Good qualitative correlations between mutation in bacteria and carcinogenic effects in laboratory animals have previously demonstrated. In vitro microbial mutation assays are now widely used in a preliminary screen for detecting possible mammalian mutagens and, by implication, possible carcinogens. In order to investigate the relationship between mutation and carcinogenesis in vivo, a method was developed in which somatic mutation can be studied in specific organs of Chinese hamsters after acute dosing with chemical carcinogens. Using this approach, in which chemicals are subject to the full spectrum of mammalian metabolism and detoxification, changes in mutation frequency are studied in primary cultures of tissues obtained from hamsters dosed with the test chemical. Forward mutation is assayed using 8-azaguanine-resistance or ouabain-resistance as genetic markers. An increase in mutation at both loci was detected in cultures derived from lung tissue of hamsters after i.p. dosing with the direct-acting mutagen and weak carcinogen, ethyl methanesulfonate, or with diethylnitrosamine, a carcinogen that requires in vitro metabolic activation before mutagenic activity can be demonstrated. Subsequent experiments showed the induction of mutation in cells derived from the bladder of Chinese hamsters dosed with diethylnitrosamine or methyl nitrosourea, from the lungs of hamsters dosed with urethane, and in a variety of tissues from animals dosed with methanesulfonates, 2-acetylaminofluorene and 3-methylcholanthrene.
