The effect of antibiotics chloramphenicol (CML) and actinomycin-D (AMD) on the modification of the carcinogen metabolizing capacity was studied in vivo in mouse liver microsomes at different durations of treatment, for one day and three and six consecutive days. Following the administration of CML, a significant increase was observed in the activity of the low and high substrate levels of the hepatic microsomal N-nitrosodimethylamine demethylases (NDMAd I and II, respectively). On the contrary, AMD reduced the activity of NDMAd I and II in a time-dependent manner up to 3 days of treatment while no effect was observed when the drug was administered for 6 consecutive days. No effect was observed in the activity of the arylhydrocarbon (benzo(alpha)pyrene) hydroxylase either at the single or the repeated doses of CML only for 6 days of treatment with AMD. The expression of the hepatic content of cytochrome P450 revealed a significant induction at the various treatment intervals with CML. AMD, however, reduced the cytochrome P450 at 1 and 6 days treatment with induction at 3 days of repeated treatment.

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