Methyl-substituted diindolylmethanes as inhibitors of estrogen-induced growth of T47D cells and mammary tumors in rats

Diindolylmethane (DIM) is formed by acid catalyzed dimerization of the phyt ochemical indole-3-carbinol, and both compounds inhibit formation and/or gr owth of mammary tumors in rodents. In this study, we have investigated the aryl hydrocarbon receptor (AhR) agonist activity and inhibitory AhR-estroge n receptor crosstalk induced by the following methyl-substituted DIMs: 1,1' -dimethyl-, 2,2'-dimethyl-, 5,5'-dimethyl-, 6,6'-dimethyl-, and 7,7'-dimeth ylDIM and 1,1',2,2'-tetramethylDIM. The six compounds bound to the rat cyto solic AhR in a transformation assay but, at concentrations < 10 muM, exhibi ted minimal to non-detectable AhR agonist or antagonist activities associat ed with CYP1A1 induction. In contrast, the methyl-substituted DIMs inhibite d estrogen-induced T47D human breast cancer cell growth and the four most a ctive compounds (1,1'-, 2,2'-, 5,5'-dimethylDIM and 1,1',2,2'-tetramethylDI M) inhibited one or more estrogen-induced responses in the 21-day-old femal e B6C3F1 mice at a dose of 100 mg/kg/day (X3). Induction of hepatic CYP1A1- dependent activity was not observed at this high dose. The antitumorigenic activity of these compounds was examined in 7,12-dimethylbenz[a]anthracene- induced rat mammary tumor model in which the DIM analogs were orally admini stered (by gavage in corn oil) at a dose of 1 mg/kg/day (X10). 1,1'-Dimethy lDIM, 5,5'-dimethylDIM and 1,1',2,2'-tetramethylDIM significantly inhibited mammary tumor growth, and this was not accompanied by changes in organ/bod y weights or histopathology. These studies demonstrate that methyl-substitu ted DIMs are selective AhR modulators (SAhRMs) with potential for clinical treatment of breast cancer.