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

Abstract

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 <10μM, 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 100mg/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 1mg/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.