Validity and reliability of in vitro systems in safety evaluation

Changes in cytochrome P450 gene expression and enzyme activity induced by xenobiotics in rabbits in vivo and in vitro

As considerable inter-species differences exist in xenobiotic metabolism, developing new pharmaceutical therapies for use in different species is fraught with difficulties. For this reason, very few medicines have been registered for use in rabbits, despite their importance in inter alia meat and fur production. We have developed a rapid and sensitive screening system for drug safety in rabbits based on cytochrome P450 enzyme assays, specifically CYP1A1, CYP1A2 and CYP3A6, employing an adaptation of the luciferin-based clinical assay currently used in human drug screening. Short-term (4-h) cultured rabbit primary hepatocytes were treated with a cytochrome inducer (phenobarbital) and 2 inhibitors (alpha-naphthoflavone and ketoconazole). In parallel, and to provide verification, New Zealand white rabbits were dosed with 80 mg/kg phenobarbital or 40 mg/kg ketoconazole for 3 d. Ketoconazole significantly increased CYP3A6 gene expression and decreased CYP3A6 activity both in vitro and in vivo. CYP1A1 activity was decreased by ketoconazole in vitro and increased in vivo. This is the first report of the inducer effect of ketoconazole on rabbit cytochrome isoenzymes in vivo. Our data support the use of a luciferin-based assay in short-term primary hepatocytes as an appropriate tool for xenobiotic metabolism assays and short-term toxicity testing in rabbits.<p> </p>

Time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes

In the present study, time-dependent variations of drug-metabolising enzyme activities (DMEs) in primary cultures of rabbit hepatocytes, a species of economic importance in Mediterranean countries, were investigated. Cross-bred rabbits were anesthetised and their livers perfused in situ by a two-step collagenase technique; cells suspensions were filtered, seeded in collagen-coated dishes and cultivated at 37 degrees C in a controlled atmosphere for 24 and 72 h. Cytochrome P450 and b(5) contents as well as the catalytic activity of some P450-dependent monooxygenases were measured in subcellular fractions obtained by differential ultracentrifugation; microsomal proteins were also subjected to immunoblotting, using antibodies to rat P4501A, 2B, 2E1 and 3A isoforms. The activity of some microsomal hydrolytic enzymes was also determined. As regards conjugative enzymes, glutathione content and activities of glutathione S-transferase, uridindiphosphoglucuronosyl-transferase, acetyl-transferase and 1,2-epoxibuthane glutathione transferase were assayed. An overall reduction of the catalytic activity was observed 72 h after plating, reaching in certain instances the level of statistical significance. On the whole, our data confirm those previously reported with hepatocytes obtained from other species; however, the evidence that DMEs were still measurable after 72 h supports the usefulness of this in vitro method for drug metabolism studies in the rabbit as well.

Use of precision-cut rat liver slices for studies of xenobiotic metabolism and toxicity: comparison of the Krumdieck and Brendel tissue slicers

1. In this study we have compared freshly cut and cultured precision-cut rat liver slices produced by the Krumdieck and Brendel-Vitron tissue slicers. 2. No significant differences were observed in levels of protein, potassium, total glutathione (i.e. GSH and GSSG), reduced glutathione (GSH) and cytochrome P450 and activities of 7-ethoxyresorufin O-deethylase and 7-benzoxyresorufin O-debenzylase in freshly cut rat liver slices produced by the two tissue slicers. However, levels of oxidized glutathione (GSSG) were significantly greater in liver slices produced with the Brendel-Vitron tissue slicer. 3. Precision-cut rat liver slices produced with both tissue slicers were cultured for 0 (i.e. a 1-h preincubation), 24 and 72 h in a dynamic organ culture system in an atmosphere of either 95% 02/5% CO2 or 95% air/5% CO2. 4. Apart from small differences in glutathione levels in 0 and 24 h cultured liver slices, no significant differences were observed in the parameters measured between liver slices prepared with both tissue slicers and cultured in both gas phases. 5. With liver slices produced by both tissue slicers 50 microM sodium arsenite produced a greater induction of heat shock protein 70 levels in slices cultured for 24 h in a high oxygen than in an air atmosphere. 6. These results suggest that both tissue slicers can readily produce precision-cut liver slices for studies of xenobiotic metabolism and toxicity. However, the data suggest that for any given application of precision-cut tissue slices it is desirable to establish optimal culture conditions

Induction of CYP3A isoforms in cultured precision-cut human liver slices

1. The effect of rifampicin on cytochrome P450 isoforms in the CYP1A and CYP3A subfamilies has been studied in 72-h cultured precision-cut human liver slices. 2. In cultured human liver slices 50 microM rifampicin induced testosterone 6 beta-hydroxylase activity, but had no effect on 7-ethoxyresorufin O-deethylase and 7-methoxyresorufin O-demethylase activities. 3. Western immunoblotting of liver slice microsomes was performed with antibodies to rat CYP1A2 and human CYP3A4. Compared with control (dimethyl sulphoxide only treated) liver slice microsomes, rifampicin increased levels of CYP3A4 but had no effect on CYP1A2. 4. These results demonstrate that rifampicin induces CYP3A isoforms, but not CYP1A2, in cultured human liver slices. Some variability in the magnitude of induction by rifampicin was observed in the six human liver samples examined. 5. These results demonstrate that cultured human liver slices may be used to evaluate the effects of xenobiotics on CYP3A isoforms.