Inhibition of DMBA-induced mammary cancer by the monoterpene d-limonene

We investigated whether the addition of d-limonene to the diets of rats would modify the process of mammary tumor induction by 7,12-dimethylbenz[a]anthracene (DMBA). When diets containing 1000 or 10000 p.p.m. d-limonene were fed to rats from one week before DMBA administration to the end of the experiment (27 weeks post DMBA treatment), a significant reduction in mammary cardnogenesis was observed at each level. The inhibition of cardnogenesis was mainly due to an increase in latency; however, major differences in incidence could be seen during the follow-up period. For example, rats fed 10000 p.p.m. of d-limonene had a 72% reduction in mammary tumors when compared to controls at 18 weeks post DMBA treatment. In addition to inhibiting the appearance of mammary tumors, d-limonene was also found to cause the regression of frank mammary tumors. No toxicity was evident in these rats even at the highest d-limonene dose. Control and all treated rats gained weight at the same rate throughout the experiment. In addition, no differences were found in their peripheral blood counts (Hct, total white cell and differential count).