Inhibitory Effect of Antioxidants Ethoxyquin and 2(3)-tert-Butyl-4-hydroxyanisole on Hepatic Tumorigenesis in Rats Fed Ciprofibrate, a Peroxisome Proliferator

The objective of this study was to test the hypothesis that hepatocarcinogenesis by peroxisome proliferators, a novel class of chemical carcinogens, is mediated either directly by carcinogenic H₂0₂, generated by peroxisomal oxidase(s) or indirectly by free radicals produced from H₂0₂, and that antioxidants could retard or inhibit neoplasia by scavenging active oxygen (superoxide radicals 0₂^-_, hydrogen peroxide, hydroxyl radicals HO^., and singlet oxygen ¹0₂). Accordingly, the effect of synthetic antioxidants 2(3)-tert-butyl-14-hydroxyanisole and ethoxyquin on the peroxisome proliferator 2-[4-(2,2-dichlorocyclopro-pyl)phenoxy]2-methyl-propionic acid (ciprofibrate)-induced hepatic tumorigenesis has been examined in male Fischer 344 rats. Rats were fed either a 2(3tert-butyl-4-hydroxyanisole (0.5% w/w)- or ethoxyquin (0.5% w/w)-containing diet with or without ciprofibrate (10 mg/kg of body weight) for 60 weeks. Rats fed ciprofibrate (10 mg/kg of body weight) in the diet or fed a diet with no added chemicals served as controls. Results of this study demonstrated that ethoxyquin markedly inhibited the hepatic tumorigenic effect of ciprofibrate, as evidenced by a decreased incidencel of tumors, a decreased number of tumors per liver, and a reduced tumor size. 2(3)-tert-Butyl-4-hydroxyanisole also caused a significant decrease in the incidence and number of hepatocellular carcinomas that were larger than 5 mm. The present data suggest that the inhibitory effect of antioxidants on ciprofibrate-induced hepatic tumorigenesis may be due to H₂0₂ and free radical-scavenging property of ethoxyquin and 2(3)-tert-butyl-4-hydroxyanisole, since these antioxidants do not prevent peroxisome proliferation and induction of H₂0₂-generating peroxisomal enzymes in livers of rats fed ciprofibrate. Whether the inhibitory effect of antioxidants is exercised on the presumptive H₂0₂ initiation process and/or on the postinitiation growth phase of foci and nodules in liver is, at present, unknown.