The metabolism of albendazole (ABZ), a benzimidazole anthelminthic, was studied in either microsomal preparations of human liver biopsies or cultured human hepatoma cell lines. Metabolites were analyzed by HPLC. Our data show that microsomes from human biopsies and two human cell lines, HepG2 and Hep3B, oxidize the drug to the sulfoxide very efficiently, whereas the third cell line tested, SK-HEP-1, does not. Both cytochrome P-450 dependent monooxygenases and flavin-containing monooxygenases appear to be involved in human ABZ metabolism. Using the cell line displaying the highest ABZ-metabolizing activity, HepG2, the cytotoxic and the inducing effects of the parent drug ABZ and of two primary metabolites, the sulfoxide and the sulfone were studied. These three chemicals provoked a rise in mitotic index resulting from cell division blockage at the prophase or at the metaphase (ABZ metabolites) stage, and ABZ was more cytotoxic than its metabolites. With regard to enzyme-inducing effects, our data clearly demonstrate that the sulfoxide and, to a lesser degree, the sulfone are potent inducers of some drug metabolizing enzymes (i.e., cytochrome P-488 dependent monooxygenases and UDP glucuronyltransferase), whereas ABZ fails to increase and even slightly decreases these enzymatic activities. In conclusion, the HepG2 human hepatoma cell line appears to be suitable for the study of many parameters of metabolism and action of ABZ and other structurally related compounds in humans.

NADPH:cytochrome P-450 (c) reductase is a microsomal enzyme which is involved in the cytochrome P-450-dependent biotransformation of many exogenous agents as well as of some endogenous molecules. Using cytochrome c as a substrate, the kinetic parameters of this enzyme were determined in brain microsomes. The comparison of the NADPH:cytochrome P-450 reductase's Vmax values and cytochrome P-450 contents in both fractions, suggests a role of cerebral NADPH:cytochrome P-450 reductase in cytochrome P-450 independent pathways. This is also supported by the different developmental pattern of brain enzyme as compared to the liver enzyme, and by the presence of a relatively high NADPH:cytochrome P-450 reductase activity in immature rat brain and neuronal cultures, while cytochrome P-450 was hardly detectable in these preparations. The enzyme activity was not induced by a phenobarbital chronic treatment neither in the adult brain nor in cultured neurons, suggesting a different regulation of the brain enzyme expression.

The cytochrome P-450 content found in human livers obtained post mortem was between 0.21-0.42 nmol/mg protein. The kinetic parameters of the mono-oxygenase activities--Km and Vmax--were determined in liver microsomes for N-demethylation (aminopyrine, benzphetamine, ethylmorphine), O-demethylation (4-nitroanisole), O-deethylation (7-ethoxycoumarin) and hydroxylation (benzo[a]pyrene), in an attempt to establish an inter-species comparison between man and the six animal species studied. The four substrates studied (aminopyrine, benzphetamine, ethylmorphine, benzo[a]pyrene) were shown to be less active in humans than the male rat, which is the most commonly used model. However, other animal species, such as the female Sprague-Dawley rat and the pig, are much more similar to man. From a procedural point of view, the optimal substrate concentrations vary from one experimental species to another. Due to the apparent Km observed, for example, the activities of the guinea-pig require a higher substrate concentration.

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