The metabolism of cyclophosphamide by isolated rat hepatocytes

Microcarrier-attached rat hepatocytes as a xenobiotic-metabolizing system in cocultures

A method for the primary culture of rat liver cells on collagen-coated dextran microcarriers is described. Ethoxycoumarin deethylase (EOD) activity 24 hr after inoculation was comparable for liver cells cultured on microcarriers and on collagen-coated dishes. Cells were cultured on microcarriers for up to 48 hr and retained 25% of the initial EOD-activity that was seen in freshly isolated liver cells. Microcarrier-attached hepatocytes were cocultured with BALB/c 3T3 cells to study the metabolism-mediated cytotoxicity of cyclophosphamide (CPA). In the absence of hepatocytes, growth of 3T3 cells was not affected by CPA at concentrations up to 3600 microM. In coculture with hepatocytes, cytotoxicity of CPA was expressed in a time- and concentration-dependent manner. At high concentrations, CPA slightly depressed the EOD-activity of hepatocytes. Our results indicate that cocultivation of microcarrier-attached rat liver cells with target cells represents a valuable approach to the study of the metabolism-mediated toxicity of xenobiotics in vitro.

Toxic effects of cyclophosphamide in differentiating chicken limb bud cell culture using rat liver 9,000 g supernatant or rat liver cells as an activation system: an in vitro short-term test for proteratogens

Cells from 4-day chicken embryo limb buds plated as micromass cultures differentiate and form cartilage nodules after a 5- to 6-day growth period. The innate ability of these cells to biotransform compounds, such as cyclophosphamide (CP), into reactive metabolites is apparently inadequate. Protocols used rat liver S9 from Aroclor 1254-pretreated (PCB) rats or hepatocytes from control rats or polychlorinated biphenyls (PCB)-pretreated rats and were added to micromass cultures with CP causing concentration-related toxicity in the cell cultures. Coculturing chicken limb bud cells with PCB-hepatocytes was the most efficient method, yielding an IC50 of 2 micrograms CP per ml. Toxic CP metabolites accumulated in the medium of PCB-hepatocyte cultures and were quite stable. The toxicity of bioactivated CP was nearly identical for both proliferation and cartilage proteoglycan accumulation.

Sex differences in the biotransformation of 2-acetylaminofluorene in cultured rat hepatocytes

Sex-related differences in susceptibility to 2-acetylaminofluorene (2-AAF) hepatocarcinogenicity and in vivo biotransformation of 2-AAF have been observed. In order to determine the contribution of hepatocytes to these differences, 2-AAF biotransformation was investigated in monolayer cultures of hepatocytes freshly isolated from male and female F-344 rats. In cultured hepatocytes from both sexes, ring and N-hydroxylated, deacetylated and conjugated metabolites were formed. The half-life of 2-AAF was similar at concentrations of 5 x 10(-6) and 10(-5) M; however, at 10(-4) M a slower rate was observed in cultures from males. Although the total formation of aqueous metabolites was similar, the ratio of sulfate to glucuronide conjugates of 2-AAF formed by hepatocytes from male and female rats differed. Sulfate conjugates predominated in hepatocytes from male rats, whereas in females, glucuronides predominated. The demonstration of sex-dependent variations in the rate of metabolism at a high concentration of 2-AAF and in conjugation provides evidence that in vivo differences are a function, at least in part, of the biotransformation characteristics of hepatocytes.

Comparison of S9 mix and hepatocytes as external metabolizing systems in mammalian cell cultures: cytogenetic effects of 7,12-dimethylbenzanthracene and aflatoxin B1

Two external metabolizing systems, S9 mix from Aroclor-induced rat livers and freshly isolated hepatocytes, were used for activation in cultures of human lymphocytes and V79 cells. 7, 12-dimethylbenzanthracene (DMBA) and aflatoxin B1 (AFB1) were employed as indirectly acting reference mutagens. Mutagenic effects were measured by induction of sister chromatid exchange (SCE). With DMBA, SCE-inducing effects were found to be quite similar after activation by S9 mix and activation by hepatocytes. In human lymphocytes nearly the same dose-effect relationships were found with both metabolizing systems; in V79 cells the hepatocyte-mediated induction of SCE was detectable at slightly lower concentrations than the S9-mediated SCE induction. In contrast with AFB1, S9 activation led to a stronger SCE induction than hepatocyte activation in both target cells. The induction of chromosomal aberrations by AFB1 after activation by the two metabolizing systems was also analysed in V79 cells. This experiment again revealed that AFB1 was more efficiently activated by S9 mix than by hepatocytes, and it appeared that AFB1 is a more potent inducer of chromosomal aberrations than of SCE. The different activation capacities of the two metabolizing systems for AFB1 may be due to the maintenance of inactivation mechanisms in hepatocytes or to the Aroclor induction of the S9 fraction. Our experiments have shown that the suitability of hepatocytes as an activation system is not restricted to microbial or eukaryotic point mutation assays, but that hepatocyte metabolism can also be successfully included in cytogenetic tests with short- and long-term cultures of mammalian target cells.

