Estrogen receptor (antagonist)

Class

next-generation selective estrogen receptor modulator (SERM); the active Z-isomer of endoxifen, the principal active metabolite of tamoxifen; also inhibits protein kinase C beta 1 (PKCβ1)

Pathway

blocks estrogen receptor (ERα/ERβ) signaling and promotes receptor degradation; PKCβ1 inhibition and retained activity against ESR1-mutant receptors address two distinct drivers of endocrine resistance; delivered as the active metabolite itself, bypassing the CYP2D6-dependent hepatic activation tamoxifen requires

The estrogen receptor (ER) is a nuclear hormone receptor that, when activated by estrogen, turns on genes that drive cell division. In many breast cancers, tumor growth remains dependent on this signaling, making ER a central pharmacological target. Antagonists that prevent estrogen from activating the receptor, and that also promote its breakdown, can slow or halt tumor growth. This mechanism describes an antagonist that is itself the active form of an older, well-established antiestrogen drug, rather than a precursor that the body must convert. That distinction matters because the parent drug's effectiveness depends on a liver enzyme that varies widely between people; some individuals convert it inefficiently, which can blunt clinical benefit. Delivering the active form directly is intended to make the drug's effect more consistent across patients. This particular molecule also retains activity against certain receptor mutations that can emerge during treatment and inhibits a signaling kinase linked to resistance, addressing two distinct routes by which tumors can stop responding to estrogen-lowering therapy. Together, these properties aim to extend how long endocrine therapy remains effective in hormone-receptor-positive breast cancer.