Kinetics of diethylnitrosamine hepatocarcinogenesis in the infant mouse.

Kinetics of hepatocarcinogenesis was evaluated in 15-day-old male C57BL/6J X C3HeB/FeJ F1 mice using a nontoxic carcinogenic dose range of diethylnitrosamine (DEN). The carcinogen was injected i.p. once, and the animals were killed according to the protocol. Two studies were carried out sequentially over a period of 4 years. In the first study, groups of mice were treated with 0.625, 1.25, 2.5, and 5.0 micrograms of DEN per g of body weight, and subgroups of eight mice were killed at 10-week intervals, the first at 10 weeks following carcinogenic treatment. The dose-response relationship, transformation probabilities, and the dose versus time to 50% incidence of the early (basophilic foci) and later appearing focal and nodular hepatocellular lesions were evaluated. In the second study, groups of mice were treated with 0.312, 0.625, 1.25, 2.5, and 5.0 micrograms of DEN per g of body weight, and subgroups of 8 to 20 animals were killed at 10, 16, 20, 24, 30, 34, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, and 110 weeks after carcinogenic treatment. The numbers of induced basophilic foci and hepatocellular carcinomas per number of liver cells at risk (transformation probabilities) were used to evaluate dose-response, time-response, and time-dose kinetics. In both studies, the kinetics of hepatocarcinogenesis was evaluated from data plotted on the double logarithmic scale. Regardless of the dose used, DEN induced four distinct morphological entities: basophilic (glucose-6-phosphatase deficient) foci; hyperplastic nodules; hepatocellular adenomas; and hepatocellular carcinomas in all animals. The first study demonstrated a positive dose-response relationship and constancy (k) of the product of single dose (d) and the time to 50% (t50%) incidence (d . tn50% = k) for each of the four morphological entities. The numerical value of the power of time (n) increased from 2.6 to 2.7, 3.4, and 5.7 for the above four lesions, respectively. The second study showed first-order kinetics regarding the induction of basophilic foci and hepatocellular carcinomas. The transformation probability of development of basophilic foci was up to three orders of magnitude greater than that observed for development of hepatocellular carcinomas, suggesting a qualitative difference between these two types of hits. The time-response kinetics showed that the development of basophilic foci and carcinomas was related to the time factor by powers of 2 and 4 for these two lesions, respectively. The time-dose relationship to a fixed number of lesions per number of liver cells at carcinogenic risk showed a negative slope with an n value of 2 for the induction of basophilic foci (d . t28/10(7) = k) and an n value of 4 for the induction of hepatocellular carcinomas (d . t40.08/10(7) = k). The data indicated that at least two critical events are needed for the induction of basophilic foci and at least four events are required for the induction of hepatocellular carcinomas...