Evaluation of a luciferase-based reporter assay as a screen for inhibitors of estrogen-ERα-induced proliferation of breast cancer cells

Estrogens, acting through estrogen receptor α (ERα), stimulate breast cancer proliferation, making ERα an attractive drug target. Since 384-well format screens for inhibitors of proliferation can be challenging for some cells, inhibition of luciferase-based reporters is often used as a surrogate end point. To identify novel small-molecule inhibitors of 17β-estradiol (E₂)-ERα-stimulated cell proliferation, we established a cell-based screen for inhibitors of E₂-ERα induction of an estrogen response element (ERE)₃-luciferase reporter. Seventy-five "hits" were evaluated in tiered follow-up assays to identify where hits failed to progress and evaluate their effectiveness as inhibitors of E₂-ERα-induced proliferation of breast cancer cells. Only 8 of 75 hits from the luciferase screen inhibited estrogen-induced proliferation of ERα-positive MCF-7 and T47D cells but not control ERα-negative MDA-MB-231 cells. Although 12% of compounds inhibited E ₂-ERα-stimulated proliferation in only one of the ERα-positive cell lines, 40% of compounds were toxic and inhibited growth of all the cell lines, and ∼37% exhibited little or no ability to inhibit E₂-ERα-stimulated cell proliferation. Representative compounds were evaluated in more detail, and a lead ERα inhibitor was identified.