Depletion of estrogen receptor in human breast tumor cells by a novel substituted indole that does not bind to the hormone binding domain.

Steroidal antiestrogens appear to have at least two major modes of action in breast cancer cells, direct antagonism of estrogen binding to its receptor and depletion of estrogen receptors (ER) due to inhibition of dimerization of the receptor and a resultant destabilization of the receptor protein. In a search for other classes of compounds which would act as dimerization inhibitors, a novel substituted indole (8-{2-[1-(4-chlorobenzoyl)-5-hydroxy-2-methyl-1H-indol-3-yl]-acetylamino} octanoic acid butyl-methyl amide, MDL 101,906) was synthesized. Binding of the ER to its consensus response element (ERE) was apparently decreased in nuclear extracts from MCF-7 human breast cancer cell treated with MDL 101,906. This decreased binding was found to be due to depletion of ER based on direct measurement of ER using an enzyme-linked immunoassay. Other transcription factors were apparently unaffected by MDL 101,906 treatment. Whereas depletion of ER with a steroidal antiestrogen was almost complete after 3 h of treatment of MCF-7 cells, the effect of MDL 101,906 took significantly longer to occur, suggesting a fundamental difference in the mechanisms of action of the two drugs. This was also evident in the lack of binding of MDL 101,906 to the hormone binding domain of ER. MDL 101,906 treatment also caused depletion of ER mRNA in MCF-7 cells. Depletion of ER mRNA was noted by 3 h of drug treatment and was apparently almost complete after 24 h of treatment. Depletion of ER from MCF-7 cells led to a dose-dependent decrease in the expression of luciferase by an ERE-driven luciferase reporter gene assay system. The mechanism of MDL 101,906 appears to be unique and additional studies with this chemical class seem to be warranted to assess the potential for therapeutic utility.