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 phytochemical indole-3-carbinol, and both compounds inhibit formation and/or growth of mammary tumors in rodents. In this study, we have investigated the aryl hydrocarbon receptor (AhR) agonist activity and inhibitory AhR-estrogen receptor crosstalk induced by the following methyl-substituted DIMs: 1,1′-dimethyl-, 2,2′-dimethyl-, 5,5′-dimethyl-, 6,6′-dimethyl-, and 7,7′-dimethylDIM and 1,1′,2,2′-tetramethylDIM. The six compounds bound to the rat cytosolic AhR in a transformation assay but, at concentrations ≤ 10 μM, exhibited minimal to non-detectable AhR agonist or antagonist activities associated with CYP1A1 induction. In contrast, the methyl-substituted DIMs inhibited estrogen-induced T47D human breast cancer cell growth and the four most active compounds (1,1′-, 2,2′-, 5,5′-dimethylDIM and 1,1′,2,2′-tetramethylDIM) inhibited one or more estrogen-induced responses in the 21-day-old female 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 administered (by gavage in corn oil) at a dose of 1 mg/kg/day (X10). 1,1′-DimethylDIM, 5,5′-dimethylDIM and 1,1′,2,2′-tetramethylDIM significantly inhibited mammary tumor growth, and this was not accompanied by changes in organ/body weights or histopathology. These studies demonstrate that methyl-substituted DIMs are selective AhR modulators (SAhRMs) with potential for clinical treatment of breast cancer.