Estrogen receptor alpha (covalent antagonist)

Class

first-in-class selective estrogen receptor covalent antagonist (SERCA); covalently modifies Cys530 of ERα to enforce a unique antagonist conformation, active against both wild-type and mutant ERα; developed by H3 Biomedicine

Pathway

inactivates ERα-dependent transcription in wild-type and ESR1-mutant breast cancer models, including mutations that confer partial resistance to existing endocrine therapies, reducing tumor cell proliferation

Estrogen receptor alpha (ERα) is a nuclear hormone receptor that, upon binding estrogen, moves to the nucleus and switches on genes that drive proliferation of certain tissues, including a large fraction of breast cancers classified as ER-positive. Blocking ERα signaling is a long-established strategy in breast cancer, traditionally achieved with drugs that compete with estrogen for the receptor's binding pocket or that promote its degradation. A limitation of these approaches is that tumors can acquire mutations in the ESR1 gene encoding ERα, producing a receptor that stays active even without estrogen and that binds many existing antagonists less effectively, leading to treatment resistance. A covalent antagonist approach addresses this by forming a permanent chemical bond to a specific amino acid within the receptor's ligand-binding region, locking it into an inactive shape regardless of whether estrogen or the mutant conformation is present. This durability of inactivation is designed to maintain effectiveness against both unaltered and mutated forms of the receptor, extending the reach of endocrine therapy into settings where resistance has emerged. The broader rationale mirrors that of other ER-directed therapies: interrupting a key growth-driving transcriptional program in hormone-dependent breast cancer while sparing pathways unrelated to estrogen signaling.