Mechanism of Protection against Aflatoxin Tumorigenicity in Rats Fed 5-(2-Pyrazinyl)-4-methyl-1,2-dithiol-3-thione (Oltipraz) and Related 1,2-Dithiol-3-thiones and 1,2-Dithiol-3-ones

Abstract

1,2-Dithiol-3-thiones, reported constituents of cruciferous vegetables, are five-membered cyclic sulfur-containing compounds with antioxidant, chemotherapeutic, and chemoprotective activities. The effects of dietary administration of a substituted 1,2-dithiol-3-thione, oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione], a potent antischistosomal agent, on aflatoxin B₁ (AFB₁) metabolism, DNA adduct formation, and hepatic tumorigenesis were examined in male F344 rats. Rats were fed graded doses of oltipraz (0.01–0.1%) for 4 wk. During the second and third wk of oltipraz feeding rats were gavaged with 250 µg of AFB₁/kg five times a wk. Rats were finally restored to control diet 1 wk after cessation of AFB₁ dosing. At 4 months focal areas of hepatocellular alteration were identified and quantitated by staining sections of liver for γ-glutamyl transpeptidase activity. Treatment with oltipraz at all doses reduced by >90% the volume of liver occupied by γ-glutamyl transpeptidase-positive foci. Levels of AFB₁ bound to hepatic DNA were reduced between 40 and 80% in animals fed increasing doses of dietary oltipraz (0.01–0.1%) for 1 wk prior to a single exposure to AFB₁. Feeding of the higher levels of oltipraz led to marked increases in the specific activity of glutathione S-transferases, presumably serving to facilitate the detoxication of the ultimate electrophilic form of AFB₁, the 8,9-oxide. At low dietary concentrations of oltipraz (0.01%), the only inductive effects seen were on the activities of selected cytochrome P-450 monooxygenases. Therefore, the protection afforded by oltipraz may be due to both the enhancement of electrophile detoxication pathways as well as modified oxidative metabolism of AFB₁. In in vitro metabolism studies with hepatic post-mitochondrial supernatant, low-dose oltipraz pretreatment facilitated the oxidative production of aflatoxins P₁ and Q₁, but not M₁, from AFB₁. High-dose (0.1%) oltipraz pretreatment enhanced the primary metabolism of AFB₁ to aflatoxins P₁, M₁, and Q₁ as well as the formation of chloroform-insoluble metabolites. Feeding studies with a series of 1,2-dithiol-3-thione and 1,2-dithiol-3-one derivatives of oltipraz demonstrated that the inductive activity for cytochrome P-450-dependent monooxygenases and electrophile detoxication enzymes, such as glutathione S-transferases, could be readily separated by minor modifications of the 1,2-dithiol-3-thione structure. The unsubstituted 1,2-dithiol-3-thione nucleus strongly induced electrophile detoxication enzymes, but not the monooxygenases, and was the most effective inhibitor of the binding of AFB₁ to hepatic DNA in vivo. The high potency, low toxicity, and selectivity of enzyme inductive effects may render some of the 1,2-dithiol-3-thiones as excellent compounds for chemoprotection in humans.