The effect of treatment with antineoplastic drugs on the modification of the carcinog en-metabolizing capacity was studied in mice liver microsomes at different durations as a single and as a repeated dose treatment. It is generally demonstrated that there is a commonality of influence for each specific antineoplastic group on the expression of the mixed function mono-oxygenases. The antineoplastic alkylating agents (chlorambucil, melphalan and busulfan) significantly increased the activity of carcinogen metabolizing enzymes in contrary to the effects observed for the antibiotic actinomycin-D. The antimetabolite agents (5-flourouracil and methotrexate), on the other hand, although decreased these activities when administered as a single dose, a pronounced increase was observed at the repeated dose treatment. Significant increases in the activity of N-nitrosodimethylamine demethylases I and II and aryl hydrocarbon (benzo[alpha]pyrene) hydroxylase were observed with chlorambucil when administered as a single or repeated doses. Similar effects was observed when the animals were treated with repeated doses of melphalan, busulfan and 5-flourouracil. Contrary to these effects, inhibition in the enzyme activity was exhibited when actinomycin-D was administered for either single or repeated dose treatment. The hepatic content of cytochrome P450 was significantly increased with all the administered drugs, except busulfan, when applied repeatedly. The implication of such alterations in the capacities of carcinogen metabolizing enzymes for antitumour-induced toxicity and carcinogenicity are discussed.

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