ESR1 and breast cancer: As for antiproliferative activity in the ER-negative MDA-MB-231 human breast cancer cells, drug conjugates 8 and 9 displayed, as expected, only micromolar IC50-values (2.67 μM and 2.48 μM) being 8-fold more potent than tamoxifen and (Z/E)-4-OH-tamoxifen (20 μM and 18 μM).