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Human breast cancer cell line MCF-7 was used for activity evaluation. Stock cells of the MCF-7 clone E3 in the passage range of 160 to 190 were maintained in estrogen “rich” media and then withdrawn into an estrogen “free” media 7 days prior to seeding for growth response experiments.41 The mitogenic activity of each compound was evaluated at concentrations of 10^-12, 10^-11, 10^-10, 10^-9, 10^-8, 10^-7, and 10^-6 M. All cell growth determinations included positive controls treated with 10^-11 M E2 as well as negative controls devoid of estrogen treatment. In addition, a check of estrogen contamination of culture conditions for each experiment was made by including cells treated only with 10^-7 M ICI. Treatments with this “pure” antiestrogen served to validate the negative control flasks since only experiments with negative control cell counts within 10% of ICI treated flasks at the end of the treatment period (indicating no significant estrogen contamination) were considered valid assays of each compound’s mitogenic potential.