ER degrader 10 (Compound 51) is a selective, orally active degrader and antagonist for estrogen receptor ( ER ) with a DC 50 of 0.43 nM and an IC 50 of 0.56 nM. ER degrader 10 inhibits the proliferation of ER-positive cells with IC 50 s of 0-15 nM. ER degrader 10 exhibits a weak inhibitory activity against hERG channel with an IC 50 >40 μM. ER degrader 10 is blood-brain barrier penetrable with a brain/plasma ratio (Kp) of 3.05. ER degrader 10 exhibits antitumor efficacy in mice model.