Prevention by synthetic phenolic antioxidants of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)- or activated MelQx-induced mutagenesis and MelQx-induced rat hepatocarcinogenesis, and role of antioxidant activity in the prevention of carcinogenesis

Effects of synthetic phenolic antioxidants l-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butylhydroquinone (TBHQ) and propyl gallate (PG) on 2-amino-3,8-dimethylimidazo[4,5-F]quinoxaline (MelQx)- or activated MelQx-induced mutagenesis and rat hepatocarcinogenesis were compared, and the association between antioxidative activity and inhibition of carcinogenesis was examined. When the antimutagenic activity of five antioxidants against MelQx- or activated MelQx-induced mutagenesis was compared in the Ames assay using the Salmonella strain TA 98, HTHQ showed the greatest effect, followed by BHA, BHT, PG and TBHQ, in that order. In a rat hepatocarcinogenesis study, 6-week-old male F344 rats were given a single ip injection of 200 mg/kg bw of diethylnitrosamine (DEN) and starting 2 weeks later, groups of 15 animals received a diet containing 0.03% MelQx alone, MelQx together with each antioxidant at a dietary dose of 0.25%, each antioxidant alone, or basal diet alone for 6 weeks. Three weeks after the DEN injection, animals were subjected to 2/3 partial hepatectomy. Liver tissues obtained at partial hepatectomy were processed for the measurement of 8-hydroxydeoxyguanine (8-OHdG) and lipid peroxidation. The average number and areas of glutathione 5-transferase placenta! form (GST-P) positive foci were increased by the treatment with MelQx (27.2 ± 6.5 per cm2 and 3.17 ± 0.96 mm2/cm2, respectively). A significant decrease in these parameters was found with the simultaneous antioxidant treatment, HTHQ demonstrating the greatest effect, followed by BHA, BHT and TBHQ, and PG. Without MelQx, a weak increase in the number of foci was observed in the BHT treatment case. Examination of 8-OHdG levels in liver DNA, as well as malondialdehyde (MDA) and 4-hydroxyalkenals, did not reveal any inter-group variation. These results indicate that antimutagenic activity of antioxidants against MelQx roughly parallels their anticarcinogenic activity, with HTHQ as the most powerful chemopreventor, but that oxidative stress and antioxidative activity may not be responsible for MelQx-induced hepatocarcinogenesis and its inhibition, respectively