Stereoselective metabolism of benoxaprofen in rats. Biliary excretion of benoxaprofen taurine conjugate and glucuronide.

Benoxaprofen (BOP) was administered iv to bile duct-cannulated rats at a dose of 10 mg/kg. BOP and its metabolites in plasma, urine, and bile were quantified using HPLC. A previously unidentified BOP metabolite was found in HPLC chromatograms of rat bile, and the metabolite was isolated chromatographically. Positive-ion fast-atom bombardment (FAB) MS analysis of the compound showed [M+H]+ at m/z 409, i.e. 108 mass units greater than the molecular weight of BOP (301 mass units). In the 1H NMR spectrum of the compound, two signals assigned to two methylene groups appeared at 2.53 ppm and 3. 30 ppm, in addition to BOP signals. Analysis of FAB mass spectra and 1H-1H and 1H-13C correlated NMR spectra of the isolated metabolite suggested that the new metabolite was a BOP taurine conjugate (BOP-T). A BOP-T standard was chemically synthesized, and physicochemical data were compared with those for the isolated metabolite. Identical results, i.e. RF values from TLC, RT values from HPLC, and FAB MS and 1H-13C correlated NMR findings, were obtained, establishing that the new metabolite found in rat bile was BOP-T. In five rats, mean values for per cent excretion of the dose in bile over 12 hr for BOP glucuronide (BOP-G), BOP-T, and unchanged BOP were 13.2 +/- 2.3, 2.54 +/- 0.80, and 0.33 +/- 0.09%, respectively. Furthermore, the optical isomers of BOP and its metabolites in plasma and bile were analyzed using a chiral HPLC column. (R)-BOP showed rapid plasma elimination, whereas the plasma elimination of (S)-BOP was very slow. The amounts of BOP, BOP-G, and BOP-T enantiomers excreted into the bile were as follows: (S)-BOP-G and (R)-BOP-G, 12.5 +/- 1.8 and 2.1 +/- 0.6% of the dose; (R)-BOP-T and (S)-BOP-T, 2.0 +/- 0.6 and 0.3 +/- 0.05% of the dose; (R)-BOP and (S)-BOP, 0.02 +/- 0.03 and 0.2 +/- 0.1% of the dose, respectively. (S)-BOP was metabolized mainly to BOP-G, and BOP-T excreted into the bile was produced mainly from (R)-BOP.