Mutagenicity of procarbazine for V79 Chinese hamster fibroblasts in the presence of various metabolic activation systems

Abstract

Even though procarbazine is mutagenic in most in vivo systems, a number of in vitro assays failed to indicate a positive effect. Since inappropriate metabolic activation in vitro is one explanation for these findings, the effects of procarbazine on V79 Chinese hamster fibroblasts in the absence and in the presence of either an S9 liver homogenate or hepatocytes from male rats were evaluated. The influence of enzyme induction was studied by performing experiments using S9 and hepatocytes both from non-treated and from Aroclor 1254-treated rats. Serum from procarbazine-treated rats was also tested for mutagenicity. Cytotoxicity was not strongly influenced by the presence of either S9 or hepatocytes, irrespective of whether Aroclor-induced or non-induced preparations were used. Without a metabolic activation system, and in the presence of S9 from non-induced animals, a weak mutagenic effect, i.e. an increased frequency of thioguanine-resistant clones, was observed in the cytotoxic dose range of 6000 .mu.g/ml. In the hepatocyte-mediated assay, 2-6 .mu.g/ml procarbazine proved to be mutagenic. Using the hepatocyte-medicated assay, a decrease in mutagenicity in the cytotoxic dose range was observed, reaching, at 6000 .mu.g/ml in the case of hepatocytes from non-induced animals, a value almost identical to that observed without a metabolic activation system. Thus testing chemicals in the cytotoxic dose range only might lead to false conclusions, particularly if hepatocytes were used for metabolic activation. The use of liver preparations from Aroclor-pretreated rats led to much stronger mutagenic responses than those caused by non-induced material. Although S9 from induced animals clearly led to an activation of procarbazine, the corresponding hepatocytes were much more effective, thus confirming the data obtained with non-induced preparations. Serum from rats treated with 50-150 mg procarbazine/kg was not cytotoxic, but induced a mutagenic effect which was intensified by the presence of hepatocytes.