Differences in skin carcinogenesis by methylnitrosourea between mice of several strains

To compare the susceptibilities of the skin of different strains of mice to the carcinogenic effect of a directly acting alkylating agent, groups of 20 mice were treated twice a week with 25 microliters of a solution of methylnitrosourea in methanol. The solution was 0.04M and was applied to the shaved back of female BALB/c, Sencar, CD-1 and Swiss mice for 25 weeks. Four groups of 20 mice of each strain were 8 weeks old at the beginning of treatment. Another four groups were 58 weeks old when treatment began. More of the BALB/c mice developed skin tumors than the other three strains, the Sencar mice somewhat less. Few CD-1 mice developed skin tumors and about one third of the Swiss mice. In all four strains, there were fewer animals with skin tumors among those begun at 58 weeks than in the young mice, but the difference was small. Survival was poor among CD-1 mice, but there was not a large difference between the strains in time of appearance of first tumor, or in average latent period of skin tumors, almost all of which were carcinomas. The Sencar mice were not outstandingly more sensitive to skin carcinogenesis by MNU, as they were to UV radiation-induced skin carcinogenesis. In a comparable study in Swiss mice neither dimethylnitrosourea nor diethylnitrosourea induced skin tumors by painting and both showed only a weak systemic carcinogenic effect in the lungs, although they are directly acting mutagens.