PGR and breast cancer: ESE-16 was cytotoxic to all of the breast cancer cell lines investigated and revealed nanomolar GI50 concentrations in MDA-MB-231 (ER/PR-;Her2/Neu-), MCF-7 (ER/PR+; HER2/Neu-) and BT-20 (ER/PR-; HER2/Neu-) cells.