In vitro metabolism and toxicity assessment of N-methylcarbazole in primary cultured rat hepatocytes

Predictive value of in vitro model systems in toxicology

The application of in vitro model systems to evaluate the toxicity of xenobiotics has significantly enhanced our understanding of drug- and chemical-induced target toxicity. From a scientific perspective, there are several reasons for the popularity of in vitro model systems. From the public perspective, in vitro model systems enjoy increasing popularity because their application may allow a reduction in the number of live animals employed in toxicity testing. In this review, we present an overview of the use of in vitro model systems to investigate target organ toxicity of drugs and chemicals, and provide selective examples of these model systems to better understand cutaneous and ocular toxicity and the role of drug metabolism in the hepatotoxicity of selected agents. We conclude by examining the value and use of in vitro model systems in industrial development of new pharmaceutical agents.

Metabolism of N-methylcarbazole by rat lung microsomes

N-methylcarbazole (NMC) is a procarcinogenic component of tobacco smoke particulate matter. It is metabolized by liver microsomes into some hydroxylated metabolites such as the potent mutagen N-hydroxymethylcarbazole (NHMC). Lung metabolism and toxicity of NMC is not known. Since the lung is the primary organ of inhalation of tobacco smoke, NMC metabolism by lung microsomes was studied in comparison with the metabolism by liver microsomes. Liver or lung microsomes (1 mg/mL) were incubated with 0.5 mM NMC for 30 min at 37 degrees C. NMC metabolites were extracted with ethyl acetate and analyzed by reversed-phase high-performance liquid chromatography. Rat lung microsomes metabolized NMC with a similar profile to liver microsomes, although lung microsomes produced greater number of metabolites. The potent mutagen NHMC was also the major NMC metabolite produced by lung microsomes, as confirmed by particle beam mass spectrometry. However, lung microsomes produced only 10% of NHMC produced by liver microsomes. Metabolism of NMC by lung microsomes also led to depletion of the endogenous antioxidant glutathione by 34% compared to controls, indicating a significant generation of some reactive intermediates during NMC metabolism by lung microsomes. The data show that the lung participates directly in producing the potent mutagen NHMC from NMC present in tobacco smoke.

Bioassay of quinoline, 5-fluoroquinoline, carbazole, 9-methylcarbazole and 9-ethylcarbazole in newborn mice

Quinoline and carbazole are among the more prevalent aza-arenes present as components of environmental pollutants. Both of these aza-arenes are hepatocarcinogenic to mice when administered in the diet. The hepatocarcinogenic potential of quinoline is consistent with its mutagenic activity in Salmonella typhimurium TA100 and potential to induce unscheduled DNA synthesis (UDS) in rat hepatocytes. Structure-activity studies with fluorinated quinolines indicate that the presence of a fluorine atom at the 5-position of quinoline may inhibit detoxification and result in enhanced genotoxic potency. Quinoline and 5-fluoroquinoline were assayed in newborn CD-1 mice at a total dose of 1.75 mumol to establish their relative tumorigenic activity. Liver tumours developed in 60 and 90% of the male newborn mice treated with quinoline and 5-fluoroquinoline, respectively. The majority of liver tumours observed among the quinoline-treated mice were classified as adenomas. In contrast, liver carcinomas developed in most of the male mice treated with 5-fluoroquinoline. Unlike the well established genotoxic potential of quinoline, there is limited evidence for carbazole having either genotoxic or carcinogenic activity. Whereas carbazole is not mutagenic towards several strains of S. typhimurium, both 9-methylcarbazole and 9-ethylcarbazole are active as mutagens in S. typhimurium TA100. Carbazole, 9-methylcarbazole and 9-ethylcarbazole were assayed in primary rat hepatocytes to assess their relative potential to induce UDS in rat hepatocytes; only 9-ethylcarbazole did so. These carbazole derivatives were also assayed in newborn CD-1 mice at a total dose of 1.75 mumol. Neither carbazole nor either of these 9-alkylcarbazoles produced a significant tumorigenic response in this bioassay system.

Microbial models of mammalian metabolism: involvement of cytochrome P450 in the N-demethylation of N-methylcarbazole by Cunninghamella echinulata

1. As previously reported (Yang and Davis 1992), N-methylcarbazole (NMC) is converted to N-hydroxymethylcarbazole (NHMC), and 3-hydroxy-N-hydroxymethylcarbazole (3-OH-NHMC), two relatively stable carbinolamine metabolites by the fungus Cunninghamella echinulata (ATCC 9244). Decomposition of these two carbinolamines yields the corresponding dealkylated metabolites, carbazole and 3-hydroxycarbazole. In the present study, the possible involvement of cytochrome P450 in the requisite N-alkyl hydroxylation reaction was examined. 2. Carbon monoxide, a classical P450 inhibitor, markedly inhibited the formation of NHMC, as did potassium cyanide. 1-Benzylimidazole, piperonyl butoxide and SKF-525A inhibited the formation of both NHMC and 3-OH-NHMC, while beta-naphthoflavone (5,6-benzoflavone) induced their formation. 3. The source of the oxygen atom in the metabolite NHMC was examined by GC/MS analysis of NHMC formed during incubation of NMC in H218O-enriched medium which resulted in no incorporation of labelled oxygen into the metabolite. 4. An intermolecular isotope effect was not observed for the formation of NHMC suggesting that C-H bond cleavage is not a rate limiting step in the formation of this metabolite under the conditions examined. 5. It was concluded that P450 enzymes may be involved in the N-demethylation of NMC catalyzed by this fungal model of mammalian metabolism, and provides further support for biochemical and mechanistic parallels between mammalian metabolism and microbial systems catalyzing phase-1 biotransformations.

Production of a toxic, novel mammalian metabolite of N-methylcarbazole predicted by a fungal cell model of mammalian metabolism

The formation of N-hydroxymethylcarbazole (NHMC), carbazole, 1-hydroxy-N-methylcarbazole, 2-hydroxy-N-methylcarbazole, and 3-hydroxy-N-methylcarbazole as products of mammalian liver microsomal metabolism of N-methylcarbazole (NMC) has been documented by several investigators. In previous studies in our laboratory, the fungus Cunninghamella echinulata (ATCC 9244) produced two new metabolites, 3-hydroxy-N-hydroxymethylcarbazole (3-OH-NHMC), and 3-hydroxycarbazole (3-OH-carbazole), in addition to the known mammalian metabolites, NHMC and carbazole. One of the two novel metabolites isolated from the microbial models, 3-OH-NHMC, was also identified and characterized in rat liver microsomes by analytical (HPLC) and spectral (UV and NMR) comparisons with a reference standard. The two metabolites, 3-OH-NHMC and 3-OH-carbazole, were shown to be cytotoxic to cultured rat hepatocytes as assessed by lactate dehydrogenase (LDH) leakage and neutral red (NR) uptake. These studies demonstrate the prospective potential of microbial models for predicting the formation of metabolites from drugs and other xenobiotics in mammalian systems.