Potent inhibition of aflatoxin-induced hepatic tumorigenesis by the monofunctional enzyme inducer l,2-dithiole-3-thione

l,2-Dithiole-3-thiones are five-membered cyclic sulfur-containing compounds with antioxidant, chemotherapeutic, radioprotective and chemoprotective properties. Several substituted l,2-dithiole-3-thiones are used medicinally and one of these, oltipraz [5-(2-pyrazinyl)-4-methyl-l,2-dithiole-3-thione], has been recently shown to be an inhibitor of aflatoxin B₁ (AFB₁) hepatocarcinogenesis in the rat. Structure-activity studies have been undertaken to probe the mechanisms by which dithiolethiones inhibit carcinogenesis. Such studies revealed that unsubstituted l,2-dithiole-3-thione was more effective than oltipraz at inhibiting aflatoxin-DNA adduct formation in vivo and at inducing electrophile detoxication enzymes in cell culture. In the present studies the effects of dietary administration of l,2-dithiole-3-thione on the induction of xenobiotic metabolizing enzymes and inhibition of aflatoxin-induced hepatic tumorigenesis were examined. Male F344 rats were fed graded doses of 1,2-dithiole-3-thione (0.001-0.03%) for 4 weeks. During the second and third weeks of l,2-dithiole-3-thione feeding, rats were dosed by gavage with 250μg of AFB₁/kg five times a week. Rats were then restored to control AIN-76A diet 1 week after cessation of AFB₁ dosing. At 4 months, focal areas of hepatocellular alteration were identified and quantified by staining sections of liver for λ-glutamyltranspeptidase (GGT) activity and glutathione S-transferase P (GST-P) expression. Treatment with l,2-dithiole-3-thione at the lowest dose (0.001%) reduced by >80% the volume of liver occupied by GGT or GST-P foci; higher dietary concentrations provided > 98% reductions in the volume per cent of these markers for presumptive preneoplastic lesions. All dietary concentrations of l,2-dithiole-3-thione resulted in significant elevations in hepatic GST activities. In accord with the protective effects against tumorigenesis, 4- to 6-fold increases in the specific activities of aflatoxin-glutathione conjugation were observed in cytosols prepared from livers of animals fed 1,2-dithiole-3-thione. By contrast, l,2-dithiole-3-thione did not have any detectable inductive effects on hepatic mkrosomal cytochrome P450 levels or activities. Dietary administration of 1,2-di-thiole-3-thione also elevated activities of GSTs and other phase II enzymes in several extrahepatic organs. This broad pattern of induction of detoxication enzymes by 1,2-dithiole-3-thione supports the potential widespread use of this compound as a protective agent against chemical carcino-genesis and other forms of electrophile toxicity.