Spectral interaction of orphenadrine and its metabolites with oxidized and reduced hepatic microsomal cytochrome P-450 in the rat
Abstract
Product inhibition has been suggested to be a determinant in orphenadrine pharmacokinetics. Two possibilities for the mechanism of product inhibition in orphenadrine metabolism are explored in this study. Orphenadrine and its metabolites may compete for cytochrome P-450 catalytic binding sites. Therefore the interaction of orphenadrine and some of its metabolites with hepatic microsomal ferricytochrome P-450 of the rat was investigated. The spectral dissociation constant for the type I (substrate) interaction of orphenadrine and its metabolites displayed no relationship with the lipophilicity of the compounds. Orphenadrine is only partially displaced from its cytochrome P-450 binding sites by its respective metabolites. For this mechanism to be significant in vivo. the metabolites need to reach concentrations near cytochrome P-450 similar to that of orphenadrine. This is not known yet. The significance of this mechanism for the product inhibition phenomenon is therefore uncertain. In this study it is also established that during both in vitro as well as in vivo metabolism of orphenadrine, a metabolic intermediate is formed, which binds irreversibly to ferrous-cytochrome P-450 (MI complex). In vitro, both the rate and extent of the MI complex formation with orphenadrine and metabolites as precursor, decreased in the order N-hydroxytofenacine >; tofenacine > orphenadrine > bisnororphenadrine. The metabolite orphenadrine-N-oxide did not produce an MI complex, in vitro. Furthermore. in vitro, it was shown that the N-demethylation of tofenacine paralleled the concomitant MI complex formation. Together, the data suggest that the first N-demethylation step of orphenadrine occurs via α-carbon oxidation, whereas the second N-demethylation step mainly comes about via N-oxidation. Both metabolic pathways eventually lead to the MI complex forming species. These two parallel pathways also account for the complicated substrate dependency and concentration dependency in MI complex formation. Finally, the formation of the nitroxide radical (the ultimate ligand for MI complexation) has been shown to be susceptible to inhibition by its precursors.
The occurrence of MI complex formation resulting in metabolic inactive cytochrome P-450 is probably the main mechanism for the product inhibition phenomenon in orphenadrine metabolism.
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Effects of intravenous infusion of lidocaine on its pharmacokinetics in conscious instrumented dogs
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Reductase and oxidase activity of rat liver cytochrome P450 with 2,3,5,6-tetramethylbenzoquinone as substrate
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