Kinetics of sequential metabolism. I. Formation and metabolism of oxazepam from nordiazepam and temazepam in the perfused murine liver.

In murine liver, temazepam (TZ) and nordiazepam (NZ) are mainly metabolized 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 metabolism of a primary metabolite (OZ) is less than that of the preformed metabolite and is dependent on the effective intrinsic clearance (unbound fraction x intrinsic clearance) of its precursor, as predicted by the parallel tube and dispersion models of hepatic drug clearances. Mouse livers were perfused with tracer concentrations of [14C]-NZ, [14C]-TZ and [3H]NZ in a single-pass fashion (2.5 ml/min). The steady-state extraction ratio (E) of [3H]NZ, [14C]NZ and [14C]TZ were 0.29, 0.40 and 0.49, respectively (P < .01), whereas the fractional metabolism (formation rate/total elimination rate of drug) of [3H]-NZ, [14C]NZ and [14C]TZ to form OZ was 0.39, 0.79 and 0.68, respectively. Values of E of [3H]NZ and [14C]NZ and fractional metabolism for OZ formation had differed because of a kinetic isotope effect (around 3.5) that affected the C3-hydroxylation of [3H]NZ. The extraction ratios of OZ (E[OZ,P]) arising from [14C]-NZ and [14C]TZ were both 0.056, and were less than that for preformed OZ (E[OZ]), previously found to be 0.125. The parameter E[OZ,P] was poorly correlated with the extraction ratio of the precursor, was overestimated by the parallel tube and dispersion models, but was highly correlated with the effective intrinsic clearance of the precursor (unbound fraction x intrinsic clearance).(ABSTRACT TRUNCATED AT 250 WORDS)