ErSO

ErSO is an experimental small molecule anticancer drug that selectively targets estrogen receptor-positive (ER+) breast cancer cells. Its mechanism of action involves binding to the estrogen receptor (ERα) and activating the anticipatory unfolded protein response (a-UPR), a cellular stress pathway that is already upregulated in ER+ cancer cells. This activation leads to rapid and selective necrosis of tumor cells, causing them to swell and die, while sparing normal tissues. In preclinical studies, ErSO has demonstrated remarkable efficacy in eliminating both primary tumors and metastases in multiple organs—including bone, brain, liver, and lungs—in mouse models of human ER+ breast cancer. The compound was developed by researchers at the University of Illinois Urbana-Champaign as a novel approach distinct from traditional anti-estrogen therapies like tamoxifen or fulvestrant; it does not block estrogen signaling but instead exploits a vulnerability unique to ER+ tumor cells[1][2][6]. While still at an early stage of development with no current clinical approval, ErSO represents a promising proof-of-concept for targeting the unfolded protein response as an anti-cancer strategy.