Application, limitations and research requirements of in vitro test systems in toxicology

There is increasing application of in vitro-test systems for toxicological evaluation of chemicals, which became possible by increasing understanding of the biological endpoints present in such systems and their capability for metabolic activation and inactivation. This communication focuses on the capacities of metabolic activation and inactivation in mutagenicity test systems, using bacteria, mammalian cells in culture and isolated hepatocytes. Bacterial test procedures with S-9-fraction are specific metabolic activation systems with low inactivation capacity. Mammalian cells are either deficient in metabolic activities or contain only limited activation capacity although special cell lines derived from hepatoma cells express certain metabolic activation as well as inactivation reactions. Isolated hepatocytes contain enzymatic activities similar to those in the intact liver, which however decrease at different rates. It is the goal of present research to construct cell lines with defined and sufficient activities of these enzymes. In isolated hepatocytes chemically induced DNA repair can be readily detected when a clear discrimination between mitochondrial, semiconservative and repair synthesis is provided. In such systems genotoxicity of reactive oxygen species is decreased by physiological concentrations of alpha-keto-acids, pyruvate possessing the highest antioxidant activity. It is concluded that in vitro test systems provide a suitable tool for detecting genotoxic and toxic effects of chemicals. However, many biological parameters such as metabolic activity, degree of differentiation of the cells, cofactor requirement, and composition of the medium affect the reliability of the test system. Thus, only a detailed understanding of the biology and biochemistry of such test allow production of reliable and reproducible results.

Toxicity and mutagenicity of 6 anti-cancer drugs in Chinese hamster V79 cells co-cultured with rat hepatocytes

Toxicity and induction of 6-thioguanine-resistant mutants in Chinese hamster V79 cells, co-cultured with or without isolated rate hepatocytes, by 6 anti-cancer drugs (cyclophosphamide, adriamycin, methotrexate, cytosine arabinoside, 6-mercaptopurine and vincristine) were studied. The effect of hepatocyte density on the cloning efficiency and recovery of mutants was found using dimethylnitrosamine as a positive control. In the absence of hepatocytes, this compound was neither toxic nor mutagenic to V79 cells, but in their presence it was highly mutagenic and extremely toxic. The cloning efficiency and mutation frequency of control (untreated) cells was unaffected by hepatocyte density. All the drugs were toxic to V79 cells, although different responses were found for certain of them depending upon whether hepatocytes were present or not. Cyclophosphamide and adriamycin were clearly mutagenic, and 6-mercaptopurine only weakly so. A slight mutagenic effect was seen for cytosine arabinoside, but both methotrexate and vincristine were negative. Here also, the presence or absence of hepatocytes was important.

2-Chloroacetaldehyde, a metabolite of cyclophosphamide in the rat

Following simultaneous i.v. administration of a mixture of [4-14C]cyclophosphamide (14C-CP) and [side-chain 3H]CP to rats, a metabolite containing predominantly 3H radioactivity was excreted in the urine. The 3H-labelled urinary metabolite was identified as 2-chloroacetaldehyde. Chloro[3H]acetaldehyde accounted for approx. 3.8% of urinary 3H radioactivity. The importance of chloroacetaldehyde as a toxic metabolite of CP is discussed, particularly in relation to haemorrhagic bladder disease.

Activation of nitrosamines and other carcinogens by mouse-liver S9, mouse hepatocytes and in the host-mediated assay produces different mutagenic responses in Salmonella TA1535

5 indirect alkylating carcinogens, namely, dimethylnitrosamine (DMNA), methylethylnitrosamine (MENA), diethylnitrosamine (DENA), 1,2-dimethylhydrazine (DMH) and cyclophosphamide (CP), were tested in liquid incubation assays for their mutagenic activity towards Salmonella TA1535 in the presence of mouse-liver homogenate (S9) or freshly isolated, single liver-cell preparations. The capacity of these mouse-liver preparations to activate the compounds to mutagens for TA1535 was compared with the mutagenic effect of low doses of the carcinogens in intrasanguineous host-mediated assays, with the same strain of mice as host. Although the mouse hepatocytes retained their activating capacity longer than S9 preparations did during incubation at 37 degrees C, the latter gave much higher yields of mutants with 10 mM (DMNA, MENA, DMH) and 5 mM (CP) of 4 out of the 5 compounds. DENA was not mutagenic in either assay. These differences between whole cell and disrupted cell preparations were reduced or absent when the concentrations of the test compounds were reduced by a factor of 10. It was concluded that hepatocytes at the maximal concentration of cells have a limited capacity to metabolize the mutagens. On the basis of protein concentration, hepatocytes are more effective (nitrosamines) or equally effective (CP and DMH) in activating the compounds. Compared with the host-mediated assays, both liver fractions have only a marginal potential to activate equal low amounts of the carcinogens. The present results do not indicate that hepatocytes take an 'intermediate' position between existing in vitro and in vivo activation systems, although they do suggest that these mouse hepatocyte preparations activate the nitrosamines DMNA and MENA in a quantitatively or qualitatively different way than do mouse-liver homogenates.