A Novel Dual-Targeting PROTAC Overcomes Endocrine Resistance by Engaging Orthosteric and Allosteric Sites of Estrogen Receptor α

Estrogen receptor α (ERα) remains a pivotal therapeutic target for ER-positive (ER+) breast cancer, yet resistance to endocrine therapies driven by ligand-binding pocket (LBP) mutations demands novel strategies. To integrate the complementary strengths of binding modalities of the orthosteric LBP and the allosteric coactivator binding site (CBS) against endocrine resistance, we designed a series of ERα LBP/CBS dual-targeting proteolysis-targeting chimeras (PROTACs). The lead compound P5 demonstrated high binding affinities for LBP and CBS sites, potent antiproliferative activity, and efficient ERα degradation efficacy across a panel of breast cancer cell lines via the ubiquitin-proteasome system. Mechanistically, P5 triggered S-phase cell cycle arrest and apoptosis in LCC2 cells. In vivo, P5 exhibited significant tumor growth inhibition in the LCC2 xenograft model, with efficacy comparable to fulvestrant, and a favorable safety profile. Our findings validate the LBP/CBS dual-targeting PROTAC strategy as a promising avenue for overcoming resistance in ER+ breast cancer.