Immunogenic cell death (ICD) offers a method of stimulating the immune system to attack and remove cancer cells. We report a copper(II) complex containing a Schiff base ligand and a polypyridyl ligand, 4, capable of inducing ICD in breast cancer stem cells (CSCs). Complex 4 kills both bulk breast cancer cells and breast CSCs at sub-micromolar concentrations. Notably, 4 exhibits greater potency (one order of magnitude) towards breast CSCs than salinomycin (an established breast CSC-potent agent) and cisplatin (a clinically approved anticancer drug). Epithelial spheroid studies show that 4 is able to selectively inhibit breast CSC-enriched HMLER-shEcad spheroid formation and viability over non-tumorigenic breast MCF10 A spheroids. Mechanistic studies show that 4 operates as a Type II ICD inducer. Specifically, 4 readily enters the endoplasmic reticulum (ER) of breast CSCs, elevates intracellular reactive oxygen species (ROS) levels, induces ER stress, evokes damageassociated molecular patterns (DAMPs), and promotes breast CSC phagocytosis by macrophages. As far as we are aware, 4 is the first metal complex to induce ICD in breast CSCs and promote their engulfment by immune cells.

We investigate the anti‐cancer stem cell (CSC) properties of two copper(II) complexes containing 4,7‐diphenyl‐1,10‐phenanthroline and a S, N, X‐Schiff base ligand, where X=O (1) or S (2). Complex 1 was previously reported by us, and is the first metal complex to induce cytotoxic and immunogenic cell death of breast CSCs. Complex 2 is a structural analogue of 1, where the phenolate moiety (within the Schiff base ligand) is replaced by a thiophenolate group. Complex 2 kills bulk breast cancer cells and breast CSCs in the sub‐micromolar range, with reduced toxicity towards non‐cancerous epithelial breast cells. Remarkably, 2 is over 10‐fold more potent towards breast CSC spheroids than salinomycin (an established anti‐breast CSC agent) and cisplatin (a clinically approved anticancer drug). Complex 2 readily enters breast CSCs, accumulates in the cytosol, and increases intracellular reactive oxygen species (ROS) levels upon short exposure (1 h). The latter is likely to be the mechanism by which 2 induces breast CSC death.

Strategies for reducing CRS in CAR-T cells capitalize on the specificity between CAR-T cells, cytokines, and their receptors, the role of macrophages in cytokine release, and genetic constructs to ablate CAR-T function reversibly and irreversibly.