Stereospecificity and stereoselectivity of flobufen metabolic profile in male rats in vitro and in vivo: phase I of biotransformation.

Flobufen (F) is the original nonsteroidal antiinflammatory drug (NSAID) containing two enantiomers. The aim of this investigation was to elucidate the biotransformation pathway of F at chiral level in phase I of biotransformation. Stereoselectivity and stereospecificity of the respective enzymes were studied in male rats in vitro (microsomal and cytosolic fractions, hepatocytes suspension) and in vivo. The rac-F, (+)-R-F and (-)-S-F were used as substrates. Amounts of F enantiomers, 4-dihydroflobufen diastereoisomers (DHF) and other metabolites (M-17203, UM) were determined with a chiral HPLC method in two chromatographic runs on R,R-ULMO and allyl-terguride bonded columns. Stereoselective biotransformation of the two enantiomers of F was observed at all tested levels and significant bidirectional chiral inversion of enantiomers of F was observed in hepatocytes. Mean enantiomeric ratios of F concentrations (S-/R-), after rac-F incubations, ranging from 1.09 in cytosolic fraction to 18.23 in hepatocytes. Stereospecificity of the respective F reductases was also observed. (2R;4S)-DHF and (2S;4S)-DHF are the principal metabolites of F in microsomes and hepatocytes. Neither DHF diastereoisomers nor M-17203 were found in cytosolic fraction. Only the nonchiral metabolite, M-17203, was found in all urine and feces samples after oral administration of F.