A demonstration of the in vitro bacterial mutagenicity of procarbazine, using the microtitre fluctuation test and large concentrations of S9 fraction

The anti-neoplastic agent procarbazine is genetically active in a variety of short term mutagenicity tests, and it also possesses carcinogenic and teratogenic potential. This compound has consistently yielded false negative results in in vitro microbial mutagenicity tests in the presence and absence of mammalian metabolic activation. In this study, procarbazine was not mutagenic in standard and preincubation Ames tests using large S9 concentrations and bacterial test strains devoid of the rfa mutation. The microtitre fluctuation test is a sensitive technique for the detection of bacterial mutagens. Using this assay, procarbazine proved to be a bacterial mutagen after in vitro metabolic activation at concentrations as low as 200 μg/ml. Activity was dependent upon liver S9-concentrations which were higher than those usually attained within agar assays, and was only observed when such fractions were derived from aroclor-treated or phenobarbitone plus β - naphthoflavone treated rat livers. The excision-repair proficient strain E. coli WP2 was equally, if not more, sensitive than the repair deficient strain E. coli WP2 uvrA. Furthermore, the differential mutagenic effects obtained using S. typhimurium strains TA1535, TA1530 and his G46 indicated that the absence of an rfa mutation was essential for procarbazine mutagenesis. Procarbazine was also mutagenic for E. coli D494 in a forward mutation fluctuation assay measuring resistance to ampicillin. In this assay lower concentrations of S9-fraction were sufficient, reflecting the increased sensitivity of the test strain towards the mutagemc metabolites of the drug. In conclusion, the results suggest that rat liver S9-fraction contains only low levels of enzymes capable of activating procarbazine to a mutagen. This may be a result of rapid breakdown during storage, or due to intrinsically low levels in rat liver extracts. Inducing agents appear to increase these levels. The low concentrations of mutagenic species formed in vitro , may only be detectable using a highly sensitive assay such as the microtitre fluctuation test.