KINETICS OF SEQUENTIAL METABOLISM .1. FORMATION AND METABOLISM OF OXAZEPAM FROM NORDIAZEPAM AND TEMAZEPAM IN THE PERFUSED MURINE LIVER

In murine liver, temazepam (TZ) and nordiazepam (NZ) are mainly metabo lized via N-demethylation and C3-hydroxylation, respectively, to form a common metabolite, oxazepam (OZ), which is then glucuronidated. With these precursors, we tested the hypotheses that the sequential metabo lism of a primary metabolite (OZ) is less than that of the preformed m etabolite and is dependent on the effective intrinsic clearance (unbou nd fraction x intrinsic clearance) of its precursor, as predicted by t he parallel tube and dispersion models of hepatic drug clearances. Mou se livers were perfused with tracer concentrations of [C-14]-NZ, [C-14 ]-TZ and [H-3]NZ in a single-pass fashion (2.5 ml/min). The steady-sta te extraction ratio (E) of [H-3]NZ, [C-14]NZ and [C-14]TZ were 0.29, 0 .40 and 0.49, respectively (P < .01), whereas the fractional metabolis m (formation rate/total elimination rate of drug) of [H-3]-NZ, [C-14]N Z and [C-14]TZ to form OZ was 0.39, 0.79 and 0.68, respectively. Value s of E of [H-3]NZ and [C-14]NZ and fractional metabolism for OZ format ion had differed because of a kinetic isotope effect (around 3.5) that affected the C3-hydroxylation of [H-3]NZ. The extraction ratios of OZ (E{OZ,P}) arising from [C-14]-NZ and [C-14]TZ were both 0.056, and we re less than that for preformed OZ (E{OZ}), previously found to be 0.1 25. The parameter E{OZ,P} was poorly correlated with the extraction ra tio of the precursor, was overestimated by the parallel tube and dispe rsion models, but was highly correlated with the effective intrinsic c learance of the precursor (unbound fraction x intrinsic clearance). Th e identical E{OZ,P} with both [C-14]NZ and [C-14]TZ precursors may be explained by the stereochemical nature of the reactions and/or by acin ar heterogeneity of enzymes. The C3-hydroxylation of prochiral NZ pred ominantly forms (S)-OZ, the favored enantiomer for glucuronidation, wh ereas OZ generated from (racemic) TZ is likely an equimixture of (R) a nd (S)-OZ. The overestimation of E{OZ,P} based on an even distribution of enzymatic activities may be ameliorated with a more posterior dist ribution of oxidative enzymes (zone 3) and an evenly distributed glucu ronidation system.