A method to quantitate the relative initiating and promoting potencies of hepatocarcinogenic agents in their dose-response relationships to altered hepatic foci

The relative response to various initiating doses of diethylnitrosamine(DEN) and dimethylbenza]anthracene of the induction of numbersand size (vol. % of liver) of altered hepatic foci (AHF) inlivers of adult female rats of the Sprague-Dawley and Fischer344 (F-344) strains was studied by methods of quantitative stereologyin the presence and absence of the promoting agent, phenobarbital(PB, 0.05% in the diet). In all cases, a relatively linear responsewith dose, even at the lowest doses employed, was obtained exceptfor the numbers of AHF at the highest dose of DEN (30 mg/kg),which was not significantly different from that at a dose of10 mg/kg in F-344 female rats. Similar dose-response data wereobtained at various doses of two promoting agents effectivein hepatocarcinogenesis, PB and 2,3,7,8-tetrachloro-dibenzo-p-dioxin(TCDD), in livers of F-344 female rats following initiationwith DEN (10 mg/kg) 24 h post-70% hepatectomy. The responseto these agents exhibited threshold levels below which no increasein number or vol. % of liver of AHF was noted in comparisonwith that in livers of animals not treated with the promotingagents. At several subthreshold doses of both PB and TCDD aninhibition of AHF formation and growth (measured as vol. % ofliver) was observed. Based on quantitative stereologic calculations,parameters for the estimation for the relative potency of chemicalsas initiating or promoting agents have been established. Theseare defined as: initiation index = no. of foci induced x liver^–1x mmol/kg body wt]^–1 and promotion index = V_f/V_c x mmol^–1x weeks^–1 where V_f is the total volume fraction (%) occupiedby AHF in the livers of rats treated with the test agent andV_c is the total volume of AHF in control animals which haveonly been initiated. These parameters were calculated for anumber of agents based on data published in the literature andfrom those reported herein. Neither parameter varied significantlywith the dose of the initiating agent based on the data in thispaper. The range of promotion indices extended over more thaneight orders of magnitude, whereas that of initiation indiceswas much less variable. Such parameters may be useful as quantitativeestimates of the potency of hepatocarcinogenic agents, suchvalues having potential application to risk estimations.