KINETICS OF SEQUENTIAL METABOLISM .2. FORMATION AND METABOLISM OF NORDIAZEPAM AND OXAZEPAM FROM DIAZEPAM IN THE PERFUSED MURINE LIVER

Pharmacokinetic theory dictates that the extent of ensuing metabolism of a formed metabolite during drug transit through the liver is influe nced by the number of consecutive reactions required for its genesis a nd the total intrinsic clearances of the precursors. This hypothesis w as tested in the perfused murine liver by examining the successive con version of the precursor diazepam (DZ) to its primary metabolite nordi azepam (NZ), and then the secondary metabolite oxazepam (OZ) and, fina lly, the tertiary metabolite, the oxazepam glucuronides. The concomita nt C3-hydroxylation of DZ to temazepam, which can also be N-demethylat ed to form OZ, was minimal. The hepatic extraction ratios of NZ (E{NZ, DZ}) and OZ (E{OZ,DZ}) after administration of [C-14]DZ were compared to those obtained previously from [C-14]NZ (E{NZ} and E(OZ,NZ}) and [H -3]OZ (E{OZ}). The ability of three hepatic clearance models, the well -stirred, parallel-tube and dispersion models, to predict the experime ntal E{NZ,DZ} and E{OZ,DZ} was evaluated. DZ was highly extracted by t he murine liver (E{DZ} = 0.95). The metabolism of NZ, generated in sit u from DZ, was greater than that of preformed NZ(E{NZ,DZ} = 0.51; E{NZ } = 0.4), whereas E{OZ,DZ} (0.066) was similar to E{OZ,DZ} (0.056) and less than E{OZ} (0.125). The unexpected observation of E{NZ,DZ} > E{N Z} may be explained by the coupling of N-demethylation and C3-hydroxyl ation/glucuronidation reactions or by a sequestration of hydrophobic s ubstrates within the enzymic space, favoring sequential metabolism of products formed in situ. The atypical kinetic behavior of generated NZ may have also influenced the ensuing metabolic fate of its product, O Z, such that E{OZ,NZ} almost-equal-to E{OZ,DZ}. The data E{OZ,DZ} almo st-equal-to E{OZ,NZ} < E{OZ} support the contention that the extent of sequential metabolism recedes as the number of generation steps incre ases. Both the parallel-tube and dispersion models adequately predicte d this finding, but the well-stirred model did not. The phenomenon of coupling of enzymatic reactions or definition of an enzymic space need s to be investigated further.