ERX-11, also known as ERα coregulator-binding modulator-11, is a novel antiestrogen and experimental hormonal antineoplastic agent which is being researched for the potential treatment of estrogen receptor-positive breast cancer.

[1][2] It is not a competitive antagonist of the estrogen receptor (ER) like conventional antiestrogens such as tamoxifen or fulvestrant; instead of binding to the ligand-binding site of the ER, ERX-11 interacts with a different part of the ERα and blocks protein–protein interactions of the ERα with coregulators that are necessary for the receptor to act and regulate gene expression.

[1][2] The compound mimics a nuclear receptor binding motif that appears to be critical for the interaction of the ERα with its coactivators.

[1][2] ERX-11 blocked estradiol-induced proliferation in 8 of 8 ER-positive breast cancer cell lines, with IC50Tooltip half-maximal inhibitory concentration values ranging between 250 nM and 500 nM, and was as effective as tamoxifen and fulvestrant in inhibiting the growth of the ZR-75 and MCF-7 breast cancer cell lines.

[1][2] In addition, it disrupted interactions between the ERα and many ERα-binding coregulators not affected by conventional antiestrogens like tamoxifen (33 of 88 proteins, or 37.5%).