Previous studies have demonstrated that dietary administration of the schistosomicidal drug 5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione (oltipraz) ameliorates the hepatotoxicity of aflatoxin B1 (AFB1). Notably, mortality, altered hepatic function, hepatic AFB1-DNA adduct levels, and expression of hepatic enzyme-altered foci were markedly reduced in the rat by concurrent feeding of oltipraz during exposures to AFB1. Collectively, these studies prompted us to evaluate the chemoprotective properties of oltipraz against AFB1-induced liver cancer. In addition, preliminary molecular dosimetry studies were undertaken to determine the utility of measurements of urinary aflatoxin-N7-guanine excretion as a marker of relative risk for hepatocarcinogenesis in AFB1-exposed rats. For the carcinogenesis studies, 5-wk-old male F344 rats were randomly divided into two groups. One group (55 rats) received the AIN-76A diet, and the other group (56 rats) received the AIN-76A diet supplemented with 0.075% oltipraz. The oltipraz-supplemented diet was fed for 4 wk. Beginning 1 wk after starting the experimental diets, all rats in both groups received 25 µg of AFB1/rat/day by gavage for 5 days per wk over the next 2 wk. One wk following cessation of dosing with AFB1, oltipraz was removed from the diet, and all rats were fed the AIN-76A diet for the remainder of the experiment. At 3 mo after dosing, livers of ten sentinel rats from each group were analyzed for the burden of γ-glutamyltranspeptidase-positive foci. In accord with previous findings, rats fed the oltipraz-supplemented diet exhibited substantial reductions in the focal burden (97% reduction; P < 0.05) of these AFB1-induced lesions. The remaining rats were maintained for the cancer study until they became moribund or the termination of the experiment at 23 mo. Gross liver lesions were identified at autopsy and confirmed by microscopic evaluation. An 11% incidence of hepatocellular carcinoma was observed in the AFB1-treated, control diet-fed rats. An additional 9% of this group had hepatocellular adenomas. Oltipraz afforded complete protection against both AFB1-induced hepatocellular neoplasms. Using Kaplan-Meier survival analyses, rats in the oltipraz group had a significantly (P < 0.02) longer life span and an increased survival free of liver tumors (P < 0.0002). Molecular dosimetry studies used rats fed either the oltipraz-supplemented or control diet for 1 wk and then challenged with a single dose of AFB1 to examine the initial rates of 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 excreted in the urine. Levels of aflatoxin-N7-guanine adducts in the livers of the oltipraz-fed animals were reduced 76 and 65% at 2 and 24 h after dosing, respectively. While elimination of total urinary aflatoxins was indistinguishable in the 2 groups, oltipraz pretreatment led to a 67% reduction in the urinary elimination of aflatoxin-N7-guanine over the 24-h postdosing period. Protection afforded by oltipraz against hepatocarcinogenesis induced by AFB1 presumably results from the marked decrease in levels of hepatic DNA adducts. Thus, measurement of integrated levels of aflatoxin-N7-guanine adducts excreted in the urine may provide a facile, noninvasive, short-term method for monitoring the efficacy of chemoprotection by agents like oltipraz in individuals at high risk for aflatoxin exposure.
Supported in part by Grant CA 39416 from the USPHS. J. D. G. and T. W. K. are recipients of Research Career Development Awards KO4 CA 01517 and KO4 CA 01230, respectively. Presented in abstract form at the 81st Annual Meeting of the American Association for Cancer Research, Inc. (1), and submitted as partial fulfillment for a M.A. degree, Dartmouth College, by Y-L. L.