Two-stage models of carcinogenesis, classification of agents, and design of experiments.

The implications of a clonal two-stage model of carcinogenesis on the design and analysis of 2-year in vivo tumorigenesis experiments are addressed. Using a simple classification scheme for labelling test agents as initiators, promoters, and completers, it is shown that the standard experimental design has very little ability to differentiate between these different modes of action. Even when chemicals in one class (e.g., promoters) follow a highly nonlinear dose-response relationship and chemicals in another class (e.g., initiators) follow a linear dose-response relationship, it is difficult to reject one mode of action versus the other. A simple modification of the design using age-dependent dosing schemes produces patterns of tumor incidence which are unique to the particular class, making it slightly easier to differentiate between the assumed mechanisms of action. The implications of this finding on study design are discussed.