Y134 is an estrogen receptor inhibitor that is selective and orally active, exhibiting strong antagonist activity against ERα and Erβ with Ki of 0.09 nM and 11.31 nM, respectively. The selectivity of Y134 for ERα is 121.1 times higher than that for ERβ. Y134 inhibited the proliferation of ER positive human breast cancer cells stimulated by estrogen.

ERβ:11.31 nM (Ki), ERα:0.09 nM (Ki)

Y134 displays robust antagonistic effects on estrogen receptors (ERs) in CV-1 cells co-transfected with plasmids carrying ERα or ERβ along with estrogen-response element-driven luciferase, resulting in a significant decrease in luciferase activity.[1]
Treatment with Y134 (0.01 nM-10 μM; 6 d) effectively suppresses the proliferation of ER-expressing breast cancer cells (MCF-7 and T47D) stimulated by estrogen.[1]

Administration of Y134 (1-3 mg/kg/day; p.o. for 3 days) completely eliminates the estrogen-induced growth of terminal end buds (TEB) in the mammary glands of ovariectomized rats. Y134 exhibits a dose-dependent inhibition of uterine cell proliferation induced by estrogen (E2). [1]