Distinct differences were observed in the biologic response elicited by the promoting agents croton resin and its active fractions on the one hand and croton oil on the other. With the active fractions of croton resin, in initiation-promotion experiments, there is a high incidence of malignancy and a low incidence of tumor regression. Applied alone, croton resin gives rise to very few tumors. In contrast, promotion with croton oil is reported to elicit a low incidence of malignancy, a markedly higher incidence of tumor regressions, and when applied alone is notably tumorigenic. These and other factors are discussed in the light of chemical composition, animal strain, degree of skin damage, and dose. It is shown that the croton oil used contains alkylating chemicals and it is concluded that the chemical composition of croton oil and of the active materials plays an important role in determining biologic activity.
This work was supported by Grants CA-05946, CA-06989, ES-00014, and Contract PH43-64-938 from the Department of Health, Education, and Welfare, USPHS.