Approximately 75% of breast cancers are estrogen receptor alpha-positive (ERα+), and targeting ERα directly with ERα antagonists/degraders or indirectly with aromatase inhibitors is a successful therapeutic strategy. However, such treatments are rarely curative and development of resistance is universal. We recently reported ErSO, a compound that induces ERα-dependent cancer cell death through a mechanism distinct from clinically approved ERα drugs, via hyperactivation of the anticipatory unfolded protein response. ErSO has remarkable tumor-eradicative activity in multiple ERα+ tumor models. While ErSO has promise as a new drug, it has effects on ERα-negative (ERα-) cells in certain contexts. Herein, we construct modified versions of ErSO and identify variants with enhanced differential activity between ERα+ and ERα- cells. We report ErSO-DFP, a compound that maintains antitumor efficacy, has enhanced selectivity for ERα+ cancer cells, and is well tolerated in rodents. ErSO-DFP and related compounds represent an intriguing new class for the treatment of ERα+ cancers.