b. plasma concentration range (0.2-2 g samples) for both T E and TE-CO. The assay showed no bias and less than 8% relative standa rd deviation over the entire range. The method was used to determine p lasma levels of TE and TE-CO in four dogs receiving 2.5:2.5 mg/kg TE:T E-CO, intravenously. The pharmacokinetics of both isotopomers proved t o be identical, indicating no isotope effect and verifying the chemica l stability of the O-18-carbonyl label under these dosing conditions. In addition, the applicability of this analytical approach to the dete rmination of TE peroral bioavailability was initially tested in dogs.